Stormwater Modeling of the Sterns Creek Watershed and Master Drainage Planning of its City of Portion
The Sterns Creek, a tributary to the Western Branch Elizabeth River in Virginia, and its watershed with a drainage area of 2,350 acres covers 1,360 acres in the City of Chesapeake and 990 acres in the City of Portsmouth. The watershed in both the cities with residential and commercial properties suffers from recurring nuisance or property-damaging stormwater flooding at various locations. The City of Chesapeake hired AECOM to conduct a study of the watershed and develop a master drainage plan (MDP) of its City of Chesapeake portion. AECOM has completed the study in November 2022 by developing a model of the watershed and using it to develop the MDP. The PCSWMM software of the Computational Hydraulics International (CHI) which is based on the U.S. Environmental Protection Agency’s (USEPA) Storm Water Management Model (SWMM) was used to develop the model and the MDP.
Geographic Information System (GIS) data containing storm sewer and watershed boundaries, various site development plan sets and data files from both the cities, supplemented by LiDAR digital elevation model (DEM) from Virginia Geographic Information Network (VGIN), and limited field survey data were used to build the model. Model hydrologic and hydraulic parameters are based on City of Chesapeake’s model calibration studies done at similar watersheds. The one-dimensional (1D) SWMM was employed to analyze the existing (2020) flow conditions in the drainage network of the watershed under six Natural Resources Conservation Service (NRCS) design storms (2-, 5-, 10-, 25-, 50-, and 100-year) with a constant 4.3 feet tailwater boundary condition that accounts for a mild sea-level rise. The model-predicted flooding at various locations in Chesapeake under different storm events were compared with city’s flooding records through citizens’ reporting and issued-work orders for temporarily mitigating the flooding. Based on those analyses, four areas in the City of Chesapeake were selected to improve the storm sewer or ditch system to be able to handle either a 5-year storm or a 10-year storm for the city to choose for implementation based on the year the system was built. This is because, the drainage systems in older neighborhoods were built to handle a 5-year storm and more recent neighborhoods a 10-year storm. The City of Portsmouth has also developed drainage improvement plans in their portion of the watershed. Both the cities worked cooperatively to include those improvements into the model.