Case 1 — Existing imperviousness is less than or equal to 50%

Implement a stormwater management plan that prevents the post-development peak discharge rate and quantity from exceeding the pre-development peak discharge rate and quantity for the one- and two-year 24-hour design storms.

OR

Implement a stormwater management plan that protects receiving stream channels from excessive erosion by implementing a stream channel protection strategy and quantity control strategies.

OR

Case 2 — Existing imperviousness is greater than 50%

Implement a stormwater management plan that results in a 25% decrease in the volume of stormwater runoff from the two-year 24-hour design storm.

For projects that are part of a master plan development

The credit requirements may be met using a centralized approach affecting the defined project site and that is within the master plan boundary. Distributed techniques based on a watershed approach are then required. A master planning approach to storm water management and overall impervious surface management that is overall project-wide or based on the local watershed is preferred over stormwater management planning limited to one project site at a time. The master plan setting with larger boundaries and settings allows comprehensive stormwater management techniques to be applied on a larger scale and with more flexibility. This provides economies of scale and affords greater opportunities for clustering buildings, increasing natural settings, and applying distributed management techniques cost effectively. Phasing of projects may affect when a Master Plan is implemented and how the specific building(s) under consideration will be accommodated.

Stormwater design: quantity control