Challenges in Developing a Resilience Plan for Coastal Zone Urban Watershed

A stormwater “Resilience Plan” identifies specific drainage improvement projects that could be constructed to improve flood resiliency – specifically for extreme storm events. Resiliency planning addresses how fast and efficiently a system recovers after a major event. Here, resiliency specifically refers to the ability of streets and land parcels to recover from flooding. Improving resiliency means reducing the depth and duration of flooding at specific locations. Floodproofing, buyouts, elevating roadways, terrain modifications, adding culvert capacity, improving drainage system conveyance capacities, lengthening or raising bridges, constructing tidal surge barriers, and providing additional flood storage are all additional means of improving resiliency.

There are myriad challenges in developing a resiliency plan for an urban coastal watershed. Common among all of them are low-lying areas; high groundwater; high tides; heavy rainfall due to nor’easters and hurricanes; inadequate drainage; the age of the drainage system and the design standards used in the distant past; and maintenance issues. In developing a plan for future conditions, we need to account for sea-level rise & land subsidence too. Both natural and man-made constructs have cumulatively resulted in recurring nuisance flooding and any attempt to re-design gravity drainage in the region is not only a challenge and cost-prohibitive, but it may not be feasible in many instances.

Recently engineers from the City of Newport News and GKY have prepared a resilience plan for the Salters Creek watershed, located at the southeast end of the city spanning approximately 2,517 acres with two separate outfalls. Nearly 72% of the total watershed is located within the City of Newport News boundary and the remaining portion lies in the City of Hampton. The plan quantifies existing flooding and describes 14 specific improvement projects.

The project engineers used 2D finite element modeling to quantify existing condition flooding and then to develop potential improvement projects to reduce that flooding. Initially, Rain-on-Grid HEC-RAS modeling was completed. However, the HEC-RAS results demonstrated that the watershed depends more upon drainage pipes for flood relief than any other mechanisms, and a more powerful modeling approach was warranted. GKY then prepared a combined 1D/2D model using PCSWMM, at no additional cost to the City. This paper presents our modeling approach and the resilience plan.