Quantifying the Stormwater Impacts from Incremental Increases of Imperviousness in Newport News, VA Residential Neighborhoods

The amount of impervious cover in residential neighborhoods tends to gradually increase over time.  Property improvements on individual parcels, such as detached garages, storage sheds, and extended driveways can significantly increase the imperviousness of individual parcels.  However, the associated land disturbance associated with individual property improvements often falls below the minimum threshold size to require stormwater management consideration.  The City of Newport News has experienced just these types of incremental impervious cover increases in its residential neighborhoods.  While any individual improvement has a minimal impact on drainage and infrastructure needs to manage that drainage, these incremental increases in the aggregate can lead to more significant drainage and stormwater management challenges.

As part of its stormwater management, floodplain management, and resilience & climate change management masterplan development, the City of Newport News directed GKY to evaluate the collective impact on stormwater from minor property improvements, and the associated, incremental increases in impervious cover, in the City’s residential neighborhoods.  GKY established criteria to distinguish between baseline impervious cover (associated with original construction) and incremental impervious cover (associated with property improvements). Using these criteria, GKY conducted a digital analysis of 12 residential subdivisions varying in age, overall size, and lot size.  Based on the results, GKY then estimated the differences in water quality and water quantity requirements between the standards in place at the time of original development and the VSMP standards that represent current requirements.  Based on these findings, GKY provided the City with potential considerations to address the impacts associated with incremental increases in residential impervious cover.  Considerations included: 1) Make no changes in the current regulatory process; 2) Reduce the minimum land disturbance size to require land disturbing permits; 3) integrate stormwater management into the zoning approval and building permit processes; and 4) lowering the stormwater utility billing unit to capture smaller increases in impervious cover and applying it uniformly across residential and non-residential properties.