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

The hydraulics of side-channel spillways can be complex due to numerous potential flow regimes at the varying flow rates, energy losses from a wide variety of physical conditions, and the complexities associated with spatially varied flow with increasing discharge within the side channel. The analysis can become even more complex in the event that water levels in the side-channel begin to cause tailwater impacts or full submergence of the upstream spillway. Lake Skinner is owned and operated by the Metropolitan Water District of Southern California (Metropolitan) and this existing dam spillway utilizes a unique side-channel configuration that includes all of the above mentioned complexities. When originally designed in the 1960s, a reduced scale physical model of the ultimate spillway configuration was constructed and utilized along with mathematical computer models to predict how the spillway would operate under various flow conditions. Since the dam was constructed 40 years ago, the upstream watershed has realized changes in land use resulting in potential changes to the drainage basin runoff characteristics. In 2006, Metropolitan began to investigate if these changes to the drainage basin could affect the adequacy of the Lake Skinner dam spillway and retained Black & Veatch hydraulic engineers and hydrologists to perform the evaluation. Although the reduced scale physical model and mathematical computer model from the original design where not available, reports documenting the past findings were located. Utilizing the documented characteristics of the spillway as calibration data, modern day computer tools were utilized to construct a computer model of the complex spillway 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, and ogee spillway evaluations procedures from the U.S. Bureau of Reclamation's Design of Small Dams (1987). A Visual Basic Program integrated with an Excel Spreadsheet was developed to link the input and output variables of the various modeling packages and to perform logical iterations as required to converge on realistic results.