WESLEY E. SEALE DAM FIELD TRIP

BACKROUND INFORMATION

Present: Wesley E. Seale Dam at Lake Corpus Christi Reservoir serves as the primary water source for residents in the Corpus Christi and South Texas region. Daily releases of water range from approximately 65 million gallons during the winter months to 110 million gallons during the summer months. The dam operates 24 hours a day, 365 days a year and is staffed by 12 individuals assigned to the following areas: operations (4), maintenance (5), inspections (2), and reservoir supervisor (1).

Duties of City personnel at Wesley E. Seale Dam at Lake Corpus Christi Reservoir include the sanitary patrol and environmental inspection of some 18,800 acres of lake surface, upkeep of all rip- rap, check dams, earthen embankments, and the maintenance of City-owned land and facilities. Environmental inspection personnel are also responsible for the inspection of septic systems and other facilities constructed under water permits such as piers, bulkheads, excavation, recreation units, and domestic withdrawals from the lake, picking up water samples for analysis within the lake and at various points on the Nueces, Frio, and Atascosa rivers, and periodic inspection of all oil and mining operations in the watershed.

History: The Lower Nueces River Water Supply District (LNRWSD) was created by State legislation for the purpose of providing a larger raw water reservoir for Corpus Christi and the South Texas Region. Two bond issues were voted upon in 1952 and 1957 for $15,500,000 and $2,500,000, respectively, for this reservoir project. Later, an additional $3,000,000 bond issue was financed through the Texas Water Development Board to complete the land acquisition and relocations. This loan, plus other revenue, financed the dam at a total cost of $21,750,000. (This equates to an investment in current dollars of approximately $133,000,000.) Wesley Seale Dam was completed in 1958. This structure is located some 1,000 feet downstream from the previous "La Fruta" dam built in 1935; it is twenty feet higher, which inundates the original dam.

The dam was named in honor of Wesley E. Seale, former mayor and city council member, and Chairman of the Lower Nueces River Water Supply District. The dam is an "Ambursen" type and was originally equipped with counterweight operated crest gates. It consists of two concrete spillways with a combined length of some 2,400 feet and 3,580 feet of earthen embankment. The spillways contain a total of 60 crest gates. The dam has a normal pool elevation of 94 feet.

The City managed daily operations and maintenance, while the Lower Nueces River Authority managed major projects. The City was obligated to repay these funds by the purchase of water from the reservoir over a 30 year period. At the end of fiscal year 1985-86, the total indebtedness for the dam was paid in full. As a result, the Lower Nueces River Water Supply District was disestablished and the dam and all District-owned property was transferred to the City.

During the first six years of operation, the dam was maintained at 88 feet, or a capacity of 185,922 acre-feet, to allow for depletion of oil fields located in the reservoir basin. The crest gates were closed on July 1, 1964, bringing the lake elevation to 94 feet, thereby increasing the lake capacity to 302,000 acre-feet.

In September 1967, an ordinance was passed making the land area downstream between the dam and State Highway 359, a wildlife and bird sanctuary. This land was formerly owned by the Lower Nueces River Water Supply District and is located in San Patricio and Jim Wells counties.

The outlet works are located at the south end of the structure and the gates are used in conjunction with the 48-inch "Bunger" valve to make normal releases by way of the Nueces River downstream to the O. N. Stevens Water Treatment Plant. The reservoir at elevation 94 feet was predicted to satisfy the water demand for the area until the early 1980's, with an estimated yield of some 130,000 acre-feet per annum. Siltation has reduced the current yield to an estimated 113,000 acre-feet per annum, and storage capacity to 241,241 acre-feet.

Crest Gate Failure: After the crest gates were raised in July, 1964 the lake filled to maximum capacity on October 5, 1964. Later, on February 23, 1965, some of the automatic gate mechanisms consisting of counterweights, cables, and eccentrics were destroyed by surge action resulting from high waves caused by a severe cold front. Consequently, the lake immediately emptied to the top of the crest beam (88 foot elevation). The Lower Nueces River Water Supply District commenced planning the necessary repairs and alterations.

The LNRWSD engaged the consulting firm of Forrest and Cotton to redesign the crest gates and their hoisting mechanisms. The engineering firm proposed a change from the cable and counterweight type of operation to a positive screw type hoist that would utilize either electrical motors, auxiliary gasoline engines, or manual power for operation. Controls for the hoisting equipment was set at a minimum operating speed to avoid rapid gate movement during operation. Forrest and Cotton's engineers further recommended that the gate sections be strengthened to withstand the force of high wave action.

Plans were completed and a contract was awarded to Gardner Engineering Corporation of Houston, Texas, on November 30, 1965 in the amount of $1,108,000. The gates were raised and in position to store water on September 7, 1966.

The 27 gates located on the south service spillway of the dam were equipped with a positive screw type hoist with electric motors for operation. The motors can be automatically operated from a central control panel in the control tower. A diesel operation generator serves as a standby electrical power source and, in addition, all gates can be hoisted manually or by auxiliary portable gasoline engine power.

Crest Gate Rehabilitation: Inspections of the dam are generally conducted on an annual basis. The inspections determined that a major rehabilitation of the spillway crest gates and associated equipment needed to be carried out on a priority basis. In August, 1995, a contract in excess of $4.4 million was awarded to Abhe & Svoboda, Inc. for crest gate rehabilitation. Rehabilitation efforts included removal of lead based paint and sandblasting gate surface for rust removal. The gates were resurfaced with a polyurethane moisture resistance coating. The gate arms were replaced and the gates were strengthened to withstand potential loadings for probable maximum flooding safety criteria established by the Texas Natural Resource Conservation Commission (TNRCC). The work was designed and overseen by the engineering consulting firm, Freese and Nichols, Inc. based in Fort Worth. The structural work on the gates was recently completed, and electrical system renovation for the south spillway is near completion.

Wesley Scale Dam Stability Analysis: Periodic inspections of Wesley Scale Dam over the years have observed a number of small anomalies that, taken collectively, suggested the need for investigation in greater depth. Anomalies observed included spalling of concrete at some buttresses, seepage at the downstream base of the north embankment, and a 3-inch "heave" in the flat section of the spillway below the Dam. In the annual inspection report issued in April 1993, Russell-Veteto engineering inspectors recommended that an engineering consultant be engaged for a special study of a heave observed in the horizontal concrete slab which serves as the north spillway adjacent to the dam. (The spillway slab's function is to absorb the energy from water falling on it when water flows over the north spill gates.) Freese and Nichols, Inc. was awarded a contract in October, 1996 for spillway data collection, inspection, and structural/stability analysis services. Alignment surveys conducted in December, 1996 and January, 1997 indicated some deflection of the concrete structure, and this confirmed that a thorough review of the dam's stability should be promptly undertaken.

Freese and Nichols proceeded with a comprehensive engineering study. Preliminary soil testing was completed in April 1997, allowing the firm to complete a preliminary stability analysis. Freese and Nichols' representative met with City Manager Bill Hennings on April 24 and provided an oral report that the safety factor of the dam was probably not as high as it should be, and that a number of additional actions should be initiated. The City Manager authorized and directed the consultant to continue expeditiously.

Based on the results of a soils test in April, 1997 and a subsequent estimate of the safety factor for the stability of the dam, the City's outside engineering consultants recommended on May 25, 1997 that the reservoir level at Lake Corpus Christi Reservoir not be allowed to exceed 91 feet as an interim measure. The consultants recommended that a detailed engineering investigation be undertaken to determine what modifications can be made to improve the safety factor for the dam so that the reservoir may be allowed to fill to 94 feet.

Today, spillway gates are set at 94 feet which is the reservoir pool level. The lake level limit, however, is set at 91 feet by controlled water releases via the "Bunger" valve. If heavy inflows occur such that the lake level can not be maintained by the "Bunger" valve, the gates are lowered. An independent consultant, John Focht, P. E., reviewed the work of Freese and Nichols and agreed with the recommendation to operate the reservoir at a maximum level of 91 feet. The dam safety section of the Texas Natural Resource Conservation Commission also concurred with this lower lake level strategy.

On May 20, 1997, the City Council ratified the City Manager's action to authorize the consultants to proceed expeditiously and on June 24, the City Council authorized the consultants to continue the work so that there will be no delays awaiting approval of the formal contract amendment. The amendment to the contract with Freese and Nichols for the installation of present instrumentation was approved by the City Council on July 15, 1997.

Seventeen extensometers were installed to monitor horizontal and vertical movements of select buttresses in the north and south spillways. The relative horizontal displacement of the spillway foundation in the north and south spillways is monitored by four inclinometers. In addition to the twenty existing piezometers, 51 piezometers have been installed to monitor pore water pressure.

The bulge in the north spillway hearth floor does not impair the function of the dam or the spillway in any way, nor, will it be the principle focus of future dam stability improvement efforts. Future construction repairs will be made by either adding weight to increase sliding friction or by adding restraining anchors to increase the sliding factor of safety.

ABUTMENT - That part of a valley side against which a dam is constructed. An artificial abutment is sometimes constructed as a concrete gravity section, to take the thrust of an arch dam where there is no suitable natural abutment. Right and left abutments are those on respective sides of an observer looking downstream.

AXIS OF DAM - A plane or curved surface, arbitrarily chosen by a designer, appearing as a line in a plan or cross section to which the horizontal dimensions of the dam can be referred.

BLANKET (Drainage Blanket) - A drainage layer placed directly over the foundation material.

Upstream Blanket - An impervious layer placed on the reservoir floor upstream of a dam. In the case of an embankment dam, the blanket may be connected to the impermeable element in a dam.

BUNGER VALVE - A valve which is designed to control the release of water. Approximately, 98 percent of the - water at Wesley Scale Dam is released through the 48 inch Bunger Valve.

BUTTRESS DAM - A dam consisting of a watertight upstream face supported at intervals on the downstream side by a series of buttresses.

CREST GATE - A gate on the crest of a spillway to control overflow or reservoir water level. Crest gates are used at Wesley Scale Dam.

CREST OF DAM - The crown of an overflow section of a dam. In the United States, the term "crest of dam" is often used when "top of dam" is meant. To avoid confusion, the terms "crest of spillway" and "top of dam" may be used to refer to the overflow section and the dam proper, respectively.

DRAINAGE WELLS (Relief Well) - A vertical well or borehole, usually downstream of impervious cores, grout curtains, or cutoffs, designed to collect and direct seepage through or under a dam to reduce uplift pressure under or within a dam. A line of such wells forms a drainage curtain.

EARTH DAM OR EARTHFILL DAM - See Embankment Dam.

EMBANKMENT - A slope of fill material, usually earth or rock, that is longer than it is high. The sloping side of a dam.

Embankment Dam (Fill Dam) - Any dam constructed of excavated natural materials or of industrial waste materials.

EMERGENCY ACTION PLAN - A predetermined plan of action to be taken to reduce the potential for property damage and loss of lives in an area affected by a dam break.

EXTENSOMETER - Consists of a anchor, rod, and a reference head. The anchor with rod attached is installed down a hole. The reference head is installed at the bore hole collar. The rod spans the distance from the anchor to the reference head. A change in this distance indicates that a change in ground movement has occurred.

FACE - The external surface of a structure, e.g., the surface of a wall of a dam.

FAILURE - The uncontrolled release of water from a dam.

GATE - A device in which a leaf or member is moved across the waterway from an external position to control or stop the flow.

Crest Gate (Spillway Gate) - A gate on the crest of a spillway to control overflow or reservoir water level. Crest gates are used at Wesley Scale Dam.

HYDRAULIC HEIGHT - The height to which water rises behind a dam and the difference between the lowest point in the original streambed at the axis of the dam and the maximum controllable water surface.

INCLINOMETER (Inclometer) - An instrument, usually consisting of a metal or plastic tube inserted in a drill hole and a sensitized monitor either lowered into the tube or fixed within the tube. This measures at different points the tube's inclination to the vertical. By integration, the lateral position at different levels of the tube may be found relative to a point, usually the top or bottom of the tube, assumed to be fixed. The system may be used to measure settlement.

MINIMUM OPERATING LEVEL - The lowest level to which the reservoir is drawn down under normal operating conditions.

NORMAL WATER LEVEL - For a reservoir with a fixed overflow sill the lowest crest level of that sill. For a reservoir whose outflow is controlled wholly or partly by movable gates, siphons or other means, it is the maximum level to which water may rise under normal operating conditions, exclusive of any provision for flood surcharge.

OUTLET - An opening through which water can be freely discharged from a reservoir.

PHREATIC SURFACE - The free surface of groundwater at atmospheric pressure.

PIEZOMETER - An instrument for measuring pore water pressure within soil, rock, or concrete.

PIPING - The progressive development of internal erosion by seepage, appearing downstream as a hole or seam discharging water that contains soil particles.

RIPRAP - A layer of large uncoursed stones, broken rock, or precast blocks placed in random fashion on the upstream slope of an embankment dam, on a reservoir shore, or on the sides of a channel as a protection against wave and ice action. Very large riprap is sometimes referred to as armoring.

SEEPAGE - The interstitial movement of water that may take place through a dam, its foundation, or its abutments.

SLUICEWAY - An opening at a low level from a reservoir generally used for emptying or for scouring sediment and sometimes for irrigation releases.

SPILLWAY - A structure over or through which flood flows are discharged. If the flow is controlled by gates, it is a controlled spillway; if the elevation of the spillway crest is the only control, it is an uncontrolled spillway.

Auxiliary Spillway (Emergency Spillway) - A secondary spillway designed to operate only during exceptionally large floods. The north spillway at Wesley Scale Dam is an auxiliary spillway.

Primary Spillway (Principal Spillway) - The principal or first-used spillway during flood flows. The south spillway at Wesley Scale Dam is the principal spillway used during flood flows.

STILLING BASIN - A basin constructed to dissipate the energy of fast-flowing water, e.g., from a spillway or bottom outlet, and to protect the river bed from erosion.

UPSTREAM BLANKET - A drainage layer placed directly over the foundation material.

WATERSTOP - A strip of metal, rubber, or other material used to prevent leakage through joints between adjacent sections of concrete.

WEIR - (a) A low dam or wall built across a stream to raise the upstream water level, termed fixed-crest weir when unconuolled. (b) A structure built across a stream or channel for the purpose of measuring flow, sometimes called a measuring weir or gauging weir. Types of weir include broad-crested weir, sharp-crested weir, drowned weir, and submerged weir.