Floodplain and Dual Drainage Analysis using an Integrated 1D-2D SWMM Model - Simi Valley, California

Two-dimensional (2D) flood models are quickly becoming key tools for assessing flood risk and evaluating potential mitigation options. In this study, PCSWMM 2D was evaluated as a tool for adjusting Simi Valley’s existing HEC-2 based flood insurance rate maps. PCSWMM 2D is a GIS-integrated, spatial decision support system for the Environmental Protection Agency’s Storm Water Management Model (EPA SWMM) engine, capable of extending the fully dynamic 1D approach to 2D free surface flow. The 500 hectare pilot area modeled includes 10 km of storm drains and channels, 6 detention basins, 2000 houses and structures, and 30 km of streets and roadways. A 1D SWMM model representing all storages, open drainage channels and underground stormwater pipes was integrated with a topography based 2D overland mesh (8m|25ft resolution) representing street and gutter flow.

Floodplains generated for 24 hour design storms using PCSWMM 2D were compared to floodplains generated with HEC-RAS and two other 2D modeling packages (TUFLOW and FLO-2D). Although the technical bases for the three 2D models differ, all produced similar results to each other and the HEC-RAS model. All four models indicated similar adjustments should be made to the currently used floodplain. In the absence of data available for calibration the agreement of these four models has increased confidence in the results produced.

The study results have also indicated that 1D-2D integrated modeling can be used for purposes in addition to standard flood insurance mapping. A 1D-2D integrated model, when complete with all drainage system infrastructure, including the underground storm drains and inlets, channels and detention basins, roadways and buildings, and other essential details, can be a critical tool for accurately determining the extent of hydraulic deficiencies in a community’s minor and major drainage systems. Identification of insufficient overland flow capacities and vulnerable elements of the drainage infrastructure can also result in more meaningful Capital Improvement Programs (CIPs). Furthermore, the knowledge obtained gives emergency responders practical and important information for response to flood emergencies, evacuation and flood control planning. Floodplain and dual drainage analysis based on 1D-2D integrated models can therefore be a valuable decision support and management instrument for public safety as well as property protection.