20111013 Ver 1_Hydraulic Assessment_20120106

Strickland, Bev
From:
Mcmillan, Ian
Sent:
Friday, January 06, 2012 3:00 PM
To:
Strickland, Bev
Subject:
FW: FW:]RE: hydraulic assessment
Attachments:
Sweet and Geratz.pdf; Hupp 2000.pdf; simon channel evolution model.pdf
DWQ Project Number 11 -1013
Ian J. McMillan, PWS, GISP
NCDENR/Division of Water Quality - Wetlands and Stormwater Branch
1650 Mail Service Center
Raleigh, NC 27699 -1650
Office: (919) 807 -6364
Fag: (919) 807 -6494
Email: ian.mcmillan.denr(a mmail.com SENT TO MY PHONE
Email: ian.mcmillan(a�,ncdenr.gov
E -mail correspondence to and from this address may be subject to the
North Carolina Public Records Law and may be disclosed to third parties.
From: Heather [mailto:riverkeeper @ptrf.org]
Sent: Friday, January 06, 2012 1:10 PM
To: Mcmillan, Ian
Subject: FW:]RE: hydraulic assessment
For your consideration.
Heather Jacobs Deck
Pamlico -Tar RIVERKEEPER
Pamlico -Tar River Foundation
P.O. Box 1854
Washington, NC 27889
(252) 946 -7211 (office)
(252) 946 -9492 (fax)
(252) 402 -5644 (cell)
www.ptrf.org
Follow us on Facebook: http : / /www.facebook.com /pamlicotar
Follow us at Twitter: www.twitter.com /ptrfriverkee per'
From: O'Driscoll, Michael [mailto:ODRISCOLLM @ecu.edu]
Sent: Tuesday, January 03, 2012 4:09 PM
To: Heather
Subject: [Norton AntiSpam]RE: hydraulic assessment
Hi Heather,
I took a quick look, I think generally what could happen if the UT2 segments were to receive too much discharge they
may respond by widening and deepening (especially if new sediment inputs do not increase with the discharge

increases). Usually if the deepening occurs then the slope will increase and a knickpoint will develop that would result in
erosion that occurs and the knickpoint would migrate upstream, just like a waterfall moves upstream over geologic time.
This has happened a bit in Greenville, where stormwater outfalls have deepened and widened some channels. Some of
the literature seems to suggest that these disturbances may take a long time to work through the system and so it could
be decades before the stream channel and its sediment and discharge regime re- equilibrate.
One thing that is a bit inaccurate in the report is that they say bankfull flooding should occur around every 1.1 -1.3 years,
those numbers are for streams in Piedmont and Mountain settings and are too high for the Coastal Plain. There is this
paper by Sweet and Geratz (attached) where they say that bankfull flooding for similar INC Coastal Plain streams occurs
several times a year. Also if streams are channelized they may be able to handle more water than a similar natural
channel, but this might put the stream segments further downstream at a greater flood risk, since the floodplain is not
really functioning along the channelized reaches. If channels are channelized they may have steeper banks than is
normal and if greater discharge is added those banks may have a tendency to slump and the bank material may add
sediment to downstream segments. If there is vegetation on the banks that can stabilize them, but if not they would
likely contribute some more sediment.
The best model for channel evolution along channelized streams, I think is what was developed by Simon (and later
Hupp) for some channelized streams in Tennessee. These papers I've attached may help. We have a new faculty
member, Eban Bean, who is a civil engineer with hydrology experience starting this semester in the Engineering
Program, he may have some insights if you need more of an engineering assessment. Also, Scott Lecce in Geography is a
fluvial geomorphologist, it might be worth asking him since he has some teaching and research experience on this topic.
I was going to commit my environmental class this semester to work on some water conservation projects on campus
this semester, but maybe if this is still an issue next fall I could set something up for them that would help with this
problem, since that is when I do the hydrology class. It might work well in one of Scott or Eban's classes, if they offer a
fluvial geomorphology or fluid dynamics class in the spring.
Hope this helps,
Mike
From: Heather [mailto:riverkeeper @ptrf.org]
Sent: Tuesday, January 03, 2012 3:16 PM
To: O'Driscoll, Michael
Subject: hydraulic assessment
Hey Mike,
I know you are super busy, but didn't know if you had any time to review a the hydraulic assessment of the receiving
streams for this proposed martin Marietta mine discharge. I've attached it. In any way, if you don't have time, this might
be a good learning tool for one of your classes..?
If you do have time, just wanted a second opinion on the methodology, conclusions... everything look sound
(assumptions, etc). or was something missed? I'm sure it's accurate, but always like to check.
Thanks!
Heather Jacobs Deck
Pamlico -Tar RIVERKEEPER
Pamlico -Tar River Foundation
P.O. Box 1854
Washington, INC 27889

(252) 946 -7211 (office)
(252) 946 -9492 (fax)
(252) 402 -5644 (cell)
www.ptrf.org
Follow us on Facebook: http : / /www.facebool<.com /pamlicotar
Follow us at Twitter: www .twitter.com /ptrfriverkeeper"

HYDROLOGICAL PROCESSES
Hydrol. Process. 14, 2991 -3010 (2000)
Hydrology, geomorphology and vegetation of Coastal
Plain rivers in the south - eastern USA
Cliff R. Hupp*
US Geological Survey, Reston, Virginia 20192, USA
Abstract:
Rivers of the coastal plain of the south - eastern USA are characteristically low- gradient meandering systems that
develop broad floodplains subjected to frequent and prolonged flooding. These floodplains support a relatively
unique forested wetland (bottomland hardwoods), which have received considerable ecological study, but
distinctly less hydrogeomorphological study. The hydroperiod, or annual period of inundation, largely controls
the development of characteristic fluvial landforms, sediment deposition and vegetation distribution patterns.
Order -of- magnitude differences in wetted perimeter, width /depth, suspended sediment load and hydraulic
roughness may exist between `dry' in- channel seasons and the hydroperiod. Substantial sediment (and adsorbed
contaminants) retention and storage through lateral and vertical accretion is common (where not heavily impacted
by flow regulation) along these Coastal Plain rivers. The present chapter summarizes our current understanding of
the hydrology, fluvial geomorphology, general and local sedimentation patterns, and related plant ecological
patterns of these Coastal Plain bottomlands.
KEY WORDS coastal plain; meandering rivers; fluvial geomorphology; fluvial landforms; sediment deposition;
woody vegetation
INTRODUCTION
The Coastal Plain physiographic province of the USA (Figure 1) lies almost entirely in the south -east. It
covers an area of about 1.2 million square kilometres (slightly larger than the combined area of Belgium,
France, Germany and the UK). This topographically distinct lowland is bounded on the east and south by
the Atlantic Ocean and the Gulf of Mexico, respectively, and landward on the west and north, at some places
less distinctly, by the Piedmont and Ozark highlands (Figure 1). Bottomlands typically are broad, alluvial
features with low gradients, meandering streams, most of which terminate downstream in tidal estuaries.
These Coastal Plain river systems have received noticeably less hydrological study than higher gradient
Piedmont and montane river systems, where such concepts as bankfull discharge and flood - return interval
were developed (Leopold et al., 1964). The floodplains of Coastal Plain rivers are typically inundated every
year for prolonged (months in some cases) periods. The forests (bottomland hardwood systems, including
southern deep -water swamps), however, have received considerable ecological study, yet the linkages
between the fluvial geomorphological processes and forest ecology remain poorly understood.
Fluvial geomorphology refers to study of the surficial landscape, geomorphological forms, and physical
processes developed or mediated by the action of flowing water, most typically in the form of streamflow.
Most geomorphological work in fluvial systems involves the erosion, entrainment, transport, deposition and
storage of sediment. At least 90% of all sediment eroded from uplands is trapped in alluvial systems before
reaching saltwater (Meade et al., 1990). Detailed spatial and historical analyses of sediment trapping, storage
* Correspondence to: C. R. Hupp, US Geological Survey, Reston, Virginia 20192, USA.
This article is a US government work and is in the public domain in the United States Received 4 ,Tune 1999
Accepted 31 August 1999

2992
Fall
Line
C. R. HUPP
0 300 600 km
Embayed
Section
1
Atlantic
Coastal Plain
Atlantic Ocean
Figure 1. The coastal plain of south - eastern USA. Potential extent of bottomland hardwood forest is shown along major streams. Note
that the BLH forests nearly match the inland extent of the Coastal Plain delineated by the Fall Line
and retention time, at both large and small scales, generally are lacking, possibly with the exception of the
lower Mississippi Valley (Saucier, 1994). Alluvial processes create and maintain a variety of landforms,
including floodplains that support bottomland hardwood (BLH) forest ecosystems in the south - eastern
USA. These forests interact with hydrological and fluvial geomorphological processes and forms (including
binding alluvium through root development and enhancing deposition by increasing stream roughness) such
that the intrinsic character of each is at least partly the result of the other. The purpose of the present paper is
to provide an overview of our current understanding of the form and process linkage between lowland
meandering streams on the Coastal Plain of the south - eastern USA and their characteristic BLH forests.
Several definitions of BLH have been published recently (Wharton et al., 1982; Mitsch and Gosselink,
1993; Sharitz and Mitsch, 1993; Shepard et al., 1998). All definitions confine BLH to the riparian zone, thus
associating BLH systems with the bottomland adjacent to streams. The riparian zone can be defined as that
part of the landscape supported by and including recent fluvial landforms and inundated or saturated by the
bankfull discharge (Hupp and Osterkamp, 1996). Although this definition is not as inclusive of bottomland
features as others (e.g. Malanson, 1993) it is quantitative in that the bankfull discharge typically occurs at
least once every 1 to 3 years (Leopold et al., 1964). Bottomland hardwood systems are usually considered
forested wetlands; many definitions also require inundation or saturation of the soil at least once annually
during the growing season ( Mitsch and Gosselink, 1993). The riparian zone, in particular the floodplain,
reaches its greatest development in the USA along the many low- gradient rivers originating on or flowing
across the Coastal Plain physiographic province. Thus, BLH systems are, in large part, an extensive and
characteristic feature of south - eastern rivers (Figure 1). Here and throughout the text the term vegetation
refers to the woody vegetation of BLH systems; it is beyond the scope of the present paper to discuss
herbaceous vegetation, although many of the interpretations would apply as well.
Since the late 1970s there has been considerable focus on the function and value of wetlands (Greeson et
al., 1979; Carter, 1986, 1996; Landin, 1992), summarized recently by Brinson and Rheinhardt (1998).
Riparian wetlands (including BLH) have been cited to be of particular value for several reasons.
Biologically, the riparian zone is the ecosystem with the greatest biodiversity in most regions of the world
(Nilsson, 1992; Naiman et al., 1993). Riparian areas also provide critical habitat for many plants and game
and non -game species of fish and wildlife. Environmentally, these wetlands function as an important, if not
Hydrol. Process. 14, 2991 -3010 (2000)

COASTAL PLAIN RIVERS 2993
critical, natural element in the maintenance of water quality. Properly functioning BLH systems annually
trap and store enormous amounts of sediment, nutrients and contaminants (Hupp et al., 1993; Kleiss 1996).
Unfortunately, BLH systems have decreased tremendously in areal extent, principally through conversion
to agriculture (Mitsch and Gosselink, 1993; Kress et al., 1996) following World War 11. Missouri, Arkansas
and Louisiana alone lost 21 000, 57 000 and 24 000 ha, respectively, between 1960 and 1975 (Turner et al.,
1981). Their continuing rapid loss is of imminent concern because of concomitant losses of water quality and
habitat functions (Sharitz and Mitsch, 1998). In addition, the majority of remaining BLH forests have been
modified as a result of lumbering or agricultural usage and /or more recent activities such as highway
construction and channelization (Bazemore et al., 1991; Hupp, 1992; Lockaby and Walbridge, 1998). Many
of these forests, particularly those near the Fall Line (Figure 1) along the Atlantic Coastal Plain, experienced
substantial aggradation in the eighteenth and nineteenth centuries following deforestation and poor
agricultural practices by European settlement ( Wolman, 1967; Costa, 1975; Jacobson and Coleman, 1986).
Subsequent reforestation and better agricultural practices substantially reduced sediment loads, which has
led to channel incision leaving floodplains and terraces relatively `high and dry' along many east coast
streams. Heightened post - settlement deposition has been documented on at least two Coastal Plain streams,
namely along the Congaree River in South Carolina (Patterson et al., 1985) and along the Roanoke River in
North Carolina (Hupp et al., 1999a). The lower Roanoke River aggraded by as much as 4 m near the Fall
Line; this deposit attenuates downstream toward the estuary but nevertheless affects channel depths, point
bar development, levee formation and vegetation patterns (Hupp et al., 1999b).
GEOLOGICAL CONTROLS
The Coastal Plain (including the subaqueous continental shelf) from the Chesapeake Bay to eastern Texas
(Figure 1) is largely the result of sediment deposition, both alluvial and marine, from the adjacent eroding
mountains and Piedmont since at least the Cretaceous Period. This broad plain has been sculpted by
hydrological and fluvial geomorphological processes that vary in their effect in response to changes in sea
level and climate. During oceanic regressions (sea -level retreat to a low stand) streams on the coastal plain
and adjacent Piedmont tended to entrench into their valley (degradation) owing to higher stream gradients.
Oceanic transgressions (sea -level rise to a high stand) with lower stream gradients typically led to valley
filling and widening (aggradation). Coupling this rather simple conceptual model with concomitant
variation in climate and tectonics has led to a deceptively complex modern landscape. For instance, many of
the rivers on the Atlantic Coastal Plain are underfit, that is, the present channel does not carry sufficient
discharge or sediment to have created the broad alluvial floodplain within which the river now flows. This
underfit condition may result from stream capture (loss of drainage area), or perhaps more typically, from a
reduction in rainfall. Rainfall during a pluvial period 18 000 to 10 000 years ago may have been 18 times
greater than at present (Dury, 1977). Melting continental glaciers undoubtedly increased discharge and
sediment load on some of the larger rivers. Further, Pleistocene frost action and variation in plant cover
probably made available considerable amounts of sediment for transport and deposition.
The modern Coastal Plain bottomlands of most south - eastern rivers are largely a result of fluvial processes
during the last low stand (about 14 000 to 18 000 years ago) and the subsequent oceanic transgression toward
a new high stand. The lower Mississippi River incised into its bottomlands (the largest contiguous area of
BLH), during the last low stand, upstream to about the vicinity of Vicksburg, Mississippi. This relatively
short distance is probably due, in part, to a concomitant lengthening of the channel (Saucier, 1994). With
rising sea level over the past several thousand years, the bottomlands rapidly filled, widened, and formed
large anabranches associated with deltaic processes, forming the aggrading Atchafalaya Basin. Smaller rivers
of the Gulf Coastal Plain probably did not incise deeply as a result of a high sediment load and relatively low
discharges during the past low stand (Saucier, 1994). Many of these streams, such as those on the Atlantic
Coastal Plain, are now underfit following a pluvial period (18 000 to 10 000 years ago, Dury, 1977) and
subsequent oceanic transgression and aggradation. Additionally, several stream captures may have occurred,
Hydrol. Process. 14, 2991 -3010 (2000)

2994 C. R. HUPP
including the Cache River, AR (Bennett and Saucier, 1988), which contributed to the commonly underfit
nature of many BLH systems. Similar processes affected streams of the Atlantic Coastal Plain, excepting
major tributaries of the Chesapeake Bay. All large tributaries of the Chesapeake Bay that arise in the
Appalachian Mountains incised to the Fall Line and remain in an embayed condition today, possibly due
largely to the Bay's principal tributary, the Susquehanna River. The extremely high discharge and ocean
proximity of the Susquehanna River may have lowered the regional base level to such a degree during the last
glacial retreat that aggradation in the other major Bay tributaries has not kept pace with rising sea level.
However, streams of the region that arise on the piedmont and coastal plain but with substantially less
discharge and erosive capacity than the Susquehanna River did not incise like the major tributaries during
the last low stand and now support BLH systems on relatively broad bottomlands.
HYDROLOGIC PROCESSES
Water is of singular importance in BLH systems, as in all wetland systems, as the medium for biogeochemical
processes (in living and physical systems), as an ecological limiting factor, and as the force that controls the
movement and storage of sediment and associated material. All wetland functions can be described entirely
or in part by hydrological processes (Nestler and Long, 1994) and agreement on the primary influence of
hydrology in wetland ecosystems is pervasive in wetland hydrological literature (Carter, 1986, 1996; Walton
et al., 1996). Yet our basic understanding of most hydrological processes remains somewhat tenuous owing
in part to the relatively recent emergence of hydrology as a science and because most hydrological processes
are subject to the vagaries of precipitation and climate. Slight hydrological changes may result in only loosely
predictable, yet often substantial, wetland responses (Mitsch and Gosselink, 1993). Defining the relation
among hydrological processes and other wetland phenomena in BLH systems is further hampered by the
lack of more than a few rigorous studies in this wetland type (Walton et al., 1996).
A strong seasonal variation in discharge occurs along most medium and large streams on the south-
eastern Coastal Plain (Figure 2). Annual variation in evapotranspiration and rainfall produce two distinctly
different hydrological seasons; a low -flow season from about June through to October, when streamflow is
confined largely to a meandering main channel and a high -flow season from about November through to
May, when large parts of the wooded bottomland may be inundated. This period of inundation is referred to
70
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Figure 2. Mean - monthly discharge (1963 to 1977) for the Cache River near Patterson, Arkansas. The discharge necessary to flood
sloughs (8.4 m3 /s) and to inundate the floodplain (52.5 m3 /s) is indicated. Floodplains along this river are typically inundated from late
December until mid -April
Hydrol. Process. 14, 2991 -3010 (2000)

COASTAL PLAIN RIVERS
2995
Hydrol. Process. 14, 2991 -3010 (2000)
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Hydrol. Process. 14, 2991 -3010 (2000)

2996
C. R. IICPP
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Figure 3. (A) Obvious annual high -water mark (darkened tree bases) in a southern deep -water swamp (Taxodium —Nyssa forest),
Chickahominy River, Virginia. (B) Bottomland hardwood forest subjected to high sedimentation rates along the Big Sandy River,
Tennessee. Note `buried' tree trunks
Hydrol. Process. 14, 2991 -3010 (2000)

COASTAL PLAIN RIVERS 2997
as the hydroperiod (Figure 2). Order -of- magnitude differences in wetted perimeter, width /depth ratio and
roughness may occur seasonally along the same reach. This bimodal hydrology leaves an indelible signature
on the biotic and abiotic landscape (Figure 3A and B) and complicates environmental interpretations.
Two major types of streams, classified according to suspended- sediment load, form the bottomlands that
support BLH systems. They are:
(1) alluvial rivers that arise in uplands (typically mountainous areas or the Piedmont) and transport
substantial amounts of eroded mineral sediment;
(2) black -water rivers that arise on the Coastal Plain typically with low gradients that transport relatively
little mineral fines (Table I).
Alluvial rivers can be subdivided further into brown -water systems and red -water systems. The former
usually are large systems with initially high gradients arising in the mountains, whereas the latter tend to be
smaller, lower gradient systems that arise in the piedmont and derive their red colour from iron oxides that
characterize the Piedmont residuum. Black -water systems tend to be smaller than either brown- or red -water
systems, principally as a result of their limited potential to develop large watersheds. Water in black -water
systems, with little gradient, flows relatively slowly and has limited ability to erode sediment. This slow
moving water leaches tannins from the typically highly organic bottomlands or riparian wetlands, which
stains the water and lowers the pH ( from 7 to 6, Wharton et al., 1982) relative to alluvial systems. However,
drainage may affect the mineral content of black -water streams such that better drained systems develop soils
with a relatively high mineral content. The origin of streams flowing on the Coastal Plain also affects the
dissolved load (chemical characteristics). Alluvial rivers have relatively high concentrations of inorganic
ions, including several macronutrients, and relatively low concentrations of total organic carbon (Table I);
the converse is true for most black -water rivers (Wharton and Brinson, 1979). Thus, pH, hardness and
specific conductance tend to be higher in alluvial rivers than in black -water streams.
Alluvial rivers develop a fairly abrupt reduction in gradient after crossing the Fall Line and contacting the
relatively flat Coastal Plain. Coincident with reduction in gradient are greater frequencies of overbank flows,
a flatter hydrograph and longer periods of inundation. These tendencies are partly due to the relict nature of
the Coastal Plain and the stream - regime shift, without a reduction in discharge, from high energy, at least
partly bedrock - controlled, relatively straight upland reaches to low- energy meandering reaches. The broad
bottomlands of the Coastal Plain often do not fit empirical hydrological concepts such as bankfull discharge
(Leopold et al., 1964), developed for upland streams. These floodplains tend to be flooded much more
frequently than every year and a half and for much longer durations. During leaf -off seasons with high
discharges and little transpiration some BLH systems are regularly inundated for months each year,
effectively increasing channel width as much as an order of magnitude during the hydroperiod. Streamflow
during the hydroperiod is less meandering and in intimate contact with the riparian zone that supports BLH
forests. Many functional attributes of BLH systems including sediment and associated - material trapping
(Figure 3), are most prominent during the hydroperiod. Unfortunately anthropogenic features such as
nineteenth century agricultural levees may reduce the effective floodplain surface area and reduce residence
time of out -of -bank flow by increasing velocities.
GEOMORPHOLOGICAL FEATURES
Fluvial geomorphological processes create a variety of widely recognized landforms, from small channel
bedforms to extensive floodplains. The latter typically support BLH systems in the south - eastern Coastal
Plain. Floodplains, like most fluvial landforms, are dynamic features almost constantly eroding in some
places while aggrading in others. Meandering channel dynamics (typical of most streams in BLH systems)
provide the energy necessary to erode and transport floodplain sediments. Meanders typically extend,
eroding accreted sediments until they are cut -off by an avulsion (channel cut -off) leaving an ox -bow lake and
a new channel (Figure 4). Entire meander loops, additionally, tend to migrate downstream. Thus, over
Hydrol. Process. 14, 2991 -3010 (2000)

2998
C. R. HUPP
Figure 4. Generalized fluvial landforms on a coastal plain bottomland. Note greater levee development along straight reaches and on
the downvalley side of the stream
geomorphological time, nearly all alluvium in BLH systems is in a state of flux, even though the transport
distance for eroded alluvium may not extend much past one meander loop downstream (Saucier, 1994).
Floodplains on the Coastal Plain tend to have net sediment storage during periods of high or rising sea
level, such as the conditions over the past several thousand years. Floodplains aggrade in two ways. First, by
lateral accretion or point -bar extension, where coarse (sand) material is deposited on the inside bank of
channel bends; a corresponding volume is typically eroded on the opposite, or cut bank (Figure 4). Second,
by the vertical accretion of suspended sediment (typically fines) over the floodplain during overbank flows.
Lateral accretion is an episodic process that occurs during high flows, building the point bar into a typically
crescent - shaped ridge. Over time, a series of high -flow events produce the ridge- and -swale topography
Transport of sediments and contaminants
Water level from river channel to floodplain
during
hydroperiod
1
Flood lain LeveeFlooding Vertical accretion
Erosion of
cutbanks Lateral accretion
on point bar
Water level
during low
flow
Figure 5. Transport paths of sediment and associated contaminants for both lateral and vertical accretion
Hydrol. Process. 14, 2991 -3010 (2000)

COASTAL PLAIN RIVERS 2999
(Figure 4) associated with meander scrolls. The establishment of ruderal woody vegetation during
intervening low -flow periods on fresh scroll surfaces creates bands of increasingly younger vegetation toward
the main channel (McKenney et al., 1995). These bands of vegetation may accentuate the ridge- and -swale
topography by creating distinct microdepositional environments during subsequent high flows, but the
hydraulics necessary to produce meander scroll topography and the role of vegetation in its development are
poorly understood (Nanson, 1980, 1981).
Fine - sediment deposition is facilitated by the typically striking reduction in flow velocity as water leaves
the main channel and enters the hydraulically rough floodplain environment (Figure 5). As rising flood
waters overtop the bank the coarse (or heavy) sediment is deposited first and relatively rapidly create natural
levees along the floodplain margin. Levees tend to be most pronounced along relatively straight reaches
between meanders and are often the highest ground on the floodplain. Levees are sometimes breached by
streamflow, resulting in a crevasse splay that may insert coarse material deep into the otherwise fine- grained
floodplain (Figure 4). Levee development and the breaches that form are poorly documented in the
literature, yet are critical in the understanding of the surface -water hydrology of most coastal plain
bottomlands. Levee height and breaches strongly affect the hydroperiod (and thus, sedimentation dynamics)
in systems dominated by surface -water flow (Patterson et al., 1985). The levee surface usually dips gently
away from the channel into the bottomland, where the surface may be extremely flat. Superimposed on this
flat bottom are internal drainage networks, overflow channels and abandoned main channels or ox -bows
that remain wet after the hydroperiod and support the more hydric BLH species. Slight differences in
elevation associated with the above floodplain geomorphological features and large woody debris (LWD)
create a complex pattern of microsite velocity regimes during the hydroperiod that ultimately affect intrasite
sedimentation regimes. Also highly correlated with these variations in elevation are the distributional
patterns of many BLH plant species (Wharton et al., 1982; Sharitz and Mitsch, 1993).
WATER QUALITY
Sediment trapping
Suspended sediment may be the most important water - quality concern in the USA today (US EPA, 1994).
Increases in suspended sediment directly affects aquatic biota by coating vegetation and clogging gills of
invertebrates and fishes. Indirectly, increased suspended sediment changes the habitat from more coarsely
grained aquatic environments to highly silted environments. Further, increased suspended sediment may
lead to high sediment - deposition rates in critical riparian areas, thus damaging living resources through
burial and suffocation. Perhaps most importantly, suspended sediment is the transport medium for
hydrophobic forms of nutrients and pesticides, and most trace elements (Horowitz, 1991). Mean values of
suspended sediment, percentage of suspended sediment finer than 0062 mm (clay), and dissolved and total
phosphorus, nitrogen and carbon are provided in Table I for a selected set of Coastal Plain streams that
support BLH systems along their lower reaches (Alexander et al., 1998).
Deposits of fine sediment typically contain large concentrations of adsorbed, associated contaminants
(particularly nutrients, trace elements and hydrophobic pesticides) from agriculture and urban areas
(Johnston et al., 1984; White and Tittlebaum, 1985; Phillips, 1989a; Puckett et al., 1993). This sediment and
contaminant- trapping function of wetlands is commonly acknowledged (Kadlec and Kadlec, 1979;
Lowrance et al., 1984; Phillips, 1989b; Brinson, 1993; Hupp et al., 1993; Lowrance et al., 1995; Brinson et al.,
1995; Kleiss, 1996), despite limited understanding of the transport and deposition of sediment and
associated contaminants and the lack of consistent mass - balance studies (Boto and Patrick, 1979; Winter,
1981; Carter, 1986; Labaugh, 1986; Mitsch and Gosselink, 1993). Biogeochemical cycles within forested
wetlands are particularly complex and difficult to study (Walbridge and Lockaby, 1994; Lockaby and
Walbridge, 1998). Although alluvial material may be considered in transit over geomorphological time, most
bottomlands, especially those on the Coastal Plain, exhibit net aggradation through sediment deposition
Hydrol. Process. 14, 2991 -3010 (2000)

3000 C. R. HUPP
Table 11. Mean sediment deposition rates (mm /year) for Coastal Plain rivers; data from dendrogeomorphological
analyses. The Cache River was investigated twice in different studies and locations
River
Type
Rate
Reference
Hatchie, TN
Alluvial
5.4
Bazemore et al., 1991
Forked Deer, TN
Alluvial
3.5
Bazemore et al., 1991
Chicahominy, VA
Alluvial
3.0
Hupp et al., 1993
Obion, TN
Alluvial
3.0
Bazemore et al., 1991
Patuxent, MD
Alluvial
2.9
Schening et al., 1999
Cache, AR
Alluvial
2.7
Hupp and Schening, 1997
Roanoke, NC
Alluvial
2.3
Hupp et al., 1999
Cache, AR
Alluvial
1.8
Hupp and Morris, 1990
Wolf, TN
Alluvial
1.8
Bazemore et al., 1991
Mattaponi /Pamunkey, VA
Alluvial
1.7
Schening et al., 1999
Coosawhatchie, SC
Blackwater
1.6
Hupp and Schening, 1997
Choptank, MD
Blackwater
1.5
Schening et al., 1999
Pocomoke, MD
Blackwater
1.5
Hupp et al., 1999
from two initially distinct sources: (i) runoff from adjacent uplands (riparian buffer) and (ii) streamflow
during inundation of bottomlands (riparian retention). Although the former has received some study
(Brockway, 1977; Karr and Schlosser, 1978; Peterjohn and Correll, 1984; Correll, 1986; Johnston et al., 1984;
Lowrance et al., 1984, 1986, 1998) the latter has received far less (Kleiss et al., 1989; Hupp and Morris, 1990;
Hupp and Bazemore, 1993; Hupp et al., 1993; Kleiss, 1996).
Geomorphological analyses (Leopold et al., 1964; Jacobson and Coleman, 1986) verify that riparian
retention of sediment is a common and important fluvial process, yet retention time of sediment may be the
most poorly understood, generally unquantified aspect of sediment budgets (R. B. Jacobson, written
communication, 1996). As of the early 1990s, only four published accounts of vertical accretion rates or mass
accumulation for mineral fines in the USA could be located by Johnston (1991) for any type of wetland.
Since then, vertical accretion rates have been reported for BLH systems in West Tennessee, eastern Arkansas,
South Carolina, North Carolina, and along tributaries to the Chesapeake Bay in Maryland and Virginia
0.7
0.6
2
_¢
Q >- 0.5
¢ir
Z N 0.4
F-F-
w 2 0.3
5 F
Z
N U 0.2
Z
0.1
Hatchie River
Approximate time of y
initial channelization
1
f
Big Sandy River
1880- 1890- 1900- 1910- 1920- 1930- 1940- 1950- 1960- 1970-
1889 1899 1909 1919 1929 1939 1949 1959 1969 1979
AGE GROUP
Figure 6. Sedimentation rates by age class of trees sampled for rate determination along the Hatchie and Big Sandy Rivers, Tennessee.
Sedimentation rates are consistently higher along the unchannelized Hatchie River; note sharp increase in deposition beginning around
1950
Hydrol. Process. 14, 2991 -3010 (2000)

COASTAL PLAIN RIVERS 3001
(Table II). In a rare, fairly exhaustive, BLH sediment retention study, Kleiss (1996) reported that the Cache
River, Arkansas carries more than 90% of its total annual sediment load during the high -flow period and
that more than 14% (about 800 g /m� /year) of the load is trapped along a 23 km wide, 49 river -km long
reach. The results of the Cache River study (Kleiss, 1996) support current efforts to rehabilitate /restore BLH
forests. In addition to sediment trapping, restored systems also improve the water quality of adjacent river
reaches and reduce the sediment burden on existing downstream BLH systems (Kleiss, 1996). The sediment
and contaminant trapping function in Coastal Plain fluvial systems is especially important because these
floodplain surfaces are the last areas for sediment storage (and biogeochemical cycling) before entering
estuaries and their critical nurseries for marine biological production.
Some of the highest concentrations of suspended sediment occur in the Mississippi Embayment because of
channel instability, highly erodible uplands ( fine alluvium and loess), and extensive agriculture (Trimble and
Carey, 1984; Simon and Hupp, 1987). Many streams of this region, particularly in West Tennessee, south-
eastern Missouri, and northern Mississippi, have been channelized, which has led to severe upstream channel
incision with concomitant channel erosion (Simon and Hupp, 1987; Simon et al., 1996). Channel incision
facilitates runoff and peak flows, which reduce the hydroperiod and trapping function in BLH systems that
have been channelized (Hupp, in press), ironically in a region where the payoff in water quality benefits of
intact, functioning BLH systems would be great. A comparison between the unchannelized Hatchie River
and the channelized Big Sandy River in West Tennessee (Hupp and Bazemore 1993) showed that sediment
deposition rates were significantly higher on the unchannelized stream as far back as the time of initial
channelization (Figure 6), particularly after large expanses of the basins and bottomlands were cleared for
agriculture after World War II (Figure 6).
Like intensive agricultural areas, urbanizing areas tend to generate high suspended sediment and
contaminant loads, particularly trace elements (White and Tittlebaum, 1985). Both deposited and suspended
sediments contain significantly higher concentrations of most trace elements and hydrophobic contaminants
than are dissolved in water (Horowitz, 1991). Thus, patterns of sediment transport and deposition largely
control the fluvial deposition of most trace elements and many contaminants. Water - quality concerns in
south - eastern Virginia along the Chickahominy River, which arises in the urbanizing Richmond area, are
high because the river is the source of a water - supply reservoir for the densely populated Hampton Roads
area. A study of sediment and trace - element trapping along Coastal Plain reaches of the Chickahominy
(Hupp et al., 1993) has shown that large amounts of sediment and associated trace elements are trapped in
the adjacent BLH system (Table III). Additionally, changes in the gradient of the river, from relatively steep
Table 111. Summary of estimated amounts (kilograms) of sediment and trace elements deposited annually at eight
sites along the Coastal Plain reaches of the Chickahominy River between Richmond, Virginia and Providence Forge,
Virginia; site number ascends downstream. Area at each site is calculated from forested wetlands delineated from a 2
km reach centred at the site. Site 3 (greatest amount of sediment) is located near the confluence of several tributaries
draining urbanizing areas around Richmond; site 4 has the highest stream gradient of the study reaches and nearly
the least amounts of sediment and trace elements
Site
Sediment
Zn
Cu
Ni
Pb
Cd
Cr
Sn
1
670 000
118
6
8
43
< 1
23
< 1
2
1 400 000
205
33
17
155
1
60
2
3
7 600 000
1269
76
76
426
2
274
4
4
840 000
235
8
12
41
< 1
26
1
5
2 200 000
446
29
33
130
1
110
2
6
1 600 000
125
5
10
37
< 1
42
< 1
7
2 000 000
259
14
17
54
1
51
1
8
860 000
68
5
7
25
< 1
28
< 1
Total
17 170 000
2725
176
180
911
5
614
9
Hydrol. Process. 14, 2991 -3010 (2000)

3002
c. R. i i L PP
Cache River Deposition and Elevation
14
_
77
i
Deposition, mm
183
182
12
Elevation (ft)
181
L
21 10
:
_
E
E..
180
E
179
.�
r 8
O
u
178 m
>
6
m
a
G.
s
177 W
q
c _
176
M
175
2
- t
m
174
0
173
m m 00 m n n ° m n Goo m n m
Plot number
Figure 7. Relationship between elevation and sediment deposition along the Cache River, Arkansas. Deposition rates were determined
from dendrogeomorphic analyses (tree -ring) and are averages from trees ranging in age from 180 to 25 years
straight - channel reaches to low- gradient anastomozing reaches, strongly control deposition rates of both
sediment (0.7 and 5.7 mm /year, respectively) and associated trace elements; low stream gradients facilitate
the development of broad bottomlands and long hydroperiods, which, in turn, enhance sediment trapping.
Local sediment deposition
It may be intuitive that as sediment -laden flow leaves the main channel and enters a forested wetland,
velocities slow owing to the hydraulically rough nature of a forested bottom (also a dramatic increase in
wetted perimeter) and subsequently sediment deposition occurs ( Kleiss, 1996). Yet, as previously stated,
until quite recently only a handful of attempts have been made to quantify sediment deposition in any
wetland system. Thus, it should not be surprising that there are even fewer published accounts and
interpretations of factors affecting local variation in deposition rate (Hupp and Schening, 1997). The amount
of suspended fines available rather obviously, pervasively affects deposition potential. Variation in local
elevation (Figure 7) across a bottomland and correlated length of hydroperiod also have been cited as
important factors affecting deposition rate (Hupp and Morris, 1990; Hupp and Bazemore, 1993; Kleiss,
1993, 1996). Several other factors, distinct and correlated, may play an important role in local sediment
deposition, including: flow velocity, distance in line of flow from main channel, hydraulic connection to main
channel, internal flow paths, ponding (typically in backswamps or behind levees), roughness from standing
vegetation and LWD, and beaver activity.
The US Geological Survey, in association with the US Forest Service and several universities, is
conducting research on the functioning of BLH through the Southern Forested Wetlands Initiative (Burke
and Eisenbies, 1999). Two sites chosen for this initiative are the Coosawhatchie River, SC, a black -water
Hydrol. Process. 14, 2991 -3010 (2000)

COASTAL PLAIN RIVERS 3003
stream, and the Cache River, AR, an alluvial stream. They have been sampled intensely in order to
investigate several factors that may affect local sediment deposition (Hupp and Schening, 1997; Hupp et al.,
1999b). The Coosawhatchie and Cache Rivers annually trap substantial amounts of sediment, 24.5 kg /ha/
year and 1876 kg /ha /year, respectively, reinforcing the water - quality functions of both black -water and
alluvial forested wetlands. Deposition rates were estimated along multiple transects, normal to the
downvalley axis, using dendrogeomorphological (tree ring) techniques and clay -pad marker horizons. These
rates were then related to several physical parameters, including velocity, elevation, LWD and hydraulic
connectivity at each sampling station and to dominant woody vegetation. The transects were closely spaced
so that the sampling points were arrayed in a grid -like fashion, permitting a three- dimensional analysis of
data. Many of the following observations could not have been made using widely spaced transects
(essentially two - dimensional analysis).
Mean sediment deposition rates on the black -water Coosawhatchie River ranged from 0.02 to 0.20 cm/
year and from 0.20 to 0.36 cm /year on the Cache River. The Cache River carries a suspended load of about
100 to 350 mg /1, whereas the Coosawhatchie River carries about 5 to 25 mg /1. Thus, a greater amount of
fourth
ti
Flow Flow
250 500 750 1000 1250 1500
Distance from River in meters
15
5 .° E
D
Figure 8. Sediment deposition patterns across the Cache River, Arkansas bottomland. Data shown along grid lines first through fourth,
individual points are separated in straight lines by 250 m. Deposition rates tend to be greatest along flow paths: (A) indicates area of
concentrated large woody debris and relatively high sediment deposition rate; (B) indicates a stagnant area that is poorly hydraulically
connected to sediment -laden river water and has a relatively low sediment deposition rate
Hydrol. Process. 14, 2991 -3010 (2000)

3004 C. R. HUPP
sediment deposition on the brown -water Cache River was expected and measured. Major sloughs bifurcating
through the sites affect both study areas. Hydraulic connectivity (degree of flow -path connections to the
main channel) appears to strongly affect sedimentation rates (Figure 8), with highest deposition occurring
near sloughs and their anabranches with a direct flow path to the river. In contrast, relatively low deposition
rates occur in stagnant areas poorly connected to the channel or unaffected by sloughs, presumably due to
diminished replenishment of suspended sediment during the hydroperiod (Figure 8); this occurs despite
nearly complete inundation during the hydroperiod at both sites. Smaller sloughs associated with crevasse
splay areas near the main channels (Figure 8), similarly experience high deposition rates.
Woody vegetation, including LWD, may also play an integral part in directing and concentrating flow
paths across bottomlands through variation in surface roughness (sloughs tend to be more open).
Sedimentation tends to be high on the upstream faces of `ridges' (Figure 8), areas that also accumulate
considerable LWD, whereas adjacent downstream areas tend to have less sediment accumulation.
Deposition rates vary inversely with velocity on the Coosawhatchie River but vary directly with velocity
on the Cache. Velocities at both sites are relatively low, except in the main channel and major sloughs. Low
velocities facilitate deposition of fines, particularly organic material, however, relatively moderate velocities
may ensure a continuous supply of sediment -laden water, particularly mineral fines. Fines deposited over the
clay pads on the Coosawhatchie River contained substantial amounts of organic material with a mean of
nearly 40% after loss on ignition as opposed to 22% on the Cache River. Mineral fines were concentrated
largely on the levees and near sloughs.
VEGETATION
The likelihood of a given species vigorously growing on a particular landform, including the various fluvial
landforms, is a function of (i) the suitability of the site for germination and establishment (ecesis), and (ii) the
ambient environmental conditions that permit persistence at least until reproductive age (Hupp and
Osterkamp, 1996). The distributional pattern may be limited by the tolerance of a species for specific
disturbance or stress regimes, as well as by tolerance for other more diffuse interactions including
competition. In fluvial systems, the distribution of vegetation across landforms may be driven largely by the
tolerance of species to specific geomorphological processes (hydroperiod and sedimentation dynamics in
BLH systems) at the severe end of a stress - equilibrium gradient and by competition with other bottomland
species at the other end.
Variations in hydroperiod (and, perhaps, to a lesser degree sedimentation /erosion) and plant adaptive
strategies largely explain the complex patterns of BLH species distributions (Bedinger, 1971; Leitman et al.,
1984; Wharton et al., 1982; Mitsch and Gosselink, 1993; Sharitz and Mitsch, 1993); however, specific
patterns of BLH species distribution and their quantitative relations with water level and sediment dynamics
remain incompletely understood. For example, streamflow of varying magnitude and duration and sediment
deposition /erosion dynamics affect vegetation by creating new areas for establishment, such as point bars
(lateral accretion), the subsequent ridge- and -swale topography, and by creating hydroperiod /sediment -size
clast gradients (vertical accretion) across the floodplain. Although, these fluvial processes are yet
insufficiently understood to allow for reasonably accurate prediction at a specific site, even less understood
is the role riparian vegetation plays in affecting fluvial processes (Hupp and Osterkamp, 1996). We know that
vegetation increases flow resistance (and thus facilitates deposition), increases bank strength, and provides
LWD in the form of log jams in channels and as debris piles (rack) across floodplains (Hickin, 1984; Hupp,
1992; Hupp and Osterkamp, 1996). However, separating factors that simultaneously influence both
vegetation patterns and geomorphological processes is difficult because most are distinctly interdependent,
and consistent definitions of landform and process generally are lacking within the geomorphological
sciences, and particularly between the geomorphological and plant - ecological sciences. Where conformity
occurs between sciences it is usually in the common belief that hydrological processes control most aspects of
Hydrol. Process. 14, 2991 -3010 (2000)

COASTAL PLAIN RIVERS
3005
Figure 9. (A) High water on the alluvial Cache River, Arkansas. Levee forest is nearly inundated. (B) Slough through a Taxodium forest
on the black -water Coosawhatchie River, South Carolina
Hydrol. Process. 14, 2991 -3010 (2000)

3006 C. R. HUPP
the fluvial BLH ecosystem. Indeed only hydrological characteristics provide independent parameters
consistent on all perennial streams.
Despite the difficulty to demonstrate quantitative relationships among hydrology, geomorphology and
vegetation, the striking vegetation zonation across most BLH systems (Figure 9) has tempted several
researchers to develop a classification of vegetation patterns (Kellison et al., 1998). Small differences in
elevation, often measured in centimetres, may lead to pronounced differences in hydroperiod and, thus, to
community composition (Mitsch and Gosselink, 1993). As a result, most classification systems infer that
length of hydroperiod is the most influential factor in controlling species patterns, most probably due to
anaerobic conditions associated with flooding (Wharton et al., 1982). Anaerobic respiration within the roots
of plants leads to the production of toxic byproducts and limits the uptake of nutrients and water. Plants
tolerant of varying degrees of flooding have developed physical and /or metabolic adaptations to withstand
inundation and anoxia (Wharton et al., 1982). Presumably the degree to which individual species have
adapted to anoxia - related stresses controls the distinct and striking changes in vegetation composition across
very short (metres) lateral distances on many floodplains in this region (Huffman and Forsyth, 1981).
A zonal classification system (Figure 10) of coastal plain bottomlands, described by Clark and Benforado
(1981) and adopted by the National Wetland Technical Council, has been the basis for most subsequent
BLH vegetation - community classifications. This classification is based largely on hydrological regime and is
Zone
Name
Water modifier
Flooding frequency,
of years
Flooding duration,
of growing season
Figure 10. Zonal classification of bottomland hardwood forests showing average hydrological conditions for each zone (after Sharitz
and Mitsch, 1993. Reproduced by permission of John Wiley & Sons, Ltd.)
Hydrol. Process. 14, 2991 -3010 (2000)
r
Aquatic
Bottamland hardwood
Bottomland
#ecosystem
ecosystem
0-
upland
transition
Floodplaln
1
II
III
IV
V
V1
Open water
Swamp
Lower hard-
Medium hard-
Higher hard-
Transition to
wood wetland
wood wetland
,wood wetland
upland
Continuously
Intermittently
Semipermanently
Seasonally
Temporarily
Intermittently
flooded
exposed
flooded
flooded
flooded
flooded
100
-10D
51 -144
51 -100
11 -54
1.10
100
-104
>25
12.5 -25
2 -12.5
4
Figure 10. Zonal classification of bottomland hardwood forests showing average hydrological conditions for each zone (after Sharitz
and Mitsch, 1993. Reproduced by permission of John Wiley & Sons, Ltd.)
Hydrol. Process. 14, 2991 -3010 (2000)

COASTAL PLAIN RIVERS 3007
highly generalized; much of the geomorphological detail, described earlier, is lacking and of limited use at
specific sites. However, this classification serves as a useful framework for interpreting vegetation patterns, as
long as the deceptively complex local hydraulic patterns, particularly in plan view (Hupp, 1999), are not
ignored. With the possible exception of the lower Mississippi (Saucier, 1994), hydrogeomorphological
patterns across these bottomlands have received far less study than the vegetation.
The species found in BLH systems are remarkably similar throughout the bottomlands of the Coastal
Plain (Sharitz and Mitsch, 1993). Forty -two tree species occur commonly and 13 of these are ubiquitous
(Kellison et al., 1998). A typical pattern of vegetation distribution is shown in Figure 11. Point bars, typically
the most recently created surfaces, tend to support shade- intolerant ruderal species such as Salix nigra,
Betula nigra and Populus deltoides. Inward from the channel, levee surfaces and scroll `ridges' frequently
stand in considerable relief relative to the rest of the bottomland and tend to be well drained owing the
typically sandy substrate. These fluvial features generally support a mixture of older point -bar individuals
and other relatively high and dry species, such as Platanus occidentalis, Quercus laurifolia, Q. phellos, Q.
nigra, Fraxinus pennsylvanica and Liquadambar styraciva. The often broad floodplain is a mosiac of flats
punctuated by sloughs, ox -bows and swales, which may be only tens of centimetres (or less) lower than the
surrounding flats. These lower and thus wetter features support the most hydric species, such as Taxodium
distichum and Nyssa aquatica. Just outside these areas are slightly less moist surfaces dominated by Quercus
lyrata, Carya aquatica and Gleditsia aquatica. The flats support a diverse forest that may include the levee
species in addition to Quercus michauxii, Q. pagoda, Ulmus americana, Acer negundo, A. rubrum, Celtis
laevigata, Pinus taeda and Fagus grandifolia. These species display distributional patterns, often in
association with other species (Figure 11), along often virtually imperceptible variations in elevation across
the floodplain. Along alluvial rivers, the floodplains tend to become increasingly more hydric from the Fall
Bald cypress, Water tupelo
Water elm, Buttonbush
River birch
Black willow
Overcup oak,
Red maple
Willow and Pin
oaks, Sweetgum
Swamp
chestnut oak
Figure 11. Cross - section of a bottomland hardwood forest showing species distribution relative to a perennial stream and ox -bow; (A)
is the highest flow elevation, (B) is the mean annual high -water elevation and (C) is the mean annual low -water elevation (after Sharitz
and Mitsch, 1993. Reproduced by permission of John Wiley & Sons, Inc.)
Hydrol. Process. 14, 2991 -3010 (2000)

3008 C. R. HUPP
Line to the estuary. Extensive tidal BLH systems near estuaries are rarely flooded relative to other
floodplains but are wetted daily during wind and /or lunar high tides; these systems are largely unstudied.
SUMMARY
The Coastal Plain of the south - eastern USA is characterized by broad, frequently inundated low- gradient
fluvial systems that support a characteristic forest ecosystem, bottomland hardwoods. These systems have
received considerable ecological study, but distinctly less hydrogeomorphological study; quantitative process
linkages among hydrology, geomorphology and ecology remain largely undocumented. Although heavily
impacted by land use, these floodplains and their bottomland hardwood systems remain a critical landscape
element for the maintenance of water quality by trapping and storing large amounts of sediment and
associated contaminants.
Alluvial and black -water fluvial systems within the region typically are flooded annually for prolonged
periods, creating two distinct hydrological seasons. Order -of- magnitude differences in wetted perimeter,
width /depth, suspended sediment load and hydraulic roughness exist between the in- channel `dry' season
and the inundated season or hydroperiod. Vertical sediment accretion rates may be among the highest of any
fluvial system. Channel processes and sedimentation dynamics in these low- gradient systems result in the
extensive development of levees, point bars and scroll topography, avulsion and associated back channels or
sloughs and ox -bows, and broad extremely flat floodplains. Variation in hydroperiod, velocity and
suspended sediment may largely control sediment deposition rates. These factors are controlled locally by
elevation, levee breaches, amount of large woody debris, and degree of hydraulic connectivity to sediment -
laden inundating water. Diverse woody vegetation of this ecosystem has adapted to prolonged inundation
(anaerobic conditions) creating unique characteristic riparian vegetation patterns revealed in a mosaic of
associations ranging from relatively mesic levee and high floodplain associations to hydric slough and low
floodplain associations. Nearly imperceptible changes in elevation may result in distinct pervasive changes in
species composition and zonation, strongly suggesting a rigorous relationship between vegetation and
hydrogeomorphological processes. These critical fluvial systems and their attendant forests are ripe for
future multidisciplinary research.
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EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 14,11-26 (1989)
A MODEL OF CHANNEL RESPONSE IN DISTURBED ALLUVIAL
CHANNELS
ANDREW SIMON
U.S. Geological Survey, Nashville, Tennessee, U.S.A.
Received 1 November 1986
Revised 21 March 1988
ABSTRACT
Dredging and straightening of alluvial channels between 1959 and 1978 in West Tennessee caused a series of morphologic
changes along modified reaches and tributary streams. Degradation occurred for 10 to 15 years at sites upstream of the
area of maximum disturbance and lowered bed - levels by as much as 6.1 m. Following degradation, reaches upstream of
the area of maximum disturbance experienced a secondary aggradation phase in response to excessive incision and
gradient reduction. Aggradation downstream of the area of maximum disturbance reached 0.12 m per year with the
greatest rates occurring near the stream mouths.
The adjustment of channel geometry and phases of channel evolution are characterized by six process- oriented stages of
morphologic development -- premodified, constructed, degradation, threshold, aggradation, and restabilization. Down -
cutting and toe removal during the degradation stage causes bank failure by mass wasting when the critical height and
angle of the bank material is exceeded (threshold stage). Channel widening continues through the aggradation stage as the
`slough line' develops as an initial site of lower -bank stability. The bank profile develops three dynamic elements (1)
vertical face (70° to 90 °), (2) upper bank (25° to 50 °), and (3) slough line (20° to 251). Alternate channel bars form during the
restabilization stage and represent incipient meandering of the channel.
KEY WORDS Unstable channels Degradation Aggradation Channelization Empirical model
INTRODUCTION
Changes imposed on a fluvial system, be they natural or man - induced, tend to be absorbed by the system
through a series of channel adjustments (Gilbert, 1880; Mackin, 1948; Lane, 1955; Hack, 1960; Schumm, 1973;
Bull, 1979). Due to the drastic changes in energy conditions imposed by channel dredging, enlarging, and
straightening, a rejuvenated condition is established within the fluvial system much like that imposed by uplift
or a natural lowering of base level. The predominant difference is one of time scale. Channel adjustments
caused by climatic changes or uplift may be exceedingly slow, and progressive, practically imperceptible by
man's standards. Conversely, large -scale channel modifications by man result in a sudden and significant
shock to the fluvial system that causes migrating knickpoints and observable morphologic changes. This
shortened time scale presents an opportunity to document successive process -- response mechanisms through
the course of fluvial adjustment over time and space. Channel modifications from 1959 to 1978 through much
of three major river systems in West Tennessee create a natural laboratory for the study of channel
adjustments in rejuvenated fluvial networks (Table I).
PURPOSE AND SCOPE
The U.S. Geological Survey in cooperation with the Tennessee Department of Transportation (TDOT) began
this study in 1981 as a result of bridge failures on modified channels in West Tennessee. The purpose was to
0197- 9337/89/010011- 16$08.00
1989 by John Wiley & Sons, Ltd.

12 A. SIMON
document and empirically model stream - channel changes and process— response mechanisms following large -
scale channelization projects in West Tennessee. TDOT was interested in obtaining estimates of channel
changes in alluvial - channel morphology throughout West Tennessee for the purpose of designing adequate
river - crossing structures.
BACKGROUND
West Tennessee is an area of 27500 km2 bounded by the Mississippi River on the west and the Tennessee
River divide on the east (Figure 1).The region is characterized by unconsolidated, highly erosive formations,
predominantly of Quaternary age (U.S. Department of Agriculture, 1980). The major rivers of the region
(Obion, Forked Deer, and Hatchie) flow in channels comprising medium -sand beds and silt —clay banks. In
contrast, many small tributary streams flow through extensive silt deposits of reworked Wisconsin loess and
have strikingly similar bed- and bank - material properties (Simon, in press). Those tributaries (such as Cub
and Porters Creeks) that head in the eastern sandier Tertiary and Cretaceous formations flow on medium -
sand beds. A lack of bedrock control of base level assures unrestricted bed -level adjustment.
Accounts of river conditions following land clearing throughout the region (mid to late 1800s) describe
extremely sinuous, sluggish sediment - choked streams that frequently overflowed their banks for 3 to 10 days
at a time (Hidinger and Morgan, 1912). This channel infilling, attributed to major deforestation and severe
upland erosion in the late 1800s, prompted channel dredging and straightening in West Tennessee near the
EXPLANATION
Basin boundary
................
Clearing and snagging
Channelization
Year work was completed F
0 10 20 30 MILES
0 10 20 30 KILOMETERS W
36 °_�.
35°
90
C�
Base from U.S. Geological Survey,
State base map, 1967,
revised 1973
7 r-r�3 a 1
—q'T'11
ER BASIN�q�/
F
\i\ .
FORKED DEER RIVER BASIN�
HATCHIE RIVER
Figure 1. Extent of channel modifications in West Tennessee

CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS 13
turn of the century. The most recent channel modification programme in West Tennessee occurred between
1959 and 1978 and is the subject of this study (Table 1). The modified channels listed in Table I by no means
include all of the modified streams in the region, but were selected to represent a spectrum of West Tennessee
alluvial channels based on size, location, and bed and bank properties, and to represent the magnitude and
extent of modifications. Channel lengths were shortened as much as 44 per cent, gradients were increased as
much as 600 percent, and beds were lowered as much as 5.2 m (Simon, in press).
DATA COLLECTION
Channel- morphology data collected from 1982 to 1984 were compared to previous surveys to determine
channel changes with time. Sources of these earlier surveys include the U.S. Geological Survey, Army Corps of
Engineers, Soil Conservation Service, and the Obion- Forked Deer Basin Authority. These data consist of bed
elevations and gradients, channel top - widths, and channel lengths before, during, and after modification.
Where available, gauging station records were used to record annual changes in the water- surface elevation at
a given discharge. Changes in the stage- discharge relation imply similar changes on the channel bed and can
be used to document bed -level trends (Blench, 1973; Wilson, 1979; Robbins and Simon, 1983; Williams and
Wolman, 1984).
The identification and dating of various geomorphic surfaces were diagnostic in determining relative
stability of a reach and the status of bank -slope development (Simon and Hupp, 1986; Hupp and Simon,
1986). Data collection involved dating of riparian vegetation (1) on newly stabilized surfaces to determine the
timing of initial stability for that surface, and (2) on unstable bank surfaces to estimate rates of bank retreat.
BED -LEVEL ADJUSTMENT
Adjustment of the channel bed at a site follows channel modifications immediately and can be described
through time. Channel bed adjustments through time (irrespective of the temporal and spatial scales applied)
Table I. Type, extent, and dates of recent channel modifications on studied streams (1959 -1978)
Basin
Stream
Type of modification
Length
(km)
Dates
Obion
Obion River
Enlarging and straightening
75.1
1959-66
Clearing and snagging
6.8
1976
Enlarging and straightening
—
1974 -77
North Fork
Enlarging and straightening
17.5
1967
Obion River
Clearing and snagging
17.4
1974 -76
Hoosier Creek
Enlarging and straightening
11.9
1967
Rutherford Fork
Enlarging
11.9
1967
Obion River
Clearing and snagging
28.8
1973 --78
South Fork
Enlarging
9.6
1967, 1969
Obion River
Clearing and snagging
27.5
1976 -78
Forked Deer
North Fork Forked
Enlarging and straightening
6.9
31.5
1973
1974 -77
Deer River
Clearing and snagging
Pond Creek
Clearing and snagging
21.1
1976 -78
South Fork Forked
Enlarging and straightening
7.1
1969
Deer River
Clearing and snagging
36.5
1973 -77
Meridian Creek
Enlarging and straightening
2.6
1959?
Enlarging and straightening
8.4
1969
Enlarging
2.6
1969
Cane
Cane Creek
Enlarging and straightening
52.0
1970
(lower Hatchie)
Enlarging and straightening
20.9
1978
Hyde Creek
Enlarging and straightening
1.3
1970
Upper Hatchie
Cub Creek
Enlarging and straightening
15.6
34.4
1970
1972
Porters Creek
Enlarging and straightening
tL

1.
14 A. SIMON
are best described mathematically by nonlinear functions which asymptotically approach a condition of
minimum variance. The description of channel adjustment and evolution by nonlinear decay functions is well
documented (Schumm and Lichty, 1965; Graf, 1977; Bull, 1979; Hey, 1979; Robbins and Simon, 1983).
However, there seems to be considerable disagreement as to the mathematical form of the function. Graf
(1977) used exponential functions to describe the `relaxation time' necessary to achieve equilibrium following
a disturbance. Simon and Robbins (1987) used similar equations to model gradient adjustment through time.
Williams and Wolman (1984) found that hyperbolic functions were appropriate for describing degradation in
alluvial streams, downstream from dams. In this study, both exponential and power equations were initially
fitted to the observed data. The power function gave consistently superior matches of the empirical data, and
was therefore used to describe bed -level adjustment through time. The general form is:
E =a(t)b (1)
where E= elevation of the bed for a given year, in metres above sea level;
a= coefficient determined by regression, representing the premodified elevation of the bed, in metres
above sea level;
t =time since beginning of adjustment process, in years, where to =1.0; and
b= dimensionless exponent, determined by regression and indicative of the nonlinear rate of change
on the bed.
Trends of bed -level change through time at gauging stations (where bed elevation is measured every 4 to 6
weeks) having periods of record of up to 20 years (Table II), served to support the well -known concept of
nonlinear adjustment (Figure 2). Once it was established that power functions provided the best fit to this
gauged data, data from periodically surveyed sites were similarly fitted (Figure 2). Sites with just two recorded
82
81
to
LM so
U-
w
Z
79
LAJ
�i
w
J
Q 79
* Note. Denotes specific page data used.
'South Fork Forked Deer River
River kilometer = 12.71
e
°
°
r' = 0.94
N 1964 1968 1972 1976 1980 1984
w
>
89
O
M
Q
Z
88
O
Q
J
87
w
86
85
84
T 1 —r-
° ° • Rutherford Fork Obion River
River kilometer = 7.88
r'= 0.93 �
r'= 0.96
1958 1962 1966 1970 1974 1978 1982 1986
143
°
142
°
141
°
140
139 r'= 1.00
138
Porters Creek
River kilometer = 27.51
137
1970 1974 1978 1982 1986
90
89
88
87
86
85
84
83
82
1962 1966 1970 1974 1978 1982 1986
'South Fork Obion River
° River kilometer = 9.33
°
r'= 0.93
Figure 2. Examples of fitting power equations to degradation and aggradation trends through time

CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS
Table II. Sites with calculated aggradation ( +b) and degradation ( -b)
15
Stream
b
n
r
RKM
To
Stream
b
n
r2
RKM
To
Cane Creek
- 0.01620
7
0.99
2389
1969
Obion River
- 0.02220
10
0.95
110.22
1965*
1978
-00168
2
-
23.89
1980
- •01720
-00463
10
-74
110.22
1974*
12
- -02022
4
1.00
2024
1969
-87
-00235
16
•76
10008
1968*
9.33
•01052
2
-
20.24
1980
•00908
19
-93
86.40
1965*
Obion River •00111
- •03300
3
1.00
14.46
1869
-00518
15
-84
55.03
1963*
--
•00770
2
-
14.46
1980
-00585
16
-74
3347
1960*
1975
- -03131
2
-
10.09
1969
--00372
13
-80
28.96
1972*
-00352
2
-
10.09
1980
Pond Creek
- •00828
5
-81
18.29
1977
- •04126
3
•91
6.53
1969
1965*
- •00799
4
-84
1580
1977
00303
- •02011
4
•92
4.06
1969
- -01233
4
-97
11.78
1977
•00835
2
-
4.06
1980
- •00900
5
-79
1.71
1977
Cub Creek
- -00243
3
-69
11.13
1969
- •00342
3
•87
9.22
1969
- •00565
4
•88
3.48
1969
Porters Creek
- •01069
6
1.00
27.51
1971
- -00905
5
•91
2.48
1969
- •01320
7
-99
18.02
1971
-00272
2
-
2.48
1976
--00578
6
1.00
14.30
1971
Hoosier Creek
- •00843
3
1.00
8.29
1967
Rutherford Fork
Obion River
•00149
19
-60
48.11
1965*
- •01130
4
•94
4.81
1966
- •00317
4
-91
2880
1977
- •02081
3
•67
•88
1965
- •00493
3
1.00
24.46
1977
•00274
2
--
•88
1968
- -00991
4
-79
16.73
1972
- -02630
3
•99
-02
1965
•00356
4
•99
16.73
1977
- •01728
9
-93
7.88
1965*
-00433
10
-88
7.88
1974*
Hyde Creek
•00281
2
-
3.81
1975
South Fork
Forked Deer
River
- •00895
6
•59
44.41
1976
- •00737
2
-
3.81
1969
- -00950
10
•92
26.23
1974*
- •01070
4
-92
222
1969
- -00978
5
•76
21.40
1969
- •01380
3
•99
1.19
1969
- •01264
5
•96
19.15
1969
- •02050
4
-00
-02
1969
- •01630
15
-94
1271
1969*
-01180
13
-92
5.31
1969*
Meridian Creek
- •00326
3
-99
5.94
1965
- •00580
4
-98
4.73
1964
South Fork
Obion River
•00133
13
•90
55.35
1969*
-00341
3
•99
2.41
1969
- -00054
4
•26
45.70
1972
- •00190
3
-99
1.54
1967
- -00238
6
•50
37.33
1972
North Fork
Forked Deer
River -00740
4
•95
38.46
1977
- -00661
7
-90
30.89
1977*
- •01076
5
-52
32.47
1974
- •00573
5
•87
27.03
1972
-00839
4
-96
30.28
1978
- •00932
4
•94
18.34
1972
- •01720
10
-95
8.53
1973*
- •02430
12
•87
9.33
1965*
-02297
3
-87
6.16
1972
•00544
8
•88
9.33
1975*
North Fork
Obion River •00111
15
-69
59.37
1969*
-00206
2
--
42.48
1979
-00490
2
--
33.95
1975
--00372
13
-80
28.96
1972*
- •01240
6
-93
15.83
1965*
--02470
4
-85
9.49
1965*
00303
5
-89
9.49
1967*
Note: b, nonlinear gradation rate; n, number of observations; rz, coefficient of determination; RKM, river kilometre; To, start of observed
gradation process; *, specific gauge data used; where n is low, statistical significance may be limited)

16 A. SIMON
bed elevations were included because of confidence in the general trend of adjustment at -a -site, and to
increase areal coverage of the data network.
The initial response of a channel reach (degradation or aggradation) depends on the location of the reach
relative to the imposed channel modifications. Reaches located upstream from the disturbance will eventually
go through an initial phase of general degradation, followed by a period of general aggradation. In these cases,
the data set was split, and Equation 1 was fitted to the degradation and aggradation periods separately. The
last data point of the degradation curve (lowest elevation) was also used as to for the aggradation curve.
The exponent `b' (Equation 1) is a measure of the nonlinear rate of degradation (negative b) or aggradation
(positive b), and was used as the primary variable in identifying longitudinal adjustment trends. Values of b
used in this study are listed by stream. in Table II. The data described by Equation 1 imply that bed -level
response at a site is initially rapid and then diminishes as bed elevation asymptotically approaches a condition
of no net change (Begin et al., 1981).
Because adjustments of channel width are functionally related to changes that occur on the channel bed
(Table III), a bed -level model was developed and is summarized using the Obion River system as an example
(Figure 3). The area of maximum disturbance (AMD) is defined as the upstream terminus of channel work.
However, if a stream is channelized throughout its length, stream power is greatest at the mouth, and the
AMD is located there (Simon, in press).
Stream reaches are divided into the following four groups based on the location of the reach relative to the
imposed AMD in the fluvial network, and the dominant process occurring on the channel bed: (1)
downstream aggradation, (2) migrating degradation, (3) secondary aggradation, and (4) premodified
aggradation. These reach types in the Obion River drainage system are represented in Figure 3 and denote not
only temporal data at sites in the network but also longitudinal bed response through time, with distance from
the AMD.
Maximum amounts of degradation (largest negative b values) occurred just upstream of the AMD (as a
response to the significant increase in stream power imposed by the channel work here in 1967, A in Figure 3)
and decreased nonlinearly with distance upstream (curve C in Figure 3). Note that curve C also represents the
headward migration of the degradation process and therefore has a temporal as well as spatial component (To
in Table 11). Rates of migration on the Obion River Forks are 1.6 km year -1. Degradation occurred for 10 to
15 years at sites just upstream of the AMD and has lowered bed levels as much as 6.1 m. Effects of the imposed
disturbance decreased with distance upstream resulting in minimal degradation rates at about river kilometre
150 (b = 0.0). The absence of net erosion or deposition on the channel bed (b = 0.0) is analagous to Bull's (1979)
threshold of critical stream power. Further upstream (E in Figure 3), channel beds of the Obion River system
(including upstream reaches of the North, South, and Rutherford Forks) remain unaffected by the
downstream channel work and aggrade at low, `background' rates.
Sites downstream of the AMD (line B in Figure 3) aggraded immediately following channel modification
with material delivered from eroding reaches upstream. Maximum recorded rates of aggradation are
0.12 m year -1 close to the mouth of the Obion River. Aggradation near `D' in Figure 3 occurs at previously
degraded sites where gradient has been significantly reduced by incision and knickpoint migration. Flows
become incapable of transporting the greater bed - material loads being generated from upstream channel
Table III. Coefficient of determination (r') for
relations between changes in top width (D W) and
the absolute value of nonlinear degradation rates
(b)
Basin
r
n
Cane Creek
0.49
9
Forked Deer River
0.60
13
Obion River
0.80
18
Upper Hatchie River
0.64
7

0.010
N 0.005
w
V
� 0.000
O`
z
O —0.005
Q
< —0.010
w
O —0.015
w
Q
D: —0.020
Of
Q
w
Z -0.025
J
Z
Z —0.030
u_
O
—0.035
O
Q
V —0.040
0
Z
—0.045
CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS
■ B
la ®
THRESHOLD OF CRITICAL STREAM POWER
n _
— — —
E
■
■
C
■
A Area of maximum
disturbance (1967)
B Line representing downstream
aggradation
C Curve representing migrating
degradation A
D Location of secondary
aggradation
E Location of premodified
aggradation
■ Observed data (b)
25 45 65 85 105 125 145 165
DISTANCE ABOVE MOUTH OF OBION RIVER, IN KILOMETERS
Figure 3. Model of bed -level response for the Obion River system
17
beds. The channel aggrades and thereby, increases gradient and transporting capacity. Schumm and Parker
(1973) describe similar results in an experimental watershed. This secondary aggradation migrates headward
with time and is apparently a response to excessive lowering (overadjustment) by the degradation phase
(Simon, in press). Hey (1979) and Alexander (198 1) similarly argue for alternating phases of degradation and
aggradation following rejuvenation of a channel.
With the short time scales involved in the application of the disturbance and the channel's consequent
response, the evolution of channel form over time and space can be monitored. Numerical results such as
those plotted in Figure 3 can be derived for most alluvial systems where even limited data are available.
Results could be applied over longer time scales with the inclusion of dendrochronologic data (as in Graf,
1977), or to experimental and conceptual models of channel evolution such as Schumm and Parker (1973) and
Hey (1979), respectively. It has been shown that bed -level adjustment does follow nonlinear trends, both over
time at -a -site (Figure 2), and over time with distance upstream (Figure 3).
WIDTH ADJUSTMENT AND (BANK -SLOPE DEVELOPMENT
Channel widening by mass wasting processes is common in adjusting channels in West Tennessee. Bank
failure in the loess- derived alluvium is induced by the overheightening and oversteepening of the bank by
degradation and by undercutting and seepage forces at the toe of the bank (Table II1). Piping and tension
cracks in the bank materials enhance bank failures by internally destabilizing the bank (Simon, in press).
Channel bank material of West Tennessee streams is completely alluvial; comprised of loess - derived
Quaternary sediments which are predominantly nonplastic, dispersive silts of low cohesion (Simon, in press).
In situ shear strength tests confirm the existence of low cohesive strengths; generally less than 14 kilopascals
(kPa). These highly erodible materials tend to maintain high moisture contents due to the relatively slow
downward adjustment of the water table. The mean degree of saturation for studied channel banks is

18 A. SIMON
91 per cent (182 tests) even though most samples were taken during periods of low flow (Simon and Hupp,
1987). Moderate rises in river stage are sufficient to complete saturation of the channel banks, and result in
mass failures upon recession of river stage. Figure 4 shows the frequency distributions of the primary soil
mechanics variables; cohesion, friction angle, and field density. Note that the histograms do not display a
normal distribution, but are markedly skewed towards lower strength characteristics. Friction angle and
cohesion values vary within accepted limits for the types of materials tested (Lohnes and Handy, 1968).
Maximum values of cohesion represent tests taken at depth in localized clay rich deposits. Statistical
properties of the soil mechanics data are given in Table IV.
Changes in channel width following the most recent major modifications of West Tennessee stream
channels are summarized in Table V. Mean widening data serve only to provide a point of reference as to
magnitude and variability of width changes. Minimum and maximum values of channel widening (Table V)
do reflect a realistic range of width changes throughout the stream lengths studied.
70
00
ao
40
30
20
to
W
U
W 45
OC 40
99
35
V 30
O 25
W 20
O
} 15
Z 10
t =J
0
W
w
3 1 15 21 V
COHESION, IN KILOPASCALS
f0
eo
70
so
so
40
30
20
10
10 15 20 25 30 33 40
FRICTION ANGLE, IN DEGREES
13 10 " 15 21
FIELNSITY, IN KILONEWTONS
D DE
Figure 4. Frequency distributions of soil mechanics data for studied streams
Table IV. Statistical properties of selected soil mechanics data
Standard
n Mean error Minimum
Maximum
Cohesion (kPa) 168 8.59 0.59 0.14
54.3
Friction angle 168 30.1 0.62 8.30
42.4
Field Density (kN) 182 18-6 0-22 11-3
48-1
[kPa, kilopascals; kN, kilonewtons]

CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS 19
Table V. Range of changes in channel top width (DW) by stream
Years since most
DW recent major
(m) channelization
Stream Mean Min Max n (from 1983)
Cane Creek
18.2
14
25
6
13
Cub Creek
2.1
0.0
6.7
4
13
Hoosier Creek
14.2
7.0
2
3
16
Hyde Creek
3.1
0.3
5.4
3
18
Meridian Creek
3.4
2.4
4.3
2
16
North Fork Forked Deer River
10.3
3.0
25
4
10
North Fork Obion River
10.4
0.6
25
5
16
Obion River
37.8
20
59
4
17
Pond Creek
2.9
2.1
4.3
4
5
Porters Creek
7.7
3.7
12
3
11
Rutherford Fork Obion. River
72
0.0
18
5
14
South Fork Forked Deer River
13.6
4.9
27
6
14
South Fork Obion River
11.9
0.0
36
7
14
Processes and stages of bank retreat and slope development
The processes and successive forms of bank retreat and bank -slope development reflect the interaction of
hillslope and fluvial processes. Interpretations of these processes are based largely on bed -level adjustment
trends and botanical evidence. A complete `cycle' of slope development from the premodified condition
through stages of adjustment, to the eventual reestablishment of stable bank conditions, assumes that
degradation is of sufficient magnitude to instigate unstable bank conditions and a 20- to 40 -year period of
mass wasting. However, in grossly unstable channels comprised of silt -clay alluvium, much longer amounts of
time may be required for the complete restabilization of the channel. The idealized representation of six stages
of bank retreat and bank -slope development (Figure 5) represent distinguishable bank morphologies
characteristic of the various reach types describing bed -level adjustment (Table VI). By applying a space for
time substitution, longitudinal variation in bank -slope development can then be used to denote morphologic
change through time. Sand -bed channels will require 20 to 40 years of channel adjustment to pass through the
first five stages of the model. In these types of channels, a total of 50 to 100 years is assumed necessary for the
restabilization of the channel banks (stage VI) and for the development of incipient meanders.
Premodified Stage (1). Premodified bank conditions are assumed to be the result of `natural' fluvial
processes and land -use practices. Bank failure by mass wasting processes generally does not occur and banks
are considered stable. Data referring to stable bank conditions along West Tennessee channels are available
from the literature, agency work plans, the upstream -most reaches of present day (1987) adjusting networks,
and the nonchannelized Hatchie River.
Premodified bank conditions (stage I in Figure 5) are characterized generally by low -angle slopes (20° to
25 °), convex upper -bank and concave lower -bank shapes, and established woody vegetation along the top
bank and downslope towards the low -flow channel. Channel reaches with these characteristic bank
conditions generally have width -depth ratios (as measured from the flood -plain surface) between 6 and 10,
and narrow slowly with time due to limited aggradation and bank accretion.
Premodified channel widths (prior to 1915) along the downstream ends of the forks of the Obion and
Forked Deer Rivers ranged from 15 to 28 in depending on drainage area (Morgan and McCrory, 1910;
Hidinger and Morgan, 1912). The present premodified sites (upstream -most) on the forks of the Obion River
are approximately 23 m wide.
Constructed Stage (11). Construction of a new channel involves reshaping the existing channel banks or
repositioning the entire channel. In either case, the banks are generally steepened, heightened, and made linear
(stage II in Figure 5). West Tennessee channels are generally constructed as trapezoids with bank- slopes
A,

Slab and rotational
failures
IVa
j
\--Previous profile
V
\— Previous profile
VI
- -- —\ Substantial bed-
level recovery
Non dispersive
materials
Slough line
Note: Scale is relative.
RESTABILIZATION STAGE
Figure 5. The six stages of bank -slope development
Substantial bed -level
recovery
Moderately dispersive
\ materials
ranging from 18° to 34 °. Channel widths are increased and vegetation is removed in order to increase channel
conveyance. It is appropriate to consider the constructed condition (stage II) as a transition from the stable,
premodified state to the more unstable degradation stage, when further heightening and steepening of the
banks occur by fluvial processes.
Degradation Stage (III). The degradation stage (stage III in Figure 5) is characterized by rapid erosion of
the channel bed and the consequent increase in bank heights. Downcutting generally does not steepen bank
slopes directly but results in bank angles that approximate the angle of internal friction (Skempton, 1953;
Carson and Kirkby, 1972; Simon and Hupp, 1986). Steepening occurs when moderate flows attack basal
surfaces and remove toe material (stage IIIa in Figure 5). Pop -out failures, due to excess pore pressure at the
bank toe, similarly cause steepening. Ideally, widening by mass wasting does not occur because the critical
bank height and angle have not yet been exceeded. Bed degradation with constant channel width (Pickup,
1975) is thus possible in these materials if the amount of incision is limited.
The degradation stage is probably the most important in determining the magnitude of channel widening
that will occur because the amount of incision partly controls the bank - failure threshold. Sites close to the
AMD (where degradation is greatest) are therefore expected to widen appreciably.
Threshold Stage (IV). Stage IV (Figure 5) is the result of continued degradation and basal erosion that
further heightens and steepens the channel banks. The critical bank height is exceeded, and bank slopes and
shapes become the product of mass wasting processes.

a
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CHANNEL
RESPONSE IN DISTURBED ALLUVIAL CHANNELS
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21

22
A. SIMON
Degraded reaches of West Tennessee streams have mean bank angles ranging from 55 to 65 degrees.
However, toe removal often creates almost vertical banks. Given the low cohesive strengths and the ambient
high moisture contents of the channel banks, critical bank heights at saturation rarely exceed 5 m (Simon and
Hupp, 1987). Degraded reaches of the studied streams, having bank heights commonly in excess of 8 m,
therefore fail readily.
Slab failures occur due to excessive undercutting and the loss of support for the upper part of the bank,
Pop -out failures at the base of the bank due to ground -water saturation and excess pore -water pressure
further oversteepen the bank and lead to slab failure.
Deep- seated rotational failures shear along an are and often become detached from the top bank surface by
piping. Water drains into the bank mass along the pipes, thereby reducing shear strength and increasing pore -
water pressure and lubrication along the potential failure surfaces. Failures can occur as a saturated mass of
bank material that leave 1.8 to 3.0 -m -long slickensides along the vertical section of the failure surface (often a
pipe) and come to rest in a lobate form at the base of the bank. The failed material loses shear strength and, in
its saturated state, can be removed easily by moderate flows. Rotational failures and debris flows under
saturated conditions along overheightened reaches of the Rutherford and South Forks Obion River and Cane
Creek tend not to reduce bank angles as much as rotational failures under drier conditions. These failure
mechanisms are considered worst -case conditions in dispersive loess - derived materials, as occurs in the Cane
Creek basin.
Rotational failures also occur in less dispersive materials where piping is not as prevalent. Banks fail as a
single mass of material along an arc, leaving elliptical scars 15 to 60 m long. The failed material does not come
to rest at the base of the bank but forms a definable surface higher on the bank profile at slopes ranging from
25° to 50° (stage IV in Figure 5). This surface, the `upper bank' (Simon and Hupp, 1986), is often identified by
tilted and fallen vegetation. A second definable surface, the `vertical face', representing the top section of the
major failure- plane, also is developed during the threshold stage (stage IV in Figure 5 and Table VI).
Bank retreat by slab and rotational failures continues through the threshold stage, developing the vertical
face and upper bank as two distinguishable bank forms (stage IV in Figure 5). The threshold stage is the first of
two that are dominated by active channel widening (Table VI). Failed material is generally removed by
moderate to high flows, thereby retaining the overheightened and oversteepened bank profile, and giving the
banks a scalloped appearance.
Aggradation Stage (V). Stage V is marked by the onset of aggradation on the channel bed and often can be
identified by sand deposited on bank surfaces. Bank retreat dominates the vertical face and upper bank
sections because bank configurations still exceed the critical conditions of the material. The failed material on
the upper bank is subject to secondary, low -angle slides resulting from frequent wetting by rises in stage and
from the added weight of fluvially deposited materials. The low -angle failures reduce the angle of the upper
bank and extend it downslope (stage V in Figure 5).
Previously failed material on the upper bank also moves downslope by low -angle slides and shows --victence
of fluvial reworking and deposition. This combination of mass wasting and fluvial processes creates a low -
angle surface (20° to 25 °) termed the `slough line', extending downslope from the upper bank (Simon and
Hupp, 1986). Woody vegetation reestablishing on the slough line shows only mild tilting from secondary slips
and can be used to date the timing of renewed bank stability (stage V in Figure 5; Hupp and Simon, 1986).
Rates of deposition on the slough line can be obtained by dividing the depths of burial above the root collar of
trees, by the age of the trees.
Bank stability, as indicated by establishing vegetation on the slough line, extends upslope, away from the
channel with time. Bank angles on the upper bank and slough line surfaces continue to flatten through stage V
due to secondary low -angle slides and fluvial reworking. The range of bank angles given for the vertical face
and the upper bank (Table VI) are in general agreement with values reported in previous studies of unstable
slopes comprised of loess (Lohnes and Handy, 1968), and loess- derived alluvium (Throne et al., 1981).
Three classes of slope angles were identified by Lohnes and Handy (1968) in their classic study of Iowa and
West Tennessee loess; 10 to 41 degrees, 50 to 52 degrees, and 68 to 85 degrees. The second and third classes
relate to the upper bank and vertical face as described in this report respectively. The angles reported by
Lohnes and Handy (1968) for their second class (50 to 52 degrees) are steeper than those observed in West

CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS 23
Tennessee during this study (25 to 50 degrees). The reason is that with the addition of water from seepage or
from rises in river stage, effective strength, and therefore slope angles, are reduced (Lutton, 1974). The
differences reported here suggest that mass wasting and bank -slope development along streams of loess -
derived alluvium occur by processes similar to those that occur on loess slopes. However, the addition of
fluvial and fluvial - related processes result in slope forms that are consistent, but slightly different from those of
upland loess slopes. The genesis and range of angles of the slough line further support this argument. These
low -angle surfaces (20 to 25 degrees) are not explicable in the Lohnes and Handy (1968, p. 254) model because
their study did not deal with channels, and the processes of fluvial reworking and low -angle sliding in
saturated materials of residual strength. In fact, the 10 to 41 degree class identified by Lohnes and Handy
(1968) from topographic maps may indeed represent old slough lines, emanating downslope from what are
now terraces.
Assuming that the critical height of the entire bank is still exceeded and (or) dispersion and piping continue
to weaken the vertical face, parallel retreat along the vertical face and flattening of the upper bank and slough
line may continue as the channel creates a new flood plain at an elevation lower thar�,the previous one. Highly
disturbed channels such as Cane Creek, which is cut through loess- derived sediments and lacks a coarse
sediment -load for aggradation, tend to aggrade extremely slow following degradation. The lack of bed -level
recovery leads to an extended period when bank profiles exceed the critical bank height and angle, allowing
for continued bank retreat and flood plain development. It is estimated that the development of a new and
distinct flood plain on channel such as Cane Creek will occur over 1000 years or more. Stage V would then
represent the final stage of bank -slope development in these types of channels.
In sand -bed channels, aggradation may be of sufficient magnitude to stabilize the vertical face and upper
bank through a reduction in bank height below the critical height. Bed levels of sand -bed channels, such as the
forks of the Obion and Forked Deer Rivers and the lower reaches of Cub, Porters, and Meridian Creeks rise,
and become stage VI channels over time spans that can approximate 20 to 40 years.
The aggradation stage (stage V) of bank -slope development occurs first in downstream reaches and
progresses upstream with trends of secondary aggradation. A meandering low -water thalweg and alternate
channel bars also begin to develop during late stage V, further reducing gradient and stream power.
Restabilization Stage (VI). The restabilization stage is marked (1) by a significant reduction of bank heights
by aggradation on the channel bed and (2) by fluvial deposition on the upper bank and slough line surfaces.
Bank retreat along the vertical face by intense mass wasting processes subsides because bank heights no
longer exceed critical heights. Woody vegetation extends upslope towards the base of the vertical face and the
former floodplain surface becomes a terrace (stage VI in Figure 5).
In channels where bank material is only moderately resistant and bed -level has sufficiently aggraded to
cause more frequent wetting of vertical face, the uppermost section of the bank may take a convex shape due
to fluvial reworking and deposition (stage VI in Figure 5). In heavily aggrading reaches of the Obion, and
North and South Forks Forked Deer Rivers, the floodplain continues to be a conduit for moderately high
flows. Woody vegetation is reestablished at the top of the bank and on the flood plain surface.
Bank -slope development through stage VI assumes either significant bed aggradation which occurs along
downstream reaches of sand -bed channels, and (or) limited initial downcutting. Flattening of the upper bank
and slough line surfaces by secondary slides continues due to weaker residual strengths, increased moisture
contents, and the additional weight of fluvially deposited material.
Summary of slope development and channel evolution
The six stages of bank -slope development represent a conceptual model of width adjustment. Stages
(premodified, constructed, degradation, threshold, aggradation, and restabilization) are induced by a
succession of interactions between fluvial and hillslope processes (Table VI). By associating the six bank -slope
development stages with the dominant hillslope and fluvial processes, and with characteristic channel forms, a
conceptual model of channel evolution over time and space was developed (Table VII). The model does not
suggest that each adjusting reach will undergo all six stages but implies that specific trends of bed -level
response will result in a series of mass wasting processes and definable bank and channel forms. However, the

24
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A. SIMON
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CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS 25
conceptual framework of the simultaneous retreat of the vertical face and flattening along surfaces below is
supported by the observations of other investigators (Carson and Kirkby, 1972, p. 184).
The model described in this report makes the following assumptions (1) there is no local bedrock control of
bed - level, (2) overadjustment and secondary response are active processes, (3) the bed and banks are free to
adjust to imposed changes, and (4) successive stages of evolution are not interrupted by other disturbances.
Extrapolation of the six -stage conceptual model of bank -slope development and channel evolution should be
particularly appropriate for areas of the Mississippi embayment and the central United States. Application
over a broader geographical area is conceptually justified on the basis that similar processes can create similar
forms. Variations in time scales and forms from the idealized model will occur due to local bedrock control
and variations in relief, soil properties, and climatic conditions. Still, the model reflects the overadjustments
inherent to fluvial response and may be useful in determining expected changes in alluvial channel
morphology over the course of a major adjustment cycle.
SUMMARY
Channel modifications between 1959 and 1978 in West Tennessee caused a series of morphologic changes
along modified reaches and tributary streams. Degradation occurred for 10 to 15 years at sites upstream of the
area of maximum disturbance (AMD) and lowered bed - levels by as much as 6.1 metres. Aggradation occurs
downstream of the AMD with the greatest rates near the stream mouths. Initially degraded sites experience a
secondary aggradation phase in response to excessive incision and gradient reduction.
Adjustments of channel geometry and phases of channel evolution are characterized by six process -
oriented stages of morphologic development— premodified, constructed, degradation, threshold, aggra-
dation, and rest abilization. Downcutting and toe removal occur immediately following construction along
stream reaches upstream of the AMD (degradation stage). Bank failure by mass wasting begins during the
threshold stage when the critical height and angle of the bank material is exceeded. Top -bank widening
continues concurrently with deposition on the channel bed during the aggradation stage as the slough line
develops and becomes an initial site of lower -bank stability. The development of the bank profile is defined in
terms of three observable and dynamic surfaces (1) vertical face (70" to 90 "), (2) upper bank (25" to 50 ''), and (3)
slough line (20" to 25'). Alternate channel bars form during the restabilization stage and represent incipient
meandering of the channel. The vertical face may concurrently take a convex shape from continual reworking
of materials by stream flow.
REFERENCES
Alexander, D. 1981. 'Threshold of critical power in streams, Discussions and reply', Bulletin of the Geological Soviet y nJ'America Bulletin,
92, 310 -312.
Begin, Z. B., Meyer, D. F., and Schumm, S. A. 1981. 'Development of longitudinal profiles of alluvial channels in response to base -level
lowering', Earth Surface Processes, 6, 49 -68.
Bull, W. B. 1979. 'Threshold of critical power in streams', Bulletin gl'the Geological Society q /'America, part 1, 90, 453 -464.
Carson, M. A. and Kirkby, M. J. 1972. Hillslope Form and Process, Cambridge University Press, London, 475 pp.
Gilbert, G. K. 1880. 'Report on the geology of the Henry Mountains (2)', Geographical and Geological Survey of the Rocky Mountain
Region, United States Government Printing Office, Washington, 170 pp.
Graf, W. L. 1977. 'The rate law in fluvial geomorphology', American Journal of Science +, 277, 178-191.
Hack, J. T. 1960. 'Interpretation of erosional topography in humid temperate regions', American Journal gl Science, 258 -A, 80 -97.
Hey, R. D. 1979. 'Dynamic process - response model of river channel development', Earth Surface Processes, 4, 59 -72.
Hidinger, L. L. and Morgan, A. E. 1912.'Drainage problems of Wolf, Hatchic, and South Fork of Forked Deer Rivers, in west Tennessee',
in The Resources of Tennessee. Tennessee Geological Survey, 2, No. 6, 231 -249.
Hupp, C. R. and Simon, A. 1986. 'Vegetation and bank -slope development', Proceedings of the 4th Federal Interagency Sedimentation
Conference, Las Vegas, Nevada, March 1986, 2, 5 -83 to 5--92.
Lane, E. W. 1955. 'The importance of fluvial morphology in hydraulic engineering', Proceedings gf the American Society o/' Civil
Engineering, 81, No. 745, 17.
Lohnes, R. A. and Handy, R. L. 1968. 'Slope angles in friable loess', Journal gf,yeology, 76, No. 3, 247 - -258.
Lutton, R. J. 1974. 'Use of loess soil for modeling rock mechanics', Miscellaneous Report S- 74 -28, U.S. Army Engineers Waterways
Experiment Station, Vicksburg, Mississippi.
Mackin, J. H. 1948. 'Concept of a graded river', Bulletin of the Geological Society of America Bulletin, 59, 463 -511.
Morgan, A. E. and McCrory, S. H. 1910.'Drainage of lands overflowed by the North and Middle Forks of the Forked Deer River and the

26 A. SIMON,
Rutherford Fork of the Obion River in Gibson County, Tennessee', Bulletin of the Tennessee State Geological Survey, 3 -B, 17-43,
Pickup, G. 1975. `Downstream variations in morphology, flow conditions and sediment transport in an eroding channel', Zeitschrift fur
Geomorphologie, 19, 443 -459.
Robbins, C. H. and Simon, A. 1983. `Man- induced channel adjustment in Tennessee streams', United States Geological, Survey Water -
Resources Investigations Report, 82 -4098, 129.
Schumm, S. A. 1973. `Geomorphic thresholds and the complex response of drainage systems', in Morisawa, M. (Ed.), Fluvial
Geomorphology, Binghamton, State University of New York, 229 -310.
Schumm, S. A. and Lichty, R. W. 1965. `Time, space, and causality in geomorphology', American Journal of Science, 263, 110 - -119.
Schumm, S. A. and Parker, R. S. 1973. `Implications of complex response of drainage systems for Quaternary alluvial stratigraphy',
Nature, 243, 99 -100.
Simon, A. In press. `Gradation processes and channel evolution in modified West Tennessee streams: process, response, and form', United
States Geological Survey Professional Paper, 1470.
Simon, A. and Hupp, C. R. 1986. `Channel widening characteristics and bank slope development along a reach of Cane Creek, West
Tennessee', in Subitzsky, Seymour (Ed.), Selected Papers in the Hydrologic Sciences, United States Geological Survey Water - Supply
Paper, 2290, 113 -126.
Simon, A. and Hupp, C. R. 1987. `Geomorphic and vegetative recovery processes along modified Tennessee streams: an interdisciplinary
approach to disturbed fluvial systems', Forest Hydrology and Watershed Management, International Association of Hydrologic
Sciences, Publication, 167, 251 -262.
Simon, A. and Robbins, C. H. 1987. `Man- induced gradient adjustment of the South Fork Forked Deer River, West Tennessee',
Environmental Geology and Water Sciences, 9, No. 2, 109 -118.
Skempton, A. W. 1953. `Soil mechanics in relation to geology', Proceedings of Yorkshire Geological Society', 29, 33 -62.
Thorne, C. R., Murphey, J. B., and Little, W. C. 1981. `Bank stability and bank material properties in the bluHiine streams of northwest
Mississippi', Stream Channel Stability, United States Department of Agriculture Sedimentation Laboratory, Oxford, Mississippi,
Appendix D, 257 pp.
United States Department of Agriculture 1980. `Summary report final: Obion- Forked Deer River Basin Tennessee', Soil Conservation
Service, 43 pp.
Williams, G. P. and Wolman, M. G. 1984. `Downstream effects of dams on alluvial rivers', U.S. Geological Survey Professional Paper 1286,
83 pp.
Wilson, K. V. 1979. `Changes in channel characteristics, 1938 -1974, of the Homochitto River and tributaries, Mississippi', U.S. Geological
Survey Open -File Report, 79 -554, 18 pp.

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EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 14,11-26 (1989)
A MODEL OF CHANNEL RESPONSE IN DISTURBED ALLUVIAL
CHANNELS
ANDREW SIMON
U.S. Geological Survey, Nashville, Tennessee, U.S.A.
Received 1 November 1986
Revised 21 March 1988
ABSTRACT
Dredging and straightening of alluvial channels between 1959 and 1978 in West Tennessee caused a series of morphologic
changes along modified reaches and tributary streams. Degradation occurred for 10 to 15 years at sites upstream of the
area of maximum disturbance and lowered bed - levels by as much as 6.1 m. Following degradation, reaches upstream of
the area of maximum disturbance experienced a secondary aggradation phase in response to excessive incision and
gradient reduction. Aggradation downstream of the area of maximum disturbance reached 0.12 m per year with the
greatest rates occurring near the stream mouths.
The adjustment of channel geometry and phases of channel evolution are characterized by six process- oriented stages of
morphologic development -- premodified, constructed, degradation, threshold, aggradation, and restabilization. Down -
cutting and toe removal during the degradation stage causes bank failure by mass wasting when the critical height and
angle of the bank material is exceeded (threshold stage). Channel widening continues through the aggradation stage as the
`slough line' develops as an initial site of lower -bank stability. The bank profile develops three dynamic elements (1)
vertical face (70° to 90 °), (2) upper bank (25° to 50 °), and (3) slough line (20° to 251). Alternate channel bars form during the
restabilization stage and represent incipient meandering of the channel.
KEY WORDS Unstable channels Degradation Aggradation Channelization Empirical model
INTRODUCTION
Changes imposed on a fluvial system, be they natural or man - induced, tend to be absorbed by the system
through a series of channel adjustments (Gilbert, 1880; Mackin, 1948; Lane, 1955; Hack, 1960; Schumm, 1973;
Bull, 1979). Due to the drastic changes in energy conditions imposed by channel dredging, enlarging, and
straightening, a rejuvenated condition is established within the fluvial system much like that imposed by uplift
or a natural lowering of base level. The predominant difference is one of time scale. Channel adjustments
caused by climatic changes or uplift may be exceedingly slow, and progressive, practically imperceptible by
man's standards. Conversely, large -scale channel modifications by man result in a sudden and significant
shock to the fluvial system that causes migrating knickpoints and observable morphologic changes. This
shortened time scale presents an opportunity to document successive process -- response mechanisms through
the course of fluvial adjustment over time and space. Channel modifications from 1959 to 1978 through much
of three major river systems in West Tennessee create a natural laboratory for the study of channel
adjustments in rejuvenated fluvial networks (Table I).
PURPOSE AND SCOPE
The U.S. Geological Survey in cooperation with the Tennessee Department of Transportation (TDOT) began
this study in 1981 as a result of bridge failures on modified channels in West Tennessee. The purpose was to
0197- 9337/89/010011- 16$08.00
1989 by John Wiley & Sons, Ltd.

12 A. SIMON
document and empirically model stream - channel changes and process— response mechanisms following large -
scale channelization projects in West Tennessee. TDOT was interested in obtaining estimates of channel
changes in alluvial - channel morphology throughout West Tennessee for the purpose of designing adequate
river - crossing structures.
BACKGROUND
West Tennessee is an area of 27500 km2 bounded by the Mississippi River on the west and the Tennessee
River divide on the east (Figure 1).The region is characterized by unconsolidated, highly erosive formations,
predominantly of Quaternary age (U.S. Department of Agriculture, 1980). The major rivers of the region
(Obion, Forked Deer, and Hatchie) flow in channels comprising medium -sand beds and silt —clay banks. In
contrast, many small tributary streams flow through extensive silt deposits of reworked Wisconsin loess and
have strikingly similar bed- and bank - material properties (Simon, in press). Those tributaries (such as Cub
and Porters Creeks) that head in the eastern sandier Tertiary and Cretaceous formations flow on medium -
sand beds. A lack of bedrock control of base level assures unrestricted bed -level adjustment.
Accounts of river conditions following land clearing throughout the region (mid to late 1800s) describe
extremely sinuous, sluggish sediment - choked streams that frequently overflowed their banks for 3 to 10 days
at a time (Hidinger and Morgan, 1912). This channel infilling, attributed to major deforestation and severe
upland erosion in the late 1800s, prompted channel dredging and straightening in West Tennessee near the
EXPLANATION
Basin boundary
................
Clearing and snagging
Channelization
Year work was completed F
0 10 20 30 MILES
0 10 20 30 KILOMETERS W
36 °_�.
35°
90
C�
Base from U.S. Geological Survey,
State base map, 1967,
revised 1973
7 r-r�3 a 1
—q'T'11
ER BASIN�q�/
F
\i\ .
FORKED DEER RIVER BASIN�
HATCHIE RIVER
Figure 1. Extent of channel modifications in West Tennessee

CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS 13
turn of the century. The most recent channel modification programme in West Tennessee occurred between
1959 and 1978 and is the subject of this study (Table 1). The modified channels listed in Table I by no means
include all of the modified streams in the region, but were selected to represent a spectrum of West Tennessee
alluvial channels based on size, location, and bed and bank properties, and to represent the magnitude and
extent of modifications. Channel lengths were shortened as much as 44 per cent, gradients were increased as
much as 600 percent, and beds were lowered as much as 5.2 m (Simon, in press).
DATA COLLECTION
Channel- morphology data collected from 1982 to 1984 were compared to previous surveys to determine
channel changes with time. Sources of these earlier surveys include the U.S. Geological Survey, Army Corps of
Engineers, Soil Conservation Service, and the Obion- Forked Deer Basin Authority. These data consist of bed
elevations and gradients, channel top - widths, and channel lengths before, during, and after modification.
Where available, gauging station records were used to record annual changes in the water- surface elevation at
a given discharge. Changes in the stage- discharge relation imply similar changes on the channel bed and can
be used to document bed -level trends (Blench, 1973; Wilson, 1979; Robbins and Simon, 1983; Williams and
Wolman, 1984).
The identification and dating of various geomorphic surfaces were diagnostic in determining relative
stability of a reach and the status of bank -slope development (Simon and Hupp, 1986; Hupp and Simon,
1986). Data collection involved dating of riparian vegetation (1) on newly stabilized surfaces to determine the
timing of initial stability for that surface, and (2) on unstable bank surfaces to estimate rates of bank retreat.
BED -LEVEL ADJUSTMENT
Adjustment of the channel bed at a site follows channel modifications immediately and can be described
through time. Channel bed adjustments through time (irrespective of the temporal and spatial scales applied)
Table I. Type, extent, and dates of recent channel modifications on studied streams (1959 -1978)
Basin
Stream
Type of modification
Length
(km)
Dates
Obion
Obion River
Enlarging and straightening
75.1
1959-66
Clearing and snagging
6.8
1976
Enlarging and straightening
—
1974 -77
North Fork
Enlarging and straightening
17.5
1967
Obion River
Clearing and snagging
17.4
1974 -76
Hoosier Creek
Enlarging and straightening
11.9
1967
Rutherford Fork
Enlarging
11.9
1967
Obion River
Clearing and snagging
28.8
1973 --78
South Fork
Enlarging
9.6
1967, 1969
Obion River
Clearing and snagging
27.5
1976 -78
Forked Deer
North Fork Forked
Enlarging and straightening
6.9
31.5
1973
1974 -77
Deer River
Clearing and snagging
Pond Creek
Clearing and snagging
21.1
1976 -78
South Fork Forked
Enlarging and straightening
7.1
1969
Deer River
Clearing and snagging
36.5
1973 -77
Meridian Creek
Enlarging and straightening
2.6
1959?
Enlarging and straightening
8.4
1969
Enlarging
2.6
1969
Cane
Cane Creek
Enlarging and straightening
52.0
1970
(lower Hatchie)
Enlarging and straightening
20.9
1978
Hyde Creek
Enlarging and straightening
1.3
1970
Upper Hatchie
Cub Creek
Enlarging and straightening
15.6
34.4
1970
1972
Porters Creek
Enlarging and straightening
tL

1.
14 A. SIMON
are best described mathematically by nonlinear functions which asymptotically approach a condition of
minimum variance. The description of channel adjustment and evolution by nonlinear decay functions is well
documented (Schumm and Lichty, 1965; Graf, 1977; Bull, 1979; Hey, 1979; Robbins and Simon, 1983).
However, there seems to be considerable disagreement as to the mathematical form of the function. Graf
(1977) used exponential functions to describe the `relaxation time' necessary to achieve equilibrium following
a disturbance. Simon and Robbins (1987) used similar equations to model gradient adjustment through time.
Williams and Wolman (1984) found that hyperbolic functions were appropriate for describing degradation in
alluvial streams, downstream from dams. In this study, both exponential and power equations were initially
fitted to the observed data. The power function gave consistently superior matches of the empirical data, and
was therefore used to describe bed -level adjustment through time. The general form is:
E =a(t)b (1)
where E= elevation of the bed for a given year, in metres above sea level;
a= coefficient determined by regression, representing the premodified elevation of the bed, in metres
above sea level;
t =time since beginning of adjustment process, in years, where to =1.0; and
b= dimensionless exponent, determined by regression and indicative of the nonlinear rate of change
on the bed.
Trends of bed -level change through time at gauging stations (where bed elevation is measured every 4 to 6
weeks) having periods of record of up to 20 years (Table II), served to support the well -known concept of
nonlinear adjustment (Figure 2). Once it was established that power functions provided the best fit to this
gauged data, data from periodically surveyed sites were similarly fitted (Figure 2). Sites with just two recorded
82
81
to
LM so
U-
w
Z
79
LAJ
�i
w
J
Q 79
* Note. Denotes specific page data used.
'South Fork Forked Deer River
River kilometer = 12.71
e
°
°
r' = 0.94
N 1964 1968 1972 1976 1980 1984
w
>
89
O
M
Q
Z
88
O
Q
J
87
w
86
85
84
T 1 —r-
° ° • Rutherford Fork Obion River
River kilometer = 7.88
r'= 0.93 �
r'= 0.96
1958 1962 1966 1970 1974 1978 1982 1986
143
°
142
°
141
°
140
139 r'= 1.00
138
Porters Creek
River kilometer = 27.51
137
1970 1974 1978 1982 1986
90
89
88
87
86
85
84
83
82
1962 1966 1970 1974 1978 1982 1986
'South Fork Obion River
° River kilometer = 9.33
°
r'= 0.93
Figure 2. Examples of fitting power equations to degradation and aggradation trends through time

CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS
Table II. Sites with calculated aggradation ( +b) and degradation ( -b)
15
Stream
b
n
r
RKM
To
Stream
b
n
r2
RKM
To
Cane Creek
- 0.01620
7
0.99
2389
1969
Obion River
- 0.02220
10
0.95
110.22
1965*
1978
-00168
2
-
23.89
1980
- •01720
-00463
10
-74
110.22
1974*
12
- -02022
4
1.00
2024
1969
-87
-00235
16
•76
10008
1968*
9.33
•01052
2
-
20.24
1980
•00908
19
-93
86.40
1965*
Obion River •00111
- •03300
3
1.00
14.46
1869
-00518
15
-84
55.03
1963*
--
•00770
2
-
14.46
1980
-00585
16
-74
3347
1960*
1975
- -03131
2
-
10.09
1969
--00372
13
-80
28.96
1972*
-00352
2
-
10.09
1980
Pond Creek
- •00828
5
-81
18.29
1977
- •04126
3
•91
6.53
1969
1965*
- •00799
4
-84
1580
1977
00303
- •02011
4
•92
4.06
1969
- -01233
4
-97
11.78
1977
•00835
2
-
4.06
1980
- •00900
5
-79
1.71
1977
Cub Creek
- -00243
3
-69
11.13
1969
- •00342
3
•87
9.22
1969
- •00565
4
•88
3.48
1969
Porters Creek
- •01069
6
1.00
27.51
1971
- -00905
5
•91
2.48
1969
- •01320
7
-99
18.02
1971
-00272
2
-
2.48
1976
--00578
6
1.00
14.30
1971
Hoosier Creek
- •00843
3
1.00
8.29
1967
Rutherford Fork
Obion River
•00149
19
-60
48.11
1965*
- •01130
4
•94
4.81
1966
- •00317
4
-91
2880
1977
- •02081
3
•67
•88
1965
- •00493
3
1.00
24.46
1977
•00274
2
--
•88
1968
- -00991
4
-79
16.73
1972
- -02630
3
•99
-02
1965
•00356
4
•99
16.73
1977
- •01728
9
-93
7.88
1965*
-00433
10
-88
7.88
1974*
Hyde Creek
•00281
2
-
3.81
1975
South Fork
Forked Deer
River
- •00895
6
•59
44.41
1976
- •00737
2
-
3.81
1969
- -00950
10
•92
26.23
1974*
- •01070
4
-92
222
1969
- -00978
5
•76
21.40
1969
- •01380
3
•99
1.19
1969
- •01264
5
•96
19.15
1969
- •02050
4
-00
-02
1969
- •01630
15
-94
1271
1969*
-01180
13
-92
5.31
1969*
Meridian Creek
- •00326
3
-99
5.94
1965
- •00580
4
-98
4.73
1964
South Fork
Obion River
•00133
13
•90
55.35
1969*
-00341
3
•99
2.41
1969
- -00054
4
•26
45.70
1972
- •00190
3
-99
1.54
1967
- -00238
6
•50
37.33
1972
North Fork
Forked Deer
River -00740
4
•95
38.46
1977
- -00661
7
-90
30.89
1977*
- •01076
5
-52
32.47
1974
- •00573
5
•87
27.03
1972
-00839
4
-96
30.28
1978
- •00932
4
•94
18.34
1972
- •01720
10
-95
8.53
1973*
- •02430
12
•87
9.33
1965*
-02297
3
-87
6.16
1972
•00544
8
•88
9.33
1975*
North Fork
Obion River •00111
15
-69
59.37
1969*
-00206
2
--
42.48
1979
-00490
2
--
33.95
1975
--00372
13
-80
28.96
1972*
- •01240
6
-93
15.83
1965*
--02470
4
-85
9.49
1965*
00303
5
-89
9.49
1967*
Note: b, nonlinear gradation rate; n, number of observations; rz, coefficient of determination; RKM, river kilometre; To, start of observed
gradation process; *, specific gauge data used; where n is low, statistical significance may be limited)

16 A. SIMON
bed elevations were included because of confidence in the general trend of adjustment at -a -site, and to
increase areal coverage of the data network.
The initial response of a channel reach (degradation or aggradation) depends on the location of the reach
relative to the imposed channel modifications. Reaches located upstream from the disturbance will eventually
go through an initial phase of general degradation, followed by a period of general aggradation. In these cases,
the data set was split, and Equation 1 was fitted to the degradation and aggradation periods separately. The
last data point of the degradation curve (lowest elevation) was also used as to for the aggradation curve.
The exponent `b' (Equation 1) is a measure of the nonlinear rate of degradation (negative b) or aggradation
(positive b), and was used as the primary variable in identifying longitudinal adjustment trends. Values of b
used in this study are listed by stream. in Table II. The data described by Equation 1 imply that bed -level
response at a site is initially rapid and then diminishes as bed elevation asymptotically approaches a condition
of no net change (Begin et al., 1981).
Because adjustments of channel width are functionally related to changes that occur on the channel bed
(Table III), a bed -level model was developed and is summarized using the Obion River system as an example
(Figure 3). The area of maximum disturbance (AMD) is defined as the upstream terminus of channel work.
However, if a stream is channelized throughout its length, stream power is greatest at the mouth, and the
AMD is located there (Simon, in press).
Stream reaches are divided into the following four groups based on the location of the reach relative to the
imposed AMD in the fluvial network, and the dominant process occurring on the channel bed: (1)
downstream aggradation, (2) migrating degradation, (3) secondary aggradation, and (4) premodified
aggradation. These reach types in the Obion River drainage system are represented in Figure 3 and denote not
only temporal data at sites in the network but also longitudinal bed response through time, with distance from
the AMD.
Maximum amounts of degradation (largest negative b values) occurred just upstream of the AMD (as a
response to the significant increase in stream power imposed by the channel work here in 1967, A in Figure 3)
and decreased nonlinearly with distance upstream (curve C in Figure 3). Note that curve C also represents the
headward migration of the degradation process and therefore has a temporal as well as spatial component (To
in Table 11). Rates of migration on the Obion River Forks are 1.6 km year -1. Degradation occurred for 10 to
15 years at sites just upstream of the AMD and has lowered bed levels as much as 6.1 m. Effects of the imposed
disturbance decreased with distance upstream resulting in minimal degradation rates at about river kilometre
150 (b = 0.0). The absence of net erosion or deposition on the channel bed (b = 0.0) is analagous to Bull's (1979)
threshold of critical stream power. Further upstream (E in Figure 3), channel beds of the Obion River system
(including upstream reaches of the North, South, and Rutherford Forks) remain unaffected by the
downstream channel work and aggrade at low, `background' rates.
Sites downstream of the AMD (line B in Figure 3) aggraded immediately following channel modification
with material delivered from eroding reaches upstream. Maximum recorded rates of aggradation are
0.12 m year -1 close to the mouth of the Obion River. Aggradation near `D' in Figure 3 occurs at previously
degraded sites where gradient has been significantly reduced by incision and knickpoint migration. Flows
become incapable of transporting the greater bed - material loads being generated from upstream channel
Table III. Coefficient of determination (r') for
relations between changes in top width (D W) and
the absolute value of nonlinear degradation rates
(b)
Basin
r
n
Cane Creek
0.49
9
Forked Deer River
0.60
13
Obion River
0.80
18
Upper Hatchie River
0.64
7

0.010
N 0.005
w
V
� 0.000
O`
z
O —0.005
Q
< —0.010
w
O —0.015
w
Q
D: —0.020
Of
Q
w
Z -0.025
J
Z
Z —0.030
u_
O
—0.035
O
Q
V —0.040
0
Z
—0.045
CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS
■ B
la ®
THRESHOLD OF CRITICAL STREAM POWER
n _
— — —
E
■
■
C
■
A Area of maximum
disturbance (1967)
B Line representing downstream
aggradation
C Curve representing migrating
degradation A
D Location of secondary
aggradation
E Location of premodified
aggradation
■ Observed data (b)
25 45 65 85 105 125 145 165
DISTANCE ABOVE MOUTH OF OBION RIVER, IN KILOMETERS
Figure 3. Model of bed -level response for the Obion River system
17
beds. The channel aggrades and thereby, increases gradient and transporting capacity. Schumm and Parker
(1973) describe similar results in an experimental watershed. This secondary aggradation migrates headward
with time and is apparently a response to excessive lowering (overadjustment) by the degradation phase
(Simon, in press). Hey (1979) and Alexander (198 1) similarly argue for alternating phases of degradation and
aggradation following rejuvenation of a channel.
With the short time scales involved in the application of the disturbance and the channel's consequent
response, the evolution of channel form over time and space can be monitored. Numerical results such as
those plotted in Figure 3 can be derived for most alluvial systems where even limited data are available.
Results could be applied over longer time scales with the inclusion of dendrochronologic data (as in Graf,
1977), or to experimental and conceptual models of channel evolution such as Schumm and Parker (1973) and
Hey (1979), respectively. It has been shown that bed -level adjustment does follow nonlinear trends, both over
time at -a -site (Figure 2), and over time with distance upstream (Figure 3).
WIDTH ADJUSTMENT AND (BANK -SLOPE DEVELOPMENT
Channel widening by mass wasting processes is common in adjusting channels in West Tennessee. Bank
failure in the loess- derived alluvium is induced by the overheightening and oversteepening of the bank by
degradation and by undercutting and seepage forces at the toe of the bank (Table II1). Piping and tension
cracks in the bank materials enhance bank failures by internally destabilizing the bank (Simon, in press).
Channel bank material of West Tennessee streams is completely alluvial; comprised of loess - derived
Quaternary sediments which are predominantly nonplastic, dispersive silts of low cohesion (Simon, in press).
In situ shear strength tests confirm the existence of low cohesive strengths; generally less than 14 kilopascals
(kPa). These highly erodible materials tend to maintain high moisture contents due to the relatively slow
downward adjustment of the water table. The mean degree of saturation for studied channel banks is

18 A. SIMON
91 per cent (182 tests) even though most samples were taken during periods of low flow (Simon and Hupp,
1987). Moderate rises in river stage are sufficient to complete saturation of the channel banks, and result in
mass failures upon recession of river stage. Figure 4 shows the frequency distributions of the primary soil
mechanics variables; cohesion, friction angle, and field density. Note that the histograms do not display a
normal distribution, but are markedly skewed towards lower strength characteristics. Friction angle and
cohesion values vary within accepted limits for the types of materials tested (Lohnes and Handy, 1968).
Maximum values of cohesion represent tests taken at depth in localized clay rich deposits. Statistical
properties of the soil mechanics data are given in Table IV.
Changes in channel width following the most recent major modifications of West Tennessee stream
channels are summarized in Table V. Mean widening data serve only to provide a point of reference as to
magnitude and variability of width changes. Minimum and maximum values of channel widening (Table V)
do reflect a realistic range of width changes throughout the stream lengths studied.
70
00
ao
40
30
20
to
W
U
W 45
OC 40
99
35
V 30
O 25
W 20
O
} 15
Z 10
t =J
0
W
w
3 1 15 21 V
COHESION, IN KILOPASCALS
f0
eo
70
so
so
40
30
20
10
10 15 20 25 30 33 40
FRICTION ANGLE, IN DEGREES
13 10 " 15 21
FIELNSITY, IN KILONEWTONS
D DE
Figure 4. Frequency distributions of soil mechanics data for studied streams
Table IV. Statistical properties of selected soil mechanics data
Standard
n Mean error Minimum
Maximum
Cohesion (kPa) 168 8.59 0.59 0.14
54.3
Friction angle 168 30.1 0.62 8.30
42.4
Field Density (kN) 182 18-6 0-22 11-3
48-1
[kPa, kilopascals; kN, kilonewtons]

CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS 19
Table V. Range of changes in channel top width (DW) by stream
Years since most
DW recent major
(m) channelization
Stream Mean Min Max n (from 1983)
Cane Creek
18.2
14
25
6
13
Cub Creek
2.1
0.0
6.7
4
13
Hoosier Creek
14.2
7.0
2
3
16
Hyde Creek
3.1
0.3
5.4
3
18
Meridian Creek
3.4
2.4
4.3
2
16
North Fork Forked Deer River
10.3
3.0
25
4
10
North Fork Obion River
10.4
0.6
25
5
16
Obion River
37.8
20
59
4
17
Pond Creek
2.9
2.1
4.3
4
5
Porters Creek
7.7
3.7
12
3
11
Rutherford Fork Obion. River
72
0.0
18
5
14
South Fork Forked Deer River
13.6
4.9
27
6
14
South Fork Obion River
11.9
0.0
36
7
14
Processes and stages of bank retreat and slope development
The processes and successive forms of bank retreat and bank -slope development reflect the interaction of
hillslope and fluvial processes. Interpretations of these processes are based largely on bed -level adjustment
trends and botanical evidence. A complete `cycle' of slope development from the premodified condition
through stages of adjustment, to the eventual reestablishment of stable bank conditions, assumes that
degradation is of sufficient magnitude to instigate unstable bank conditions and a 20- to 40 -year period of
mass wasting. However, in grossly unstable channels comprised of silt -clay alluvium, much longer amounts of
time may be required for the complete restabilization of the channel. The idealized representation of six stages
of bank retreat and bank -slope development (Figure 5) represent distinguishable bank morphologies
characteristic of the various reach types describing bed -level adjustment (Table VI). By applying a space for
time substitution, longitudinal variation in bank -slope development can then be used to denote morphologic
change through time. Sand -bed channels will require 20 to 40 years of channel adjustment to pass through the
first five stages of the model. In these types of channels, a total of 50 to 100 years is assumed necessary for the
restabilization of the channel banks (stage VI) and for the development of incipient meanders.
Premodified Stage (1). Premodified bank conditions are assumed to be the result of `natural' fluvial
processes and land -use practices. Bank failure by mass wasting processes generally does not occur and banks
are considered stable. Data referring to stable bank conditions along West Tennessee channels are available
from the literature, agency work plans, the upstream -most reaches of present day (1987) adjusting networks,
and the nonchannelized Hatchie River.
Premodified bank conditions (stage I in Figure 5) are characterized generally by low -angle slopes (20° to
25 °), convex upper -bank and concave lower -bank shapes, and established woody vegetation along the top
bank and downslope towards the low -flow channel. Channel reaches with these characteristic bank
conditions generally have width -depth ratios (as measured from the flood -plain surface) between 6 and 10,
and narrow slowly with time due to limited aggradation and bank accretion.
Premodified channel widths (prior to 1915) along the downstream ends of the forks of the Obion and
Forked Deer Rivers ranged from 15 to 28 in depending on drainage area (Morgan and McCrory, 1910;
Hidinger and Morgan, 1912). The present premodified sites (upstream -most) on the forks of the Obion River
are approximately 23 m wide.
Constructed Stage (11). Construction of a new channel involves reshaping the existing channel banks or
repositioning the entire channel. In either case, the banks are generally steepened, heightened, and made linear
(stage II in Figure 5). West Tennessee channels are generally constructed as trapezoids with bank- slopes
A,

Slab and rotational
failures
IVa
j
\--Previous profile
V
\— Previous profile
VI
- -- —\ Substantial bed-
level recovery
Non dispersive
materials
Slough line
Note: Scale is relative.
RESTABILIZATION STAGE
Figure 5. The six stages of bank -slope development
Substantial bed -level
recovery
Moderately dispersive
\ materials
ranging from 18° to 34 °. Channel widths are increased and vegetation is removed in order to increase channel
conveyance. It is appropriate to consider the constructed condition (stage II) as a transition from the stable,
premodified state to the more unstable degradation stage, when further heightening and steepening of the
banks occur by fluvial processes.
Degradation Stage (III). The degradation stage (stage III in Figure 5) is characterized by rapid erosion of
the channel bed and the consequent increase in bank heights. Downcutting generally does not steepen bank
slopes directly but results in bank angles that approximate the angle of internal friction (Skempton, 1953;
Carson and Kirkby, 1972; Simon and Hupp, 1986). Steepening occurs when moderate flows attack basal
surfaces and remove toe material (stage IIIa in Figure 5). Pop -out failures, due to excess pore pressure at the
bank toe, similarly cause steepening. Ideally, widening by mass wasting does not occur because the critical
bank height and angle have not yet been exceeded. Bed degradation with constant channel width (Pickup,
1975) is thus possible in these materials if the amount of incision is limited.
The degradation stage is probably the most important in determining the magnitude of channel widening
that will occur because the amount of incision partly controls the bank - failure threshold. Sites close to the
AMD (where degradation is greatest) are therefore expected to widen appreciably.
Threshold Stage (IV). Stage IV (Figure 5) is the result of continued degradation and basal erosion that
further heightens and steepens the channel banks. The critical bank height is exceeded, and bank slopes and
shapes become the product of mass wasting processes.

a
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CHANNEL
RESPONSE IN DISTURBED ALLUVIAL CHANNELS
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22
A. SIMON
Degraded reaches of West Tennessee streams have mean bank angles ranging from 55 to 65 degrees.
However, toe removal often creates almost vertical banks. Given the low cohesive strengths and the ambient
high moisture contents of the channel banks, critical bank heights at saturation rarely exceed 5 m (Simon and
Hupp, 1987). Degraded reaches of the studied streams, having bank heights commonly in excess of 8 m,
therefore fail readily.
Slab failures occur due to excessive undercutting and the loss of support for the upper part of the bank,
Pop -out failures at the base of the bank due to ground -water saturation and excess pore -water pressure
further oversteepen the bank and lead to slab failure.
Deep- seated rotational failures shear along an are and often become detached from the top bank surface by
piping. Water drains into the bank mass along the pipes, thereby reducing shear strength and increasing pore -
water pressure and lubrication along the potential failure surfaces. Failures can occur as a saturated mass of
bank material that leave 1.8 to 3.0 -m -long slickensides along the vertical section of the failure surface (often a
pipe) and come to rest in a lobate form at the base of the bank. The failed material loses shear strength and, in
its saturated state, can be removed easily by moderate flows. Rotational failures and debris flows under
saturated conditions along overheightened reaches of the Rutherford and South Forks Obion River and Cane
Creek tend not to reduce bank angles as much as rotational failures under drier conditions. These failure
mechanisms are considered worst -case conditions in dispersive loess - derived materials, as occurs in the Cane
Creek basin.
Rotational failures also occur in less dispersive materials where piping is not as prevalent. Banks fail as a
single mass of material along an arc, leaving elliptical scars 15 to 60 m long. The failed material does not come
to rest at the base of the bank but forms a definable surface higher on the bank profile at slopes ranging from
25° to 50° (stage IV in Figure 5). This surface, the `upper bank' (Simon and Hupp, 1986), is often identified by
tilted and fallen vegetation. A second definable surface, the `vertical face', representing the top section of the
major failure- plane, also is developed during the threshold stage (stage IV in Figure 5 and Table VI).
Bank retreat by slab and rotational failures continues through the threshold stage, developing the vertical
face and upper bank as two distinguishable bank forms (stage IV in Figure 5). The threshold stage is the first of
two that are dominated by active channel widening (Table VI). Failed material is generally removed by
moderate to high flows, thereby retaining the overheightened and oversteepened bank profile, and giving the
banks a scalloped appearance.
Aggradation Stage (V). Stage V is marked by the onset of aggradation on the channel bed and often can be
identified by sand deposited on bank surfaces. Bank retreat dominates the vertical face and upper bank
sections because bank configurations still exceed the critical conditions of the material. The failed material on
the upper bank is subject to secondary, low -angle slides resulting from frequent wetting by rises in stage and
from the added weight of fluvially deposited materials. The low -angle failures reduce the angle of the upper
bank and extend it downslope (stage V in Figure 5).
Previously failed material on the upper bank also moves downslope by low -angle slides and shows --victence
of fluvial reworking and deposition. This combination of mass wasting and fluvial processes creates a low -
angle surface (20° to 25 °) termed the `slough line', extending downslope from the upper bank (Simon and
Hupp, 1986). Woody vegetation reestablishing on the slough line shows only mild tilting from secondary slips
and can be used to date the timing of renewed bank stability (stage V in Figure 5; Hupp and Simon, 1986).
Rates of deposition on the slough line can be obtained by dividing the depths of burial above the root collar of
trees, by the age of the trees.
Bank stability, as indicated by establishing vegetation on the slough line, extends upslope, away from the
channel with time. Bank angles on the upper bank and slough line surfaces continue to flatten through stage V
due to secondary low -angle slides and fluvial reworking. The range of bank angles given for the vertical face
and the upper bank (Table VI) are in general agreement with values reported in previous studies of unstable
slopes comprised of loess (Lohnes and Handy, 1968), and loess- derived alluvium (Throne et al., 1981).
Three classes of slope angles were identified by Lohnes and Handy (1968) in their classic study of Iowa and
West Tennessee loess; 10 to 41 degrees, 50 to 52 degrees, and 68 to 85 degrees. The second and third classes
relate to the upper bank and vertical face as described in this report respectively. The angles reported by
Lohnes and Handy (1968) for their second class (50 to 52 degrees) are steeper than those observed in West

CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS 23
Tennessee during this study (25 to 50 degrees). The reason is that with the addition of water from seepage or
from rises in river stage, effective strength, and therefore slope angles, are reduced (Lutton, 1974). The
differences reported here suggest that mass wasting and bank -slope development along streams of loess -
derived alluvium occur by processes similar to those that occur on loess slopes. However, the addition of
fluvial and fluvial - related processes result in slope forms that are consistent, but slightly different from those of
upland loess slopes. The genesis and range of angles of the slough line further support this argument. These
low -angle surfaces (20 to 25 degrees) are not explicable in the Lohnes and Handy (1968, p. 254) model because
their study did not deal with channels, and the processes of fluvial reworking and low -angle sliding in
saturated materials of residual strength. In fact, the 10 to 41 degree class identified by Lohnes and Handy
(1968) from topographic maps may indeed represent old slough lines, emanating downslope from what are
now terraces.
Assuming that the critical height of the entire bank is still exceeded and (or) dispersion and piping continue
to weaken the vertical face, parallel retreat along the vertical face and flattening of the upper bank and slough
line may continue as the channel creates a new flood plain at an elevation lower thar�,the previous one. Highly
disturbed channels such as Cane Creek, which is cut through loess- derived sediments and lacks a coarse
sediment -load for aggradation, tend to aggrade extremely slow following degradation. The lack of bed -level
recovery leads to an extended period when bank profiles exceed the critical bank height and angle, allowing
for continued bank retreat and flood plain development. It is estimated that the development of a new and
distinct flood plain on channel such as Cane Creek will occur over 1000 years or more. Stage V would then
represent the final stage of bank -slope development in these types of channels.
In sand -bed channels, aggradation may be of sufficient magnitude to stabilize the vertical face and upper
bank through a reduction in bank height below the critical height. Bed levels of sand -bed channels, such as the
forks of the Obion and Forked Deer Rivers and the lower reaches of Cub, Porters, and Meridian Creeks rise,
and become stage VI channels over time spans that can approximate 20 to 40 years.
The aggradation stage (stage V) of bank -slope development occurs first in downstream reaches and
progresses upstream with trends of secondary aggradation. A meandering low -water thalweg and alternate
channel bars also begin to develop during late stage V, further reducing gradient and stream power.
Restabilization Stage (VI). The restabilization stage is marked (1) by a significant reduction of bank heights
by aggradation on the channel bed and (2) by fluvial deposition on the upper bank and slough line surfaces.
Bank retreat along the vertical face by intense mass wasting processes subsides because bank heights no
longer exceed critical heights. Woody vegetation extends upslope towards the base of the vertical face and the
former floodplain surface becomes a terrace (stage VI in Figure 5).
In channels where bank material is only moderately resistant and bed -level has sufficiently aggraded to
cause more frequent wetting of vertical face, the uppermost section of the bank may take a convex shape due
to fluvial reworking and deposition (stage VI in Figure 5). In heavily aggrading reaches of the Obion, and
North and South Forks Forked Deer Rivers, the floodplain continues to be a conduit for moderately high
flows. Woody vegetation is reestablished at the top of the bank and on the flood plain surface.
Bank -slope development through stage VI assumes either significant bed aggradation which occurs along
downstream reaches of sand -bed channels, and (or) limited initial downcutting. Flattening of the upper bank
and slough line surfaces by secondary slides continues due to weaker residual strengths, increased moisture
contents, and the additional weight of fluvially deposited material.
Summary of slope development and channel evolution
The six stages of bank -slope development represent a conceptual model of width adjustment. Stages
(premodified, constructed, degradation, threshold, aggradation, and restabilization) are induced by a
succession of interactions between fluvial and hillslope processes (Table VI). By associating the six bank -slope
development stages with the dominant hillslope and fluvial processes, and with characteristic channel forms, a
conceptual model of channel evolution over time and space was developed (Table VII). The model does not
suggest that each adjusting reach will undergo all six stages but implies that specific trends of bed -level
response will result in a series of mass wasting processes and definable bank and channel forms. However, the

24
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CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS 25
conceptual framework of the simultaneous retreat of the vertical face and flattening along surfaces below is
supported by the observations of other investigators (Carson and Kirkby, 1972, p. 184).
The model described in this report makes the following assumptions (1) there is no local bedrock control of
bed - level, (2) overadjustment and secondary response are active processes, (3) the bed and banks are free to
adjust to imposed changes, and (4) successive stages of evolution are not interrupted by other disturbances.
Extrapolation of the six -stage conceptual model of bank -slope development and channel evolution should be
particularly appropriate for areas of the Mississippi embayment and the central United States. Application
over a broader geographical area is conceptually justified on the basis that similar processes can create similar
forms. Variations in time scales and forms from the idealized model will occur due to local bedrock control
and variations in relief, soil properties, and climatic conditions. Still, the model reflects the overadjustments
inherent to fluvial response and may be useful in determining expected changes in alluvial channel
morphology over the course of a major adjustment cycle.
SUMMARY
Channel modifications between 1959 and 1978 in West Tennessee caused a series of morphologic changes
along modified reaches and tributary streams. Degradation occurred for 10 to 15 years at sites upstream of the
area of maximum disturbance (AMD) and lowered bed - levels by as much as 6.1 metres. Aggradation occurs
downstream of the AMD with the greatest rates near the stream mouths. Initially degraded sites experience a
secondary aggradation phase in response to excessive incision and gradient reduction.
Adjustments of channel geometry and phases of channel evolution are characterized by six process -
oriented stages of morphologic development— premodified, constructed, degradation, threshold, aggra-
dation, and rest abilization. Downcutting and toe removal occur immediately following construction along
stream reaches upstream of the AMD (degradation stage). Bank failure by mass wasting begins during the
threshold stage when the critical height and angle of the bank material is exceeded. Top -bank widening
continues concurrently with deposition on the channel bed during the aggradation stage as the slough line
develops and becomes an initial site of lower -bank stability. The development of the bank profile is defined in
terms of three observable and dynamic surfaces (1) vertical face (70" to 90 "), (2) upper bank (25" to 50 ''), and (3)
slough line (20" to 25'). Alternate channel bars form during the restabilization stage and represent incipient
meandering of the channel. The vertical face may concurrently take a convex shape from continual reworking
of materials by stream flow.
REFERENCES
Alexander, D. 1981. 'Threshold of critical power in streams, Discussions and reply', Bulletin of the Geological Soviet y nJ'America Bulletin,
92, 310 -312.
Begin, Z. B., Meyer, D. F., and Schumm, S. A. 1981. 'Development of longitudinal profiles of alluvial channels in response to base -level
lowering', Earth Surface Processes, 6, 49 -68.
Bull, W. B. 1979. 'Threshold of critical power in streams', Bulletin gl'the Geological Society q /'America, part 1, 90, 453 -464.
Carson, M. A. and Kirkby, M. J. 1972. Hillslope Form and Process, Cambridge University Press, London, 475 pp.
Gilbert, G. K. 1880. 'Report on the geology of the Henry Mountains (2)', Geographical and Geological Survey of the Rocky Mountain
Region, United States Government Printing Office, Washington, 170 pp.
Graf, W. L. 1977. 'The rate law in fluvial geomorphology', American Journal of Science +, 277, 178-191.
Hack, J. T. 1960. 'Interpretation of erosional topography in humid temperate regions', American Journal gl Science, 258 -A, 80 -97.
Hey, R. D. 1979. 'Dynamic process - response model of river channel development', Earth Surface Processes, 4, 59 -72.
Hidinger, L. L. and Morgan, A. E. 1912.'Drainage problems of Wolf, Hatchic, and South Fork of Forked Deer Rivers, in west Tennessee',
in The Resources of Tennessee. Tennessee Geological Survey, 2, No. 6, 231 -249.
Hupp, C. R. and Simon, A. 1986. 'Vegetation and bank -slope development', Proceedings of the 4th Federal Interagency Sedimentation
Conference, Las Vegas, Nevada, March 1986, 2, 5 -83 to 5--92.
Lane, E. W. 1955. 'The importance of fluvial morphology in hydraulic engineering', Proceedings gf the American Society o/' Civil
Engineering, 81, No. 745, 17.
Lohnes, R. A. and Handy, R. L. 1968. 'Slope angles in friable loess', Journal gf,yeology, 76, No. 3, 247 - -258.
Lutton, R. J. 1974. 'Use of loess soil for modeling rock mechanics', Miscellaneous Report S- 74 -28, U.S. Army Engineers Waterways
Experiment Station, Vicksburg, Mississippi.
Mackin, J. H. 1948. 'Concept of a graded river', Bulletin of the Geological Society of America Bulletin, 59, 463 -511.
Morgan, A. E. and McCrory, S. H. 1910.'Drainage of lands overflowed by the North and Middle Forks of the Forked Deer River and the

26 A. SIMON,
Rutherford Fork of the Obion River in Gibson County, Tennessee', Bulletin of the Tennessee State Geological Survey, 3 -B, 17-43,
Pickup, G. 1975. `Downstream variations in morphology, flow conditions and sediment transport in an eroding channel', Zeitschrift fur
Geomorphologie, 19, 443 -459.
Robbins, C. H. and Simon, A. 1983. `Man- induced channel adjustment in Tennessee streams', United States Geological, Survey Water -
Resources Investigations Report, 82 -4098, 129.
Schumm, S. A. 1973. `Geomorphic thresholds and the complex response of drainage systems', in Morisawa, M. (Ed.), Fluvial
Geomorphology, Binghamton, State University of New York, 229 -310.
Schumm, S. A. and Lichty, R. W. 1965. `Time, space, and causality in geomorphology', American Journal of Science, 263, 110 - -119.
Schumm, S. A. and Parker, R. S. 1973. `Implications of complex response of drainage systems for Quaternary alluvial stratigraphy',
Nature, 243, 99 -100.
Simon, A. In press. `Gradation processes and channel evolution in modified West Tennessee streams: process, response, and form', United
States Geological Survey Professional Paper, 1470.
Simon, A. and Hupp, C. R. 1986. `Channel widening characteristics and bank slope development along a reach of Cane Creek, West
Tennessee', in Subitzsky, Seymour (Ed.), Selected Papers in the Hydrologic Sciences, United States Geological Survey Water - Supply
Paper, 2290, 113 -126.
Simon, A. and Hupp, C. R. 1987. `Geomorphic and vegetative recovery processes along modified Tennessee streams: an interdisciplinary
approach to disturbed fluvial systems', Forest Hydrology and Watershed Management, International Association of Hydrologic
Sciences, Publication, 167, 251 -262.
Simon, A. and Robbins, C. H. 1987. `Man- induced gradient adjustment of the South Fork Forked Deer River, West Tennessee',
Environmental Geology and Water Sciences, 9, No. 2, 109 -118.
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Thorne, C. R., Murphey, J. B., and Little, W. C. 1981. `Bank stability and bank material properties in the bluHiine streams of northwest
Mississippi', Stream Channel Stability, United States Department of Agriculture Sedimentation Laboratory, Oxford, Mississippi,
Appendix D, 257 pp.
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Williams, G. P. and Wolman, M. G. 1984. `Downstream effects of dams on alluvial rivers', U.S. Geological Survey Professional Paper 1286,
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Survey Open -File Report, 79 -554, 18 pp.

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
AUGUST AMERICAN WATER RESOURCES ASSOCIATION 2003
BANKFULL HYDRAULIC GEOMETRY RELATIONSHIPS AND RECURRENCE
INTERVALS FOR NORTH CAROLINA'S COASTAL PLAINT
William V. Sweet and Jens W. Geratz2
ABSTRACT: Bankfull hydraulic geometry relationships relate
stream channel geometry to watershed size for specific physio-
graphic regions. This paper presents bankfull hydraulic geometry
relationships and recurrence intervals for the Southeastern Plain
ecoregion and the flatwoods subtype of the Middle Atlantic Coastal
Plain ecoregion found within North Carolina's Coastal Plain phys-
iographic province. Cross - sectional and longitudinal survey data
from gaged and ungaged streams were used to compute channel
dimension and profile information. Power functions were devel-
oped, relating drainage area to bankfull discharge, cross - sectional
area, width, and mean depth. Recurrence intervals of bankfull
events were estimated from gaged streams using both a Log -Pear-
son Type III distribution of peak annual discharge and a partial -
duration series of average daily discharge. Results from both
methods indicate that average bankfull recurrence intervals for the
study area are below one year. Determinations of recurrence inter-
vals by the Log- Pearson Type III distribution were for the most
part inconclusive (less than one year), while a partial duration
series estimated a 0.19 year average, ranging from 0.11 to 0.31
years.
(KEY TERMS: bankfull; environmental engineering; flood frequen-
cy; partial duration series; bankfull hydraulic geometry relation-
ships; surface water hydrology.)
Sweet, William V. and Jens W. Geratz, 2003. Bankfull Hydraulic Geometry Rela-
tionships and Recurrence Intervals for North Carolina's Coastal Plain. J. of the
American Water Resources Association (JAWRA) 39(4):861 -871.
INTRODUCTION
Bankfull hydraulic geometry relationships allow
comparisons of bankfull flows (channel forging
events) and associated dimensional parameters to
various watershed sizes (Lindley, 1919; Leopold and
Maddock, 1953; Wolman, 1955; Nixon, 1959; Myrick
and Leopold, 1963; Emmett, 1972, 1975; Dunne and
Leopold, 1978). Hydraulic geometry relationships are
region specific, describing areas of similar physiogra-
phy and climate. Bankfull hydraulic geometry rela-
tionships are valuable tools that aid in the prediction
of bankfull channel characterization on ungaged
reaches, both natural and modified, and for guiding
"natural" channel design.
Bankfull is associated with flows that fill a stream
channel to the elevation of the active floodplain.
These events are most effective at moving sediment
loads over time (Wolman and Miller, 1960; Andrews,
1980; Carling, 1988) and are essentially responsible
for sizing the channel dimensions of a stable system
( Rosgen, 1994). The recurrence interval for bankfull
events is traditionally determined by a Log- Pearson
Type III distribution of annual peak discharge and
has been estimated to occur between a 1.4 and 1.6
year period throughout much of the United States
and the Mountain and Piedmont physiographic
provinces of North Carolina (Dunne and Leopold,
1978; Leopold, 1994; Rosgen, 1996; Harman et al.,
1999, 2000; Castro and Jackson, 2001).
With stream restoration currently receiving much
attention, it is imperative that bankfull hydraulic
geometry relationships be developed for individual
physiographic provinces for channel design purposes.
In addition, accurate recurrence interval and flood -
duration estimates of bankfull events are important
to agricultural interests, developments situated on
flood prone lands, and for stream and floodplain
restoration activities. This paper presents regional
bankfull hydraulic geometry relationships for the
Coastal Plain of North Carolina and results from a
flood frequency analysis of both a Log - Pearson Type
1Paper No. 02013 of the Journal of the American Water Resources Association. Discussions are open until February 1, 2004.
2Respectively, Project Scientist and Senior Scientist, EcoScience Corporation, 1101 Haynes Street, Suite 101, Raleigh, North Carolina
27604 (E- Mail/Geratz: geratz @ecosciencenc.com).
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 861 JAWRA

SWEET AND GERATZ
III distribution and a partial duration series to deter-
mine an average recurrence interval for bankfull
events.
STUDY AREA
North Carolina is commonly divided into three
major physiographic provinces: the Mountains, Pied-
mont, and Coastal Plain. These provinces are separat-
ed by dominant geographic delineations that have
unique topography, soils, climate, and vegetation.
Recent efforts (Griffith and Omernik, 2000) have sub-
divided the physiographic provinces into ecoregions,
which depict ecological systems on regional scales
that have distinct geology, hydrology, climate, and
biology. Within a given ecoregion, additional subdivi-
sions further classify specific subtypes that are depen-
dent upon locally dominant characteristics. The
Coastal Plain physiographic province (Coastal Plain)
of North Carolina is separated into the Southeastern
Plains (SP) and Middle Atlantic Coastal Plain
(MACP) ecoregions (Figure 1). For the current study,
the MACP ecoregion was further segmented following
delineations depicted by Griffith and Omernik (2000)
into a flatwoods subtype or a lowlands, swamps, and
floodplains subtype.
The Coastal Plain province is characterized as a
predominately sandy and flat, low elevation plain.
Stream gradients are quite low and become increas-
ingly small from the SP to the MACP ecoregion, with
slopes ranging from 0.5 percent to less than 0.01 per-
cent. Unconsolidated sediment formations thicken
toward the coast, with surficial deposits progressively
changing from sand in the SP ecoregion and flatwoods
MACP subtype to poorly drained, loamy soils within
the lowlands, swamps, and floodplains MACP sub-
type. Within the lowlands, swamps, and floodplains
MACP subtype, bottomlands border alluvial systems
whose floodplains contain clay and organic deposits
that form restrictive lenses with very low permeabili-
ty (Griffith and Omernik, 2000). Mean annual precipi-
tation throughout the entire Coastal Plain ranges
from 48 to 58 inches, with the highest amounts occur-
ring near the coast (Hirth, 1998).
The SP ecoregion is characterized by gently rolling
topography that supports a mosaic of pasture, wood-
land, cropland, and forested areas. Natural vegetation
is mostly oak/hickory /pine forest on upland slopes
with bottomland hardwood species in isolated flood -
plains and low terraces. The flatwoods MACP subtype
is dominated by loblolly shortleaf pine forests in well
drained soils and oak, gum, and cypress stands along
streams and terraces. The lowlands, swamps, and
floodplains MACP subtype is characterized by low
Figure 1. Locations of Gaged and Ungaged Stream Reaches in the Southeastern Plain (SP) and Middle Atlantic Coastal Plain (MACP)
Ecoregions With MACP Subtypes (modified from Griffith and Omernik 2000) for the Coastal Plain of North Carolina.
JAWRA 862 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

BANKFULL HYDRAULIC GEOMETRY RELATIONSHIPS AND RECURRENCE INTERVALS FOR NORTH CAROLINA'S COASTAL PLAIN
relief, poorly drained flats interspersed with broad,
shallow valleys, and tidally influenced streams near-
ing the estuaries. Naturally vegetated communities
include high and low pocosins, pond/pine woodlands,
cypress tupelo forests, coastal marshes, and barrier -
dune systems (Griffith and Omernik, 2000).
METHODS
Regional bankfull hydraulic geometry relationships
were developed for the SP ecoregion and the flatwoods
subtype of the MACP ecoregion of North Carolina's
Coastal Plain (Figure 1). The hydraulic geometry
relationships were composed of data from U.S. Geo-
logical Survey (USGS) gaged streams and ungaged
streams located within remote portions of the above
mentioned regions. Since the majority of USGS gage
sites within the Coastal Plain tend to monitor larger
watersheds, the ungaged streams represented smaller
watersheds to supplement the data set.
Site Selection and Criteria
Since most USGS gages are located immediately
downstream of roadway crossings, careful attention
was given to avoid sites with significant, historical
disturbances, usually in the form of straightened and
incised channel banks. Gaged watersheds included
current and recently discontinued stations that fit the
following criteria: a minimum eight year record of
hydrological data (flood frequency analysis only on
sites with a >10 year record), minimal land use
changes in the last 10 years, no immediate upstream
impoundments, and less than 10 percent impervious
area in the watershed.
Ungaged streams identified for the study included
reaches with small drainage areas that had natural
channel conditions — largely undeveloped and unal-
tered by agricultural, logging, or other anthropogenic
disturbances. Site selection criteria for the ungaged
streams were the same as the gage sites, minus the
flow record requirements. In addition, a survey was
made to assess the channel bed, banks, and vegeta-
tion for signs of either channel or stream bank insta-
bilities. The stream bed was observed for scouring or
deposition including the occurrence of bars or islands
in the channel, cut faces on bars, cutoffs and flood
chutes, and unusually steep riffles. Channel incision
evaluations using low bank height ratios (i.e., low
bank height /maximum bankfull depth) were per-
formed along each stream profile to assess stability.
Low bank height ratios, those approaching 1.0, were
considered indicative of a stable channel, particularly
when observed over a significant length of stream
( Rosgen, 1997).
Field Protocol and Analysis
Prior to each gage survey, the following data were
obtained from the USGS for the site: stage discharge
rating tables, established reference marks, gage sum-
maries (Form 9 -207), and peak annual discharge
series. Peak annual discharge was graphed using the
USGS accepted (Dalrymple, 1960) Weibull plotting
position (Gordon et al., 1992) to obtain flood recur-
rence interval information for the site. In addition,
aerial photography (1998; three -foot resolution) for a
one -mile stretch of stream was centered on the gage
site, allowing stream channel sinuosity calculations
for the local reach. Watershed sizes for gage sites
were provided by the USGS.
During each gage survey, the protocol for calibrat-
ing bankfull discharge at USGS stream gage stations
was followed (Leopold, 1994; Rosgen, 2001). Gage sur-
vey procedures included the centering of the longitu-
dinal survey, which spanned greater than 20 bankfull
widths about the gage plate location. A least square
fit of the data from the bankfull survey determined
bankfull stage at the gage plate location, which
identified bankfull discharge from the rating table.
Bankfull hydraulic geometry was obtained by analy-
sis of cross - sectional surveys performed along stream
riffles found within the boundary of the longitudinal
profile. The bankfull hydraulic geometry determina-
tion was based on elevations of consistent bankfull
indicators, which included the top of banks, back of
point bars, significant breaks in bank slopes, and
highest scour lines (Leopold, 1994). The bankfull
hydraulic geometry relationships were developed
without regard to recurrence intervals associated
with bankfull discharge.
Prior to each ungaged stream survey, watershed
sizes were estimated using USGS 1:24,000 digitized
topographic maps and USGS 14 -digit hydrologic unit
mapping provided by the North Carolina Center for
Geographical Information Analysis (CGIA). For each
ungaged stream, a longitudinal profile survey, includ-
ing measurements of bankfull, low bank, water level,
and thalweg were performed over distances greater
than 20 bankfull widths. Cross - sectional surveys were
performed at representative riffle locations to deter-
mine bankfull hydraulic geometry. Bankfull identifi-
cation was based upon indicators that included the
back of point bars and the top of banks. Plan form
surveys were used to calculate stream sinuosity.
Stream roughness coefficients (n) were estimated for
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SWEET AND GERATZ
each of the ungaged streams using a version of Arce-
ment and Schneider's (1989) weighted method for
Cowan's (1956) roughness component values and
applied to the following equation (Manning, 1891) to
obtain bankfull discharge (Qbkf) estimates.
Qbkf = [1.486 / n] * [A * R2/3 * S1 /2]
where, A equals bankfull area, R equals bankfull
hydraulic radius, and S equals average water surface
slope measured over for the reach during surveying
efforts.
Post - survey analysis for the gaged and ungaged
streams compared water surface and bankfull eleva-
tions over the surveyed reach to ensure that there
was consistency in profiles, which would indicate
proper bankfull identification (Leopold, 1994). Also,
samples of surficial bed deposits that were collected
at all sites were dried, sieved, and weighed (Gordon
et al., 1992), allowing for preliminary bed material
assessments for stream classification purposes only.
Each stream was summarily categorized under the
Rosgen method (1994) for stream classification pur-
poses.
Recurrence Interval Analysis
Bankfull recurrence intervals were estimated by
two methods for nine gaged streams. One method
computed recurrence intervals (Pope et al., 2001) for
various floods (Q2, Q5, Q10 ... Q500) by traditional
procedures (USGS, 1982) utilizing Log - Pearson Type
III distributions of peak annual discharge (conven-
tional method). To determine bankfull recurrence
intervals for the current study, the Pope et al. (2001)
results were graphed onto a semi -log plot (recurrence
interval [logged x -axis] versus discharge [y -axis] ), and
fit with a least square, regression line (R2 > 0.95 in all
cases) that was extrapolated to bankfull discharge
values. The equation of this line was empirically
solved, yielding site specific, recurrence intervals of
bankfull discharge. If the extrapolated values were
below the one -year period, the results were deemed
inconclusive (Langbein, 1949) and reported as less
than one year.
Recurrence intervals were also computed by a par-
tial duration series of historical, average daily dis-
charge (USGS, 2001). The partial duration method
determines flood frequency by counting the number of
independent, maximum, daily flows above a specified
level (bankfull) and dividing by the total number
of years of record for that station. Only discrete bank-
full events separated by at least a one -day period
between successive floods were included in the series.
The resultant series was graphed using the Weibull
plotting position to allow graphical representation
and numeric interpolation of the bankfull recurrence
interval. The average duration of events at or above
bankfull levels was also calculated for each gage site,
and equaled the total number of days a site's flow was
above bankfull discharge, divided by the total number
of events. It should be noted that the vast majority of
bankfull events at any given gage site were separated
by more than a week.
RESULTS
Bankfull hydraulic geometry data for 24 gaged and
ungaged streams within the Coastal Plain of North
Carolina are shown in Table 1. Results represent
watershed sizes ranging from 0.6 to 182 square miles,
and include bankfull measurements, recurrence inter-
vals calculated by both the conventional and partial -
duration methodologies, and stream classification
(Rosgen 1994). In general, the streams surveyed for
the current study were located in well developed, allu-
vial floodplains with dense, mature forests and
streamside communities. Most of the surveyed
streams had sandy bed material, with a few main-
taining elevated levels of organic muck within the
channel and floodplain. The streams exhibited low
gradient, riffle pool structures with low width/depth
ratios (< 12) indicative of "E" stream types (Rosgen,
1994). Several streams exhibited slightly higher
width/depth ratios (> 12), as well as point bars, which
are traits characteristic of "C" stream types (Rosgen,
1994).
The bankfull hydraulic geometry relationships
shown in Figures 2(a through d) are computed from a
least square, power function regression of bankfull
characteristics versus watershed area for 22 of the 24
surveyed sites. The hydraulic geometry relationships
do distinguish between data points from the SP ecore-
gion and flatwoods subtype of MACP ecoregion, but
no additional stratification was performed. The
regression equations for the bankfull hydraulic geom-
etry relationships, along with R- squared values based
on a natural log distribution, are shown below.
(a) Abkf = 9.43 (Aw)0.74 R2 = 0.96
(b) Wbkf = 9.64 (Aw)0.31 R2 = 0.95
(c) Dbkf = 0.98 (Aw)0.36 R2 = 0.92
(d) Qbkf = 8.79 (Aw)0.76 R2 = 0.92
where, Abkf equals bankfull cross - sectional area (sq
ft), Aw equals watershed drainage area (sq mi), Wbkf
JAWRA 864 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

W
a
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BANKFULL HYDRAULIC GEOMETRY RELATIONSHIPS AND RECURRENCE INTERVALS FOR NORTH CAROLINA'S COASTAL PLAIN
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JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 865 JAWRA

1000
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SWEET AND GERATZ
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10000
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Figure 2. Bankfull Hydraulic Geometry Relationships (a through d) With 95 Percent Prediction Limits (dash) Composed of Gage
(triangles) and Ungaged Sites (circles) for the Southeastern Plain Ecoregion (hollow symbols) and Flatwoods Subtypes
of the Middle Atlantic Coastal Plain Ecoregion (filled symbols) Within the Coastal Plain of North Carolina.
equals bankfull width (ft), Dbkf equals bankfull mean
depth (ft), and Qbkf equals bankfull discharge (cfs).
It should be noted that the original effort attempt-
ed to create bankfull hydraulic geometry relation-
ships valid for the entire Coastal Plain of North
Carolina. However, two gage sites (Cashie River and
Van Swamp) had bankfull hydraulic geometry signifi-
cantly distinct from the rest of the surveyed Coastal
Plain sites. A subsequent, GIS mapping investigation
found that the above mentioned sites were uniquely
located within the lowlands, swamps, and floodplains
MACP subtype. Due to temporal constraints and a
lack of additional, long term gaging stations, further
surveys within the lowlands, swamps, and floodplains
MACP subtype were prohibitive. Therefore, the
hydraulic geometry relationships developed within
the current study exclude these two data sets, as well
as all subsequent flood frequency analyses. Even
though the presented bankfull hydraulic geometry
relationships may not be applicable within the low-
lands, swamps, and floodplains MACP subtype, the
results from the two surveys are considered impor-
tant to current and future stream restoration efforts,
and are therefore included and briefly discussed.
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BANKFULL HYDRAULIC GEOMETRY RELATIONSHIPS AND RECURRENCE INTERVALS FOR NORTH CAROLINA'S COASTAL PLAIN
The average recurrence interval of bankfull events
for the gaged streams is estimated to be 0.19 year,
with a range of 0.11 to 0.31 year as estimated by the
partial duration series method (Table 1). A determi-
nation of an average bankfull recurrence interval by a
Log - Pearson Type III distribution of peak annual dis-
charge was not possible, since all but one site had a
recurrence interval below one year. The average dura-
tion of bankfull events for the gaged streams ranges
from 1.4 to 8.8 days (Table 1).
DISCUSSION
The bankfull hydraulic geometry relationships for
the Coastal Plain of North Carolina exhibit strong
relationships (R2 > 0.9) that are considered valid for
the majority of the physiographic province. Variability
within the bankfull hydraulic geometry relationships
may be attributed to differences of regional land uses
(Leopold, 1994; Rosgen, 1994), and presumably water -
surface gradients, quantity of instream debris and
bank vegetation, and underlying geology that affects
water storage capacity within a given watershed.
Bank full Recurrence Interval
Flood frequency analysis reveals that the recur-
rence interval of bankfull events in the Coastal Plain
is much shorter than 1.5 -year interval estimated by
previous state and nationwide investigations (Dunne
and Leopold, 1978; Leopold, 1994; Harman et al.,
1999, 2000; Castro and Jackson, 2001). The Log -
Pearson Type III distribution of peak annual dis-
charge for the Coastal Plain indicates subannual
bankfull recurrence intervals. The partial duration
series further refines the recurrence interval, estimat-
ing a 0.19 -year average, which implies that bankfull
occurs more than five times a year. The shortened
recurrence interval of bankfull in the Coastal Plain is
attributed to high precipitation inputs onto a wide
expanse of near level topography with large surface
storativity, an elevated water table, and slow flushing
rates.
The substantially shorter recurrence interval of
bankfull events in the Coastal Plain has a couple of
important implications. Besides providing reliable
flood frequency information, recurrence intervals can
influence the development of bankfull hydraulic
geometry relations. Presently, standard practice by
Rosgen (2001) utilizes recurrence intervals from
either a Log- Pearson Type III distribution or the
Weibull plotting position of peak annual discharge
data to verify correct field identification of bankfull
indicators along often modified channels at USGS
gaging sites. Rosgen (2001) states that appropriately
chosen bankfull elevations along a given reach will
have a stage derived (USGS rating table) discharge
with an associated recurrence interval between the
one- to two -year span. Unfortunately, without an
enhanced range of accurate bankfull event recurrence
intervals, relationship developers within low gradient
regions similar to the Coastal Plain may overestimate
actual bankfull elevations in the field to be in line
with the currently accepted one- to two -year range.
A shortened recurrence interval for bankfull events
also indicates that stream systems in the Coastal
Plain are spilling their banks more frequently than
other regions. This fact has important repercussions
for agricultural and urban planning interests, and
might also affect the sizing of land acquisitions during
stream restoration efforts that attempt to create nat-
urally shaped stream channels with active flood -
plains. To more precisely determine the frequency of
bankfull events, the current study chose to analyze a
partial duration series of average daily discharge for
each gage station. This analysis was actually an
afterthought, decided upon after a number of field vis-
its to remote gage sites revealed that discharges asso-
ciated with the tops of bank (bankfull) had a
recurrence interval of less than a one -year period as
computed by methods utilizing the peak annual dis-
charge series.
General acceptance of the conventional method by
most governmental and private agencies was intend-
ed to permit a consistent and proven technique to
estimate flood frequencies at USGS gaging stations.
However, the method was intended to tackle the
issues of peak flood/flow frequency estimations for the
national flood damage abatement program (USGS
1982), not for determining accurate magnitude and
frequency estimates of relatively small scale phe-
nomenon such as bankfull events. The conventional
method is preferred because of its effective treatment
of positive skew, which provided a reasonably good fit
to empirical flood distributions from a wide range of
watersheds throughout the United States. As with
most probability distributions, the fit provides most
accurate estimates near the middle values for the
range of the historical data (Gordon et al., 1992).
On the other hand, a partial duration series allows
assessments of recurrence intervals for bankfull flows
with discharges less than the minimum annual peak
flow for a given data set (NERC, 1975; USGS, 1982;
IAE, 1987). Unlike the conventional method, which
attempts to determine future probabilities of exceed-
ing a particular flood level based only on the maxi-
mum flood peak from each year of a given data set,
the partial duration series method analyzes the entire
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SWEET AND GERATZ
historical daily data set to more precisely isolate the
frequency of particular flood magnitudes. Compar-
isons between the two methods for a 41 -year flow
record from the Swift Creek gage site highlight the
enhanced resolution of the partial duration series for
high frequency events (Figure 3). Figure 3 also illus-
trates the intended purpose and strength of the con-
ventional method — to smoothly fit the peak annual
series graphed using the Weibull plotting position,
which is another method that cannot resolve recur-
rence intervals below a one -year period.
There are advantages associated with each of the
above strategies used to generate flood frequency
information. Inherent properties of Log - Pearson Type
III distribution lends itself well to flood data that tend
to have a lower limit but no upper limit (Gordon et
al., 1992). The distribution permits fairly straightfor-
ward extrapolation beyond the temporal boundaries
of the data set (Chow, 1964), allowing frequency pre-
dictions of large scale floods previously unobserved at
or around a gage location. However, smaller floods
will occur more frequently than indicated by distribu-
tions or plotting positions based on peak annual
7000
6000
5000
w
4000
a�
3000
0
2000
1000
0
series, because several peaks in one year may be
higher than the highest flood in others. The National
Research Council (NRC, 1999) recognizes this report-
ing bias of the conventional method and cautions
against the use of Log - Pearson Type III distributions
to describe high frequency floods, including annual
and multidecadal events. In any case, the longer the
available data set, the more accurate results from
either method become, provided that no significant
climatic events or management actions occur (USGS,
1982; NRC, 1999).
Bankfull Hydraulic Relationship Comparisons
As noted in the result section, surveyed results
from two gaged streams, the Cashie River and Van
Swamp sites, substantially deviate from bankfull
trends and were excluded during relationship devel-
opment (Table 1). These two sites are located adjacent
to drowned river valleys and swampy wetlands of the
Croatan, Albemarle, and Pamlico Estuarine System
(CAPS), and uniquely positioned within the lowlands,
0.1
1
10
Average Recurrence Interval (years)
100
Figure 3. Recurrence Interval Method Comparison for Swift Creek Gage Site With Bankfull Discharge (dashed line), Weibull Plotting
Position for Both the Partial Duration Series of Average Daily Flow (circle) and for the Peak Annual Series (triangle), Which is
Overlaid by the Log- Pearson Type III Distribution (square) of the Peak Annual Series With a Regression Fit (solid line).
JAWRA 868 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

BANKFULL HYDRAULIC GEOMETRY RELATIONSHIPS AND RECURRENCE INTERVALS FOR NORTH CAROLINA'S COASTAL PLAIN
swamps, and floodplains subtype of the MACP ecore-
gion (Figure 1).
Undisturbed sections of the lowlands, swamps, and
floodplains MACP subtype are characterized by bot-
tomlands that are perched on ancient marine terraces
underlain by mucky histosols with extremely low per-
meability and high surface storativity (Griffith and
Omernik, 2000). These topographic flats have mini-
mal regional stream gradients as well as numerous
debris obstructions, beaver dams, and relatively close -
spatial connectivity to their receiving waters. As
bankfull flows flood into components of the CAPS
system, downstream water level may rise slightly
and reduce barotropic (i.e., gradient driven) forces
upstream from the barrier or confluence. This combi-
nation of factors further diminishes regional water -
surface gradients and allows for extensive flooding
with heightened instream and floodplain deposition.
Accordingly, the Cashie River and Van Swamp gage
sites have prolonged average bankfull event durations
(18.1 and 13.0 days, respectively) that are typical of
alluvial swamp systems (Brinson, 1993), which flood
during the early months of each year when evapo-
transpiration rates are low. Physiographic considera-
tions, with emphasis upon minimal barotropic forcing
and reduced channel flushing, as well as channel and
floodplain composition may constructively differenti-
ate bankfull hydraulic geometry for the lowlands,
swamps, and floodplains MACP subtype from the rest
10000
w 1000
U
oho
100
Q
x
10
(a)
10 100
Drainage Area (sq. mi.)
of the Coastal Plain. Distinct from the rest of the sur-
veyed sites and indicative of streams of the region,
these two streams have developed low flow channels
within their expansive floodplain, which is flooded for
extended parts of the year.
Likewise, physiographic properties unique to the
Coastal Plain, such as low water surface gradients
and sandy soils with shallow aquacludes, render
bankfull hydraulic geometry relationships distinct
from the other North Carolina provinces. Figure 4
shows the Coastal Plain relationships (from the cur-
rent study) with those produced by Harman et al.
(1999, 2000) for North Carolina's Mountain and Pied-
mont provinces. For comparative purposes, the rela-
tionships have been truncated and averaged over
watershed sizes ranging between 1 and 100 square
miles. Comparisons reveal that average bankfull dis-
charge of streams within the Coastal Plain is approxi-
mately 10 percent of that of the Mountain and
Piedmont provinces (Figure 4a). Also, the average
bankfull cross - sectional area of Coastal Plain streams
is approximately 50 percent of the other two provinces
(Figure 4b), while its width is approximately 50 and
75 percent, and its depth is 100 and 70 percent of the
Mountain and Piedmont provinces, respectively.
1000
a 100
a�
10
W
(b)
10 100
Drainage Area (sq. mi.)
Figure 4. Truncated Bankfull Hydraulic Geometry Relationships of (a) Bankfull Discharge and (b) Bankfull
Area for the Coastal Plain (circle) and the Overlapping Mountain (square) and Piedmont
(triangle) Provinces of North Carolina (Harman et al., 1999, 2000).
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 869 JAWRA

SWEET AND GERATZ
CONCLUSIONS
The bankfull hydraulic geometry relationships for
the Coastal Plain physiographic province of North
Carolina provide valuable templates that can guide
stream channel classification and restoration efforts
within the SP ecoregion and the flatwoods subtype of
the MACP ecoregion. However, caution is advised for
restoration efforts near pocosins, bottomlands, and
wetlands within or adjacent to the lowlands, swamps,
and floodplains subtype of the MACP ecoregion. For
these cases, stream channel parameters may vary sig-
nificantly, and reference reaches located within the
surrounding vicinity that share similar physiography
should be mimicked.
Bankfull events occur much more frequently within
the Coastal Plain of North Carolina than reported in
other state and nationwide surveys. Bankfull recur-
rence intervals were computed by a Log - Pearson Type
III distribution of peak annual discharge and by a
partial duration series of average daily flow. Results
from the former method indicated a subannual recur-
rence interval for bankfull, but was inconclusive in
estimating values with any more precision. The lad-
der method estimated a 0.19 -year average recurrence
interval for the Coastal Plain, with a 0.11 to 0.31 -year
range. Future development of regional bankfull
hydraulic relationships within other low gradient
regions along the Southeast Atlantic and Gulf Coast
should consider this lower range of naturally occur-
ring recurrence intervals. In particular, the step -wise
procedures guiding the development of regional
hydraulic geometry relationships (Leopold, 1994; Ros-
gen, 2001) should be refined to include an adjusted
range of bankfull recurrence intervals. Findings from
the current study suggest that when the Log- Pearson
Type III distribution of peak annual discharge is
unable to discern recurrence intervals below one year,
a 0.1 to 0.3 -year interval should be chosen using a
partial duration series of daily discharge.
Development of additional bankfull hydraulic
geometry relationships may be possible for the low-
lands, swamps, and floodplains subtype of the MACP
ecoregion within the Coastal Plain of North Carolina.
Appropriate delineation for regional relationship
development should give close consideration to water
surface slopes, localized geology, and water storativity
within the watershed. With bankfull hydraulic geome-
try relationships developed for all of North Carolina's
physiographic provinces, a more spatially continuous,
statewide relationship might be possible through a
deterministic modeling effort that integrates all data
sets.
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