Impact of Artificial Reservoir Size on Probable Maximum Flood (PMF): The case of Folsom Dam on American River

Artificial reservoirs are created with the construction of dams on a given river. The design of these dams usually considers available historical data. However, the impact of future flood extremes that may change due to the local feedback from reservoir is not considered in conventional dam design approach. Recent study using a regional atmospheric model has shown that the size (surface area) of reservoir can physically increase the probable maximum precipitation (PMP). Probable Maximum Flood (PMF), which is the key design parameter for hydraulic features of a dam, is estimated from PMP and the hydrology of the basin. Hence, it is important to understand the impact of reservoir size on PMF. Given the non-linearity of the rainfall-runoff process, a key question that needs to be asked by water managers now is, "what is the dependency of PMF on the size of an artificial reservoir given the strong local atmospheric feedbacks?" Using the American River Watershed (ARW) as a representative example of an impounded watershed with a large artificial reservoir (Folsom Dam), this study sets up the distributed Variable Infiltration Capacity (VIC) model to simulate the PMF from the atmospheric feedbacks for various reservoir sizes. The atmospheric feedbacks were simulated in the form of PMP in a previous study using the Regional Atmospheric Modeling System (RAMS). The RAMS-generated PMP scenarios were propagated through VIC model to simulate the PMFs. The study overcomes several logistic and conceptual hurdles to generating the basin's natural response to PMF given the extensive nature of upstream impoundment on various tributaries and incomplete stream flow record for adequate calibration. The results obtained from this study will be used to show how large artificial reservoirs should be managed and operated for flood control in a changing climate with strong local feedbacks triggered by the dam itself.