20070812 Ver 1_Salisbury Conditions Comments_20070831

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August 24, 2007
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Mrs. JohnCDorney E TO [phone] YVFT(AND NV ST7q~D~H
NCDENR Division of Water Quality
1617 Mail Service Center
Raleigh, NC 27699-1617
Re: Proposed Conditions in 401 Water Quality Certification (DWQ #2007-0812)
Yadkin Hydroelectric Project (FERC Project No. 2197)
Dear Mr. Dorney,
Thank you for the letter dated August 8, 2007 regarding the Water Quality
Certification ("WQC") for the Yadkin Hydroelectric Project and a draft USGS report that
reviews some of the studies commissioned by the City of Salisbury. We appreciate this
opportunity to provide additional information and comments.
We understand the August 8 letter requests suggested wording for a dredging
condition to protect our intakes, but tentatively concludes that DWQ may not require a
condition to protect our water supply use from Project-caused interruption by flooding.
We address these matters below.
REQUEST FOR FLOOD MITIGATION CONDITION
The August 8 letter explains that the 401 WQC rules are intended to protect
Salisbury's water supply use, as follows:
DWQ's 401 Certification Rules... are predicated in part on the need to
protect the existing use of the water - in this case, the water supply use by
the City of Salisbury.
We understand the letter as indicating that DWQ's tentative negative conclusion
regarding mitigation of Project-caused interruption of water supply use by flooding is
based on the following:
1) Questions about the sufficiency of the data to provide a basis for a
certification condition.
217 S. Main Street P.O. Box 479 Salisbury, N.C. 28!45 Phone: [phone] Fax: [phone]

2) The first of the projected, recurring interruptions of our water supply use
by Project-caused flooding has not occurred yet.
3) Some of the projected, recurring interruptions of our water supply use by
Project-caused flooding will be accompanied by damage to our water
supply infrastructure.
Sufficient Data
The studies and reports commissioned by the City of Salisbury represent the best
available science regarding the sedimentation and flooding effects of the Yadkin Project.
We have not spared expense in retaining top experts to conduct the most scientifically
advanced studies. The extensiveness and high quality of the studies and the reports on
those studies provide DWQ with an unusually strong fact basis for its decision on
conditions for the sedimentation and flooding effects on our water supply use. The
studies and reports include the following:
o Mobile Boundary Hydraulics, PLLC, Numerical Sedimentation
Investigation, Yadkin River, North Carolina (February 20, 2007)
o High Rock Dam and Sediment Delta Flooding and Sedimentation Effects
(1927-2058) On City of Salisbury Critical Infrastructure, Dr. Martin
Doyle (February 2007)
o Technical Report: High Rock Dam and High Rock Lake Sedimentation
Flooding Effects as Estimated Using HEC-RAS Modeling, Salisbury-
Rowan Utilities, with Hazen and Sawyer, Environmental Engineers and
Scientists (January 2006)
o Response to "Consolidated Answer of Alcoa Power Generating, Inc. ",
Mobile Boundary Hydraulics, PLLC (May 8, 2007) (provided to
DENR in May 2007)
o Large Flood Relief Channel, Mobile Boundary Hydraulics, PLLC
(May 10, 2007) (provided to DENR in May 2007)
o Equilibrium Analysis of Yadkin and South Yadkin Rivers, Dr. Martin
Doyle (August 21, 2007) (copy enclosed under Tab 1)
o Effects of Bridges over Yadkin River on Water Surface Elevation
Profiles, Dr. Martin Doyle (August 2007) (copy enclosed under Tab 2)
Four of the reports listed above (in bold font) have not been considered or
included in the USGS draft report; two of them were mailed and e-mailed to DENR on
May 14, 2007 and were listed in our letter to DWQ dated July 18, 2007.' We believe that
consideration of the omitted reports will resolve the issues raised in the USGS report
regarding the studies commissioned by Salisbury.
We note, as one example, that one of the reports omitted by the USGS draft
report, Response to "Consolidated Answer of Alcoa Power Generating, Inc. ", Mobile
Boundary Hydraulics, PLLC (May 8, 2007), compares the dam and no-dam conditions,
~ We e-mailed copies of these reports to you on August 16, 2007, in advance of this letter. We understand
that the reports were forwarded to USGS. We appreciate your assistance.
2

thereby isolating the effect of the dam on sedimentation and flooding. The same report
includes the following discussion:
41. These additional sriidy restdts address the desire for scientific verification of the
effect of High Rock Damon sediment accumulation and increased flood water stuface
elevation ui the Yadkui River upstream from High Rock Reservoir. We have acco~uited
for the sediment that would have occturred in the absence of High Rock Dam. These
results confirm that the presence of High Rock Dam is responsible for vu~h~ally all of the
sediment acciunulation and increase iv flood water stu•face elevation at the pinup station
and the wastewater heauneut plant.
A primary purpose for Salisbury incurring the cost of the extensive studies and
reports on the sedimentation and flooding effects of the Yadkin Project is to provide a
robust factual basis for DWQ's 401 WQC decision as it relates to Salisbury's water
supply use of the Yadkin River. The data are sufficient. In fact, although not required for
a WQC decision, the data are overwhelming. The Project-caused flooding is a thoroughly
proven and documented water supply and water use issue. The best available science
makes it clear that Project-caused flooding will severely disrupt and severely interrupt
our long-established water supply use. Flood mitigation ought to be a condition of the
401 WQC for the Yadkin Project.
Interruption of Water Supply Use by Flooding
The first of the projected, recurring interruptions of our water supply use by
Project-caused flooding has not occurred yet. However, it is clear that Project-caused
flooding will interrupt our water supply use in two ways:
1) Necessitating shut-down of the pump station when flood waters reach elevation
641 NGVD29 (within two feet of entering the raised pump station equipment
rooms); and
2) Entering the pump station raised equipment rooms (at elevation 643 NGVD29)
and destroying switching and pumping equipment.
The sediment delta continues to grow and continues to increase flood levels. At
the end of a new 50-year FERC license a 100-year flow will put a foot of floodwaters in
our raised equipment rooms and a flow slightly greater than a 10-year flow will require
us to shut down the pumping station. See Mobile Boundary Hydraulics, PLLC,
Numerical Sedimentation Investigation, Yadkin River, North Carolina (February 20,
2007) at page 49.
In contrast, no foreseeable Yadkin River flow could cause the interruption of our
water supply use if not for the effects of High Rock Dam and the High Rock Sediment
Delta. When the pump station was built (10 years before High Rock Dam was built and
the High Rock Sediment Delta began to accumulate) the, ow of record would produce a
water elevation of 635.5 NGVD29. This flood elevation is more than five feet below the
elevation that would require shut-down.

In March of 2003, flood waters reached an elevation of 640.7 NGVD29. See
Mobile Boundary Hydraulics, PLLC, Numerical Sedimentation Investigation, Yadkin
River, North Carolina (February 20, 2007) at page 36. The City of Salisbury barely
escaped having to shut down the pump station. This event was caused by a flow with a
return frequency of roughly 10 years. See Mobile Boundary Hydraulics, PLLC,
Numerical Sedimentation Investigation, Yadkin River, North Carolina (February 20,
2007) at page 49.
In our experience, DWQ's approach to conditions in water quality certifications is
proactive. For example, it is our understanding that DWQ requires the implementation of
buffer requirements for new sewer lines based on a projection, without the benefit of site
specific studies, that an impact of the new sewer line will be an increase in impervious
surfaces in the vicinity of the sewer line. In contrast to this projected impact, the
interruption of our water supply use by Project-caused flooding is supported by unusually
well-documented and verified site-specific scientific studies. We request that DWQ
require a flood relief channel as a WQC condition based on the scientific studies without
waiting for documentation of the first instance of the recurring Project-caused
interruptions of Salisbury's water supply use.
Damage to Critical Water Supply Infrastructure
Project-caused flooding will interrupt our water supply use. Such interruption of
our water supply use will be recurring and will become worse over the life of a new
FERC license. In some cases, the interruption will probably not be accompanied by
"property damage." In some cases, the interruption of our water supply use will probably
be accompanied by severe damage to our critical water supply infrastructure. We trust
that Project-caused infrastructure damage does not somehow exempt the Project from
mitigating the Project-caused interruption of our water supply use.
Flood Mitigation Condition
The water levels that interrupt our water supply use are projected to be avoided if
the High Rock Sediment Delta is partially removed (i.e., a flood relief channel is
established and maintained). We respectfully request that DWQ exercise its authority to
protect our water supply use from the most severe adverse effects of the Project and
include a condition that requires the establishment and maintenance of a flood relief
channel through the High Rock Sediment Delta, as follows:
A flood relief channel with a downstream section 500 feet wide and 10.1
miles long and an upstream section 300 feet wide and 2.3 miles long, as
described in Large Flood Relief Channel, Dr. Ronald Copeland, Mobile
Boundary Hydraulics, PLLC (May 10, 2007) shall be completed no later
than October 25, 2009, and its dimensions and function shall be re-
established at least annually by dredging, which may include sand mining,
beginning no later than October 25, 2010.
4

REQUESTED WORDING OF CONDITION REQUIRING DREDGING
In response to the request for proposed text for the condition for dredging to
protect the intakes from sedimentation, we request the use of the following text:
The removal of sand and sediment down to an elevation of 607 (f 1 ft) in
Area 1 and down to an elevation of 607 (f 2 ft) in Area 2 shall be
completed no later than July 25, 2008 and shall be repeated at least
annually in order to maintain the specified elevations. Area 1 shall be the
area within a 100-foot radius of the Salisbury intakes. Area 2 shall be the
Yadkin River channel from a point 1,000 feet upstream of the Salisbury
intakes to a point 1,000 feet downstream of the Salisbury intakes.
We request an opportunity to review, contemporaneously with the applicant, any
draft WQC that DWQ may provide to the applicant. Thank you again for this opportunity
to comment on the tentative conclusions regarding the Water Quality Certification for the
Yadkin Hydroelectric Project. If we can be of assistance in any way, please feel free call
me at [phone] or our environmental counsel, V. Randall Tinsley, at (336) 373-
8850.
Sincerely, ,~-~
~~'~'
~~,
Matt' Bernhardt
Assistant City Manager for Utilities
Enclosures
cc: William G. Ross, Jr., Secretary, DENR
Marc Bernstein, Assistant Attorney General
Susan Kluttz, Mayor
David W. Treme, City Manager
V. Randall Tinsley, Environmental Counsel for Salisbury
5

August 21 2007
EQUILIBRIUM ANALYSIS OF YADKIN AND SOUTH YADKIN RIVERS
Martin Doyle, PhD, Associate Professor
Department of Geography & Department of Environmental Science & Engineering (joint
appointment)
University of North Carolina -Chapel Hill
1. EXECUTIVE SUMMARY
The purpose of this analysis was to examine any long-term trends in the channel equilibrium
conditions of the Yadkin and South Yadkin Rivers as indicated by available long-term USGS
stream gauge and USGS survey records. Data were available for two Yadkin River sites and one
South Yadkin River site. No statistically significant temporal trends were evident, indicating
that based on available information, the Yadkin River and South Yadkin River are in geomorphic
equilibrium.
2. CHANNEL STABILITY
Over the course of analyzing the impacts of reservoir sedimentation on the Yadkin River in the
vicinity of the High Rock Reservoir, questions arose regarding whether the Yadkin River and/or
the South Yadkin River were in equilibrium. If these channels were out of equilibrium, then this
should be evidenced in long-term stream data.
To quantify any changes in channel stability, USGS stream gauge station surveys were used. We
used those sites on the Yadkin or South Yadkin with comparable drainage areas, upstream of
major impoundments, and with long-term data that would allow statistical time-series analysis.
The gauges used are indicated in Table 1. USGS Gauge station (02121000), Yadkin River at
Salisbury did not have field data (i.e., actual field measurements) to permit using these early data
for equilibrium analysis (only discharge data were available).
A simple statistical test was run by correlating the date with stream width, without controlling for
discharge outliers. The bulk of measurements taken by USGS were at low-flow conditions, and
results did not vary when outliers were removed.
Channel equilibrium is indicated by changes in channel width, which can be 10 times changes in
channel depth, i.e., channel width changes are amplified in comparison to changes in channel
depth for out-of-equilibrium channels (Simon, 1992). Thus, changes in channel width through
time are an appropriate indicator of changes in channel equilibrium.
There were divergent trends as some sites in the Yadkin watershed had slightly positively
increasing widths with time, and others slightly negative trending (Table 1). However, in all
cases, the temporal trends of channel width through time show no trends over time (i.e., the
slopes of the time-series graphs were not statistically different from zero.

Table 1. Site locations and temporal changes through time.
Site USGS Gauge Duration of Number of Slope of Statistically
Number records observations width (ft) significant?
vs ear ( < 0.05)
02118000 South Yadkin 1938-2007 652 -0.53 No
River near
Mocksville, NC
02115360 Yadkin River at 1964-2007 250 +0.76 No
Enon, NC
02116500 Yadkin River at 1928-2007 513 -0.64 No
Yadkin College,
NC
Simon, A., 1992. Energy, time, and channel evolution in catastrophically disturbed fluvial
systems. Geomorphology 5: 345-372.
2

August 2007
EFFECTS OF BRIDGES OVER YADKIN RIVER ON WATER SURFACE ELEVATION
PROFILES
Martin Doyle, PhD, Associate Professor
Department of Geography
Department of Environmental ScienceBngineering (joint appointment)
University of North Carolina -Chapel Hill
1. EXECUTIVE SUMMARY
The purpose of this report is to quantify the hydraulic effects of bridges on the Yadkin River
flood profiles relative to the effects of sediment accumulation associated with the downstream
High Rock Dam and reservoir. The study used HEC-RAS modeling and 1917 and 1997 channel
conditions with and without a series of bridges in the simulated channel. The HEC-RAS model
was calibrated and validated with two available high water marks.
Hydraulic analysis show that there would be an increase of 0.28 ft at the WWTP (RM 16.7) and
0.03 ft at the water intakes (RM 19.4) that can be solely attributed to the bridges. Increase in
water surface elevations associated with sediment accumulation, in contrast, exceed 7 ft.
Further, the railroad bridge, which did not block flow under 1917 channel conditions at 121,000
cfs, is now a potential blockage to flows as low as the 10-yr flood event. Therefore, the
influence of the bridges on the upstream water surface elevations has increased because of
sediment accumulation, and this blockage will occur more frequently than it did prior to
reservoir sediment accumulation.
2. BRIDGES ON YADKIN RIVER
Over the course of analyzing the impacts of reservoir sedimentation on the Yadkin River in the
vicinity of the High Rock Reservoir, questions arose regarding the potential hydraulic influence
of bridges upstream of the reservoir and downstream of the Salisbury/Rowan Utilities
Wastewater Treatment Plant and Water Intake Structure.
The bridges are located approximately 14 miles upstream of the High Rock Dam. These bridges
were constructed in the following sequence':
• 1818: single lane bridge, which was rebuilt in 1899.
• 1856: Railroad Bridge; second track added in 1918
• 1921: Wil-Cox bridge currently carrying US 29 South
1 http://hdl.loc. ovg /loc.gmd/ 3~ 861 p.rr001350 and http://www.ncrr.com/ncrr-history html;. Southern
Railway Company, Office of Chief Engineer MW&S, Charlotte, NC 1918 - 1927;

1951: US 29 North Bridge (located between the Wil-Cox and the railroad bridge)
1960: I-85 bridge
Because of the timeline of bridge construction, the railroad bridge should be taken to represent
the `existing conditions' prior to the construction of High Rock Dam.
The channel topography for the 1997 and 1917 HEC-RAS models for the Yadkin River have
been described at length in previous reports. The 1917 model is taken to represent pre-dam and
reservoir conditions, and the 1997 model is taken to represent the conditions under accumulated
sediment conditions. Within both of these models, all of the bridges were added. The data
availability for the I-85 bridge was the least certain, and as such the 1997 model was done with
and without the I-85 bridge in order to test the sensitivity of the results to this bridge.
The pier dimensions of the Railroad Bridge are 3 piers equally spaced (approx 163 ft) across the
river; the piers are 15 ft wide and 50 ft long. The pier dimensions of the Interstate Bridge are 9
piers equally spaced (approx 80 ft) across the river as well as two 10 ft x 10 ft columns with 40 ft
between columns. The pier dimensions of the Hwy 29 Bridges are 4 piers equally spaced
(approx 163 ft) across the stream and are 10 ft wide and 30 ft long.
For the RR bridge, the elevations of the platform were known to be 630.6 at the base and 657.35
for the top of the platform (i.e., elevation of the tracks). Elevations of the other bridge platforms
were all higher than the RR bridge, but were unknown. For conservative estimates, base
elevations for all of the other bridge platforms were all placed at 630.6. Thus, all estimates of
the effects of the bridges are considered conservative, particularly when flood elevations
exceeded 630.6.
3. MODELS USED AND ASSUMPTIONS
3.1. Initial Model Setup
The purpose of the modeling was to use already applied HEC-RAS models to assess the increase
in water surface elevations that can be attributed to the bridges. HEC-RAS model for the Yadkin
River used standard parameters where possible (e.g., expansion and contraction coefficients were
set at default values). Hydraulic roughness was initially assumed to be 0.035 for the main
channel and 0.05 for the left and right out of bank areas. For the 1997 channel conditions, step-
backwater calculations were conducted using a known water surface elevation of 623.9 at the
dam (i.e., downstream boundary condition). For 1917. channel conditions, downstream
boundary condition was calculated using normal depth calculations assuming a slope of 0.00048
(average bed slope of pre-dam downstream reach); this downstream assumption would not affect
the calculations further upstream near the bridges. For calculating energy losses associated
bridge piers, the Energy method (i.e., standard step) was used for both low flow and high flow
conditions.
3.2. Model Validation and Calibration
Using the l 997 channel configuration with all bridges present represented the actual channel and
bridge conditions and thus allowed validating and calibrating the model output. Two high water
marks were available at the water intake structure (RM 19.4):
2

• January 28, 1998, Q = 31,906 cfs and High water mark = 633.8
• March 21, 2003, Q = 67,926, and High water mark = 640.7
Under the initial channel roughness assumptions the model under-predicted the observed water
surface elevations. The channel roughness was thus adjusted to be 0.04 for the main channel and
0.1 for the floodplain. Under these values, the model predicted the water surface within < 1 ft of
the observed water surface elevation:
• January 28, 1998, Q = 31,906 cfs; observed = 633.8; modeled = 633.0
• March 21, 2003, Q = 67,926, and High water mark = 640.7; modeled = 640.2
While not exact, these values show that the model is predicting the water surface elevations
within 1 ft of the observed values with minimal adjustment of the roughness parameters. This
level of accuracy is appropriate given scale of this model. Importantly, using these roughness
values (0.04 and 0.1) matches the roughness values that were used by Mobile Boundary
Hydraulics in their sediment transport and hydraulic modeling. Thus, these values were used for
all modeling of bridge effects, and the model is considered validated.
3.3. Sensitivity Analysis of Water Elevations to Bridges and Discharges
Many discharges were modeled, but only three are presented here for brevity:
• 65,000 cfs = 10-yr recurrence interval flood event
• 101,000 cfs = 100-yr recurrence interval flood event
• 121,000 cfs =design flood
The following models were run to quantify the relative influence of the bridges relative to
channel conditions on the water surface flood profiles:
• 1917 Channel Conditions No Bridges
• 1917 Channel Conditions All Bridges
• 1997 Channel Conditions No Bridges
• 1997 Channel Conditions All Bridges
• 1997 Channel Conditions All Bridges except I-85
The final model run was done because of the current lack of information on the elevation of the
I-85 bridge platform. Again, the bridge elevation is higher than this, but was set at this lower
value to provide a conservative estimate.
Of primary interest here are the maximum changes in waters surface elevation, the change in
elevation at the WWTP (R1VI 16.7) and the change in elevation at the water intakes (lZM 19.4).
4. RESULTS
4.1. Relative to 1917 No Dam and No Bridge Conditions
Comparing the results of the different model scenarios with the 1917 No Dam Conditions allows
quantifying the change in water surface elevation relative to the most extreme condition, i.e., if
no dam and no bridges existed and no sediment had accumulated (Fig 1, and Tables 1, 2, and 3
below). The model results for a discharge of 121,000 cfs show that there would be an increase of
0.42 ft at RM 15.4, immediately upstream of the bridges that can be attributed solely to the

bridges (i.e., comparing 1917 all bridges to 1917 no bridges), and this decreases to 0.28 ft at the
WWTP (RM 16.7) and 0.03 ft at the water intakes (RM 19.4). That is, the maximum water
surface increase at the water intakes that can be solely attributed to all the bridges is 0.03 ft.
121,000 cfs
-~ 1917 w/ bridges
~ -.~ 1997 No bridges
•°- 14 ,-~-------- - ~ - _ x 1997 All bodges but I-85
X -~ -_~
V 12 ~~- -~--1997 all bridges
m
'a 10
m` ~~*..
z° ~ 8
°' 6
E
0
w 4
m
v
c
m 2
~_
~ 0
14 15 16 17 18 19 20
River Miles upstream of Dam
Figure 1. Difference in water surface elevation between model scenarios and 1917 No Bridge
Condition. Only data for upstream of RM 14 are shown. Water Surface Profiles are shown at
end of report.
4.2. Relative to 1997 No Bridge Conditions
Comparing the results of the different 1997 model scenarios allows quantifying the effects solely
of the bridges under relatively recent channel topography, i.e., channel under accumulated
sediment conditions. This comparison shows that for a discharge of 121,000 cfs, the bridges are
responsible for increasing the water surface elevation a total of 0.43 ft (4 inches) at the water
intakes and 0.89 ft at the WWTP. These effects are not independent of the accumulated
sediment.
4.3. Sedimentation Has Increased the Backwater Effects of the Bridges
The effects of reservoir sedimentation have increased the backwater effects of the bridges,
particularly the railroad bridge. In 1917, assuming all bridges in place (untrue but conservative
condition), the water surface elevation during the design flood (121,000 cfs, > 100-yr flood) at
the Railroad bridge was 625.44, or ~ 5 ft below the elevation of the base of the bridge platform
(630.6). However, under 1997 channel conditions (accumulated sediment), the water surface
elevation reaches 632.35 ft at the 10-yr flood, and >638 at the design flood. Thus, the platform
of the railroad bridge, which did not ever block flow under 1917 conditions, is now a potential
blockage to flows as low as the 10-yr flood event. Therefore, the influence of the bridges on the
upstream water surface elevations has increased, and this blockage will occur more frequently
than it did prior to reservoir sediment accumulation.
4

In short, the sediment accumulation in the High Rock Reservoir and in the main channel of the
upstream Yadkin River has increased the probability of the Railroad bridge platform being
affected by flow from < 1% per year to > 10% per year, given the conservative estimates for
model conditions discussed above. This has in turn increased the upstream backwater effects of
the bridges under current channel conditions.
5. TABLES AND FIGURES
Table 1. Water surface elevation (ft) under different modeling scenarios. Distance from dam of
15.4 miles is immediately upstream of the bridge crossings, 16.7 miles is at the W WTP, and 19.4
is at the water intake structures.

Q
Distance
From Dam
1917 no
brid es
1917 with
brid es
1997 No
brid es
1997 all
brid es 1997
Bridges
exce t I-85
65000 15.4 617.88 618.13 632.98 633.25 633.15
65000 16.7 620.65 620.81 635.29 635.46 635.4
65000 19.4 632.22 632.23 639.61 639.69 639.66

101000 15.4 623.75 624.09 636.66 637.52 637.22
101000 16.7 626.41 626.65 639.48 640.05 639.85
101000 19.4 637.88 637.91 644.94 645.21 645.11

121000 15.4 626 626.42 638.38 639.72 639.26
121000 16.7 628.76 629.04 641.47 642.36 642.04
121000 19.4 640.44 640.47 647.47 647.9 647.74
Table 2. Difference in water surface elevations (ft) with 1917 No Bridge Conditions. Distance
from dam of 15.4 miles is immediately upstream of the bridge crossings, 16.7 miles is at the
WWTP, and 19.4 is at the water intake structures.

Q

Distance From
Dam miles
1917 with
bridges - 1917 No
brid es

1997 all bridges-
1917 No brid es 1997 Bridges
except I-85-
1917 No
brid es
1997 No
bridges- 1917
No brid es
65000 15.4 0.25 15.37 15.27 15.1
65000 16.7 0.16 14.81 14.75 14.64
65000 19.4 0.01 7.47 7.44 7.39

101000 15.4 0.34 13.77 13.47 12.91
101000 16.7 0.24 13.64 13.44 13.07
101000 19.4 0.03 7.33 7.23 7.06

121000 15.4 0.42 13.72 13.26 12.38
121000 16.7 0.28 13.6 13.28 12.71
121000 19.4 0.03 7.46 7.3 7.03

Table 3. Difference in water surface elevations (ft) with 1997 No Bridge Conditions. Distance
from dam of 15.4 miles is immediately upstream of the bridge crossings, 16.7 miles is at the
W WTP. and 19.4 is at the water intake structures.

Q Distance From
Dam miles 1997 all Bridges -
1997 No Brid es 1997 Bridges except I-85
- 1997 No Brid es
65000 15.4 0.27 0.17
65000 16.7 0.17 0.11
65000 19.4 0.08 0.05

101000 15.4 0.86 0.56
101000 16.7 0.57 0.37
101000 19.4 0.27 0.17

121000 15.4 1.34 0.88
121000 16.7 0.89 0.57
121000 19.4 0.43 0.27
65,000 cfs
0 620
.~
640 jx
~` •-*"
• t
630 x * x•
X! Y Y X A ~ Y * E
d
610 t 1917 no bridges

v ~ 1917 w/ bridges
~ 600 -~ 1997 No bridges
w x 1997 All bridges but I-85
m 590 0 1997 all bridges

580
570
0 2 4 6 8 10 12 14 16 18 20
River Miles upstream of Dam
Figure 2. Water surface elevations under different modeling scenarios at the 10-yr flood event
(65,000 cfs). Note that bridges are located at ~ RM 14-15.
6

I.
101,000 cfs
650
640 ~~i~-~
Y
630
O Y x A Y
- i s a x a x
> 620
m
d
~ 610
~ t 1917 no bridges
H 600 ~-1917 w/ bridges
°7 --x-1997 No bridges
io
~ 590 x 1997 All bridges but I-85
n 1997 all bridges
580
570
0 2 4 6 8 10 12 14 16 18 20
River Miles upstream of Dam
Figure 3. Water surface elevations under different modeling scenarios at the 100-yr flood event
(101,000 cfs). Note that bridges are located at ~ RM 14-15.
1sl,ooo cfs
650
640
c 630
.~
~ 620
d
~ 610
r'n 600
d
~ 590
580
570
0
x a Y-
Y
Y Y ! Y
• Y t Y E t _
2 4
~ 1917 no bridges
~ 1917 w/ bridges
-.-1997 No bridges
x 1997 All bridges but I-85
e 1997 all bridges

6 8 10 12 14 16 18 20
River Miles upstream of Dam
Figure 4. Water surface elevations under different modeling scenarios at the design flood event
(121,000 cfs). Note that bridges are located at ~ RM 14-15.
7