Side-Channel Spillway Hydraulics (Case Study: Lake Skinner Spillway Adequacy Evaluation)

摘要

The hydraulics of side-channel spillways can be complex due to numerous potentialflow regimes at the varying flow rates, energy losses from a wide variety of physicalconditions, and the complexities associated with spatially varied flow with increasingdischarge within the side channel. The analysis can become even more complex inthe event that water levels in the side-channel begin to cause tailwater impacts or fullsubmergence of the upstream spillway.Lake Skinner is owned and operated by the Metropolitan Water District of SouthernCalifornia (Metropolitan) and this existing dam spillway utilizes a unique sidechannelconfiguration that includes all of the above mentioned complexities. Whenoriginally designed in the 1960s, a reduced scale physical model of the ultimatespillway configuration was constructed and utilized along with mathematicalcomputer models to predict how the spillway would operate under various flowconditions. Since the dam was constructed 40 years ago, the upstream watershed hasrealized changes in land use resulting in potential changes to the drainage basinrunoff characteristics.In 2006, Metropolitan began to investigate if these changes to the drainage basincould affect the adequacy of the Lake Skinner dam spillway and retained Black &Veatch hydraulic engineers and hydrologists to perform the evaluation. Although thereduced scale physical model and mathematical computer model from the originaldesign where not available, reports documenting the past findings were located.Utilizing the documented characteristics of the spillway as calibration data, modernday computer tools were utilized to construct a computer model of the complexspillway system.The computer modeling included the use of the Hinds side-channel spillway equation,the U.S. Army Corps of Engineers HEC-RAS and HEC-HMS modeling tools, andogee spillway evaluations procedures from the U.S. Bureau of Reclamation's Designof Small Dams (1987). A Visual Basic Program integrated with an Excel Spreadsheetwas developed to link the input and output variables of the various modelingpackages and to perform logical iterations as required to converge on realistic results.