Stormwater BMP Design Study
A practical engineering study evaluating, sizing, and documenting best management practices for stormwater runoff control — translating regulatory requirements into real on-the-ground solutions for a developing site in North Carolina.
Background
Stormwater runoff from developed land is one of the leading causes of water quality degradation in North Carolina's streams and watersheds. When land is graded and hardened for construction, natural infiltration drops and peak flows spike — carrying sediment, nutrients, and pollutants directly into local waterways.
This study focuses on evaluating best management practice (BMP) options for a developing site, working through the full design process: calculating impervious cover, modeling pre- and post-development hydrology, and sizing control structures to meet NCDEQ Phase II standards.
Approach
The work began with a site visit and GIS analysis to map drainage areas, slopes, and soil types using ArcGIS Pro and the NRCS Web Soil Survey. From there, runoff volumes were calculated using the Rational Method and SCS Curve Number approach for both pre- and post-development conditions.
BMP alternatives were evaluated against three criteria: pollutant removal efficiency, site feasibility, and long-term maintenance burden. Candidates included wet detention ponds, bioretention cells, and vegetated swales. Final sizing was completed per the NCDEQ Stormwater BMP Manual design standards.
"The goal isn't just to check the regulatory box — it's to design something that will actually function and be maintainable 20 years from now."
Key Details
- Drainage area delineated using DEM data in ArcGIS Pro — contributing area of approximately 8.4 acres.
- Post-development impervious cover calculated at 42%, triggering Phase II BMP requirements.
- Wet detention pond selected as primary BMP — designed for 85% TSS removal and 1-inch runoff treatment volume.
- Secondary bioretention cell sized for rooftop runoff from the proposed main structure (~3,200 sq ft).
- All designs referenced against NCDEQ Stormwater BMP Manual, Chapter 8 (Wet Detention Ponds).
Field Photos
Outcomes & Status
The BMP sizing calculations and alternative analysis are complete. The wet detention pond design is finalized and has been drafted in AutoCAD Civil 3D with grading, outlet structure details, and planting specifications. The bioretention cell design is currently in progress.
Next steps include compiling the full design package and preparing a brief report summarizing the hydrology calculations, BMP selection rationale, and maintenance requirements.