Design site features to minimize erosion and runoff from the building site.
Note: Certain surface water management strategies may be regulated, restricted, or even prohibited by local water authorities or code requirements.
4.1 Permeable lot for mid-rise (maximum 2 points, as specified in Table 8). Design the lot such that at least 70% of the buildable land, not including area under roof, is permeable or designed to capture water runoff for infiltration on-site. Area that can be counted toward the minimum includes the following:
- Vegetative landscape (e.g., grass, trees, shrubs).
- Permeable paving, installed by an experienced professional. Permeable paving must include porous above-ground materials (e.g., open pavers, engineered products) and a 6-inch porous subbase, and the base layer must be designed to ensure proper drainage away from the home.
- Impermeable surfaces that are designed to direct all runoff toward an appropriate permanent infiltration feature (e.g., vegetated swale, on-site rain garden, or rainwater cistern).
4.2 Permanent erosion controls (1 point). Design and install one of the following permanent erosion control measures:
- If portions of the lot are located on a steep slope, reduce long-term runoff effects through use of terracing and retaining walls.
- Plant one tree, four 5-gallon shrubs, or 50 square feet of native groundcover per 500 square feet of disturbed lot area (including area under roof).
4.3 Stormwater quality control for mid-rise (2 points).
Implement a stormwater management plan that reduces impervious cover, promotes infiltration and captures and treats the stormwater runoff from 90% of the average annual rainfall3 using acceptable best management practices (BMPs).
BMPs used to treat runoff must be capable of removing 80% of the average annual post development total suspended solids (TSS) load based on existing monitoring reports. BMPs are considered to meet these criteria if:
- They are designed in accordance with standards and specifications from a state or local program that has adopted these performance standards,
- There exists infield performance monitoring data demonstrating compliance with the criteria. Data must conform to accepted protocol (e.g., Technology Acceptance Reciprocity Partnership [TARP], Washington State Department of Ecology) for BMP monitoring.
|Table 8. Permeable Area|
|Percentage of buildable lot (excluding area under roof) that is permeable||Points|
- Humid Watersheds — 1 inch of rainfall
- Semiarid Watersheds — 0.75 inches of rainfall
- Arid Watersheds — 0.5 inches of rainfall