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21st Century Dam Design —

Advances and Adaptations

31st Annual USSD Conference

San Diego, California, April 11-15, 2011

On the CoverArtist's rendition of San Vicente Dam after completion of the dam raise project to increase local storage and provide

a more flexible conveyance system for use during emergencies such as earthquakes that could curtail the region’s

imported water supplies. The existing 220-foot-high dam, owned by the City of San Diego, will be raised by 117

feet to increase reservoir storage capacity by 152,000 acre-feet. The project will be the tallest dam raise in the

United States and tallest roller compacted concrete dam raise in the world.

The information contained in this publication regarding commercial projects or firms may not be used for

advertising or promotional purposes and may not be construed as an endorsement of any product or

from by the United States Society on Dams. USSD accepts no responsibility for the statements made

or the opinions expressed in this publication.

Copyright © 2011 U.S. Society on Dams

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To be the nation's leading organization of professionals dedicated to advancing the role of dams

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• Advancing the knowledge of dam engineering, construction, planning, operation,

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• Enhancing practices to meet current and future challenges on dams; and

• Representing the United States as an active member of the International Commission on

Large Dams (ICOLD).

Restricted Reservoir Operating Plan 1631

DEVELOPMENT OF A RESTRICTED RESERVOIR OPERATING PLAN AT WEST POINT DAM, GEORGIA

Kevin Fagot, P.E.1

Jamie M. Bartel, P.G.2 Andy Ashley, P.E.3

Michael Schmidt, P.E. BCEE4

ABSTRACT

Currently, the spillway gates at West Point Dam, a U.S. Army Corps of Engineers project operated by the Mobile District, on the Chattahoochee River in Georgia have trunnion rod issues and need to be repaired. These structural issues with the spillway gates may cause two of the six gates to be taken off line. If this occurs, stoplogs would be placed in front of the two non-operating gates. The top of the stoplogs is at elevation 638.0 feet. This would become the new limiting elevation, and the reservoir would need to be kept at or below this elevation using only four of the six spillway gates. Because of this, a restricted reservoir operating plan (RROP) needed to be developed for West Point Dam. For this analysis, several historical events ranging from 1919 to 2009 along with the Standard Project Flood (SPF) and Spillway Design Flood (SDF) were analyzed. The effect of dropping the top of the conservation pool to a lower elevation as well as altering the induced surcharge operation for a more aggressive release schedule was studied. This paper covers the development of the restricted reservoir operating plan as well as explaining how the Hydrologic Engineering Center (HEC) program, Res-Sim, was used to perform this analysis.

INTRODUCTION

West Point Dam is located on the Chattahoochee River about 201.4 miles above the mouth of the river. It is about 3 miles north of West Point, Georgia. The reservoir extends approximately 35 miles along the Alabama-Georgia state line. Figure 1 shows the location of West Point Dam. The dam is a concrete gravity type structure with rolled earthfill embankments. The total drainage area contributing flow at this location is 3,440 square miles. The outlets for the dam include a powerplant and a spillway. The spillway consists of six tainter gates that are 50 feet wide by 41 feet high. The project is part of the Apalachicola, Chattahoochee, and Flint (ACF) River system and is operated by the Mobile District of the U.S. Army Corps of Engineers (USACE, Mobile). Project purposes include flood control, hydroelectric power, recreation, fish and wildlife development, and stream flow regulation for downstream navigation.

1 Project Engineer, WEST Consultants, Bellevue, WA, kfagot@westconsultants.com 2 Task Order Manager, Geologist, CDM Federal Programs, Baton Rouge, LA, barteljm@cdm.com 3 Chief, Water Management Section, U.S. Army Corps of Engineers, Mobile District, jonathan.a.ashley2@usace.army.mil 4Project Manager, Vice President, CDM Jacksonville, FL, schmidtmf@cdm.com

21st Century Dam Design — Advances and Adaptations 1632

Figure 1. West Point Dam Location

STUDY PURPOSE

The spillway gates at West Point dam have issues with the trunnion rods which need to be repaired. Because of this, the spillway may have only four of its six gates in operation during repairs. Stoplogs would be placed in front of the two non-operational gates. The top of the stoplogs would be at elevation 638.0 ft NGVD. This is also the elevation of the

Restricted Reservoir Operating Plan 1633

top of the spillway gates in closed position and would become the new limiting pool elevation at West Point. With this new requirement, a restricted reservoir operating plan needed to be developed to maintain pool levels at or below elevation 638.0 ft.

STUDY METHODOLOGY

The Corps of Engineers software, HEC Res-Sim was used to perform this analysis. HEC Res-Sim is the reservoir simulation program developed by the Hydrologic Engineering Center of the Corps of Engineers (USACE, HEC 2003). The data requirements for this model included the physical and operational characteristics of West Point Dam and Reservoir. The physical reservoir data is described through the use of the storage-elevation and storage-area curves. The physical data of the dam includes the type and capacity of each outlet. The operational data includes the zone definitions along with the rules governing the operations in each zone. West Point dam has 3 major water management zones or pools. These are the inactive pool, the conservation pool, and the flood pool. The inactive pool is often referred to as dead storage since this is water that is below the elevation of the lowest outlet in the dam. The conservation pool holds water that is set aside for purposes such as water supply and hydropower production. The flood pool is storage that is set aside for the capture of inflow from precipitation events to manage potential downstream flooding. The major zones in West Point Reservoir are shown in Figure 2. It should be noted that the conservation pool varies throughout the year making this a seasonal pool. Each of these zones can have subzones where different rules of operation will apply.

The analysis for the restricted reservoir operation involved high inflow events with elevations in the flood pool in order to define operational protocols. Because of this, the two main rules involved in the analysis were the 40,000 cfs maximum release rule and the induced surcharge function. These rules limit the release from West Point Dam to 40,000 cfs until the conditions trigger the induced surcharge function. Induced surcharge occurs when flow conditions require the spillway gates to be opened and the gates are operated to force (induce) extra storage (surcharge) at the reservoir. The goal of an induced surcharge function is to reduce the peak downstream flows by keeping the peak outflow values below the peak inflow values with the extra storage that is created from having a higher top of gate elevation. The surcharge function at West Point Dam has an induced surcharge envelope curve. This curve gives the minimum required release for any given pool elevation. This minimum required release can be exceeded at West Point for certain combinations of pool elevation and inflow. These curves are shown in Figure 3. The 160,000 cfs inflow curve is showing greater minimum required releases than what is given by the envelope curve. West Point has minimum required release curves for inflows ranging from 40,000 cfs to 300,000 cfs.

21st Century Dam Design — Advances and Adaptations 1634

Figure 2. Major Zones in West Point Reservoir

For the analysis, eight different events were analyzed. The events were December 1919, January 1925, February 1961, March 1990, May 2003, September 2009, Standard Project Flood and Spillway Design Flood. These events represent very large historic and synthetic flood events at West Point. For each event, a baseline simulation was developed with all six gates in operation. This baseline was compared with event data given in the operations manual for West Point Reservoir (USACE, Mobile 1975) or from actual data when available. If the peak elevation in the baseline simulation exceeded elevation 638.0 ft, alternatives were developed to quantify the change needed in the induced surcharge function to maintain a peak elevation below 638.0 ft. In addition, the impact of dropping the top of conservation pool was also analyzed to increase pre-storm storage for potential operational flexibility during a large storm.

Figure 4 shows the comparison of pool elevations from the 1919 event for the Res-Sim simulations and the data provided in the operations manual for the West Point project. The results compare favorably for this event. This comparison was performed for all eight events. For the Res-Sim results, the current operating criteria were applied. This included the appropriate top of conservation pool for the time of year as well as a 40,000 cfs maximum release when in the flood pool. The results in the manual indicated a lower top of conservation pool and a lower flood pool release were used causing some differences in the two results. The current operating criteria were used to establish the baseline simulation.

Restricted Reservoir Operating Plan 1635

Figure 3. West Point Induced Surcharge Function

Figure 4. Comparison of 1919 Pool Elevations from Res-Sim and Operations Manual

The peak Res-Sim elevation for the 1919 event was 639.7 ft. The peak release was computed to be 65,875 cfs. Four different alternatives were analyzed to reduce the peak elevation to 638.0 ft. The first alternative involved only dropping the top of conservation

21st Century Dam Design — Advances and Adaptations 1636

pool. It was found that this action alone could not keep the peak elevation below 638.0 ft. The top of conservation pool was dropped down to the top of the inactive pool at elevation 620.0 ft. The pool peaked at elevation 638.9 ft. The peak release, however, was reduced to 49,707 cfs. It was evident that modifications would be needed to the induced surcharge curves to create a more aggressive release. For this analysis, two methods for altering the induced surcharge curve were utilized. First, the curves were dropped to a lower elevation meaning that a specific minimum required release would now be occurring at a lower elevation. Next, the values on the curves could be increased. Both of these methods ultimately serve the same purpose of giving a greater release at a specific elevation. Analyzing each of these methods separately would bracket the solution. The curves could be dropped by a certain amount or values could be increased by a certain amount or a combination of these solutions could be used.

Figure 5 shows an example of altering the induced surcharge envelope curve. It should be noted that only the envelope curve is shown in this figure, but the change is applied to the inflow curves as well. These examples show the changes to the envelope curve from dropping the curve by 2 feet as well as increasing the values by 75%. In both of these cases the values are checked to ensure that the total release capacity is not exceeded. The capacity used in the analysis of the alternatives has a reduced spillway capacity to reflect the loss of two of the gates. The amount for this outlet has been reduced by one-third.

Figure 5. Altering Induced Surcharge Curves at West Point

Restricted Reservoir Operating Plan 1637

For the 1919 event, both increasing the surcharge curves by 65% and dropping the surcharge curves by 2 feet gave peak elevations at or below 638.0 ft. It was also found that dropping the surcharge curves by 1 foot and increasing the values by 35% would give a peak elevation of 638.0 ft. These results give various combinations of altering the induced surcharge curves to lower the peak elevation to an acceptable level. It was also desirable to analyze the impact of dropping the top of conservation pool along with altering the induced surcharge curves since this will typically lead to a less aggressive required release. The 1919 event occurred in early December when the top of conservation pool is transitioning from elevation 635.0 ft on 01Nov to elevation 628.0 ft on 15Dec. The top of conservation pool was dropped to elevation 627.0 for this simulation. The simulations for the 1919 event are summarized in Table 1.

Table 1. Summary of Simulations for 1919 Event Simulation Peak Pool Elev (ft) Peak Release (cfs)

Baseline 639.7 65,875

Dropping top of conservation pool down to elev 620.0 638.9* 49,707

Increase surcharge curves by 65% 638.0 74,203

Drop surcharge curves by 2 feet 637.9 85,754

Drop surcharge curve by 1 foot and increase values by 35% 638.0 77,687

Drop top of conservation pool to a constant elev of 627.0, drop surcharge curves 1 foot and

increase values by 30%

638.0 69,742

*not possible to keep pool below 638.0 by dropping pool only

DETERMINATION OF EVENT TO BE USED FOR RESTRICTED RESERVOIR OPERATING PLAN

The alternatives needed to keep the pool elevation below 638.0 ft were summarized for six of the eight events. For the 1961 event, it was found that the baseline condition peaked below elevation 638.0 ft making alternatives unnecessary. The operations manual shows this event exceeding elevation 638.0 ft, however, the releases that are occurring prior to the induced surcharge function are closer to 16,000 cfs instead of the 40,000 cfs that is used in the current operations. This shows the importance of an aggressive release for the restricted reservoir operating plan. The spillway design flood (peak inflow = 557,800 cfs) was the largest event used for this analysis. It was not possible to keep the elevation below 638.0 ft for this event. The top of conservation pool was dropped down to the top of the inactive pool at elevation 620.0 ft, and all inflow was released up to the

21st Century Dam Design — Advances and Adaptations 1638

total release capacity. The inflow from this event exceeds the release capacity of the project causing a rise in pool elevation that exceeds the desired maximum of 638.0 ft. Using the results provided for the remaining six events, USACE, Mobile selected the standard project flood (peak inflow = 282,900 cfs) as the event to be used for development of the restricted reservoir operating plan. The initial results for the standard project flood showed a combination of top of conservation pool levels along with changes to the induced surcharge curves. It was not possible to keep the standard project flood from exceeding elevation 638.0 ft by only dropping the top of conservation pool level. The summary of simulations for the standard project flood is shown in Table 2.

Table 2. Summary of Simulations for Standard Project Flood Simulation Peak Pool Elev (ft) Peak Release (cfs)

Baseline with starting elevation = 628.0 640.8 255,129

Baseline with starting elevation = 635.0 641.0 278,510

Dropping top of conservation pool to

elevation 620.0 640.6 198,305

Increasing surcharge 260% with starting elevation =

628.0 637.9 229,040

Dropping surcharge curves down 5 feet with starting

elevation = 628.0 637.9 228,954

Dropping surcharge curves down 2 feet and increasing values by 50% with starting

elevation = 625.0

638.0 229,484

Dropping surcharge curves down 1.5 feet and increasing values by 50% with starting

elevation = 622.0

638.0 213,696

DETERMINATION OF RESTRICTED RESERVOIR OPERATING PLAN

USACE, Mobile then needed to determine the top of conservation pool elevation that would be used during the restricted reservoir operating plan. Since a seasonal pool may still be used during the restricted reservoir operating plan, five different starting pool elevations were analyzed. These elevations were 625.0 ft, 628.0 ft, 630.0 ft, 632.5 ft, and 635.0 ft. In addition, one set of curves for each top of conservation pool elevation would

Restricted Reservoir Operating Plan 1639

need to be developed for this plan. It was decided that the curves would be altered by a combination of dropping the curves and increasing the outflow values. The combinations would be modified until elevation 638.0 ft was not exceeded.

For a top of conservation pool elevation of 625.0 ft, the surcharge curves were dropped down 2 feet and the flow values were increased by 50%. The peak elevation was 638.0 ft while the peak release was 229,484 cfs. Figure 6 shows the change to the envelope curve under the restricted reservoir operating plan. The inflow curves were also changed by this same amount.

Figure 6. Modified Surcharge Envelope Curve for Standard Project Flood with Top of Conservation Pool = 625.0 ft

As higher top of conservation pool elevations were used, the envelope curve, as well as the inflow curves, needed to be moved further down and to the right to give an even more aggressive required release. Figure 7 shows the comparison between the envelope curve needed for a top of conservation pool elevation at 625.0 ft and 630.0 ft.

21st Century Dam Design — Advances and Adaptations 1640

Figure 7. Modified Surcharge Envelope Curve for Standard Project Flood with Top of Conservation Pool = 630.0 ft

If a seasonal pool is used at the West Point project as opposed to a constant top of conservation pool throughout the entire year, the appropriate set of induced surcharge curves would need to be used throughout the year. The changes to the induced surcharge curves along with the peak elevation and peak release is shown in Table 3.

Table 3. Summary of Restricted Reservoir Operating Plant Top of Conservation

Pool (ft) Change to Induced Surcharge Curves Peak elevation (ft) Peak release (cfs)

625.0 Dropped down 2 feet and increased by 50% 638.0 229,484

628.0 Dropped down 3 feet and increased by 60% 637.9 228,862

630.0 Dropped down 4 feet and increased by 40% 638.0 229,691

632.5 Dropped down 4.5

feet and increased by 30%

638.0 229,727

635.0 Dropped down 5 feet and increased by 10% 637.9 229,566

The graphical results using a top of conservation pool elevation of 630.0 ft are shown in Figure 8 and Figure 9. In the beginning of the simulation, the release is set equal to the inflow, and the pool elevation is held at top of conservation. This continues until the inflow exceeds the maximum allowable release of 40,000 cfs. The release is held constant at 40,000 cfs until the induced surcharge function is activated causing an increase in release. The pool continues to rise as the inflow exceeds the release. The pool peaks once the inflow hydrograph and the outflow hydrograph cross on the

Restricted Reservoir Operating Plan 1641

recession limb of the inflow hydrograph. This occurs when the pool elevation has reached its peak elevation of 638.0 ft.

Figure 8. Res-Sim Pool Elevation with Modified Surcharge Curves for Standard Project Flood

Figure 9. Res-Sim Releases with Modified Surcharge Curves for Standard Project Flood

The entire set of curves was developed for each top of conservation pool elevation. Table 4 shows the set of curves that was developed for the top of conservation pool elevation of 630.0 ft. The values across the top of the table are the envelope curve along with the inflow values. From this table, it can be seen that the minimum required release increases both with increasing pool elevation and increasing inflow values.

21st Century Dam Design — Advances and Adaptations 1642

Table 4. Surcharge Curves for Top of Conservation = 630.0 ft elevation (ft) envelope 40000 60000 80000 100000 140000 160000 200000 250000 300000

630 40000 44100 49000 61600 82600 109900

630.499 40000 40000 47250 52500 65800 88550 117250

630.5 40000 40000 47250 52500 65800 88550 117250

630.99 40000 40600 50400 56000 70000 94500 124600

631 40000 40000 40600 50400 56000 70000 94500 124600

632 40000 40000 40000 43400 47600 58800 63700 81200 110600 143500

632.5 40000 40000 42000 46900 51800 63000 69300 89600 120400 154000

633 41440 44100 45780 51100 56000 67900 75600 97300 130200 166600

633.5 46900 49000 50960 56000 60900 74900 84000 107800 141400 179900

634 53200 54600 56280 61600 66500 83300 94500 120400 155400 194600

634.5 60200 61600 63700 67900 73500 94500 107100 133000 170800 201500

635 69300 69300 72100 77000 82600 107100 121800 148400 187600 206767

635.5 82880 82880 85400 91000 99400 123900 140000 165200 208600 210434

636 114800 114800 114800 114800 128800 147000 162400 189000 214100 214100

636.5 184800 184800 184800 184800 184800 184800 204400 218100 218100 218100

637 222100 222100 222100 222100 222100 222100 222100 222100 222100 222100

638 230100 230100 230100 230100 230100 230100 230100 230100 230100 230100

DISCUSSION

For this analysis, eight events were modeled to determine the impact of having only four of the six spillway gates available at West Point. The current operating procedure was applied to these eight events and used as a baseline. Alternatives were developed by dropping the top of conservation pool and altering the induced surcharge curves. The standard project flood was chosen by USACE, Mobile as the event to use to determine the restricted reservoir operating plan. The plan involved five different top of conservation pool levels. This allows USACE, Mobile to continue to have the flexibility of using a seasonal pool during gate repairs. However, the higher the top of conservation pool elevation, the more aggressive the release needs to be. This analysis also demonstrated the importance of releasing inflow up to 40,000 cfs when in the flood pool. An aggressive operation to release inflow and increase storage early in the event was used in all simulations. Following the operational procedure outlined in this study will pass events up to the standard project flood at West Point while keeping the peak pool elevation below 638.0 ft. Events larger than the standard project flood would exceed this elevation as evidenced by the analysis of the spillway design flood.

Restricted Reservoir Operating Plan 1643

REFERENCES

U.S. Army Corps of Engineers, Hydrologic Engineering Center (USACE, HEC), HEC-ResSim Reservoir System Simulation, User’s Manual, Version 2.0, September 2003

U.S. Army Corps of Engineers, Mobile District (USACE, Mobile), Apalachicola Basin Reservoir Regulation Manual, Appendix E, West Point Reservoir, Chattahoochee River, Georgia, June 1975

U.S. Army Corps of Engineers, Engineer Manual 1110-2-3600, Management of Water Control Systems, November 1987