Extended drought conditions in the southeastern portion of the United States caused Lake Okeechobee to fall to historically low levels. This resulted in potentially large head to tailwater differences at water control structures operated by the South Florida Water Management District (District). Heavy flows of water, such as those caused by a hurricane, could undermine the existing structures and threaten their stability. The District elected to construct weirs downstream of three water control structures in order to eliminate the threat. An emergency authorization of $25 million was allocated for the protective measures.
Of particular concern was Structure S-65E which had developed a scour hole 200 feet by 200 feet by 10 feet deep downstream of the structure. A soil exploration and design program was instituted with the goal of constructing a protective weir prior to the beginning of hurricane season. The initial concept involved two sheet piling walls across the Kissimmee River in conjunction with four filled circular sheet piling cells and a soil embankment extending to the shores. The soil boring and testing program identified soft soil which would cause the embankments to fail. An alternate system using eight filled circular sheet piling cells was designed and prepared for bid within a two week period. The final design included eight filled circular sheet piling cells, two underwater sheet piling weirs, and two massive tremie concrete installations. The project was bid and awarded, and the contractor completed the project on schedule.
Over the last several years, Lake Okeechobee has experienced record low water levels because of an extended drought. One consequence of the low water levels is the reduced functionality of the water control structures discharging into the lake. As designed, hydraulic structures function within a certain range of headwater and tailwater elevations. Low tailwater elevations (on the downstream or lakeside) can reduce a structure’s capability to safely discharge high flows. Discharging flows with low tailwater elevation would result in large scour holes, which could possibly undermine the structure. The potential mechanism for erosion is shown schematically in Figure 1. Of particular concern was Structure S-65E which is shown in Figure 2. A scour hole 200 feet long by 200 feet wide and 10 feet deep had developed downstream of the structure. Failure of a water control structure can have disastrous effects because it could lead to lower tailwater elevations upstream, possibly causing a chain reaction of failures. The worst case could lead to failure of structures as far as 75 miles upstream with resultant catastrophic flooding of residential and agricultural property. Conceptual design of the preventive measures began in January 2008. Completion of the project was extremely urgent because hurricane season, with its associated heavy rainfall, starts at the beginning of June.
Because of the urgency of the project, a conceptual design was developed prior to completion of a site specific geotechnical investigation. Two soil borings drilled approximately nine years earlier, and located several thousand feet distant, were used for the conceptual phase of the design. A site specific investigation was conducted simultaneously with development of the conceptual design.
The anticipated preventive measures involved construction of a weir composed of two sheet piling walls approximately 90 feet apart and extending across the river, a distance slightly more than 200 feet. Between the two walls there would be a concrete slab capable of withstanding the energy of water flowing over the upstream sheet piling wall.
At the two ends of the weir, on the two shores of the river, abutments would be constructed of filled circular sheet piling cells. Each abutment would consist of two cells and one connecting arc. Earthen embankments were anticipated for closing the remaining distance between the cellular abutments and the land.