Section 4 - Grading, Drainage, and Stormwater Management
Section 4 - Grading, Drainage, and Stormwater Management
Section 4 is the most quantitative section: spot elevations, contours, cut and fill, accessible grading, and stormwater sizing.
Core rules
- Slope % = (rise ÷ run) × 100; the ratio 1:12 equals 8.33%.
- ADA: accessible routes max 1:20 (5%) without ramp conditions; ramps max 1:12 with landings and handrails where required; maximum cross slope 1:48 (2%); accessible parking spaces and aisles max 2% in all directions.
- Provide positive drainage away from buildings - typically 2% minimum on paved surfaces and 5% maximum in accessible parking.
- Spot elevations interpolate linearly between contours; cut and fill volume = area × depth change.
- Rational Method: Q = C·i·A, giving cfs when i is in/hr and A is acres. Manning's equation sizes the pipe or swale that must carry that Q, and time of concentration sets i.
Worked example
A 0.6-acre parking lot is 85% impervious, so C = 0.85. Design storm intensity is 3.5 in/hr. Q = 0.85 × 3.5 × 0.6 = 1.79 cfs. The swale or inlet system must convey that peak. Detention is then sized on the difference between pre- and post-development peaks - not on the post-development peak alone.
Traps
- Confusing percent slope with slope ratio.
- Using horizontal map distance as slope distance.
- Designing a swale for depth rather than flow depth plus freeboard.
- Sizing detention against zero pre-development release.
- Forgetting cross slope when checking accessible routes.