20151229 Ver 1_More Info Received_20160224

Homewood, Sue
From: David Kiker <[e-mail]>
Sent: Wednesday, February 24, 2016 2:08 PM
To: Homewood, Sue
Subject: RE: HAECO Site: Response to Your Comment
Attachments: HAECO Stream Stability Memo_2-24-16.pdf, Hpallfw61203
_NEW.p04.comp_msgs.zip.zip_renamed
Sue, Sorry for the delay in getting back with you regarding the downstream stream stability evaluation. Since we last
spoke, we have looked into the downstream stream stability evaluation a little deeper and have included the
recommended permissible shear stresses and velocities. The attached memorandum summarizes this evaluation. As
shown in the memorandum, the proposed shear stresses and velocities are within the permissible shear stress and
velocity ranges. Also attached is a USACE document we reference with our stream restoration and stabilization work. In
addition, we have attached a FEMA HEC -RAS model that includes 2 -year flow data for 11 tributaries to Horsepen
Creek. We included this model so you could see some of the velocities experienced in real streams in close proximity to
our stream. If you have any questions or need additional information please don't hesitate to call.
-David
David Kiker, PE
Technical Manager
WK Dickson - Raleigh Office
(919) 782- 0495
From: Homewood, Sue [mailto:[e-mail]]
Sent: Thursday, February 11, 2016 12:58 PM
To: David Kiker
Subject: RE: HAECO Site: Response to Your Comment
David,
Can we find a minute to talk this through a little further on the phone? That might help. I'll be here until about 4:30
today and then in tomorrow after 11 am if either of those dates/time work for you.
Thanks,
Sue Homewood
Division of Water Resources, Winston Salem Regional Office
Department of • Quality
336 776 9693 office
336 813 1863 mobile
Sue. Homewood(a-)ncdenr.gov
•

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From: David Kiker [mailto:[e-mail]]
Sent: Tuesday, February 09, 2016 11:00 AM
To: Homewood, Sue <[e-mail]>
Subject: RE: HAECO Site: Response to Your Comment
Picture 13: Grade control (gabion baskets) at sanitary sewer line located 40 feet upstream of Horsepen Creek
Picture 15: Horsepen Creek at confluence with Tributary (looking downstream)
David Kiker, PE
Technical Manager
WK Dickson - Raleigh Office
(919) 782- 0495
From: Homewood, Sue[mailto:sue.homewoodCa)ncdenr.Qov]
Sent: Tuesday, February 09, 2016 10:39 AM
To: David Kiker
Subject: RE: HAECO Site: Response to Your Comment
David,
Thank you for the detailed analysis. A few quick questions (I only skimmed your information but will look at it in more
detail soon)
Did you happen to take any pictures of the channel downstream and/or of Horsepen Creek in that area?
Also, can you clarify why the post construction information that you originally sent (below) has higher shear stress than
the temporary Phase 5 condition (worst case scenario)?
Thanks,
Sue Homewood
Division of Water Resources, Winston Salem Regional Office
Department of • Quality
336 776 9693 office
336 813 1863 mobile
Sue. Homewood(a-)ncdenr.gov

x
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From: David Kiker [mailto:[e-mail]]
Sent: Tuesday, February 09, 2016 10:31 AM
To: Homewood, Sue <[e-mail]>
Cc: Jason Kennedy <jken [e-mail]>, Paul Smith <
<[e-mail]>
Subject: RE: HAECO Site: Response to Your Comment
[e-mail]>; Randall, Mike
Sue, We have developed a SWMM model alternative that reflects the worst case scenario of construction (Phase
5). There will be a period of time (approximately 7 months) where the proposed high flow rate bioretention pond
remains offline and the majority of the site is covered with its final impervious cover (concrete and rooftop). In this
phase, the temporary sediment basin constructed in Phase 4 will remain in place however much of the local drainage
system is proposed to go around this temporary sediment basin. Here are the results from the updated models that
includes runoff generated from the Harris Teeter distribution site itself:
Condition
1 -Year Peak Flow
(cfs)
2 -Year Peak Flow
(cfs)
Existing
108
117
Proposed
242
328
Temporary Construction Condition (Phase 5)
283
359
We have also updated our HEC -RAS analysis of the open channel located downstream of Harris Teeter. Here are some of
the findings from a recent field walk of the downstream channel:
• The channel is extremely flat because the City is protecting its sanitary sewer line with gabion baskets lining the
section of channel where the sanitary sewer line is. This grade control is creating a large pool of water to form
behind it which will help dissipate the energy of the flows leaving the main pipe. The overall channel slope from
the culvert to the top of the gabion lined channel is 0.001 ft/ft. An overall shear stress is relatively low due to the
flat slope of the channel.
• A second rip -rap grade control crossing is located approximately 50 feet downstream of the culvert outfall which
creates a pool which will help dissipate the energy of the flow leaving the culvert. The culvert is 114" diameter.
• Horsepen Creek will also provide some dampening effect to the upstream channel during high flow events (it
creates a high tailwater condition on the tributary).
3
Existing Conditions
Storm
Design
Chan Bot
Side
Side Sloe
Design
Chan Wetted Hydraulic
Mann.
Channel
Q
Calc. C
Event
Flow (cfs)
Width
Slope
Length
Depth
Area Perim., Pw Radius
"n"
Slope
Allow.
Depth VE
1 -Year
108
77T71
5.7
� �;
64 23 2.7
U.f}5f7
` 0:'# t? 1 '
118
2=3
2 -Year
117
5.7
64 23 2.7
U.f}5f7
` Q:'#t? 1'
118
2=
10 -Year
201
777277777
5.7.,
64 1 23 2.7
U.f}5f7
` 0:'#t? 1'
118
33
100 -Year
334
" i : `1 ''
5.7�
64 23 2.7
p.f}5f7
` 0:'#t?1'
118
4=2 1
Phase 5 (During Construction Prior to Final SCM Online)
Storm
Desiqn
Chan Bot
Side
Side Slope
I Design I
Chan I Wetted I Hydraulic
I Mann.
I Channel I
Q
I Calc. C
3

Event
Flow cfs
Width
Sloe
Length
Depth
Area
Perim., Pw
Radius
"n"
Sloe
Allow.
Depth
Ve
1 -Year
283
H2
1> '
5.7
4 ' '
64
23
2.7
118
3.9
2 -Year
359
2"
:.
5.7.,
64
23
2.7
U.i}5 0:'#i? 1';
118
4=3
10 -Year
509
�2"
" i : `1 ''
5.7�
64
23
2.7
p.f}5f7 ` 0:'#i?1'
118
4=9
k
100 -Year
665
2"
" is `1 ''
5.7
64
23
2.7
p.f}5f7` 0:'#i?1'
118
5.
F
A quick summary from our end of this updated analysis:
Existing velocities are increasing as shown in the table above. Although these velocities in the Phase 5 condition
are not extremely high for an open channel.
• Shear stresses when using the overall slope of the channel are very low for an open channel.
Let me know if there is anything else that you need to complete your review.
David Kiker, PE
Technical Manager
WK Dickson - Raleigh Office
(919) 782- 0495
From: Homewood, Sue[mailto:sue.homewoodCa)ncdenr.Qov]
Sent: Thursday, February 04, 2016 9:01 AM
To: David Kiker
Subject: Re: HAECO Site: Response to Your Comment
David,
Although we are concerned with downstream velocity after the HAECO expansion is complete and the BMP is in
place, we are even more concerned with downstream issues during construction.
What I need to see is these calculations for the construction period, while the existing stormwater is routed
around the construction site. You have not indicated how long the existing stormwater will be directly
discharged with no treatment or attenuation. Will you please provide us with an analysis of the temporary
conditions, including how long they will occur.
Thanks.
From: David Kiker <[e-mail]>
Sent: Wednesday, February 3, 2016 5:21 PM
To: Homewood, Sue
Subject: RE: HAECO Site: Response to Your Comment
Sue, The numbers below were for the condition with the proposed BMP in place. One thing I should mention is that our
pre -project conditions analysis inadvertently included the detention of an area located on the HAECO site that was
previously a temporary erosion control facility that was never taken out. Removing this facility will slightly increase pre -
project flows but not enough to make a significant difference to the #s presented below.
-David

David Kiker, PE
Technical Manager
WK Dickson - Raleigh Office
(919) 782- 0495
From: Homewood, Sue[mailto:sue.homewoodCa)ncdenr.Qov]
Sent: Wednesday, February 03, 2016 4:22 PM
To: David Kiker
Subject: RE: HAECO Site: Response to Your Comment
David,
Just to make sure I understand this analysis, these calculations are for the period of time when the existing stormwater
pond at HAECO is out of service and before the new BMP is in use?
I will need to consider and research more regarding the downstream channel stability analysis. Any increase in velocity is
of concern to DWR. It may also require a site visit to see what the current state of the tributary and Horsepen Creek in
this area.
I would also like to know what length of time these conditions would be present.
Thanks,
4 e Aomewood
Division of Water Resources, Winston Salem Regional Office
Department of •
336 776 9693 office
336 813 1863 mobile
Sue. Homewood(a-)ncdenr.gov
IN -I AAM
•
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From: David Kiker [mailto:[e-mail]]
Sent: Tuesday, February 02, 2016 2:39 PM
To: Homewood, Sue <[e-mail]>
Subject: FW: HAECO Site: Response to Your Comment
Sue, We have looked at the downstream receiving waters for both flooding and channel stability. The previously
submitted 401 report documents the impacts to flooding of the Harris Teeter pond, roads and homes downstream. As
described in the original report, a review of a recent aerial showed not homes or insurable structures. Subsequent to the
November 401 report being prepared we looked at potential impacts to a parking lot located on the downstream side of
the Harris Teeter site along Horsepen Creek. There will be minimal increases to flooding at this location but this parking
lot is located outside the FEMA floodplain. Here is some additional documentation on the flooding evaluation of that
parking lot.

Downstream Flood Evaluation at Harris Teeter Parking Lot
ThefoUovvinQsummarizesthef|oodimpactztotheHarrisTeeterparkinQ|otfoundatFEK4AXS45GO3Austupstreamof
the parking lot in question):
1O-yearpeak flow increased from 1O2Ocfsto1337cfs.
100 -year peak flow increased from 1598 cfs to 1935 cfs.
Table 1: Water Surface Elevation Comparison at FEMA XS 45603 at Horsepen Creek
Flood Event Existing FEMA WSEL
Proposed Conditions
Change in WSEL
Velocity Changes in
10 -Year 824.20
824.51
0.31
4.12/4.9
100 -Year 825.53
825.76
0.23
5.0/4.91
Parking Lot Elevation =827(approximate e|evation)
Neither of the increases in Table 1 are alarming given the nature of the potential flooding (a parking lot) and relative
change from existing conditions.
Checking Bank Stability Along the Small Tributary Leaving Harris Teeter
Subsequent to the November 401 report being prepared we also performed a check on stream stability given the
increases to peak flows that result from the proposed project. This evaluation was limited to the tributary that leaves the
closed system atHarris Teeter until its outfall atHorsepenCreek. Because the drainage area for the HAECOdevelopment
is less than 10% of the drainage area at the confluence of Horsepen Creek, an evaluation of Horsepen Creek itself was not
prepared.
HorsepenCreekisastreamthatbacksupvvaterontotheopenchanne|that|eavestheHarrisTeetersite(located
downstream of the HAECO site). The following is a summary of the velocities and shear stresses obtained from a WK
Dickson prepared HEC -RAS model for the unnamed tributary to Horsepen Creek.
Table 2: Changes in Velocities

Flood Event Existing Velocity (ft/sec) Proposed Velocity Changes in Velocity
(ft/sec) Channel (ft/sec)
10 -Year 4.64 6.86 2.22
100 -Year 5.30 7.29 1.99
Table 3: Changes in Shear Stresses
Flood Event
Existing Shear Stress
(Ib/sq ft)
Proposed Shear Stress
(Ib/sq ft)
Changes in Shear Stress
(Ib/sq ft)
10 -Year
0.77
1.39
0.62
100 -Year
0.93
1.51
0.58
There are quite a few factors that affect stream stability. Stream slope, upstream sediment transport, root density
(sunlight conditions) of banks, armoring affect from gravel, cobble, rocks and large roots, soil types, meander pattern,
bank slopes, riparian vegetation (among other factors). This high tailwater condition from Horsepen Creek acts to dampen
stream hydraulics and reduces the potential for stream instabilities for the tributary leaving Harris Teeter. We have not
finalized our evaluation at this time but wanted to put this in front of you to show you that we had evaluated the
downstream receiving channel and we also wanted to solicit your thoughts. Would like to know what your thoughts are
with this evaluation. Thanks.
-David
David Kiker, PE
Technical Manager
WK Dickson - Raleigh Office
(919) 782- 0495
From: Homewood, Sue[mailto:sue.homewoodCa)ncdenr.Qov]
Sent: Thursday, January 28, 2016 10:34 PM
To: David Kiker
Subject: RE: HAECO Site: Response to Your Comment
Sorry, I should have said that Mike Randall has completed his review and these are our combined questions.
To follow up on the first question, can you provide me with an estimate of how long of a time frame the existing
stormwater will be untreated. Can you also provide any information about whether you reviewed expected flows and
downstream erosion concerns and/or what measures are being taken.
Thanks,
Sue Homewood
Division of Water Resources, Winston Salem Regional Office
Department of • Quality
336 776 9693 office
336 813 1863 mobile
Sue. Homewood(a-)ncdenr.gov

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From: David Kiker [mailto:[e-mail]]
Sent: Thursday, January 28, 2016 5:43 PM
To: Homewood, Sue <[e-mail]>
Subject: HAECO Site: Response to Your Comment
Sue, Our current 90% design plans include the erosion control sequencing for the project and a plan to divert flows
around the construction zone. The existing wet pond that treats the runoff from the existing 15 acre site to the east will
be decommissioned at the front end of construction in order to get this area dewatered. For the time period of
construction this stormwater runoff is currently proposed to go undertreated and will be diverted to the south eventually
outfalling at Radar Road. Here is an image of that plan:
T.
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Can you clarify your second comment about the request for water quality credits for oversizing the SCM? Was curious
where things stood with your review and Mike's review in terms of how far along the review was and when you
anticipated wrapping it up. I may call you tomorrow just to touch base since I told you that I would call you back. Thanks
ahead of time for your feedback.
David Kiker, PE
Technical Manager
WK Dickson - Raleigh Office
(919) 782- 0495
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a. -a: :. ;.E E II E - :.. _T vA ET -L '.
- rLE m I¢r .
Can you clarify your second comment about the request for water quality credits for oversizing the SCM? Was curious
where things stood with your review and Mike's review in terms of how far along the review was and when you
anticipated wrapping it up. I may call you tomorrow just to touch base since I told you that I would call you back. Thanks
ahead of time for your feedback.
David Kiker, PE
Technical Manager
WK Dickson - Raleigh Office
(919) 782- 0495

M E M O R A N D U M
720 Corporate Center Drive Raleigh, North Carolina 27607
TO: Sue Homewood & Mike Randall
FROM: David Kiker, PE
DATE: February 24, 2016
�N&WK
"�r
W DICKSON
community Infrastructure consultants
919.782.0495 tel. [phone] fax
RE: HAECO Facility Improvements Project— Evaluation of Downstream
Stream Stability
This memorandum summarizes the WK Dickson evaluation of downstream stream stability as a
result of the onsite development of the HAECO Facility Improvements Project at the Piedmont Triad
International Airport in Greensboro, North Carolina. An overview map that shows the closed
drainage systems for the HAECO and Harris Teeter Distribution sites, and the unnamed tributary to
Horsepen Creek is provided as Attachment #1. The conclusions drawn in this memorandum are
based on the 2 -year storm event and a hydraulic evaluation that used the U.S. Army Corps of
Engineers HEC -RAS 4.1.1 model, AutoCAD Sanitary Sewer Analysis (SSA) model, and an in-house
excel spreadsheet. This 2 -year storm event is typically used in the industry when evaluating stream
stability given the assumption that a channel can repair itself in the less frequent storms. A check
was also made using the 100 -year flood event to confirm that the proposed downstream
improvements are sustainable for a larger storm event.
Evaluating stream stability is an inexact science given the many variables that affect stream stability.
Such factors as channel shape, channel slope, cohesiveness of bank material, riparian bank
vegetation, bank armoring, bed composition, in -stream sediment load, downstream tailwater
conditions, meander pattern and other variables can all affect a stream's stability. A common
approach to evaluating stream stability is to determine the stream's velocity and shear stress and
compare these values to published permissible shear stress and stream velocities for streams of
similar conditions. The permissible shear stress and stream velocity presented in this memorandum
are based on the U.S. Army Corps of Engineers document titled Stability Thresholds for Stream
Restoration Materials, dated May 2001. As shown in this memorandum, the post -project conditions
along the unnamed tributary to Horsepen Creek will remain stable as the post -project velocities and
shear stresses are within the permissible ranges for a stable channel.
7

Existing Downstream Open Channel Conditions
The existing downstream open channel is approximately 295 feet in length prior to its mouth at
Horsepen Creek. Horsepen Creek is a FEMA mapped stream with a drainage area of 2.1 square
miles at Radar Road. The channel invert at Horsepen Creek is approximately 1 foot below the 114
inch diameter CMP leaving the Harris Teeter site. For this reason, during a significant storm event
the entire length of the unnamed tributary to Horsepen Creek and the Harris Teeter culvert itself
will be under the backwater effect from Horsepen Creek. This high tailwater condition will have the
effect to dampen the energy of the flow leaving the Harris Teeter closed pipe system as it passes
through the open channel prior to entering Horsepen Creek. Located at the downstream limits of the
unnamed tributary to Horsepen Creek is an open channel that is partially covered with NCDOT
Class A rip -rap with a Dso of 6 inches. This material is relatively small for rip -rap protection and the
fact that it has not mobilized downstream supports that contention that the high tailwater
conditions of Horsepen Creek will help dampen the effect of erosive flows passing through the
unnamed tributary. Table 1 shows the 2 -year water surface elevation from Horsepen Creek
interpolated from output found in the duplicate effective HEC -RAS model.
Table 1: Tailwater Condition from Horsepen Creek
Harris Teeter Culvert Invert
(ft NAVD'88)
2 -Year WSEL from Horsepen
Creek (ft NAVD'88)
Tailwater Depth (ft)
820.21
823.4
3.2
WSEL - water surface elevation
As shown in Table 1, the tailwater depth inside the Harris Teeter culvert is in excess of 3 feet. This
equates to the majority of the open channel being inundated close to the channel banks in a 2 -year
flood event. As a result, the unnamed tributary to Horsepen Creek will have its stream energy
dampened by the high tailwater condition.
Also providing protection from future erosion are two grade control structures located in the
channel bottom as shown in Attachment #1. These grade controls that were constructed for the
following reasons:
• Grade Control #1: Approximately 50 feet downstream of the Harris Teeter culvert is a rip -
rap (NCDOT Class B and Class I) lined grade control that was designed to create a pool of
water at the outfall of the culvert (see photo #1 and #2).
• Grade Control #2: Approximately 35 feet from the mouth at Horsepen Creek is a City of
Greensboro sanitary sewer line. This sewer system is being protected by a series of gabion
baskets set in the channel bottom (see photo #3 and #4). The top of these baskets are
approximately 0.25 inches below the invert of the 114 inch diameter CMP leaving the Harris
Teeter site.
Although there are a series of pools located between the outfall of the Harris Teeter culvert and the
Grade Control #2 that have localized slopes, the overall channel slope is extremely flat as shown in
Table 1.
r�

Table 2: Overall Channel Slove
Culvert Invert
Channel Invert at
Existing
112
Temporary Construction Condition (Phase 5)
359
Final Recommended Proposed Conditions
339
Overall
Elevation at Harris
Sanitary Sewer
Elevation
Channel
Channel Slope
Teeter Outfall
Line Elevation
Change (ft)
Distance (ft)
(ft/ft)
(ft NAVD '88)
(ft NAVD '88)
820.21
819.96
0.25
265
0.001
The existing downstream open channel is trapezoidal in shape with the following typical
dimensions (see photo #5):
• Bottom width: 12 feet
• Top width: 15 feet
• Bank height: 3.5 to 5.0 feet
• Manning's "n" value: 0.05
• Channel capacity flowing full: 260 cfs
The riparian corridor between the Harris Teeter culvert outfall and Horsepen Creek include small
trees, underbrush and kudzu. The kudzu vines have choked out much of the trees and other
vegetation that would typically provide a root system to protect the channel banks.
Hydrologic Evaluation
Peak flows in the model were obtained from a WK Dickson prepared AutoCAD Sanitary Sewer
Analysis model that runs off the EPA SWMM engine. The model was developed to size the pipe
infrastructure and high flow rate bioretention pond proposed for the onsite HAECO Facility
Improvements Project. The following table is a summary of the peak flows for the existing (pre -
project), the temporary during construction, originally proposed, and final recommended
conditions:
Table 3: Summary of Peak Flows
Condition
2 -Year Peak Flow (cfs)
Existing
112
Temporary Construction Condition (Phase 5)
359
Final Recommended Proposed Conditions
339
Hydraulic Evaluation
In addition to the SSA SWMM model, WK Dickson developed a HEC -RAS model and an in-house
spreadsheet to calculate shear stresses and further evaluate stream stability. The HEC -RAS model
includes seven (7) cross sections based on field measured data and City of Greensboro GIS
topographical mapping generated from LiDAR data. A copy of the in-house spreadsheet can be
found in Attachment #2.
3

The following series of table summarize the findings from the WK Dickson hydraulic evaluation:
Table 4: Summary of Calculated Overall Shear Stress for 2 -Year Storm Event
Condition
2 -Year Shear Stress (lbs/sq ft)
Existing
0.17
Temporary Construction Condition (Phase 5)
0.28
Final Recommended Proposed Conditions
0.27
Shear stresses calculated using WK Dickson in-house spreadsheet for overall channel slope
The shear stress for all the evaluated conditions are extremely low for a typical open channel in the
Piedmont region. WK Dickson typically targets a shear stress value of less than 0.50 lbs/square foot
on their natural stream restoration designs and rarely achieves a value under 0.30 lbs/square foot in
Piedmont stream. Once the vegetation is established these natural stream restoration projects with
designed shear stresses of 0.50 lbs/square foot become stable fairly quickly. The following table
summarizes the calculated channel velocity for the 2 -year storm event using both HEC -RAS and the
AutoCAD SSA model (based on the EPA SWMM model):
Table 5: Summary of Calculated Overall Stream Velocity
Condition
HEC -RAS Calculated 2 -Year
SWMM Calculated 2 -Year
Channel Velocity (ft/sec)
Channel Velocity (ft/sec)
Existing
3.5
2.7
Temporary Construction
5.4
3.2
Condition (Phase 5)
Final Recommended Proposed
5.4
3.3
Conditions
Note: HEC -RAS results were averaged over the entire reach.
The 2 -year channel velocities shown in Table 5 for all the evaluated conditions are relatively low for
a typical open channel in the Piedmont region. The average velocity calculated in HEC -RAS for the
proposed conditions would have been 4.7 feet per second if the two rip -rap lined reaches of
channel been removed from the calculation. The SSA model results were considerably lower than
HEC -RAS because the model is generating a weighted velocity that includes overbank flows. A
more detailed analysis of the results presented in Table 5 is provided in the next section of the
report.
Permissible Shear Stress and Velocity
Based on WK Dickson February 5, 2016 field walk, and the U.S. Army Corps of Engineers document
titled Stability Thresholds for Stream Restoration Materials, we are proposing the following
permissible shear stress and velocity for the 2 -year flood event:
4

Table 6: Recommended Permissible Shear Stress and Velocity for 2 -Year Flood Event
Permissible Shear Stress (lb/square foot) Permissible Velocity (ft/sec)
0.5 6.0
These recommended threshold values presented in Table 6 are relatively low given that natural
stable streams with cohesive banks in the Piedmont region very often see velocities that exceed 6
feet per second for a 2 -year flood. A review of the 11 other tributaries to Horsepen Creek found in
the FEMA duplicate effective HEC -RAS model shows that the typical 2 -year velocities range from 3
to 6.5 feet per second with maximum values approaching 10 feet per second.
When evaluating the permissible shear stress and channel velocity downstream of the HAECO site,
one must consider that the open channel is relatively short in length and has two significant grade
control structures that set the overall channel slope. The gabion baskets at Grade Control #2 are set
so that they are not exposed on the upstream face and as a result will be able to handle shear
stresses that exceed 10 lbs/square foot. While the Class I rip -rap found at Grade Control #1 can
withstand a shear stress of approximately 5 lbs/square foot. To evaluate channel bank erosion, you
must understand that typically the banks become unstable as a result of the toe of the channel
becoming unstable. This typically occurs when the channel thalweg experiences degradation. The
presence of these grade control structures will limit future channel degradation, will maintain the
existing overall channel slope and as a result help minimize and future channel bank erosion.
Other Design Considerations
The culverts at Radar Road are twin 8.9 feet by 6.6 feet CMP pipe arches that when flowing full
convey approximately 896 cfs. The primary closed pipe that is located at the Harris Teeter site is a
9.5 feet diameter CMP that convey approximately 750 cfs when flowing full. It appears that the
engineer who designed these two closed drainage systems considered ultimate build out for the
landuse conditions of the upstream watershed. The landuse may have reflected an industrial
landuse not reflective of the current airport's landuse which is primarily composed of highly
impervious pockets of industrial landuse with the majority of the drainage area flat grass infields.
The peak flows found in the existing and proposed conditions model prepared by WK Dickson are
relatively low given the 241 acre (0.4 square mile) drainage area and culvert capacity of Radar Road
and the primary Harris Teeter closed drainage system. It also appears that the existing open
channel was constructed with this same "conservative" hydrologic approach used to design the
Radar Road and Harris Teeter closed pipe systems. When evaluating channel capacity alone, the
existing open channel is adequately sized to convey 260 cfs prior to overtopping its banks. The 1 -
year peak flow is a storm event that should reach approximately the bankfull elevation. The 1 -year
proposed conditions peak flow is 259 cfs which indicates that the existing channel is appropriately
sized for the post -project conditions for the HAECO Facility Improvements Project.
Conclusions
The resultant shear stresses and velocities for the unnamed tributary to Horsepen Creek are under
the permissible values found in Table 6. Grade Control Structures #1 and #2 are locking in the
channel thalweg and as a result will limit future bank erosion. For these reasons, we are not
recommending additional onsite or offsite changes to the current design.
5

AV
le
AV

AL
At
�L
ri i

WDICKSON
community Infrastructure consultants
N Stream Stability Evaluation Map
WF
s Piedmont -Triad International Airport
HAECO Site Development
100 50
1 inch = 100 feet
100 Feet

Shear Stress Analysis of Unnamed Tributary to Horsepen Creek
Project: HAECO Facility Improvement Project
Location: Downstream Channel (Below Harris Teeter Distribution Center)
Engineer: DJK
Date: 2-23-16
Mannings Equation, Q=(A)( 1.49 Ro bb S0.5
n )I
Shear Stress, T = yds
T = shear stress in Ib/sq. ft.
y = unit weight of water, 62.4 Ib/cu. ft.
d = flow depth in ft.
s = channel slope in ft./ft.
TemporaryLiners
Material
A[[ow Shea6tress
Phase 5 (During Construction Prior to Final SCM Online)
Final Recommended Proposed Site Conditions
Tacked Mulch
Existing Conditions
Jute Net
Storm
Design
Storm
Design
Chan Bot Side Side Slope
Design
Storm
Design
Chan Bot Side Side Slope
Design
Chan
Wetted
Hydraulic
Mann.
Channel
Q
Calc.
HEC -RAS
Shear
Temp.
Perm.
Event
Flow (cfs)
Width
Slope Length
Depth
Area
Perim., Pw
Radius
"n"
Slope
Allow.
Depth
Velocity
Stress
Liner
Liner
1 -Year
84
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
2.5
3.5
0.15
NA
NA
2 -Year
112
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
2.9
3.8
0.17
NA
NA
10 -Year
187
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
3.6
4.5
0.21
NA
NA
100 -Year
312
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
4.5
5.3
0.27
NA
NA
Shear Stress, T = yds
T = shear stress in Ib/sq. ft.
y = unit weight of water, 62.4 Ib/cu. ft.
d = flow depth in ft.
s = channel slope in ft./ft.
TemporaryLiners
Material
A[[ow Shea6tress
Phase 5 (During Construction Prior to Final SCM Online)
Final Recommended Proposed Site Conditions
Tacked Mulch
0.35
Jute Net
Storm
Design
Storm
Design
Chan Bot Side Side Slope
Design
Chan
Wetted
Hydraulic
Mann.
Channel
Q
Calc.
HEC -RAS
Shear
Temp.
Perm.
Event
Flow (cfs)
Width
Slope Length
Depth
Area
Perim., Pw
Radius
"n"
Slope
Allow.
Depth
Velocity
Stress
Liner
Liner
1 -Year
259
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
4.2
5.0
0.25
NA
NA
2 -Year
339
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
4.7
5.4
0.27
NA
NA
10 -Year
518
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
5.3
6.5
0.31
NA
NA
100 -Year
675
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
5.7
7.2
0.33
NA
NA
Shear Stress, T = yds
T = shear stress in Ib/sq. ft.
y = unit weight of water, 62.4 Ib/cu. ft.
d = flow depth in ft.
s = channel slope in ft./ft.
TemporaryLiners
Material
A[[ow Shea6tress
Phase 5 (During Construction Prior to Final SCM Online)
(Ib/sgft)
Tacked Mulch
0.35
Jute Net
Storm
Design
Chan Bot Side Side Slope
Design
Chan
Wetted
Hydraulic
Mann.
Channel
Q
Calc.
HEC -RAS
Shear
Temp.
Perm.
Event
Flow (cfs)
Width
Slope Length
Depth
Area
Perim., Pw
Radius
"n"
Slope
Allow.
Depth
Velocity
Stress
Liner
Liner
1 -Year
286
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
4.5
4.9
0.26
NA
NA
2 -Year
359
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
4.8
5.4
0.28
NA
NA
10 -Year
509
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
5.3
6.2
0.31
NA
NA
100 -Year
634
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
5.7
6.8
0.33
NA
NA
Shear Stress, T = yds
T = shear stress in Ib/sq. ft.
y = unit weight of water, 62.4 Ib/cu. ft.
d = flow depth in ft.
s = channel slope in ft./ft.
TemporaryLiners
Material
A[[ow Shea6tress
Material
(Ib/sgft)
Tacked Mulch
0.35
Jute Net
0.45
Straw w/Net
1.45
SytheticMat
2.00
ClassA
1.25
ClassB
2.00
Class[
3.40
Class[I
4.50
Max. Permissibb Velocitiesfor Unproected Soilsin Ex. Channels
Material
Max Permissib6 Velocity(Us)
FincSand(noncollidl)
2.5
Sand Loam(noncollidl)
2.5
SiltLoam(noncollidl)
3.0
OrdinaryFirm Loam
3.5
FincGravel
5.0
Stiff Clay(verycollidal)
5.0
Graded,Silt toCobbles
5.0
Notes:
Side slope = horiz./vert.
Depth and Velocity calculated using WK Dickson generated HEC -RAS model for average overall
ax. Allow. Design V for Vegetative Channels
71Slope Soil
Grass Lining Pemnssibb
V 0t 0
5% Sands/Sill
Bemuda
5.0
Tall Fescue
4.5
KYBhregrass
4.5
Gra,s-legarrcaix
3.5
ClayMixcs
Bermuda
6.0
Tall Fescue
5.5
KY Bluegrass
5.5
Grass-legum,nix
4.5
10% Sands/Silt
Bermuda
4.5
Tall Fescue
4.0
xYBluegrass
4.0
Grass -leges -aux
3.0
Clay Mixes
Bermuda5.5
Tall Fescue
5.0
KY Bluegrass
5.0
Grass-legunvtnx
3.5
and velocity
Attachment #2

Stability Thresholds for
Stream Restoration Materials
by Craig Fischenich'
Complexity
Low Moderate High
Value as a Planning Tool
Low Moderate High
OVERVIEW
Stream restoration projects usually involve
some modification to the channel or the banks.
Designers of stabilization or restoration projects
must ensure that the materials placed within
the channel or on the banks will be stable for
the full range of conditions expected during the
design life of the project. Unfortunately,
techniques to characterize stability thresholds
are limited. Theoretical approaches do not
exist and empirical data mainly consist of
velocity limits, which are of limited value.
Empirical data for shear stress or stream power
are generally lacking, but the existing body of
information is summarized in this technical
note. Whereas shear thresholds for soils found
in channel beds and banks are quite low
(generally < 0.25 Ib/sf), those for vegetated
soils (0.5 — 4 Ib/sf), erosion control materials
and bioengineering techniques (0.5 — 8 Ib/sf),
and hard armoring (< 13 Ib/sf) offer options to
provide stability.
STABILITY CRITERIA
The stability of a stream refers to how it
accommodates itself to the inflowing water and
sediment load. In general, stable streams may
adjust their boundaries but do not exhibit trends
in changes to their geometric character. One
form of instability occurs when a stream is
unable to transport its sediment load (i.e.,
sediments deposited within the channel),
leading to the condition referred to as
aggradation.
May 2001
Cost
Low Moderate High
When the ability of the stream to transport
sediment exceeds the availability of sediments
within the incoming flow, and stability
thresholds for the material forming the
boundary of the channel are exceeded, erosion
occurs. This technical note deals with the latter
case of instability and distinguishes the
presence or absence of erosion (threshold
condition) from the magnitude of erosion
(volume).
Erosion occurs when the hydraulic forces in the
flow exceed the resisting forces of the channel
boundary. The amount of erosion is a function
of the relative magnitude of these forces and
the time over which they are applied. The
interaction of flow with the boundary of open
channels is only imperfectly understood.
Adequate analytical expressions describing this
interaction have not yet been developed for
conditions associated with natural channels.
Thus, means of characterizing erosion potential
must rely heavily upon empiricism.
Traditional approaches for characterizing
erosion potential can be placed in one of two
categories: maximum permissible velocity, and
tractive force (or critical shear stress). The
former approach is advantageous in that
velocity is a parameter that can be measured
within the flow. Shear stress cannot be directly
measured — it must be computed from other
flow parameters. Shear stress is a better
measure of the fluid force on the channel
boundary than is velocity. Moreover,
conventional guidelines, including ASTM
standards, rely upon the shear stress as a
USAE Research and Development Center, Environmental Laboratory, 3909 Halls Ferry Rd., Vicksburg MS 39180
ERDC TN-EMRRP-SR-29

means of assessing the stability of erosion
control materials. Both approaches are
presented in this paper.
Incipient Motion (Threshold Condition)
As flow over the bed and banks of a stream
increases, a condition referred to as the
threshold state is reached when the forces
tending to move materials on the channel
boundary are in balance with those resisting
motion. The forces acting on a noncohesive
soil particle lying on the bed of a flowing stream
include hydrodynamic lift, hydrodynamic drag,
submerged weight (FW — Fb), and a resisting
force Fr. as seen in Figure 1. The drag is in the
direction of the flow and the lift and weight are
normal to the flow. The resisting force depends
on the geometry of the particles. At the
threshold of movement, the resultant of the
forces in each direction is zero. Two
approaches for defining the threshold state are
discussed herein, initial movement being
specified in terms of either a critical velocity
(vcr) or a critical shear stress (rc,).
FE] - 7„d,
F L^w
F, ='fir, pmt c — % cd
upon this method. Considerable empirical data
exist relating maximum velocities to various soil
and vegetation conditions.
However, this simple method for design does
not consider the channel shape or flow depth.
At the same mean velocity, channels of
different shapes or depths may have quite
different forces acting on the boundaries.
Critical velocity is depth -dependent, and a
correction factor for depth must be applied in
this application. Despite these limitations,
maximum permissible velocity can be a useful
tool in evaluating the stability of various
waterways. It is most frequently applied as a
cursory analysis when screening alternatives.
Critical Shear Stress
The forces shown in Figure 1 can also be
expressed in terms of the shear stress. Shear
stress is the force per unit area in the flow
direction. Its distribution in steady, uniform,
two-dimensional flow in the channel can be
reasonably described. An estimate of the
average boundary shear stress (To) exerted by
the fluid on the bed is:
To = yDSf
(1)
where y is the specific weight of water, D is the
Fn . C pm u' e; - flow depth (— hydraulic radius), and Sf is the
friction slope. Derived from consideration of the
conservation of linear momentum, this quantity
F is a spatial average and may not provide a
q good estimate of bed shear at a point.
} F. 'ra d,'
Figure 1. Forces acting on the boundary of
a channel (adapted from Julien (1995)).
Critical Velocity
Figure 1 shows that both the lift and the drag
force are directly related to the velocity
squared. Thus, small changes in the velocity
could result in large changes in these forces.
The permissible velocity is defined as the
maximum velocity of the channel that will not
cause erosion of the channel boundary. It is
often called the critical velocity because it
refers to the condition for the initiation of
motion. Early works in canal design and in
evaluating the stability of waterways relied
Critical shear stress (,c,,) can be defined by
equating the applied forces to the resisting
forces. Shields (1936) determined the
threshold condition by measuring sediment
transport for values of shear at least twice the
critical value and then extrapolating to the point
vanishing sediment transport. His laboratory
experiments have since served as a basis for
defining critical shear stress. For soil grains of
diameter d and angle of repose � on a flat bed,
the following relations can approximate the
critical shear for various sizes of sediment:
c�Y = 0.5(C, — q,)d Tang For clays (2)
c:;. = 0.25d.-06(1� —q,)d Tamm For silts and
sands (3)
2 ERDC TN-EMRRP-SR-29

(,-,. = 0.060. - Ej, )d Tans For gravels and
cobbles (4)
Where
1/3
d. = d (G-�1)g� (5)
° J
ys = the unit weight of the sediment
yW = the unit weight of the water/sediment
mixture
G = the specific gravity of the sediment
G = gravitational acceleration
v = the kinematic viscosity of the
water/sediment mixture
The angle of repose � for noncohesive
sediments is presented in Table 1 (Julien
1995), as are values for critical shear stress.
The critical condition can be defined in terms of
shear velocity rather than shear stress (note
that shear velocity and channel velocity are
different). Table 1 also provides limiting shear
velocity as a function of sediment size. The
V, term is the critical shear velocity and is
equal to
V.0 = gRhSf
Table 1. Limiting
Shear Stress and Ve locitX
for Uniform Noncohesive Sediments
Class name
ds (in)
w(deg)
Boulder
Very large
>80
42
0.054
37.4
4.36
Large
>40
42
0.054
18.7
3.08
Medium
>20
42
0.054
9.3
2.20
Small
>10
42
0.054
4.7
1.54
Cobble
Large
>5
42
0.054
2.3
1.08
Small
>2.5
41
0.052
1.1
0.75
Gravel
Very coarse
>1.3
40
0.050
0.54
0.52
Coarse
>0.6
38
0.047
0.25
0.36
Medium
>0.3
36
0.044
0.12
0.24
Fine
>0.16
35
0.042
0.06
0.17
Very fine
>0.08
33
0.039
0.03
0.12
Sands
Very coarse
>0.04
32
0.029
0.01
0.070
Coarse
>0.02
31
0.033
0.006
0.055
Medium
>0.01
30
0.048
0.004
0.045
Fine
>0.005
30
0.072
0.003
0.040
Very fine
>0.003
30
0.109
0.002
0.035
Silts
Coarse
>0.002
30
0.165
0.001
0.030
Medium
>0.001
30
0.25
0.001
0.025
Table 1 provides limits best applied when
evaluating idealized conditions, or the stability
of sediments in the bed. Mixtures of sediments
tend to behave differently from uniform
sediments. Within a mixture, coarse sediments
are generally entrained at lower shear stress
values than presented in Table 1. Conversely,
larger shear stresses than those presented in
the table are required to entrain finer sediments
within a mixture.
(6)
Cohesive soils, vegetation, and other armor
materials can be similarly evaluated to
determine empirical shear stress thresholds.
Cohesive soils are usually eroded by the
detachment and entrainment of soil
aggregates. Motivating forces are the same as
those for noncohesive banks; however, the
resisting forces are primarily the result of
cohesive bonds between particles. The
bonding strength, and hence the soil erosion
resistance, depends on the physio -chemical
properties of the soil and the chemistry of the
ERDC TN-EMRRP SR -29 3

fluids. Field and laboratory experiments show
that intact, undisturbed cohesive soils are much
less susceptible to flow erosion than are non-
cohesive soils.
Vegetation, which has a profound effect on the
stability of both cohesive and noncohesive
soils, serves as an effective buffer between the
water and the underlying soil. It increases the
effective roughness height of the boundary,
increasing flow resistance and displacing the
velocity upwards away from the soil, which has
the effect of reducing the forces of drag and lift
acting on the soil surface. As the boundary
shear stress is proportional to the square of the
near -bank velocity, a reduction in this velocity
produces a much greater reduction in the
forces responsible for erosion.
Vegetation armors the soil surface, but the
roots and rhizomes of plants also bind the soil
and introduce extra cohesion over and above
any intrinsic cohesion that the bank material
may have. The presence of vegetation does
not render underlying soils immune from
erosion, but the critical condition for erosion of
a vegetated bank is usually the threshold of
failure of the plant stands by snapping, stem
scour, or uprooting, rather than for detachment
and entrainment of the soils themselves.
Vegetation failure usually occurs at much
higher levels of flow intensity than for soil
erosion.
Both rigid and flexible armor systems can be
used in waterways to protect the channel bed
from erosion and to stabilize side slopes. A
wide array of differing armor materials are
available to accomplish this. Many
manufactured products have been evaluated to
determine their failure threshold. Products are
frequently selected using design graphs that
present the flow depth on one axis and the
slope of the channel on the other axis. Thus,
the design is based on the depth/slope product
(i.e., the shear stress). In other cases, the
thresholds are expressed explicitly in terms of
shear stress. Notable among the latter group
are the field performance testing results of
erosion control products conducted by the
TXDOT/TTI Hydraulics and Erosion Control
Laboratory (TXDOT 1999).
Table 2 presents limiting values for shear
stress and velocity for a number of different
channel lining materials. Included are soils,
various types of vegetation, and number of
different commonly applied stabilization
techniques. Information presented in the table
was derived from a number of different
sources. Ranges of values presented in the
table reflect various measures presented within
the literature. In the case of manufactured
products, the designer should consult the
manufacturer's guidelines to determine
thresholds for a specific product.
Uncertainty and Variability
The values presented in Table 2 generally
relate to average values of shear stress or
velocity. Velocity and shear stress are neither
uniform nor steady in natural channels. Short-
term pulses in the flow can give rise to
instantaneous velocities or stresses of two to
three times the average; thus, erosion may
occur at stresses much lower than predicted.
Because limits presented in Table 2 were
developed empirically, they implicitly include
some off this variability. However, natural
channels typically exhibit much more variability
than the flumes from which these data were
developed.
Sediment load can also profoundly influence
the ability of flow to erode underlying soils.
Sediments in suspension have the effect of
damping turbulence within the flow.
Turbulence is an important factor in entraining
materials from the channel boundaries. Thus,
velocity and shear stress thresholds are 1.5 to
3 times that presented in the table for flows
carrying high sediment loads.
In addition to variability of flow conditions,
variation in the channel lining characteristics
can influence erosion predictions. Natural bed
material is neither spherical nor of uniform size.
Larger particles may shield smaller ones from
direct impact so that the latter fail to move until
higher stresses are attained. For a given grain
size, the true threshold criterion may vary by
nearly an order of magnitude depending on the
bed gradation. Variation in the installation of
erosion control measures can reduce the
threshold necessary to cause erosion.
4 ERDC TN-EMRRP-SR-29

Table 2. Permissible Shear and Velocity for Selected Lining Materials'
Soils
Fine colloidal sand
0.02-0.03
1.5
A
Sandy loam (noncolloidal)
0.03-0.04
1.75
A
Alluvial silt (noncolloidal)
0.045-0.05
2
A
Silty loam (noncolloidal)
0.045-0.05
1.75-2.25
A
Firm loam
0.075
2.5
A
Fine gravels
0.075
2.5
A
Stiff clay
0.26
3-4.5
A, F
Alluvial silt (colloidal)
0.26
3.75
A
Graded loam to cobbles
0.38
3.75
A
Graded silts to cobbles
0.43
4
A
Shales and hardpan
0.67
6
A
Gravel/Cobble
1 -in.
0.33
2.5-5
A
2 -in.
0.67
3-6
A
6 -in.
2.0
4-7.5
A
12 -in.
4.0
5.5-12
A
Vegetation
Class A turf
3.7
6-8
E, N
Class B turf
2.1
4 - 7
E, N
Class C turf
1.0
3.5
E, N
Long native grasses
1.2-1.7
4-6
G, H, L, N
Short native and bunch grass
0.7-0.95
3-4
G, H, L, N
Reed plantings
0.1-0.6
N/A
E, N
Hardwood tree plantings
0.41-2.5
N/A
E, N
Temporary Degradable RECPs
Jute net
0.45
1-2.5
E, H, M
Straw with net
1.5-1.65
1 -3
E, H, M
Coconut fiber with net
2.25
3-4
E, M
Fiberglass roving
2.00
2.5-7
E, H, M
Non -Degradable RECPs
Unvegetated
3.00
5-7
E, G, M
Partially established
4.0-6.0
7.5-15
E, G, M
Fully vegetated
8.00
8-21
F, L, M
Ripraq
6 - in. d5o
2.5
5-10
H
9- in. d5o
3.8
7-11
H
12- in. d5o
5.1
10-13
H
18 -in. d5o
7.6
12-16
H
24 -in. d5o
10.1
14-18
E
Soil Bioengineering
Wattles
0.2-1.0
3
C, I, J, N
Reed fascine
0.6-1.25
5
E
Coir roll
3 - 5
8
E, M, N
Vegetated coir mat
4 - 8
9.5
E, M, N
Live brush mattress (initial)
0.4-4.1
4
B, E, I
Live brush mattress (grown)
3.90-8.2
12
B, C, E, 1, N
Brush layering (initial/grown)
0.4-6.25
12
E, 1, N
Live fascine
1.25-3.10
6-8
C, E, 1, J
Live willow stakes
2.10-3.10
3-10
E, N, O
Hard Surfacing
Gabions
10
14-19
D
Concrete
12.5
>18
H
Ranges of values generally reflect multiple sources of data or different testing conditions.
A. Chang, H.H. (1988).
F. Julien, P.Y. (1995).
K. Sprague, C.J. (1999).
B. Florineth. (1982)
G. Kouwen, N.; Li, R. M.; and Simons, D.B., (1980).
L. Temple, D.M. (1980).
C. Gerstgraser, C. (1998).
H. Norman, J. N. (1975).
M. TXDOT (1999)
D. Goff, K. (1999).
I. Schiechtl, H. M. and R. Stern. (1996).
N. Data from Author (2001)
E. Gray, D.H., and Sotir, R.B. (1996).
J. Schoklitch, A. (1937).
O. USACE (1997).
ERDC TN-EMRRP SR -29 5

Changes in the density or vigor of vegetation
can either increase or decrease erosion
threshold. Even differences between the
growing and dormant seasons can lead to one -
to twofold changes in erosion thresholds.
To address uncertainty and variability, the
designer should adjust the predicted velocity or
shear stress by applying a factor of safety or by
computing local and instantaneous values for
these parameters. Guidance for making these
adjustments is presented in the section titled
"Application" below.
EROSION MAGNITUDE
The preceding discussion dealt with the
presence or absence of erosion, but did not
address the extent to which erosion might
occur for a given flow. If the thresholds
presented in Table 2 are exceeded, erosion
should be expected to occur. In reality, even
when those thresholds are not exceeded, some
erosion in a few select locations may occur.
The extent to which this minor erosion could
become a significant concern depends in large
measure on the duration of the flow, and upon
the ability of the stream to transport those
eroded sediments.
Flow Duration
Although not stated, limits regarding erosion
potential published by manufacturers for
various products are typically developed from
studies using short flow durations. They do not
reflect the potential for severe erosion damage
that can result from moderate flow events over
several hours. Studies have shown that
duration of flow reduces erosion resistance of
many types of erosion control products, as
shown in Figures 2 - 4. A factor of safety
should be applied when flow duration exceeds
a couple of hours.
flow duration (from Fischenich and Allen
(2000)).
Figure 3. Limiting values for bare and TRM
protected soils (from Sprague (1999))
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Figure 3. Limiting values for bare and TRM
protected soils (from Sprague (1999))
Figure 4. Limiting values for plain and TRM
reinforced grass (from Sprague (1999))
6 ERDC TN-EMRRP-SR-29
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Figure 4. Limiting values for plain and TRM
reinforced grass (from Sprague (1999))
6 ERDC TN-EMRRP-SR-29

Correlations between flow volume and amount
of erosion tend to be poor. Multi -peaked flows
may be more effective than single flows of
comparable or greater magnitude because of
the increased incidence of wetting. Flows with
long durations often have a more significant
effect on erosion than short-lived flows of
higher magnitude. Sediment transport
analysis can be used to gauge the magnitude
of erosion potential in the channel design, but
predictive capability is limited.
Sediment Transport
A number of flow measures can be used to
assess the ability of a stream to transport
sediment. The unit stream power (Pm) is one
common approach, and is related to the earlier
discussion in that stream power includes both
velocity and shear stress as components.
Sediment transport (Qs) increases when the
unit stream power (Pm) increases. Unit stream
power in turn is controlled by both tractive
stress and flow velocity:
Table 3. Factors Influencina Erosion
P,,, = v • i = v - YW' D- Sf (7)
The total power (Pt) is the product of the unit
power times the channel width (W):
Pt = Pm W = v • W • D • W Sf = v • A- yw Sf
= QW' YW' Sf (8)
Stream power assessments can be useful in
evaluating sediment discharge within a stream
channel and the deposition or erosion of
sediments from the streambed. However, their
utility for evaluating the stability of measures
applied to prevent erosion is limited because of
the lack of empirical data relating stream power
to stability. The analysis of general
streambank erosion is not a simple extension
of the noncohesive bed case with an added
downslope gravity component. Complication is
added by other influencing variables, such as
vegetation, whose root system can reinforce
bank material and increase erosion resistance.
Factors influencing bank erosion are
summarized in Table 3.
low properties lagnitude, frequency and variability of stream discharge; Magnitude and distribution of
elocity and shear stress; Degree of turbulence
'ediment composition pediment size, gradation, cohesion and stratification
:timate rainfall amount, intensity and duration; Frequency and duration of freezing
'ubsurface conditions seepage forces; Piping; Soil moisture levels
:hannel geometry Vidth and depth of channel; Height and angle of bank; Bend curvature
Iiology 'egetation type, density and root character; Burrows
nthropogenic factors Irbanization, flood control, boating, irrigation
APPLICATION
The stability of a waterway or the suitability of
various channel linings can be determined by
first calculating both the mean velocity and
tractive stress (by the previous equations).
These values can then be compared with
allowable velocity and tractive stress for a
particular ground cover or lining system under
consideration (e.g., existing vegetation cover,
an erosion control blanket, or bioengineering
treatment). Allowable tractive stresses for
various types of soil, linings, ground covers,
and stabilization measures including soil
bioengineering treatments, are listed in Table
2. Additionally, manufacturers' product
literature can provide allowable tractive
stresses or velocities for various types of
erosion control products.
An iterative procedure may be required when
evaluating channel stability because various
linings will affect the resistance coefficient,
ERDC TN-EMRRP SR -29 7

which in turn may change the estimated flow
conditions. A general procedure for the
application of information presented in this
paper is outlined in the following paragraphs
Step 1 -Estimate Mean Hydraulic Conditions.
Flow of water in a channel is governed by the
discharge, hydraulic gradient, channel
geometry, and roughness coefficient. This
functional relationship is most frequently
evaluated using normal depth or backwater
computations that take into account principles
of conservation of linear momentum. The latter
is preferable because it accounts for variations
in momentum slope, which is directly related to
shear stress. Several models are available to
aid the hydraulic engineer in assessing
hydraulic conditions. Notable examples include
HEC -2, HEC -RAS, and WSP2. Channel cross
sections, slopes, and Manning's coefficients
should be determined based upon surveyed
data and observed or predicted channel
boundary conditions. Output from the model
should be used to compute main channel
velocity and shear stress at each cross section.
Step 2- Estimate Local/Instantaneous Flow
Conditions.
The computed values for velocity and shear
stress may be adjusted to account for local
variability and instantaneous values higher than
mean. A number of procedures exist for this
purpose. Most commonly applied are empirical
methods based upon channel form and
irregularity. Several references at the end of
this paper present procedures to make these
adjustments. Chang (1988) is a good example.
For straight channels, the local maximum shear
stress can be assumed from the following
simple equation:
c,—. = 1.5c (9)
for sinuous channels, the maximum shear
stress should be determined as a function of
the planform characteristics using Equation 10:
R —o. s
aX = 2.65 (10)
where Rc is the radius of curvature and W is
the top width of the channel. Equations 9 and
10 adjust for the spatial distribution of shear
stress; however, temporal maximums in
turbulent flows can be 10 — 20 percent higher,
so an adjustment to account for instantaneous
maximums should be added as well. A factor
of 1.15 is usually applied.
Step 3- Determine Existing Stability.
Existing stability should be assessed by
comparing estimates of local and
instantaneous shear and velocity to values
presented in Table 2. Both the underlying soil
and the soil/vegetation condition should be
assessed. If the existing conditions are
deemed stable and are in consonance with
other project objectives, then no further action
is required. Otherwise, proceed to step 4.
Step 4- Select Channel Lining Material.
If existing conditions are unstable, or if a
different material is needed along the channel
perimeter to meet project objectives, a lining
material or stabilization measure should be
selected from Table 2, using the threshold
values as a guideline in the selection. Only
material with a threshold exceeding the
predicted value should be selected. The other
project objectives can also be used at this point
to help select from among the available
alternatives. Fischenich and Allen (2000)
characterize attributes of various protection
measures to help in the selection.
Step 5- Recompute Flow Values.
Resistance values in the hydraulic
computations should be adjusted to reflect the
selected channel lining, and hydraulic condition
should be recalculated for the channel. At this
point, reach- or section -averaged hydraulic
conditions should be adjusted to account for
local and instantaneous extremes.
Table 4 presents velocity limits for various
channel boundaries conditions. This table is
useful in screening alternatives, or as an
alternative to the shear stress analysis
presented in the preceding sections.
8 ERDC TN-EMRRP-SR-29

Table 4. Stability of Channel Linings for Given Velocity Ranges
Linin 0-2f s 2-4f s 4-6f s 6-8f s >8f s
Sandy Soils
Firm Loam
Mixed Gravel and
Cobbles
Average Turf
Degradable RECPs
Stabilizing
Bioengineering
Good Turf
Permanent RECPs
Armoring
Bioengineering
CCMs & Gabions
Riprap
Concrete
Ke
Appropriate
Use Caution
Not Appropriate
Step 6— Confirm Lining Stability.
The stability of the proposed lining should be
assessed by comparing the threshold values in
Table 2 to the newly computed hydraulic
conditions. These values can be adjusted to
account for flow duration using Figures 2-4 as a
guide. If computed values exceed thresholds,
step 4 should be repeated. If the threshold is
not exceeded, a factor of safety for the project
should be determined from the following
equations:
FS = max or FS = Vmax (11)
est est
In general, factors of safety in excess of 1.2 or
1.3 should be acceptable. The preceding five
steps should be conducted for every cross
section used in the analysis for the project. In
the event that computed hydraulic values
exceed thresholds for any desirable lining or
stabilization technique, measures must be
undertaken to reduce the energy within the
flow. Such measures might include the
installation of low -head drop structures or other
energy -dissipating devices along the channel.
Alternatively, measures implemented within the
watershed to reduce total discharge could be
employed.
APPLICABILITY AND
LIMITATIONS
Techniques described in this technical note are
generally applicable to stream restoration
projects that include revegetation of the riparian
zone or bioengineering treatments.
ACKNOWLEDGEMENTS
Research presented in this technical note was
developed under the U.S. Army Corps of
Engineers Ecosystem Management and
Restoration Research Program. Technical
reviews were provided by Messrs. E.A. (Tony)
Dardeau, Jr., (Ret.), and Jerry L. Miller, both of
the Environmental Laboratory.
POINTS OF CONTACT
For additional information, contact the author,
Dr. Craig Fischenich, (601-634-3449,
fischecRwes.army.mil), or the manager of the
Ecosystem Management and Restoration
Research Program, Dr. Russell F. Theriot
(601-634-2733, therior(@wes.army.mil). This
technical note should be cited as follows:
ERDC TN-EMRRP SR -29 9

Fischenich, C. (2001). "Stability Thresholds
for Stream Restoration Materials," EMRRP
Technical Notes Collection (ERDC TN-
EMRRP-SR-29), U.S. Army Engineer
Research and Development Center,
Vicksburg, MS.
www. wes. army. mil/el/emrrp
REFERENCES
Chang, H. H. (1988). Fluvial Processes in River
Engineering, John Wiley and Sons, New York
and other cities, citing Fortier, S., and Scobey,
F.C. (1926). "Permissible canal velocities,"
Transactions of the ASCE, 89:940-984.
Fischenich and Allen (2000). "Stream
management," Water Operations Technical
Support Program Special Report ERDC/EL SR-
W -00-1, Vicksburg, MS.
Florineth, F., (1982). Begrunungen von
Erosionszonen im Bereich Ober der
Waldgrenze. Zeitschrift fur Vegetation stech n i k
5, S. 20-24 (In German).
Gerstgraser, C. (1998). "Bioengineering
methods of bank stabilization," GARTEN &
LANDSCHAFT, Vol. 9, September 1998, 35-
37.
Goff, K. (1999). "Designer linings," Erosion
Control, Vol. 6, No. 5.
Gray, D.H., and Sotir, R.B. (1996).
Biotechnical and soil bioengineering: a practical
guide for erosion control. John Wiley and Sons,
New York.
Julien, P.Y. (1995). Erosion and sedimentation.
Cambridge University Press, New York.
Kouwen, N.; Li, R. -M.; and Simons, D.B.
(1980). "A stability criteria for vegetated
Waterways." Proceedings, International
Symposium on Urban Storm Runoff. University
of Kentucky, Lexington, KY, 28-31 July 1980,
203-210.
Norman, J. N. (1975). "Design of stable
channels with flexible linings," Hydraulic
Engineering Circular 15, U.S. Dept. of
Transportation, Federal Highway Adm.,
Washington, DC.
Schiechtl, H. M., and Stern, R. (1996). Water
Bioengineering Techniques for Watercourse
Bank and Shoreline Protection. Blackwell
Science, Inc. 224 pp.
Schoklitsch, A. (1937). Hydraulic structures; a
text and handbook. Translated by Samuel
Shulits. The American Society of Mechanical
Engineers, New York.
Shields, A. (1936). "Anwendung der
ahnlichkeits-mechanik and der turblenz-
forschung auf die geschiebebewegung," Mitt.
Preuss. Versuchsanst. Wasser. Schiffsbau, 26,
1-26 (in German).
Sprague, C.J. (1999). "Green engineering:
Design principles and applications using rolled
erosion control products," CE News Online,
downloaded from
http://www.cenews.com/edecp0399.html.
Temple, D. M. (1980). "Tractive force design of
vegetated channels, Transactions of the ASAE,
23:884-890.
TXDOT (1999). "Field Performance Testing of
Selected Erosion Control Products," TXDOT /
TTI Hydraulics and Erosion Control Laboratory,
Bryan, TX.
USACE TR EL 97-8
10 ERDC TN-EMRRP-SR-29