Sec2_ProjDesc

Toledo Bend Project – Part 12D Section 2 Safety Inspection Report Project Description

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SECTION 2

PROJECT DESCRIPTION

2.1 BRIEF PROJECT DESCRIPTION

2.1.1 Location and Construction of Primary Features

The Toledo Bend Dam is located in Newton County, Texas and Sabine Parish, Louisiana at river mile 156.5 on the Sabine River. Coordinates of the dam site are Latitude 31°10’ 25” and Longitude 93° 33’ 57”. The original river channel was located near the south end of the embankment at the state line between the Texas and Louisiana border. Refer to Figure 2.1-01 for a vicinity map. Figure 2.1-02 shows a schematic plan of the project.

2.1.2 Spillway

2.1.2.1 The spillway is located near the north abutment in Louisiana. The spillway structure is 838 feet long and consists of a gated ogee section about 530 feet long. The spillway contains eleven 40’ by 28’ tainter gates. The ogee crest is at elevation 145 feet and the top of spillway gates is at elevation 173 feet. For the spillway plan view, refer to Figure 2.1-03. Cross sections are shown in Figure 2.1-04.

2.1.2.2 The spillway releases are channeled by the spillway chute slab into a hydraulic jump stilling basin. An 8’- 4” by 12’-0” gated conduit through the spillway at elevation 100 feet is used for required low-flow releases.

2.1.2.3 A minimum flow of 144 cubic feet per second (cfs) must be maintained in the spillway channel according to provisions of the Power Sales Agreement. This minimum flow is normally met by releases through two 20-inch-diameter pipes that bypass the conduit gate and discharge into the conduit downstream of the gates, to be released through the chute slab. These operations are conducted according to the Operating Guide Rules as discussed in Paragraph 2.3, Summary of Standard Operating Procedures.

2.1.2.4 The spillway was designed for a total capacity of approximately 320,000 cfs with all gates open. The spillway has operated on several occasions to pass significant storm flows downstream. Recent examples include storm events in 1989 and 1999. In both of these events, the operating guide and operator judgment were used to pass storms with magnitudes of approximately one-third the Probable Maximum Flood (PMF). Spillway gates are typically exercised several times per year, including during inspections.


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2.1.2.5 See Figure 2.1-04 for a cross section of the spillway. Some photos of the spillway are provided in Appendix E.

2.1.3 Powerhouse

2.1.3.1 The powerhouse is located near the south abutment in Texas. 2.1.3.2 At the powerhouse, water enters the turbines through six 16’-9” by 29’-0”

foot penstocks, controlled by vertically operated caterpillar-type gates. The two vertical units utilize Kaplan turbines, each rated at approximately 41.5 MW at a minimum head of 60.8 feet. Refer to Figure 2.1-05 for plan and section views of the powerhouse.

2.1.3.3 The two units are capable of releasing 30,000 acre-feet of water per day (approximately 15,000 cfs) to produce approximately 205 million kilowatt-hours (KWH) of electricity per year.

2.1.3.4 Some photos of the Powerhouse are provided in Appendix E.

2.1.4 Embankment

2.1.4.1 The dam is a rolled earth-fill embankment with a concrete spillway structure and a powerhouse structure. Refer to Figure 2.1-02 for the dam plan view. The length of the dam is approximately 11,000 feet, with a maximum height of approximately 108 feet. The upstream face of the embankment has soil cement slope protection for controlling erosion from wave action. The embankment crest has a roadway along it that is paved with asphalt. Refer to Figure 2.1-06 for the typical embankment cross sections.

2.1.4.2 There are three saddle dikes with similar features as the main embankment located in Texas. The three saddle dikes rise up to 185 feet in elevation across saddles on the reservoir rim. Dike 2 has piezometers to monitor seepage pressures near the downstream toe of the dike. Refer to Figure 2.1-03 for dike locations.

2.1.5 Embankment Drainage System

2.1.5.1 A drainage system is located on the downstream face of the embankment. It consists of perforated corrugated metal pipe extending in each direction from manholes and parallel to the axis of the dam. Drain outfall pipes extend from the manholes to an open drainage ditch at the downstream toe of the road berm. This system drains the sand filter.

2.1.5.2 A second and separate drainage system, made up or relief wells with manhole covers, is located down slope from the road berm. The relief wells drain into an outfall made of corrugated metal pipes with bituminous coating extending to the drainage ditch at the downstream


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toe. This drainage system is not connected with lateral pipes and does not contain pumps.

2.1.5.3 Manholes provide access to observe the flow in the sand filter system; however, there are no flow measuring devices to monitor the drainage flow quantity. The relief well manholes provide access to observe and relieve the water flow in the natural ground foundation.

2.1.5.4 For many years, one measuring weir was located at the point of discharge from the open drainage toe ditch, however; this measuring weir was removed while cleaning the open drain of woody vegetation placed by a beaver. The plan is to replace the weir.

2.1.5.5 Piezometers are also located throughout the embankment to monitor the hydrostatic pressure in the soil underlayment at various depths and locations. Some piezometers also have settlement plates. A complete description of the instrumentation is available in Section 4.0 of this report or Section 7.0 of the Supporting Technical Information (STI).

2.1.6 Highway Access

State Highway 191 in Louisiana crosses the spillway structure at the left abutment of the dam; then goes down the downstream slope of the embankment at an angle before proceeding southward over the powerhouse tailrace channel and then continuing toward Texas where it connects to a Texas State secondary road. The berm, constructed to accommodate the road, leaves the toe of the dam near the right abutment just north of the power plant. Refer to Figure 2.1-01 which shows the roadways associated with the dam.

2.1.7 Summary of Pertinent Data

A summary of pertinent data regarding the Toledo Bend Dam is given in Table 2.1.1.


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Table 2.1.1 Summary of Pertinent Data for Toledo Bend Dam

Dam Type Rolled-earth embankment with concrete spillway Dam Slopes - Upstream 2.5:1 (H:V) Dam Slopes - Downstream 2.5:1 (H:V) Dam Height 108 feet (approx.) at Elevation 185 MSL Dam Length 11,000 feet (approx.) Crest Width 25 feet Crest Elevation 185 feet MSL Top of Clay Core Elevation 180 feet MSL Top of Embankment Chimney Drain Elevation

160 feet MSL

Conservation Pool Stage Water Surface Elevation

172 feet MSL

Reservoir Size 100 feet deep and 65 miles long Reservoir Shoreline 1200 miles Drainage Area 7178 square miles Spillway Gated near north abutment 838 feet long Gated ogee section about 530 feet long Ogee section with eleven 40’ x 28’ tainter gates Structure includes concrete non-overflow transition

abutment sections at each end of overflow section, a spillway chute, and a hydraulic jump-type energy dissipating stilling basin.

Design Probable Maximum Flood

320,000 cubic feet per second

Low Flow Outlet Approx. 8’-4” x 12’-0” incorporated into spillway Two 20” dia. pipes through the spillway provide flow to meet 144 cfs requirement of the Power Sales Agreement

Dam Foundation Layers of sand, silt and slickensided clays Powerhouse Located at the south abutment Contains two 41.5 MW vertical Kaplan units rated at a

head of 60.8 feet Flow approx. 15,000 cfs Approach channel lined with a concrete approach slab for

50 feet. Discharge channel lined with riprap for several hundred feet from concrete structure

Reservoir Surface Area 185,000 acres Reservoir Normal Storage Volume

4,477,000 acre-feet

Earthquake Hazard Low Hazard Rating High

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Figure 2.1-01Toledo Bend Vicinity Map

February 2005

Schematic Plan Of Site

February 2005

2004 FERC INSPECTION REPORTTOLEDO BEND DAM

Figure 2.1-02.dwg

Spillway Plan

February 2005


Figure 2.1-03.dwg

NOTE:

ALL THE SECTION VIEW CUTS INDICATED ON THIS DRAWING ARE BASED ON THE A TYPICAL SECTION THROUGH THE SPILLWAY, THEREFORE INTERNAL FEATURES MAY NOT BE TRUE REPRESENTATION OF THE ACTUAL SECTION

Spillway Typical Section

February 2005


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Powerhouse Plan and Sections

February 2005


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NOTE:

ALL THE SECTION VIEW CUTS INDICATED ON THIS DRAWING ARE BASED ON THE A TYPICAL SECTION THROUGH THE POWERHOUSE, THEREFORE INTERNAL FEATURES MAY NOT BE TRUE REPRESENTATION OF THE ACTUAL SECTION

Typical Embankment Sections

February 2005


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2.2 HAZARD POTENTIAL CLASSIFICATION

2.2.1 Rating

Toledo Bend Dam is a high risk dam. If a failure were to occur, the loss of human life is a strong probability. There have been no changes upstream or downstream of the Toledo Bend Dam that would affect the hazard potential classification.

2.2.2 Dam Break Analysis

2.2.2.1 A report on the effects of a hypothetical dam break was made in June 1989 in conjunction with an emergency action plan for the Toledo Bend Dam. Refer to the Emergency Action Plan released by the TBPJO and dated April 4, 1989.

2.2.2.2 A dam-break flood forecasting model (DAMBRK, D.L. Fread, June 1988, National Weather Service) was used to calculate the outflow from a dam failure and to route the flood through the downstream valley. Data from the National Weather Service (NWS) “Breach” program was used to evaluate the breach parameters entered into the DAMBRK program. The microcomputer version of the DAMBRK program, dated June 20, 1988, was utilized for the study. U.S. Army Corps of Engineers HEC-1 Flood Hydrograph Package was used to route the high flow 1945 flood through the Toledo Bend Dam and Reservoir.

2.2.2.3 Various types of breaks were examined in conjunction with different river flow rates in evaluating dam breaks. A dam break in conjunction with the PMF was calculated to produce the maximum flow of 1,030,323 cfs at Bon Weir, Texas and Merryville, Louisiana, 39 river miles downstream from the dam.


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2.3 SUMMARY OF STANDARD OPERATING PROCEDURES

2.3.1 Purpose of Project

The Toledo Bend Reservoir is operated for hydroelectric power generation and water conservation by the Toledo Bend Project Joint Operation (TBPJO), which is a combined project of the Sabine River Authority (SRA), State of Louisiana, and the Sabine River Authority of Texas. When constructed, it was the largest man-made body of water in the South, the fifth largest in surface area in the United States, and the nation’s only public water conservation and hydroelectric power project undertaken without federal participation in the permanent financing.

2.3.2 Reservoir Rule Curves by Season

The reservoir levels are adjusted as required by the power sales agreement. See Table 2.3.1 in paragraph 2.3.3, Standard Gate Operating Procedures, for a complete presentation.

2.3.3 Standard Gate Operation Procedures

a. Non-Flood Releases. The normal maximum water level is at elevation 172.50. Releases from the powerhouse are made as required by the power sales contract and as flood levels and reservoir levels dictate. Releases are made at a typical rate of 15,000 cfs to a maximum rate of about 18,000 cfs, depending on lake and tailwater levels. A low flow release of 144 cfs (required for environmental reasons) is continuously made from the low flow release structure in the spillway.

b. Flood Releases. 1) Flood releases are defined as those releases made when

the reservoir water level exceeds 172.50. When the reservoir level exceeds 172.50, flood releases are made from the spillway by opening spillway gates and by operating the hydroelectric units at full gate. Reservoir water levels and inflow to the reservoir influence gate openings. Inflow is calculated by changes in lake level, stream flow, and rainfall measurements in the upstream watershed.

2) The gate-opening schedule requires gates to be adjusted as needed to satisfy the schedule. The required gate openings are determined by the reservoir level and are influenced by inflows. A new gate opening schedule for flood releases was developed in 1994, as was a copy of the report titled, “Spillway Gate Operating Evaluation”. The gate operation schedule is provided in Table 2.3.1. Gate operation


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procedures were modified slightly in 1998 because of determinations made Brown & Root in the report titled “Operating Guidelines Re-Evaluation Study.”

Table 2.3.1 Existing Operating Guide Rule

Hydro-Electric Power Plant - Toledo Bend Reservoir Criteria: Minimum release of 144 cfs (286 ac-ft/day).

Minimum average monthly flow of 1500 cfs at Ruliff, Texas during October through April; 3000 cfs during May through September.

Approx. 1,058,000 ac-ft released from May through September. Annual average power generation of 205,000,000 KWH. Minimum level of 162.2 for power generation. Level above which gates are opened: 172.5 ft MSL Guide Rule Curve Prime Power Secondary Power

Below Power * Above Power

Full Capacity = 30,000 ac-ft/ day (930,000 ac-ft/month) Month

Level (in ft.) (ac-ft)

Level (in ft) (ac-ft)

==> 1.0 foot in 6 days, 5.0 ft/month **

January - 168.5 Up to Full February - 169.0 Up to Full

March - 169.5 Up to Full April - 170.0 Up to Full May Any 113,000 172.0 Up to Full 113,000 = 3,767 ac-

ft/day (0.61 ft/month) June Any 115,000 172.3 Up to Full 115,000 = 3,833 ac-

ft/day (0.62 ft/month) July Any 270,000 172.5 Up to Full 270,000 = 8,710 ac-

ft/day (1.45 ft/month) August Any 290,000 172.0 Up to Full 290,000 = 9,350 ac-

ft/day (1.57 ft/month) September

1-15 Any 135,000 171.0 Up to Full 135,000 = 4,355 ac-

ft/day (0.725 ft/month) September

16-30 Any 135,000 170.0 Up to Full 135,000 = 4,355 ac-

ft/day (0.725 ft/month) October - 168.0 Up to Full

November - 168.0 Up to Full December - 168.0 Up to Full

Total 1,058,000

* Numbers derived from the 1994 B&R Report. Actual releases are variable depending upon upstream and downstream conditions and upon power needs. ** One foot in 6 days assumes no inflow to reservoir.


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2.3.4 Spillway Rating Curve

The spillway rating curve is shown on the attached Figure 2.3-01. The rating curve shows what discharge could be expected with the eleven gates fully open at different reservoir surface elevations.


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2.4 MODIFICATIONS CONDUCTED FOR PROJECT SAFETY

2.4.1 Two modifications have been conducted for project safety considerations in addition to those related to security. Security issues are not discussed in this report.

2.4.2 Additions and modifications to the instrumentation monitoring program have been performed. The changes in instrumentation are described in the Supporting Technical Information, Section 3.6, Major Dam Safety Modifications.

2.4.3 Installation of French drains has been made to address minor seepage issues. These additions are described in the Supporting Technical Information, Section 3.6, Major Dam Safety Modifications.


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2.5 FLOOD HISTORY

2.5.1 Flood of Record, Probable Maximum Flood (PMF), and Inflow Design Flood (IDF)

2.5.1.1 Flood of Record 1) The gated spillway has eleven 40’ by 28’ tainter gates that

are operated to make flood flow releases. The spillway was designed to safely pass the probable maximum flood with a discharge rate of 320,000 cfs with the reservoir water level at 178.40.

2) Flows on the Sabine River have been recorded by the USGS at Burkeville, Texas below the Toledo Bend Dam since September 1955. The USGS has also maintained a flow gage on the Sabine River near Milam at river mile 194.8 (Toledo Bend Dam is at river mile 156.5). The period of record is from October 1923 to September 1925 and from January 1939 until the Toledo Bend Reservoir inundated the site. The maximum flow of record at the Milam gage was 83,400 cfs and occurred on April 12, 1945.

3) The maximum inflow into the reservoir during the January 1999 flood was calculated to be 172,783 cfs. The actual river flow rate, if the reservoir would not have been in place, would not have been greatly different. The second largest inflow into the reservoir is 235,000 cfs and was recorded in May 1989 in association with major releases from the dam made at that time.

2.5.1.2 Inflow Design Flood: One PMF analysis was made during initial design in 1961 and is found in Design Memorandum No. 1 by Forrest & Cotton. The peak inflow for the spillway design flood was 554,000 cfs based on 6-hour values.

2.5.1.3 Probable Maximum Flood 1) A second PMF analysis was made in 1988 and is

documented in the report prepared by Brown & Root titled, “Probable Maximum Flood Analysis, Toledo Bend Dam and Reservoir” dated June 1988. That report, which has been forwarded to the FERC, calculated the maximum inflow to be 759,000 cfs based on the maximum 6-hour values for the PMF.


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2) The analysis, assumptions, results, probable maximum precipitation distribution, the watershed model for converting rainfall to runoff along with runoff and flood routing are present in the Probable Maximum Flood Analysis, 1988, as referenced.

2.5.2 Zero Freeboard Spillway Capacity The report, “Probable Maximum Flood Analysis” calculated the maximum still water surface level during the PMF at elevation 178.48. The embankment crest is at elevation 185.00. Calculations showed that, with a sustained wind from a particular direction and with calculated set-up and wave run-up, waves would cause some flow over the embankment crest. The calculated over-the-crest flows caused by waves were judged by the Consultant to be acceptable at that time and were reported as such to the FERC. No additional calculations have been made since 1988.

2.5.3 Peak Spillway Discharge During Previous 5-Year Period The peak spillway discharge during the previous 5 years occurred in May 1989 and was 118,350 cfs.

2.5.4 Peak Reservoir Elevation During Previous 5-Year Period

The peak reservoir elevation during the previous 5 years also occurred in May 1989 and was 173.93 feet.


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2.6 CONCLUSIONS

2.6.1 Background

As indicated in this section, the reservoir is operated for hydroelectric power generation and water conservation by the Toledo Bend Project Joint Operation (TBPJO). The TBPJO is made up by the Sabine River Authority, State of Louisiana and the Sabine River Authority of Texas. The Toledo Bend Dam is rated as a High Risk Dam – if a failure were to occur, the loss of human life is a strong probability. There have been no changes upstream or downstream of the Toledo Bend Dam that would affect the hazard potential classification. No significant modifications have been made for project safety considerations other than those related to security.

2.6.2 Releases

The dam generates an average annual power output of 205,000,000 KWH. The minimum reservoir level elevation for power generation is 162.20 and the established normal maximum water level is 172.50 at which time the spillway gates are opened. Releases made when the reservoir water level exceeds 172.50 are defined as “flood releases”.

2.6.3 Discharges Calculated for Probable Maximum Flood

The spillway was designed to safely pass the probable maximum flood with a discharge rate of 320,000 cfs; at a reservoir water level of 178.40. To date, two evaluations have been conducted to identify the probable maximum flood (PMF) elevation. The first evaluation was performed during initial design in 1961 and is found in Design Memorandum No. 1 by Forrest & Cotton. The peak inflow for the spillway design flood was 554,000 cfs based on 6-hour values. The second PMF analysis was conducted in 1988 and estimates a maximum inflow of 759,000 cfs at the maximum 6-hour values for the PMF. These values came from the report prepared by Brown & Root titled “Probable Maximum Flood Analysis” dated June 1988. The maximum still water surface level during the PMF was calculated to be 178.48. The embankment crest is at elevation 185.00. At this water level elevation, calculations show that waves generated during sustained winds would result in flow over the embankment crest; however, the calculated over-the-crest flows were judged to be acceptable at the time of the evaluation. No additional calculations have been made since 1988.

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