1D/2D Modeling of Decentralized Stormwater Control Measures for Flood Mitigation in Austin, Texas

The City of Austin Watershed Protection Department is studying an existing flood prone neighborhood in Austin, Texas to assess the efficacy of decentralized stormwater control measures (SCMs) at improving the current level of service (i.e., frequency and severity of flooding). A previous study concluded that system-wide storm drain upsizing to meet current design criteria was cost prohibitive (on the order of $200M) and would exacerbate downstream flooding, erosion, and water quality issues. Therefore, the City is investigating a retrofit plan for the neighborhood with a mixture of public and private decentralized SCMs. Project goals include quantifying the extent to which decentralized SCMs can reduce the frequency of flooding while also reducing long-term runoff volumes, pollutant loads, erosion potential, irrigation demands, and life cycle costs of storm water conveyance upgrades. The first phase of the study focused on quantifying the benefits of only green infrastructure (GI) SCMs. The second phase of the study focused on a hybrid approach of both decentralized GI hydrologic controls and decentralized drainage grey infrastructure improvements applied at strategic locations to mitigate localized flooding. The project team is employing the use of both design event analyses and an advanced continuous simulation 1D/2D modeling approach to provide quantification of SCM performance utilizing 23 years of high resolution precipitation records. Unlike event based simulations, a continuous simulation approach simulates the system performance over a wide range of observed antecedent conditions and precipitation patterns and allows for a statistical analysis of output results. The aggregated effects of decentralized SCMs for design storms and long-term performance metrics are presented. Modeling results demonstrate that widespread and intensive deployment of decentralized SCMs can significantly reduce peak flow rates, total runoff volumes, and spatial extent of flooding.