Stormwater bmp design

The project involved design of a stormwater best management practice (BMP) for capture and treatment of post-construction stormwater runoff from a commercial development in Brunswick County, North Carolina. The stormwater BMP was designed pursuant to the requirements outlined in the North Carolina’s Coastal Stormwater Regulations and the Brunswick County Stormwater Regulations, and because post construction built-upon area exceeded 30% of the total developed area and post-development peak runoff exceeded pre-development peak runoff from a 1-year, 24-hour storm.

Stormwater runoff from the site’s impervious surfaces (roofs, parking lots, and vehicle and pedestrian travel areas) and a portion of the off-site drainage areas (existing roadways) was designed to be through surface and subsurface conveyance measures (curb-and gutter) and finished surface grade into two (2) subsurface infiltration trench systems installed on the northern and southern corners of the property. Each infiltration trench system was designed to capture and treat the first 1.5 inches of stormwater runoff from its respective drainage area, in accordance with State Coastal Stormwater Management requirements. Runoff in excess of the first 1.5 inches was designed to be routed via sheet surface flow into vegetated ditches constructed within the Right-of-Ways along the southern property line.

Each infiltration trench system consisted of ADS 30-inch diameter perforated piping. The piping was installed in a row-like fashion and manifolded into a header at the inlet end of each trench. The piping was installed on a bed of 10-inches of washed stone with a 2-inch thick stone covering. A 12-inch stone border was placed along the outer perimeter of the chamber matrix, and the chambers and stone will be installed within a filter fabric liner, and covered with a minimum 2-inch layer of compacted fill and 2 inches of asphalt pavement. The infiltration trenches were installed at a final depth of at least two feet above the seasonal high ground water table depth in accordance with State BMP requirements. The piping was designed with a traffic loading rating of ASHTO H-20 due to the thin layer of cover required to maintain final site grade and depth restrictions.

The stormwater management system was permitted through the North Carolina Division of Water Quality and Brunswick County, and was installed in the Fall of 2007.