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Title: General design for replacement of or modifications to the Lower Santa Ana River drop structures, Orange County, California : hydraulic model investigation
Authors: United States. Army. Corps of Engineers. Los Angeles District
George, John F.
Pickering, Glenn A.
Turner, Herman O.
Keywords: Drop structures
Santa Ana River
Flood control
Hydraulic structures
Grade control structures
Stilling basins
Hydraulic model investigation
Publisher: Hydraulics Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: Technical report (U.S. Army Engineer Waterways Experiment Station) ; HL-94-4.
Description: Technical Report
Abstract: The lower basin of the Santa Ana River is located in Orange County, California, and is 30.5 miles in length. Flood protection for the lower basin is provided by Prado Dam and Reservoir. When completed in 1941, Prado Dam was designed to control a 200-year flood. However, recent projections have shown that Prado Dam now offers only a 70-year flood protection based upon the followfng factors: urbanization in the drainage area, additional historical data on rainfall and runoff, and advances in predicting future flood potential. According to Santa Ana River hydraulic studies, the existing lower basin channel is inadequate to convey a large flood flow safely to the ocean. Existing flood control improvements built by local interests have reduced damages from small floods. As part of these channel improvements there are 11 existing vertical concrete drop structures. However, these improvements fail to provide protection for the highly urbanized lower Santa Ana River floodplain. About 2 million people live and work in this floodplain. Since additional channel right-of-way is not available because of urbanization, increased channel capacity can only be achieved by additional depth of flow. The increased channel flow will require drop structures designed for higher unit discharges. An improved channel design will require the removal or modification of the existing drop structures in order to implement a new drop structure configuration. The first part of this report presents the results of a model study conducted at the U.S. Army Engineer Waterways Experiment Station (WES) to investigate the possibility of designing new vertical drop structures and sloping grouted stone drop structures to replace the existing structures. The results of this model study showed that drop structures can be designed to adequately convey the increased discharges expected. Section models of drop structures constructed at a scale of 1:25 were used to determine if preliminary designs were adequate, and if not, then what modifications to the structures would be needed. Vertical drop structures with changes in elevation of the channel invert of 8, 10, and 12 ft were tested. Initial tests on the preliminary designs showed than an oscillating hydraulic jump would form at discharges expected during a flood event. Various modifications were tested on these preliminary designs without success. The drop structures were then redesigned to incorporate elements based on the sequent depths of the hydraulic jump. Modifications tested included a parabolic drop, one row and two rows of baffle blocks, and a sloping end sill. Sloping drop structures constructed of grouted stone were also tested using a 1:25-scale section model and a 1:40-scale full-width model. The initial design incorporated a 1V on 10H slope, which produced unsatisfactory results. Test results indicated that with a 1V on 2H slope, a satisfactory hydraulic jump would occur for the various flow conditions observed. The second part of this report concerns model studies conducted at WES to investigate the possibility of using the existing drop structures of the Santa Ana River. The results of these model studies showed that the existing drop structures will not adequately convey the increased discharges expected. Unless modification or replacement measures are taken, severe scour to the streambed and even structural failure could result. Several 1:25-scale section models of selected drop structures and a 1:25-scale full-width model were used to determine if the existing structures were adequate, and if not, then what modifications to the structures would be needed. A general design was developed from these tests for the other existing drop structures on the Santa Ana that were not model tested. Initial tests on the existing drop structures showed that an oscillating hydraulic jump would form at discharges expected during a flood event. A sequent depth analysis using guidance from a previous study was performed on each drop structure tested. From these sequent depths, dimensions of the modifications were sized. Modifications tested included a parabolic-shaped drop downstream from the crest, one row and two rows of baffle blocks, and a sloping end sill. The resulting general drop structure design is based upon energy head and the sequent depths. This general design has application for all drop structures that have a Froude number of less than 4.5.
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Technical Report

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