The City of Alexandria, VA, has experienced repeated and increasingly frequent flooding. The increased storm related flooding is attributable to old infrastructure, inconsistent design criteria, and climate change. The objectives of the study were to (1) update stormwater collection system modeling and improve understanding of flood extents, (2) use optimization to evaluate alternatives and maximize flood mitigation. (3) prepare risk assessments to quantify existing flood risk and evaluate cost-benefit of solutions.  

The flood modeling was conducted using both current and 2070 projected climate scenario for a range of design storms from 2-year to 500-year return periods. Existing and projected conditions were evaluated using a combination of a 1D2D SWMM model and a simplified dual drainage model for improved run times. A machine-learning based optimization process was used to evaluate 1000’s alternatives and identify the highest performing alternatives that would minimize flooding at the lowest cost, including combinations of conveyance and storage sizes and locations. A planning-level cost estimate was developed for the preferred project, accounting for capital costs and O&M. A risk assessment was conducted using FEMA Hazus tool to quantify project benefits. 

The project was conducted in two phases. A priority flooding area was evaluated first, to expedite concept design and implementation and lessons learned were carried to analysis of the remainder of the FMR East service area to identify future projects to mitigate flooding throughout the rest of the watershed. The City is now advancing detailed design for the priority area, which includes large parallel or new conveyance systems and green stormwater infrastructure.