Flooding and Heavy Rainfall in Small Urban Watersheds

Abstract

Studies of the physical processes involved in the hydrology and hydrometeorology of flash flooding in small urban watersheds are completed to advance capabilities in stormwater management, flood risk analysis, and flash flood forecasting. High-resolution data and large storm sample sizes provide the necessary tools to study these fast responding, heterogeneous watersheds.

The first study utilizes high resolution radar rainfall fields and instantaneous streamflow data to investigate the spectrum of urban hydrologic response in 9 small Baltimore area watersheds. A physically-based, minimally-calibrated distributed watershed model of Dead Run, a Baltimore urban end member watershed, is created in study two to run numerical experiments investigating the role of detention basin and soil storage in urban flood events. Flash-flood-producing storms in Moores Run, another Baltimore urban end member watershed, are the focus of the third study. Storm properties are tracked from a LaGrangian perspective to investigate the characteristics of flash flood producing storm events. The final study focuses on the flashiest watersheds in the contiguous US, based on a 1 m^3 s^-1 km^-2 threshold, and puts the results obtained in the Baltimore watersheds into a national context.

Hydrology results highlight the necessity of understanding differences in hydrologic response among urban watersheds. Connectedness of land cover, drainage network characteristics, and stormwater management infrastructure impact storage and response times in urban watersheds. Detention basins substantially decrease flood peaks and response times in small urban watersheds, and soil storage also offers a path to substantially decrease flood peaks. Policies to build detention basins further downstream and to reduce compacted urban soil layers should be considered.

Hydrometeorology results indicate a shift to spatially concentrated rainfall, evening-time warm season thunderstorms, and downwind rainfall maxima for flooding in urban watersheds. Storms which produce flash flooding in Moores Run are characterized by high reflectivities and large short-term rain rates, potentially caused by collapse of storm cells over the watershed.

Baltimore is the second flashiest city in the contiguous US and Moores Run is the fifth flashiest watershed. Small urban watersheds dominate the list of flashy watersheds. They are flooded by thunderstorms and are flashier downwind of urban areas, similarly to Baltimore watersheds.