Impact of Artificial Reservoir Size, Land Use/Land Cover Patterns and Increasing Urbanization on Probable Maximum Precipitation and Flood: The Case of American River Watershed

Design of dams considers available historical data for flood frequency analysis. The limitation in this approach is future meteorological and hydrological variability due to land-use and land-cover (LULC) change are not considered. Future flood extremes may change, among other factors, due to strong local atmospheric feedbacks from the reservoir, surrounding LULC change, and urbanization. Probable maximum flood (PMF), which is the key design parameter for a dam, is estimated from probable maximum precipitation (PMP). Given the nonlinearity of the rainfall-runoff process, the key questions that need to be answered are How do reservoir size and/or LULC modify extreme flood patterns, specifically probable maximum flood via climatic modification of PMP? and What is the contribution of urbanization in altering reservoir inflow and PMF? Selecting the American River watershed (ARW) and Folsom Dam as a case study; PMP from the regional atmospheric modeling system (RAMS) and the distributed variable infiltration capacity (VIC) model are used to simulate PMF. The PMP values are simulated from atmospheric feedbacks for various LULC scenarios (pre-dam, current scenario, non-irrigation, reservoir-double, and different urbanization percentage). Comparison of PMF results for pre-dam and current scenario conditions showed that PMF peak flow can decrease by about 105m3/s, while comparison of current scenario with non-irrigation PMF results showed that irrigation development has increased the PMF by 125m3/s. Comparison of different urbanization percentage shows that a 100% impervious watershed has the potential of generating a flood that is close to design PMF. The design PMP produces an additional 1500m3/s peak flood compared to the actual PMF when the watershed is considered 100% impervious. On the other hand, the reservoir size had virtually no detectable impact on PMP and consequently on PMF results. Where downstream levee capacity is already under designed to handle a dam's spillway capacity, such as for this case study, such increases indicate a likely impact on downstream flood risk to which any flood management protocol must adapt. The premise that modern dam design and operations should consider an integrated atmospheric-hydrologic modeling approach for estimating proactively potential extreme precipitation variation due to dam-driven LULC change and increase in urbanization is well-supported by this case study.