Toledo Bend Project, Lower Sabine River HEC-RAS Hydraulic ... · various operational scenarios of the Project on flow fluctuations extending from immediately downstream of the Toledo

TOLEDO BEND PROJECT

LOWER SABINE RIVER HEC-RAS HYDRAULIC MODEL

CALIBRATION/VERIFICATION REPORT FERC PROJECT NO. 2305

Prepared for: SABINE RIVER AUTHORITY OF TEXAS

and SABINE RIVER AUTHORITY, STATE OF LOUISIANA

Prepared by: HDR|DTA

Portland, Maine

May 2011

Toledo Bend Project Sabine River Authority FERC Project No. 2305

i

1.0 EXECUTIVE SUMMARY

The joint licensees of the Toledo Bend Project (FERC No 2305) (Project), the Sabine River

Authority of Texas (SRA-TX) and the Sabine River Authority of Louisiana (SRA-LA)

contracted with HDR|DTA to develop an unsteady flow hydraulic model of the Sabine River

downstream of the Project. The model is intended to be used to assist in evaluating the impact of

various operational scenarios of the Project on flow fluctuations extending from immediately

downstream of the Toledo Bend Dam to Shoats Creek at river mile 54. The hydraulic model was

developed using the one-dimensional U.S. Army Corps of Engineers Hydrologic Engineering

Center River Analysis System, HEC-RAS version 4.1.0 (USACE 2010a).

The purpose of this calibration/verification report is to document inputs and assumptions used in

the development of the Lower Sabine HEC-RAS model to demonstrate that the model

reasonably characterizes flow behavior downstream of the Project due to project peaking

operations, and that the model is adequate for use in evaluating the hydraulic-related effects of

alternative operating scenarios. The calibration process is used to estimate hydraulic parameters

and refine model geometry to optimize replication of observed data under a range of flow

scenarios. Verification assesses the reliability of the model under different hydrologic conditions

than those used for calibration.

HDR|DTA performed model calibration and verification using time series of water surface

elevations and flows recorded at downstream USGS gages as well as recorded river stages at

numerous levelogger locations throughout the modeled reach. The measured values were

compared to model results at the same locations. Calibration was conducted for the period of

August 8th through October 8th, 2009. The first four weeks of this period are representative of

typical Project operations, with the two Project turbine-generator units operating at full capacity

for 6 hours per day during the week and no unit operation on the weekends and a continuous

flow of approximately 150 cfs being discharged at the spillway. There was minimal impact on

flow patterns due to precipitation or unusual tributary contribution during this period. The

second half of the calibration period experienced periods of rainfall and increased tributary flows

on top of variable unit operations, which significantly impacted downstream flow patterns. The


ii

Lower Sabine HEC-RAS model was calibrated to optimize timing and magnitude of peak

elevations and the magnitudes of changes in elevation when compared to observed.

The modeled verification period extended from August 10th through September 24th, 2010.

During the first 4 weeks of this period the operations consisted of a single unit at full capacity for

2 hours on Mondays and Wednesdays and for 4 hours on Fridays, with no unit operations over

the weekends. Beginning on September 10th, both units commenced continuous operation for a

period of 11 days (continuous operating scenario). Continuous flows from the spillway occurred

throughout the verification period. The continuous operating scenario was included in the

verification run to further test the capabilities and limitations of the Lower Sabine HEC-RAS

model under varying Project operations. During the 11-day continuous operating scenario,

modeled flows began to overtop banks near river mile 77 approximately 30 hours after the

hydrograph began to rise at that location. This model is not designed to accurately simulate the

complex connectivity with the Sabine River floodplain that occurs under high tributary inflows

or unusual operation patterns. Powerhouse peaking flows under low and moderate tributary

contributions do not result in general overbank flooding. Model testing determined that flows

remained within cross section extents throughout the modeled reach for up to 20 hours of 2-unit

continuous operation during the September 2010 base flow conditions.

In the opinion of HDR|DTA, verification results show the Lower Sabine HEC-RAS model

compares favorably to measured data and is appropriate for use in evaluating the effects of

alternative operating scenarios compared to the base case normal peaking operations currently

used at the Project. As with any model, accuracy is highly dependent on input data, and

consequently, model results should be viewed in a relative, rather than an absolute context.

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TOLEDO BEND PROJECT


CALIBRATION/VERIFICATION REPORT

TABLE OF CONTENTS Section Title Page No.

1.0 EXECUTIVE SUMMARY ........................................................................................ I

2.0 INTRODUCTION ................................................................................................ 2-1

3.0 LOWER SABINE HEC-RAS MODEL DEVELOPMENT ......................................... 3-1

3.1 CHANNEL GEOMETRY ........................................................................................ 3-6 3.2 HYDRAULIC PARAMETERS ............................................................................. 3-10 3.3 MODEL HYDROLOGY ........................................................................................ 3-11

4.0 MODEL CALIBRATION AND VERIFICATION ........................................................ 4-1

4.1 CALIBRATION ....................................................................................................... 4-1 4.2 VERIFICATION SCENARIO ................................................................................. 4-6

5.0 SUMMARY AND CONCLUSIONS ........................................................................ 5-1

5.1 SUMMARY .............................................................................................................. 5-1 5.2 CONCLUSIONS ...................................................................................................... 5-1

6.0 REFERENCES .............................................................................................. 6-1

APPENDICES APPENDIX A - CALIBRATION INFLOW HYDROGRAPHS APPENDIX B - CALIBRATION RESULTS

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TOLEDO BEND PROJECT



LIST OF FIGURES Figure Title Page No.

FIGURE 2-1 MODELED REGION OF THE LOWER SABINE RIVER SYSTEM ................. 2-3

FIGURE 3-1 LOWER SABINE HEC-RAS MODEL REACH NETWORK FROM DAM TO RULIFF GAGE (USGS 08030500) ................................................................... 3-2

FIGURE 3-2 LOWER SABINE HEC-RAS MODEL REACH NETWORK IN THE VICINITY OF THE TOLEDO BEND DAM ............................................................................ 3-3

FIGURE 3-3 LOWER SABINE HEC-RAS MODEL CHANNEL PROFILE FROM SPILLWAY TO RIVER MILE 35 .......................................................................................... 3-8

FIGURE 3-4 HABITAT SURVEY TRANSECT CHANNEL GEOMETRIES – RIVER MILE 139 .............................................................................................................. 3-9

FIGURE 3-5 LOWER SABINE HEC-RAS MODEL EXAMPLE CROSS SECTION 136.43 – SABINE RIVER 04 .......................................................................................... 3-10

FIGURE 4-1 LOWER SABINE HEC-RAS MAXIMUM CHANGES IN WEEKLY STAGE . 4-4

FIGURE 4-2 LOWER SABINE HEC-RAS MAXIMUM CHANGES IN DAILY STAGE DURING WEEKLY OPERATIONS ....................................................................... 4-5

FIGURE 4-3 VERIFICATION RESULTS AT BURKEVILLE GAGE (USGS 08025360) ... 4-7

FIGURE 4-4 VERIFICATION RESULTS AT BON WIER GAGE (USGS 08028500) ....... 4-8

FIGURE 4-5 VERIFICATION RESULTS AT RULIFF GAGE (USGS 08030500) ............ 4-8

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TOLEDO BEND PROJECT



LIST OF TABLES

Table Title Page No.

TABLE 3-1 LOWER SABINE RIVER USGS GAGE INFORMATION ........... 3-4

TABLE 3-2 LOWER SABINE HEC-RAS MODEL LEVELOGGER INFORMATION ........................................................................................... 3-4

TABLE 3-3 LOWER SABINE HEC-RAS MODEL HABITAT TRANSECT LOCATIONS ................................................................................................. 3-5


2-1

2.0 INTRODUCTION

The joint licensees of the Toledo Bend Project (FERC No 2305) (Project), the Sabine River

Authority of Texas (SRA-TX) and the Sabine River Authority of Louisiana (SRA-LA)

contracted with HDR|DTA to develop a hydraulic model of the Sabine River system downstream

of the Project. The model is intended to be used to assist in evaluating the impact of various

operational scenarios of the Project on flow patterns extending from immediately downstream of

the Toledo Bend Dam to Shoats Creek near river mile 54. The model extent is shown in Figure

2-1.

The hydraulic model was developed using the one-dimensional U.S. Army Corps of Engineers

Hydrologic Engineering Center River Analysis System, HEC-RAS version 4.1.0. The Lower

Lower Sabine HEC-RAS model simulates the routing of unsteady flows, extending from the

tailrace and spillway channels at the base of the dam to the downstream model boundary at the

Ruliff Gage (USGS 08030500) near river mile 35. The downstream model boundary is at a

sufficient distance not to influence flows at Shoats Creek, the downstream limit of interest for the

study. In addition to downstream routing, the HEC-RAS model is capable of simulating

backwater flow effects of powerhouse discharge on the spillway channel upstream of the tailrace.

HDR|DTA performed model calibration and verification using time series of water surface

elevations and flows at three downstream USGS gages and relative depths at levelogger

locations throughout the river system. These measured values were compared to model results at

the same locations. The calibration process is used to estimate hydraulic parameters and refine

model geometry to optimize model replication of the observed data under a range of flow

scenarios. Verification assesses the reliability of the Lower Sabine HEC-RAS model under

different hydrologic conditions than those used to calibrate the model.

Required model input includes separate time series of reservoir outflows into both the tailrace

and the spillway, as well as estimated tributary and accretion flows throughout the modeled

reach. The Lower Sabine HEC-RAS model is intended to simulate flows within the banks of the

primary channel, connected side channels and local off-channel storage. Complex floodplain


2-2

connectivity is not modeled and simulations of operations scenarios which result in overbank

flows beyond the extents of the model cross sections are not recommended. This type of

overbank flow may occur infrequently at some locations above RM 54 when peaking operations

are combined with high tributary accretion flows. Separate studies of bottomland forest

conditions found that the Sabine River bottomland ecosystem is in very good condition and

displays the characteristics of a healthy bottomland system (HDR|DTA 2011). Model testing

determined that flows remained within cross section extents throughout the modeled reach for up

to 20 hours of non-peaking 2-unit continuous operation during the September 2010 flow

conditions.


2-3

Figure 2-1 Modeled Region of the Lower Sabine River System


3-1

3.0 LOWER SABINE HEC-RAS MODEL DEVELOPMENT

The Lower Sabine HEC-RAS Study Area extends from the Toledo Bend Dam to Shoats Creek at

RM 54. To insure no impact from model boundary conditions at Shoats Creek, the model was

extended to the Ruliff Gage (USGS 08030500) at RM 35. The model includes the Sabine River

and three major tributaries, Bayou Toro, Bayou Anacoco, and Big Cow Creek, from the location

of the USGS gage on each tributary to their confluence with the Sabine River. The tributary

channels were modeled in order to approximate hydrograph attenuation from the gaged flow

location to the confluence. In addition, the model includes the tailrace channel and the current

spillway channel, as well as the old spillway channel and connected Bayou Toro loop to account

for additional channel storage. Figures 3-1 and 3-2 illustrate the reach network and the naming

of each reach segment between junctions.

The Lower Sabine HEC-RAS model geometry is based on a Triangular Irregular Network (TIN)

digital elevation model which was developed primarily from the USGS National Elevation

Dataset 1/9th Arc-Second (NED 1/9) 3-meter resolution LiDAR data (USGS 2011a) using

Geographical Information Systems (GIS)-based methodologies. The LiDAR data were

augmented by NED 1/3rd Arc-Second 10-meter resolution USGS DEM data (USGS 2011b) near

the spillway and in the upstream portions of the tributary reaches. The model datum is the North

American Vertical Datum of 1988 (NAVD88), which differs from mean sea level by -0.26 feet

near the dam and +0.15 feet at the Ruliff Gage (USGS 08030500). HEC-GeoRAS version 4.2.93

was used to cut cross sections from the elevation model and establish reach lengths and reach

connectivity. HEC-RAS is a one-dimensional model, so river meanders are taken into account

by distinguishing main channel and left and right overbank distances between each cross section.

The NED 1/9 LiDAR-derived data provide ground surface elevation information above the water

surface, so sub-water surface channels must be added after importing the cross-section data into

HEC-RAS. When modeling within-banks channel flows, the primary function of the LiDAR

data is to accurately establish bank locations and elevations, channel shape to the extent it is

depicted above the water surface, connections to off-channel storage, and the two-dimensional

flow line geometry.


3-2

Figure 3-1 Lower Sabine HEC-RAS Model Reach Network From Dam to Ruliff Gage (USGS 08030500)


3-3

Figure 3-2 Lower Sabine HEC-RAS Model Reach Network in the Vicinity of the Toledo

Bend Dam


3-4

Cross section locations were based on locations of observed data, including the USGS gage and

levelogger locations listed in Tables 3-1 and 3-2, as well as the habitat assessment survey

transect locations in Table 3-3 from the July 2010 study. Additional cross sections were placed

to capture significant surface geometry features, such as potential off-channel storage or

locations with lower bank elevations, and to retain sufficient cross-section spacing.

Table 3-1 Lower Sabine River USGS gage information

USGS Station Station Name/Location HEC-RAS Reach

ID

HEC-RAS Cross

Section 8025360 Sabine River at Toledo Bend Reservoir near Burkeville, TX N/A N/A 8025500 Bayou Toro near Toro, LA Bayou Toro 01 76407.62 8026000 Sabine River near Burkeville, TX Sabine River 04 131.64 8028000 Bayou Anacoco near Rosepine, LA Bayou Anacoco 01 137705.9 8028500 Sabine River near Bon Weir, TX Sabine River 05 91.35 8029500 Big Cow Creek near Newton, TX Big Cow Creek 01 177822.3 8030500 Sabine River near Ruliff, TX Sabine River 06 35.3 8031000 Cow Bayou near Mauriceville, TX N/A N/A

Table 3-2 Lower Sabine HEC-RAS model levelogger information

Levelogger No. HEC-RAS Reach ID

HEC-RAS Cross

Section Location (State)

Levelogger Time

Interval 146a Sabine River 01 145.95 Spillway (LA) 5-min

145-BT1 Bayou Toro 01 877.44 Toro Bayou (LA) 5-min 145 Sabine River 03 145.03 Spillway (LA) 5-min 143 Sabine River 03 142.88 Spillway (LA) 5-min

141-TR1 Tailrace 01 141.8 Tailrace at pipeline (TX) 5-min 140b Sabine River 04 139.9 Sabine River (LA) 5-min 136 Sabine River 04 135.65 Sabine River (TX) 5-min 133 Sabine River 04 133.02 Sabine River (TX) 5-min 120 Sabine River 04 120 Sabine River (LA) 15-min 114 Sabine River 04 113.6 Sabine River (LA) 15-min 100 Sabine River 05 99.66 Sabine River (LA) 15-min 90 Sabine River 05 89.96 Sabine River (LA) 15-min 82 Sabine River 05 82.56 Sabine River (LA) 15-min 73 Sabine River 05 73.25 Sabine River (TX) 15-min 63 Sabine River 06 63.1 Sabine River (LA) 15-min 55 Sabine River 06 55.03 Sabine River (LA) 15-min


3-5

Table 3-3 Lower Sabine HEC-RAS model habitat transect locations

Habitat Transect ID HEC-RAS Reach ID HEC-RAS

Cross Section

146-1 Sabine River 01 146.68 146-2 Sabine River 01 146.55 146-3 Sabine River 01 146.44 146-4 Sabine River 01 146.30 146-5 N/A N/A 146-6 Sabine River 01 146.10 143-1 Sabine River 03 142.88 143-2 Sabine River 03 143.02 143-3 Sabine River 03 143.1 143-4 Sabine River 03 143.16 143-5 Sabine River 03 143.26 143-6 Sabine River 03 143.3 143-7 Sabine River 03 143.43 143-8 Sabine River 03 143.56 143-9 Sabine River 03 143.75 141-1 Tailrace 01 141.25 141-2 Tailrace 01 141.3 141-3 Tailrace 01 141.37 141-4 Tailrace 01 141.44 141-5 Tailrace 01 141.49 141-6 Tailrace 01 141.56 141-7 Tailrace 01 141.69 141-8 Tailrace 01 141.8 141-9 Tailrace 01 141.94 139-1 Sabine River 04 139.67 139-2 Sabine River 04 139.78 139-3 Sabine River 04 139.9 139-4 Sabine River 04 140.02 139-5 Sabine River 04 140.13 139-6 Sabine River 04 140.28 132-1 Sabine River 04 132.57 132-2 Sabine River 04 132.38 132-3 Sabine River 04 132.25 132-4 Sabine River 04 132.13 132-5 Sabine River 04 131.99 132-6 Sabine River 04 131.82 120-1 Sabine River 04 120.18 120-2 Sabine River 04 120.28 120-3 Sabine River 04 120.43 120-4 Sabine River 04 120.6


3-6

Habitat Transect ID HEC-RAS Reach ID HEC-RAS

Cross Section

120-5 Sabine River 04 120.74 120-6 Sabine River 04 120.85 105-1 Sabine River 04 104.58 105-2 Sabine River 04 104.75 105-3 Sabine River 04 104.93 105-4 Sabine River 04 105.06 105-5 Sabine River 04 105.16 105-6 Sabine River 04 105.35 90-1 Sabine River 05 91.15 90-2 Sabine River 05 91.03 90-3 Sabine River 05 90.88 90-4 Sabine River 05 90.73 90-5 Sabine River 05 90.59 90-6 Sabine River 05 90.44 72-1 Sabine River 05 71.43 72-2 Sabine River 05 71.55 72-3 Sabine River 05 71.67 72-4 Sabine River 05 71.77 72-5 Sabine River 05 71.93 72-6 Sabine River 05 72.05 64-1 Sabine River 06 64.15 64-2 Sabine River 06 64.9 64-3 Sabine River 06 65.03 64-4 Sabine River 06 65.15 64-5 Sabine River 06 65.3 64-6 Sabine River 06 65.42

3.1 CHANNEL GEOMETRY

The digital elevation model provides the basic HEC-RAS model structure. Additional details of

sub-water surface channel shapes and vertical placement of the channels must be added for each

cross-section. Because the HEC-RAS model solves one-dimensional unsteady flow equations,

cross sections are essentially evaluated based on area versus elevation, so exact details of

channel shape are not essential for an accurate model. However, imperfect knowledge of

channel cross-section geometry and the slope between cross sections, or channel profile, can lead

to over or underestimation of flow volume and momentum, resulting in local differences in water

surface elevations when compared to actual measured values. The goal of model calibration is to


3-7

minimize these differences and sufficiently simulate the overall flow behavior of the river system

under the range of flow conditions intended.

The Sabine is an alluvial river system that can substantially adjust its geometry over time in

response to changes in stream flow and sediment load, especially as a result of high flow

conditions. These changes include channel profile, the meandering river geometry in plan view,

and cross-sectional form. USGS Fact Sheet 2010-3005 (Heitmuller et al. 2010) describes

changes in the channel cross sections near the Bon Wier Gage (USGS 08028500) over time,

demonstrating the significant variation in invert depth and changes in channel area versus

elevation. It is difficult to precisely model this type of dynamic alluvial system and explicitly

account for the degree of local variability in channel invert and shape. However, it is possible to

model representative flow behavior based on calibration to key locations.

HDR|DTA estimated a smoothed average channel profile to use in the model, not accounting for

local profile roughness. The profile shape is based on elevation information provided by SRA-

TX, elevations of USGS cross sections at the three downstream gages, and model calibration.

Figure 3-3 shows the HDR|DTA model profile in comparison to measured values, estimated

average slopes based on an analysis of DEM data (derived from Phillips 2007), and prior model

profiles (KBR 2010, Brown and Root 1993). The profile was used to set the invert elevation of

each channel cross section.


3-8

-20

0

20

40

60

80

100

34 54 74 94 114 134 154

Elev

atio

n (f

eet N

AVD

88)

River Mile

HDR|DTA HEC-RAS Channel Profile

Brown and Root Study

KBR Inundation Model

SRA Field Measurements (estimates)

USGS Gage Cross Section Inverts

Philips Slopes

Figure 3-3 Lower Sabine HEC-RAS Model Channel Profile From Spillway To River Mile 35

Channel cross sections were estimated from habitat survey transects and USGS gage cross-

section data. The habitat surveys provided 11-point depth profiles and stream widths for 6 to 9

transects at each of 10 study locations throughout the Sabine River, the spillway and the tailrace.

HDR|DTA assumed a uniform distribution of the depth measurements across each transect and

reconstructed representative channel geometries from these surveys. An example of the transect

geometries is shown in Figure 3-4. These geometries were used at the associated model cross

sections and also to approximate representative channel geometry at all other cross sections

within the river, spillway and tailrace. Representative geometries were chosen for each of these

cross sections based on channel width, location relative to the measured transect, and location

relative to river bends and straight sections. Geometries at some cross sections were adjusted

during the calibration process.


3-9

Figure 3-4 Habitat Survey Transect Channel Geometr ies – River Mile 139

Model cross sections were further refined by adding components such as levees and ineffective

flow areas. Without these, HEC-RAS computes flow area versus elevation for the entire cross

section, even if there is no connection between two regions of equal elevation within the same

cross section. In this case, the entire area is considered capable of conveying water. Levees can

be added to prevent inclusion of a portion of the cross section in the flow area calculation until

the levee is overtopped. These should be added in locations where there is no connection

between the channel and low-lying areas outside of the channel. Ineffective flow areas are added

where there is a connection with the channel, but the connected area acts as storage and does not

actively contribute to downstream flow. Accurate simulation of off-channel storage requires

careful cross-section placement to capture these regions, and post-processing using elevation

maps to determine connectivity to establish ineffective flow locations. An example cross section

with ineffective flow is shown in Figure 3-5.


3-10

Figure 3-5 Lower Sabine HEC-RAS Model Example Cross Section 136.43 – Sabine River 04

The bridges at the Burkeville Gage (USGS 08025360) and Bon Wier Gage (USGS 08028500)

were included in the model, as well as the railroad bridge below the Bon Wier Gage. It was not

necessary to model the bridge at the Ruliff Gage (USGS 08030500) because the model uses the

Ruliff Gage rating curve as the downstream boundary condition. Bridge geometry at the gages

was derived from the KBR inundation model, and the railroad bridge geometry was estimated

from aerial photos.

Tributary profiles were approximated using a constant slope and model cross sections were

interpolated to reduce model instability resulting from long distances between the cross sections

cut from the digital elevation model. The primary purpose of modeling the tributaries up to the

USGS gage location was to allow the model to approximate attenuation of the inflow

hydrograph.

3.2 HYDRAULIC PARAMETERS

The physical laws which govern open-channel flow are conservation of mass and conservation of

momentum. These laws are expressed as differential equations within the model. The

momentum equation takes into account pressure forces, gravitational force, and boundary drag,


3-11

or frictional force. The solution of these equations is a function of channel geometry (area and

profile) and channel roughness. Channel roughness is represented by Manning’s n coefficients.

Manning’s n is highly variable and is a function of surface roughness, vegetation, channel

irregularities, scour and deposition, obstructions, size and shape of the channel, stage and

discharge, seasonal changes, temperature, and suspended material and bedload (USACE 2010b)

The Lower Sabine River can experience large variations in stage and discharge during typical

Project peaking operations. During the calibration process HDR|DTA determined that Manning’s

“n” varies considerably as a function of discharge. This is not unusual, as low flows result in

higher friction losses because of local variation in the channel profile resulting from pools,

riffles, and sand bars, and a much higher wetted perimeter to area ratio. HEC-RAS

accommodates the specification of roughness factors to scale Manning’s “n” as a function of

discharge. This model feature was used during calibration to optimize the Lower Sabine HEC-

RAS model, matching elevations and timing of observed hydrographs. The scaling factor ranged

from a value of 4 in the tailrace to an average of 1.6 in downstream reaches.

Additional energy losses are modeled using expansion and contraction coefficients, which are

important at locations with abrupt changes in channel conveyance, and losses related to bridge

pier configuration. The Lower Sabine HEC-RAS model was relatively insensitive to these loss

coefficients and typical values were chosen.

3.3 MODEL HYDROLOGY

The Lower Sabine HEC-RAS model requires specification of inflow boundary conditions at the

following locations:

1. The first cross section in the tailrace channel, representing unit operations and leakage.

2. The first cross section in the spillway channel, representing gate operations.

3. The first cross section of each of the three modeled tributaries: Bayou Toro, Bayou

Anacoco, and Big Cow Creek.

4. At 16 cross sections throughout the reach below the tailrace junction, representing

approximate accretion flows.


3-12

5. The first cross section of the old spillway – a minimal flow of 0.25 cfs is used to fulfill

the model requirement for flow within each reach.

For the calibration and verification runs, the tailrace and spillway flows were derived from 15-

minute flow data from the Toledo Bend Reservoir Gage (USGS 08025360). These data

represent a composite of unit flows, unit leakage and gate flows and must be separated.

HDR|DTA assumed the minimum flow occurring at the spillway of 144 cfs and an approximate

wicket gate leakage of 35 cfs when the units are not in operation.

The tributary inflows were calculated from 30-minute USGS gage data for Bayous Toro and

Anacoco, and 15-minute data for Big Cow Creek. The flows were prorated by the ratio of

drainage area at the confluence to the drainage area at the gage.

Sub-basin delineation and drainage areas were provided by SRA-TX (Figure 2-1.) One or two

small tributaries were identified within each of the 15 sub-basins, and model cross sections

upstream of these tributaries were chosen as inflow locations. The sub-basin flows were

estimated using area-prorated combinations of the average daily flows at the four USGS tributary

gages within the basin. Gaged flows used to approximate the ungaged sub-basin flows were

chosen based on the relative location of the sub-basin. Average daily flows were used rather

than the higher resolution data in order to smooth the effects of hydrograph peaks local to the

gaged basins. This method approximates accretion flows throughout the reach.

HDR|DTA developed an additional hydrology inflow data set that is representative of daily flows

for the calendar year for the gaged and ungaged subbasins. This data set is derived from

proration and combination of the average daily flows at the four gaged subbasins for the

maximum extent of the overlapping period of record, from 1955 to the present. The purpose of

this data set is to provide a common base flow scenario for assessing the relative impacts of

alternative operating schemes. A “Base Case” scenario can be run using this data set and unit

and gate inflow time series developed to represent base case operations. Alternative operations

scenarios should be run using the same base case subbasin inflow hydrology for comparison to

the Base Case operations scenario.


3-13

The downstream boundary condition is set as the rating curve at the Ruliff Gage (USGS

08030500) shown in Figure 3-6. This boundary condition specifies water surface elevation

based on the model flows calculated from the differential equations. Sensitivity testing using a

normal depth boundary condition based on channel slope confirmed that water surface profiles

throughout the entire modeled system, and of particular interest, at the downstream limit of

investigation at Shoats Creek near RM 54, are insensitive to the downstream boundary condition.

0

5

10

15

20

25

30

0 20,000 40,000 60,000 80,000 100,000 120,000 140,000

Stag

e (f

eet N

AVD

88)

Flow (cfs)

Figure 3-6 Rating Curve: Ruliff Gage (USGS 08030500)

The HEC-RAS model also requires initial conditions in terms of flow at the top of each channel,

at every reach junction, and at the downstream model boundary. Initial conditions influence

flow in the reach at the beginning of the model run, especially for cases with rapidly varying

flows. This impact can be seen in the calibration and verification scenarios discussed in the

following sections. For the Sabine River model, it is best to compare flows after an initial

weekly operations cycle has established realistic base flows throughout the model.


4-1

4.0 MODEL CALIBRATION AND VERIFICATION

4.1 CALIBRATION

The calibration process is used to estimate hydraulic parameters and refine model geometry to

optimize model replication of the observed data under a range of flow scenarios. HDR|DTA

performed model calibration using time series of stage and flow at the three Lower Sabine River

USGS gages and relative depths at levelogger locations throughout the river system. Modeled

water surface elevations and the timing of hydrograph peaks were compared to observed data at

the same locations. Differences were minimized by adjusting channel shapes, channel profile,

Manning’s n, and the discharge-dependent roughness factors. The goal was to achieve an overall

“best match” to observed data throughout the river system and to replicate changes in stage

related to daily and weekly unit operations sequences.

Calibration was conducted for the period of August 8th through October 8th, 2009. The inflow

hydrographs for the tailrace and spillway channels are shown in Appendix A, Figure A-1. The

hydrographs were developed from the combined flow data provided by the USGS for the Toledo

Bend Reservoir Gage (USGS 08025360). To split the hydrograph, HDR|DTA assumed the

continuous flow at the spillway of 144 cfs, and wicket gate leakage into the tailrace of 35 cfs

during non-operation. The tributary inflow hydrographs are shown in Figure A-2.

Calibration results at the three downstream USGS gages are shown in Figures 4-1 through 4-3.

The blue line represents the modeled stage and the red line represents the observed USGS Gage

stage data. Comparison to observed data is appropriate after the first few days of modeled

operations in order to eliminate the effects of initial conditions. Observed stages at the USGS

Gage cross sections represent actual measured elevations and these locations were emphasized

during model calibration.


4-2

Figure 4-1 Calibration Results at Burkeville Gage (USGS 08025360)

Figure 4-2 Calibration Results at Bon Wier Gage (USGS 08028500)


4-3

Figure 4-3 Calibration Results at Ruliff Gage (USGS 08030500)

The first four weeks of the calibration period are representative of typical peaking operations,

with the two Project units operating at full capacity for 6 hours per day during the week and no

operation on the weekends. There was minimal impact on flow patterns from precipitation or

tributary contribution during this period. The Lower Sabine HEC-RAS model replicated

observed results well during this portion of the calibration period. Figures 4-4 and 4-5 illustrate

a comparison of modeled and observed weekly and mid-week minimum and maximum stage and

maximum rise throughout the reach. In general, modeled results overestimate peak elevations at

all observed locations, including the tailrace. This may result from several factors, including

overestimation of inflows into the tailrace, the distribution of roughness factors relative to the

magnitude of discharge, inaccuracies related to levelogger measurements, and model geometry.


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Figure 4-5 Lower Sabine HEC-RAS Maximum Changes in Daily Stage Dur ing Weekly Operations

The second half of the calibration period experienced periods of rainfall and significant tributary

flows which impacted the upstream and downstream flow patterns resulting from operations.

This portion of the calibration period was more difficult to replicate because of uncertainties

associated with proration of tributary flows and the estimated average daily ungaged accretion

flows. Base flows underlying operations flows are not well represented, especially below Bayou

Anacoco. Differences in base flows affect peak arrival times and stages. However, the general

stage pattern resulting from operations is still apparent.


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Hydrographs comparing modeled stages and relative levelogger stages at upstream and

downstream locations are shown in Appendix B. Stages at the levelogger locations were derived

from depth data with no association to an actual elevation. These results were vertically shifted

to generally match at low-flow elevations and visually demonstrate changes in measured depth

relative to the modeled results.

Figure B-1 shows the comparison of modeled elevations to the levelogger response in the

tailrace. Some backwater effects resulting from high flows from Bayou Toro can be observed in

the second half of the calibration period. Figures B-2 through B-5 demonstrate backwater effects

of operations flows in the channel originating from the spillway and in Bayou Toro. The timing

and elevation of the backwater hydrograph peaks in the spillway channel match the levelogger

data extremely well during the weekly operations in the first half of the calibration period. The

peaks in Bayou Toro differ by approximately a foot. Figures B-6 through B-16 show the

modeled to observed hydrograph comparisons for the levelogger cross sections downstream of

the tailrace.

4.2 VERIFICATION SCENARIO

The modeled verification period extended from August 10th through September 24th, 2010.

During the first 4 weeks of this period the operations consisted of a single unit at full capacity for

2 hours on Mondays and Wednesdays and for 4 hours on Fridays, with no unit operations over

the weekends. Beginning on September 10th, both units commenced continuous operation for a

period of 11 days. This period was included in the verification run to further test the capabilities

and limitations of the model under increased flow conditions.

Figures 4-3 through 4-5 show verification results at the three USGS gages. Although exact

elevations and depth changes are not replicated at every location, the pattern of flows is well-

represented throughout the river system for the different operating scenarios. The slightly

skewed hydrographs relative to observed data at the Bon Wier Gage (USGS 08028500) resulting

from the 4-hour unit operation are a result of the discharge-dependent roughness factors. The


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peak flows fall within the range of rapidly changing Manning’s n and the HEC-RAS model

results appear to be less reliable.

The favorable comparison to observed results during the continuous operations period at the

Burkeville Gage (USGS 08025360) and Bon Wier Gage (USGS 08028500) demonstrates that the

model is capable of reliably simulating these higher flows. However, the model response at

Ruliff Gage (USGS 08030500) demonstrates the model limitation to within-bank flows. The

observed hydrograph has a slower rising limb, indicating connection to off-channel storage and

overbank flows that are not simulated in the hydraulic model. Under the continuous operating

scenario, modeled flows began to overtop banks near river mile 77 approximately 30 hours after

the hydrograph begins to rise at that location. This model is designed to simulate Project

peaking operations, and not connectivity with the floodplain that may occur when powerhouse

flows are at maximum and tributary flows are high. Model testing determined that flows

remained within cross-section extents throughout the modeled peaking operations.

Figure 4-3 Ver ification Results at Burkeville Gage (USGS 08025360)


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Figure 4-4 Ver ification Results at Bon Wier Gage (USGS 08028500)

Figure 4-5 Ver ification Results at Ruliff Gage (USGS 08030500)


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5.0 SUMMARY AND CONCLUSIONS

5.1 SUMMARY

The purpose of this calibration and verification report is to document inputs and assumptions

used in the development of the Lower Sabine HEC-RAS hydraulic model to demonstrate that the

model reasonably characterizes hydraulics within the modeled river system and is a reliable tool

for evaluating the hydraulic-related effects of alternative operating scenarios on upstream and

downstream flows and water surface elevations. As with any model, accuracy is highly

dependent on input data, and consequently, model results should be viewed in a relative, rather

than an absolute context.

5.2 CONCLUSIONS

The Lower Sabine HEC-RAS hydraulic model was calibrated and verified using two separate

periods of operation. The calibration process was used to adjust model geometry and hydraulic

parameters to best match observed stages at USGS Gages and relative changes in depths at

levelogger locations. Calibration at the three downstream USGS Gages was emphasized because

stages were tied to surveyed datums at these locations.

A separate verification scenario was run with a different weekly operating pattern to assess the

model calibration. Although exact elevations and depth changes are not replicated at every

location, the pattern of flows is well-represented throughout the river system for the different

operating scenarios. The verification scenario included a period of continuous operation of both

units. The comparison to observed results during this period demonstrates that the model is

capable of simulating higher flows adequately. However, this model is not designed to

accurately simulate the complex connectivity with the Sabine River floodplain that occurs under

high tributary inflows or unusual operation patterns. Powerhouse peaking flows under low and

moderate tributary contributions do not result in general overbank flooding. Model testing

determined that flows remained within cross section extents throughout the modeled reach for up

to 20 hours of 2-unit continuous operation during the September 2010 base flow conditions.


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HDR|DTA has developed representative average daily flows for the calendar year to provide

“Base Case” hydrology for gaged tributary and ungaged basin inflows. These flows were based

on prorated and combined discharge data from the four tributary USGS Gages. Scenarios

developed to assess relative impacts of alternative operating schemes should all be run using the

same period of Base Case hydrology.

In the opinion of HDR|DTA, verification results show the Lower Sabine HEC-RAS model

compares favorably to measured data, reasonably characterizes the upstream and downstream

flow regime resulting from Project operations, and is appropriate for use in evaluating the effects

of alternative operating scenarios compared to the base case normal peaking operations currently

used at the Project. However, appropriate use of the results is cautioned. As with any model,

accuracy is highly dependent on input data, and consequently, model results should be viewed in

a relative, rather than an absolute context.


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6.0 REFERENCES Brown & Root. 1993. Sabine River Flood Study, prepared for the Sabine Basin Task Force. HDR|DTA. 2011. Toledo Bend Relicensing Project, FERC No. 2305, Lower Sabine River Bottomland Connectivity Study Report. 116 pp. Heitmuller, F.T., Greene, L.E., and Gordon, J.D. 2010. An overview of historical channel

adjustment and selected hydraulic values in the lower Sabine and lower Brazos River Basins, Texas and Louisiana. U.S. Geological Survey Fact Sheet 2010–3005. 4 p.

KBR. 2010. Investigation and Analysis of Dam Break Floods. Technical Report. January 2010. U.S. Army Corps of Engineers’ (USACE). 2005. HEC-GeoRAS Software, Version 4.2.93.

Hydrologic Engineering Center. U.S. Army Corps of Engineers, Davis, CA. September 2005.

———. 2010a. HEC-RAS River Analysis System software, Version 4.1.0. Hydrologic

Engineering Center. U.S. Army Corps of Engineers, Davis, CA. January 2010. ———. 2010b. HEC-RAS River Analysis System Hydraulic Reference Manual Version 4.1.

Hydrologic Engineering Center. U.S. Army Corps of Engineers, Davis, CA. January 2010. U.S. Geologic Service (USGS). 2011a. 1/9 Arc-second Seamless National Elevation Dataset

(raster digital data). EROS Data Center. Sioux Falls, South Dakota. Data available from U.S. Geological Survey. [Online] URL: http://seamless.usgs.gov/index.php. (Accessed February 18, 2011.)

_______. 2011b. 1/3 Arc-second Seamless National Elevation Dataset (raster digital data).

EROS Data Center. Sioux Falls, South Dakota. Data available from U.S. Geological Survey. [Online] URL: http://seamless.usgs.gov/index.php. (Accessed February 18, 2011.)

pw:\\PWAPPTPA01:SouthEast_Tampa\Documents\Sabine_River_Authority_of_Texas\246_0001Toledo_Bend_Relicensin\Sabine_SF_Field_Support\06.00_Engineering\HEC-RAS Model\ToledoBendHEC-RASDraft-051011 (2)

APPENDICES

APPENDIX A

CALIBRATION INFLOW HYDROGRAPHS

Appendix A - Page 1

Figure A-1 Calibration Scenario Inflow Into Tailrace and Spillway

Appendix A - Page 2

Figure A-2 Calibration Scenario Inflow Into Tributaries

APPENDIX B

CALIBRATION RESULTS

Figure B-1 Calibration results at Cross Section 141.8 Tailrace

Figure B-2 Calibration results at Cross Section 142.88 Sabine River Above Tailrace

Figure B-3 Calibration results at Cross Section 145.03 Sabine River Above Tailrace

Figure B-4 Calibration results at Cross Section 145.95 Spillway

Figure B-5 Calibration results at Cross Section 877.4456 Bayou Toro

Figure B-6 Calibration results at Cross Section 139.9 Sabine River



Figure B-9 Calibration results at Cross Section 120 Sabine River






