PREDICTED VS. MEASURED SLOPE EROSIONAT L-8 RESERVOIR DURING HURRICANE JEANNE

摘要

This paper presents calculated wave heights and predicted erosion of upstream leveernslopes at the Loxahatchee Reservoir (also termed L-8 Reservoir) during Hurricane Jeanne inrnSeptember 2004. The Loxahatchee Reservoir is located in western Palm Beach County,rnFlorida, and is a key component of the Comprehensive Everglades Restoration Plan (CERP),rna Federal/State project to improve water quality and the distribution of fresh water in southrnFlorida. The reservoir will assist in restoring historic flows to the Loxahatchee River, and willrnprovide additional water supply to the city of West Palm Beach.rnThe L-8 Reservoir was constructed during the period 2003-2008 in previously-minedrnlimestone quarry cells that were deepened by hydraulic dredging to El. –42 ft (NAVD-1988).rnPerimeter embankments were constructed of sandy overburden to El. +23 ft around thernreservoir to provide overtopping protection as well as additional water storage. The reservoirrnwas constructed by the quarry owner (Palm Beach Aggregates, Inc.) to specificationsrnestablished by the South Florida Water Management District (SFWMD), the owner andrnoperator of the reservoir facility. BCI Engineers & Scientists, Inc. was the Engineer of Recordrnfor the earthwork design and construction monitoring, and Phillips & Jordan, Inc. was thernGeneral Contractor.rnThe determination of freeboard required to prevent overtopping or excessive overwashrnduring major hurricanes, and the assessment of potential erosion due to wave action, havernbeen major design issues for CERP reservoirs. This is due to the major cost implications ofrnhigher embankment heights as well as the high cost of armoring interior slopes againstrnexcessive erosion caused by wind-driven waves. Computations of wave height, wave setup,rnrun-up, overtopping/overwash, and embankment erosion are used to meet these objectives.rnThe methodology used in this study includes application of the USACE (2006) CoastalrnEngineering Manual method for wave run-up and overtopping computation, the SWAN wavernmodel for wave distribution in the reservoir, and the SBEACH erosion model.rnDuring early mining activities at the quarry, a temporary north (Phase 1) perimeter leveernwas constructed, prior to the establishment of design criteria and construction specifications byrnSFWMD. This embankment had 3H:1V upstream slopes that were grassed. The embankmentrnfill was typical sandy overburden compacted by haul trucks and bulldozers. Prior to finalrnconstruction of this levee to SFWMD specifications, which included 4H:1V slopes, and soilrncement slope protection in critical areas, Hurricane Jeanne passed through the area and thernnorth interior slopes of the temporary perimeter levee experienced significant erosion due tornwave action.rnBased on hurricane wind direction, velocity, and time of passage data from the NationalrnOceanic and Atmospheric Administration (NOAA), which were used as input to the SWANrnmodel, the appropriate wave height distribution was input to the Coastal Engineering Model,