Evaluating low impact development practices as a mitigation strategy for compound flooding in urban-coastal watersheds

Flooding along the U.S. coastline has become increasingly frequent due to the combined effects of intense rainfall and tide. In fully developed urban-coastal areas, constructing large-scale detention facilities or expanding stormwater networks is often infeasible. In this context, distributed, small-scale measures such as low impact development (LID) practices represent a complementary, community-driven approach to supplement conventional drainage infrastructure by capturing runoff on both public and private properties to enhance flood resilience. This study evaluates the potential of parcel-scale LID in mitigating compound flooding under different rainfall intensities and tidal conditions in an urban-coastal watershed. A hydrodynamic model was developed for a watershed in Norfolk, Virginia to simulate interactions between overland flow and drainage networks across a range of LID adoption scenarios. Results show the extent to which LID practices can reduce flood extent from a 1-year compound flooding event, particularly for inland areas primarily affected by pluvial compound flooding. For example, under the 1-year event, flood extent in the case study inland area was reduced by an average of 43% across scenarios, which is 3.57 times the reduction observed for the entire study area. Overall, parcel-scale LID in urban-coastal watersheds may provide a complementary mitigation strategy, especially for low return period nuisance flooding events in systems with similar hydrologic and infrastructure characteristics.