Challenges and Issues in a Coastal Watershed from Model Calibration to Master Drainage Planning
Coastal areas are prone to flooding, with natural and anthropogenic factors both contributing to the duration and severity of coastal flooding events. Natural factors include low-lying areas, little relief, high groundwater tables, heavy rains, and high tides. Anthropogenic factors include increased imperviousness, aging infrastructure, lack of maintenance, and stormwater infrastructure that does not meet current design standards or was constructed before the advent of stormwater management regulations. All these factors contribute to flooding issues and add to the challenges in alleviating the problem. A Master Drainage Plan (MDP) is essential when examining problems and solutions on a watershed scale. PCSWMM has become the modeling platform of choice for most Hampton Roads localities. The City of Chesapeake contracted GKY & Associates, Inc. (GKY), to develop the MDP for the 2,000-acre, tidally-influenced Crestwood 2&3 watersheds. GKY developed an existing conditions model by methodically processing and reviewing over 1,500 plan sheets and field survey information to update existing stormwater structure data. Thirteen calibration stations were established at critical locations within the watershed to determine peak water levels resulting from a significant rainfall event producing more than three inches of rain in twenty-four hours. GKY monitored daily precipitation at four nearby Weather Underground stations and tide gauge readings at Money Point. The existing conditions model was calibrated to the 9/18/2020 rainfall event resulting from Tropical Storm Sally. GKY calibrated the PCSWMM model for the Crestwood 2&3 watershed within an average of 0.07 feet for eleven of the thirteen high water marks. The calibrated model was then used to run design storms and develop alternative solutions to alleviate the flooding problems at known locations while considering the design storms used in designing the existing drainage systems. Significant improvements included paved ditches, multiple runs of twin elliptical pipes, and numerous special design inlets and pipe junctions. This presentation reviews the challenges and issues in developing the MDP for the Crestwood 2&3 watershed.Author Bio
Sean Bradberry is a senior hydrologist with GKY & Associates. He has 25 years of experience in all aspects of stormwater management, hydraulic modeling, and permit and regulatory guidance related to MS4 and Chesapeake Bay TMDL regulations.
