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Toledo Bend Project Part 12D Section 1 Safety Inspection Report Executive Summary

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SECTION 1

EXECUTIVE SUMMARY

1.1 GENERAL

1.1.1 Formation of the Toledo Bend Project Joint Operation

In 1961 the Sabine River Authority of Texas and the Sabine River Authority, State of Louisiana entered into a contract to construct, by joint venture, the Toledo Bend Dam and Reservoir. The Toledo Bend Project Joint Operation (TBPJO) represents the combined efforts of the two authorities on this project. The dam and powerhouse were designed by Forrest & Cotton, Inc. Construction was completed in 1967. Deliberate impoundment began in October 1966 and the reservoir level reached normal pool elevation of 172.0 feet in May 1968.

1.1.2 Reservoir Location and Purpose

The reservoir is located on the Sabine River along the border of Texas and Louisiana in Newton, Sabine, Shelby and Panola Counties in Texas, and Sabine and DeSoto Parishes in Louisiana. The reservoir was designed for water supply and hydroelectric power generation. There was no allocation of storage for flood control purposes.

1.1.3 Dam Embankment Construction

The dam is a rolled earth-fill embankment with a concrete spillway. It is approximately 11,000 feet long, with a height of 108 feet at an elevation of 185 MSL. The old river channel was located near the south end of the embankment at the state line between Texas and Louisiana. There are three saddle dikes with features similar to the main embankment, all located in Texas. The three saddle dikes rise to elevation 185 MSL across saddles on the reservoir rim. The spillway is located near the left (north) abutment in Louisiana while the powerhouse is located near the right (south) abutment in Texas.

1.1.4 Erosion Protection

The crest of the embankment and dikes are paved with asphalt for erosion protection and for project access. The upstream faces of the embankment and dikes have soil cement slope protection for controlling erosion from wave action.


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1.1.5 Roadway 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 to just south of the Texas state line. The roadway turns southeastward becoming F.M. 692 in Texas, crossing over the powerhouse tailrace channel and continuing into Texas. A berm, which was constructed to accommodate the road, leaves the toe of the dam near the right abutment just north of the power plant.

1.1.6 Dam Embankment Drainage Systems

A system is located on the downstream face of the embankment consisting 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. A second separate drainage system, made up or relief wells with manhole covers, is located down slope from the road berm. These relief wells have outfall corrugated metal pipes with a bituminous coating extending to the drainage ditch at the downstream toe. They are not connected with lateral pipes and they do not have pumps. The 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 the water flow in the natural ground foundation.

1.1.7 Spillway Structure

The spillway structure is 838 feet long and consists of a gated ogee section about 530 feet long, containing eleven 40 foot by 28 foot tainter gates. The ogee crest is at elevation 145 feet MSL and the top of the spillway gates are at elevation 173 feet MSL. The spillway releases are channeled by the spillway training walls onto the chute slab and into a hydraulic jump stilling basin. An 8-4 by 12-0 gated conduit through the spillway at elevation 100 feet MSL may be used for low-flow releases. 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 and the FERC license. This minimum flow is normally met by releases through two 20-inch-diameter conduits that bypass the conduit gate and discharge into the conduit downstream of the gates.

1.1.8 Spillway Capacity for Passing Storm Flows

The spillway was designed with a total capacity of approximately 320,000 cfs with all gates open. The spillway has operated on several occasions


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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.

1.1.9 Powerhouse Design and Power Output

At the powerhouse, water enters the turbines through six 16-9 by 29-0 foot penstocks and 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. The two units are capable of releasing 30,000 acre-feet of water per day (approximately 16,000 cfs) to produce approximately two million kilowatt-hours (KWH) of electricity per day. Typical annual power production is approximately 205 million KWH, depending on the availability of water, to provide revenues for operation of the Reservoir.


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1.2 SUMMARY ASSESSMENT OF THE PFMA REPORT

Based on the results of the PFMA, the project was found to be in generally good condition, with good maintenance and operating procedures. Twelve potential failure modes (PFMs) were identified in the potential failure modes analysis of the Toledo Bend Dam Project. Of these, two Category I PFMs, five Category II PFMs, three Category III PFMs, and two Category IV PFMs were identified.

The following PFMs were identified as Category I: Stability of Spillway due to Failure of Low Flow Discharge Bypass Pipes Piping through Spillway

The following PFMs were identified as Category II:

Spillway Stability General Gate Failure in an Open Position Stability of Embankment Slope Piping through Embankment Stability of Powerhouse

The following PFMs were identified as Category III: Failure of Spillway Chute Slab Stability of Spillway due to Tailwater Overtopping of Embankment

The following PFMs were identified as Category IV: Gate Failure, Forced Open from a Closed Position Piping at Powerhouse

1.2.1 Category I PFMs

As listed above, the PFMA found two critical items (Category I) indicating an alternative method of passing the low flow through the spillway structure is needed to reduce the hazard for spillway stability problems from possible rupture of the low-flow bypass pipes in the spillway gallery. These pipes have had to be replaced in the past because of cavitation problems that were found before the pipes ruptured. The other critical item is the discovery of soil in a key drain discharge pipe; the soil type is pending classification. If the soil turns out to be clay, that would indicate a problem with the drainage system under the spillway structure.


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1.2.2 Category II PFMs

Additionally, the PFMA found the following items that should be performed or completed to improve safety and provide for monitoring or evaluation regarding the Category II PFMs of the project:

Inspect the spillway drain pipes, test the drainage system and monitor run-time on the relief well pumps to evaluate the condition and effectiveness of the spillway drainage system.

Confirm or provide a backup power supply for the relief well pumps at the powerhouse and provide a system to monitor pump run-time.

Evaluate the trunnion anchor capacities with one or two rod failures. For the embankment drainage and monitoring system, install barricades

around the piezometer stand pipes to prevent damage, install a measuring device and monitor flow from the drainage system, and inspect the drain pipe with video equipment.

1.2.3 Category III PFMs

The PFMA found the following items requiring calculations or equipment needed to determine the appropriate category for those PFMs placed in Category III:

Calculate the negative and positive pressures produced by high-velocity flows on the spillway slab during discharge through the spillway gates.

Calculate tailwater levels and velocities inside and outside the spillway stilling basin where erosion could jeopardize the dam or spillway during passage of large floods.

Calculate the reservoir levels for leaving some gates closed during passage of the PMF (to help evaluate the required operation during the PMF).

Provide a backup motor for gate operation and store it at the site for use during an emergency (failure of gate motor during a flood).


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1.3 SUMMARY OF FIELD INSPECTION FINDINGS

1.3.1 Embankments

The main embankment, Dike 1, 2 and 3 are all well maintained and appear to be in good condition. Minor seepage is occurring in some isolated areas. The soil-cement upstream embankment protection was exhibiting continued wear. However it was readily apparent that repairs had been made since the last Part 12D inspection. For the age of the structure, the soil cement is in fair condition, but maintenance efforts should continue as explained in the main body of the report.

1.3.2 Powerhouse

There were no unusual conditions observed at the powerhouse. The emergency action plan was current and readily available. The key to maintaining stability at the powerhouse is the continued operation of the pumped relief wells.

1.3.3 Spillway

The spillway continued to display a good amount of concrete spalling at the construction joints throughout the chute slab. Repairs should be scheduled and budgeted in the near term since the situation will only worsen every time a significant release is made.

1.3.4 Gate Motor Amperage Overload

The gate motor amperage overload issue needs to be resolved. It is currently under investigation by other parties. The gate trunnion arms and gate bracing need to continue to be cleared of water and debris on a regular basis to avoid continued corrosion. The rolled J-bulb at Gate 5 needs to be replaced when possible. Minor leakage at the other gate seals continues but is not a significant issue.

1.3.5 Low Flow Bypass

In the gallery, the 20-inch pipes that provide low flow by-pass need to be watched for signs of corrosion and cavitation damage, since there have been problems with the elbows in these pipe sections before. To date, they appear to be in good condition. Also, the continued build up of calcification on the gallery walls especially in conjunction with the minor spalling that occurred on the downstream side of the gallery wall should be monitored. This problem is not yet significant but should not be ignored.


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1.3.6 Spillway Underdrain System

The spillway heel underdrain system should be fully investigated as soon as possible in light of the clay-like sediment found in the key drain discharge pipes up on the chute slab. The underdrain pipe is a 6-inch perforated cast iron pipe that is surrounded by a filter of gravel, followed by a layer of Type I sand, and finally a filter of Type II sand. The spillway base material is a bentonite clay material. The underdrain pipe is subject to corrosion and is already 35 years old. The pipe may have failed in one or more areas. If clay-like material is exiting the discharge pipes, that would lead one to believe the sand material has already been lost in one or more areas and foundation material is now being lost through the discharge pipes. At this point we do not know what is happening, but the consequences could be very serious, thus the need for monitoring and investigation. The Independent Consultant has recommended continued monitoring for sediment in the discharge pipes. Attempts should be made to collect additional sample material for testing. If the problem persists, closed circuit television exploration of the pipes may be required.


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1.4 SUMMARY OF OPERATIONS & MAINTENANCE (O&M) STATUS

1.4.1 O&M Overview

a. TBPJO personnel seem to take their responsibilities very seriously, however they need to continue their vigilant upkeep of the facility. The communication and response procedures are adequate. There exists a certain level of redundancy with power provided via two separate circuits. A backup generator is also available for gate operations.

b. The report recommends a back-up motor for tainter gate operations be procured and stored nearby for emergency installation. This item is critical because there is some indication that the motors may be overloaded during gate operations. This is still under investigation, but acquiring a back-up motor is a simple precaution to take.

c. The human factors are addressed adequately. Changes to the O&M procedures based on the PFMA are listed in the following paragraph.

1.4.2 Corrective Actions for O&M

Corrective action should be considered for the identified PFMA operation and maintenance related items indicated in this section. The following recommendations should be considered for immediate implementation upon approval of this report. These items include, but are not limited to the following:

a. Improvements to the internal drain system of the dam embankment. b. Stability improvements related to liquefaction as recommended in

the 1983 report by Rone Engineering. c. Change data collection forms to allow field personnel to compare

newly collected piezometer data with previous measurements as new data is collected in the field to ensure immediate warning of potential problems or identification of inaccurate data measurements.

d. Construct barricades around piezometers to protect exposed portion of riser from mowers.

e. Calculate water velocities at various high tailwater levels in the stilling basin beyond the spillway training wall and downstream of the stilling basin.

f. Calculate downstream tailwater level for the PMF outflow and determine the effect on stability of the stilling basin.

g. Identify potential adverse effects due to the suction effect during high flows over the ogee crest.


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h. Procure backup gate operator motor to be stored on site to minimize gate operation downtime in the event a gate motor fails.

i. Confirm flow rate and nappe depth over crest of spillway gates. j. Calculate PMF elevation for scenario where one or two spillway

gates are closed and inoperable. k. Contact transportation departments of both states regarding issue

with highway speed limit for spillway bridge. l. Develop a way to restrict flow from the spillway gallery into the

underdrain system. m. Install check valves in well pump discharge piping to prevent

backflow through the pumps and back into the wells. n. Design and implement modifications for minimizing flow into the

spillway gallery to eliminate potential for flooding of the gallery (i.e., develop alternative methods of passing low flow through the spillway structure to reduce the hazard for spillway stability problems from possible rupture of bypass pipes in the spillway gallery).


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1.5 SUMMARY ASSESSMENT OF SUPPORTING TECHNICAL

INFORMATION

1.5.1 Available Information

In general a great deal of information is available regarding the Toledo Bend Project; much of it is assembled and summarized in the accompanying document, Supporting Technical Information (STI). Numerous studies have been performed regarding stability, gate operation procedures, and PMF evaluations, to name a few. No new calculations were conducted by the Part 12D Consultant for this report. Overall there is a great deal of information available in the STI document that should be helpful to anyone studying the facility.

1.5.2 Original Design Documentation

The TBPJO has done a good job in maintaining the original design drawings and design memorandums. As discussed in Section 7.0, some information is no longer available, or missing. For example, the design specifications, the original material testing reports, and the drawings for the Powerhouse could not be located.

1.5.3 Motor Load Test Data

The STI document requests motor load test data including amperage draw, line-line voltage, etc. This information is not collected by TBPJO staff currently.

1.5.4 Instrumentation History

In addition, it is difficult to trace the history of instruments that were initially installed but for some reason readings are not longer collected. It would be helpful to have a table that lists the instrument and the associated reason for discontinuance.


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1.6 CONCLUSIONS

The conclusions of findings from the 2004 Part 12 Inspection are presented in Section 5 Field Inspection, paragraph 5.5 and in Section 8 Recommendations.


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1.7 SUMMARY OF RECOMMENDATIONS

The recommendations from the 2004 Part 12 Inspection are presented in Section 8 Recommendations.

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