City of Annapolis - 2D stormwater modeling of low-lying tidally influenced area

Effie McMillan, Civil Engineer II, AECOM

To investigate the effects of stormwater improvements (including new pump stations, stormwater bypasses and tide gates) on a small area near Annapolis harbor, an integrated 1D-2D model was developed. The effects of overland flow were pronounced in this area as tidal flooding was a consistent issue. The existing outfalls lacked functioning tide gates, resulting in reverse flow during high tides that often spilled out of inlets and across the relatively flat terrain. Some of the points of concern for the City were not directly adjacent to the flooding inlets, so it was necessary to model the terrain and track flooding that way. This approach required setting several assumptions, one of the most significant being that the planned floodwalls would sufficiently prevent any increased tides from spilling over and introduce flooding completely apart from the stormwater network. Once we implemented the planned improvements, including the addition of two pump stations, we saw significant improvements, even in the worst-case scenario (where high tide coincides with the highest precipitation, using the maximum sea-level rise projection). Nuisance flooding around the points of concern was reduced significantly or eliminated from the model results. This approach to stormwater modeling is only going to become more relevant as municipalities along coastlines navigate the challenges of climate change and aging stormwater infrastructure. Introducing complexities such as a wider area of interest or overbank flooding would require different assumptions and more intricate models, but this can serve as a microcosm of the stormwater challenges and potential solutions facing similar areas across the country.

Author Bio

Effie McMillan is a water resources engineer at AECOM, where she's been working for 2 years. Her primary focus is assisting in stormwater engineering design by creating or modifying stormwater models, performing calculations, and producing reports and figures.