Sample Stormwater Plan

Sample Written Narrative:
  • A narrative description of proposed project and the existing site conditions.
  • A description of proposed permanent stormwater control Best Management Practices (BMPs) shown on the site plan and how they were selected
  • A description of proposed temporary erosion/sediment control measures used during construction shown on the plans and how they were selected
  • A description of any necessary special studies or soils reports
The written narrative is a supporting document of both the Stormwater Site plan and the Construction Pollution Prevention Plan and typically includes the following information:

Sample Narrative for a Single Family Residence
Malene McResident Residence
7519 NE Q Street

Site & Project Description
The project is located on the eastern shoreline of Mud Bay Dock Road on a 1.69-acre lot that is zoned RA. The lot is mostly forested with a wetland on the north property line. The wetland is a Category III wetland with a minimum required buffer width of 75 feet as determined in San Juan County UDC. The lot slopes down from Mud Bay Dock Road on the east to the shoreline on the west lot line. The slope on the south portion of the property is 6-14%. The house and its septic drain fields are proposed on the southeastern portion of the lot. The driveway will be approximately 10 feet by 250 feet (2,500 square feet) of impervious surface, the parking area 638 square feet, and the house 3,750 square feet. The total proposed impervious surface is 6,888 square feet. The total proposed clearing for the house, yard, drain field areas, and driveway is 0.87 acres.

The shoreline portion of the lot that is on a 40-50% slope is a steep slope hazard area as determined by the Critical Areas Map at the San Juan County Community Development and Planning office. A 50-foot buffer and a 15-foot building setback is required, as shown on the flow control BMP plan.

According to the attached soils report for the septic system design, the soil on the project site is a sandy loam underlain by dense glacial till. Therefore, full infiltration is not applicable. This allowed for selection of basic dispersion as the flow control BMP of choice for application to the target impervious surfaces of this project. To implement basic dispersion, the roof downspouts on the west side of the proposed house will discharge via splash blocks to minimum 50-foot vegetated flow-path segments located on slopes no steeper than 15% as shown on the flow control BMP site plan. The downspouts on the east side of the house will be tight-lined to a 2 feet by 20 feet dispersion trench. No more than 700 square feet of roof area will discharge to any one splash block and vegetated flow-path segment. All of the flow-paths will be down-slope of septic tank and drain fields.

Site Plan
The northern portion of the driveway will be discharged via sheet flow over a 10-foot vegetated flow-path segment to the north, as shown on the site plan. The drive way and parking area will be discharged via sheet flow over a 2-foot-wide crushed rock strip and a vegetated flow-path segment of 10 feet as shown on the Stormwater Site plan. In order to prevent erosion and trap sediments within the project site, the following BMPs will be used approximately as shown on the Construction Pollution Prevention plan:
  • BMP C103 (PDF) - Clearing limits will be marked by fencing or other means on the ground.
  • BMP C105 (PDF) - The driveway will be constructed and graveled immediately. A rocked construction entrance will be placed at the end of the driveway.
  • BMP T5.12 (PDF) - Dispersion trenches will be placed according to sheet flow control requirements from the driveway.
  • BMP C121 (PDF) - Cleared areas accepting sheet flow from graveled areas and exposed soils will be mulched. Mulch will consist of air-dried straw and chipped site vegetation. Runoff will not be allowed to concentrate and no water will be allowed to point discharge onto the slopes.
  • BMP C233 (PDF) - Silt fencing will be placed along slope contours at the down-slope limit of clearing.