Grading, Drainage, and Stormwater Management
What this domain tests
Grading, Drainage, and Stormwater Management tests your ability to read and shape topography, calculate runoff, and design conveyance and storage. Expect contour interpolation, slope calculations, cut/fill, Rational Method runoff, swale and pipe sizing, and detention concepts.
Grading rules
- Percent slope = rise ÷ run × 100. Interpolate spot elevations between contours; contours crossing a stream point upstream as a V.
- Common minimums: 2% on paved surfaces, 1% on turf/planting, 1% in swales; accessible routes max 5% running and 2% cross slope. Design cut/fill toward balance and daylight slopes to existing grade.
Drainage and stormwater
- Rational Method: Q = CiA, with Q in cfs, C a runoff coefficient, i intensity in in/hr, and A area in acres. Use a weighted C for mixed surfaces and the design storm’s time-of-concentration intensity.
- Conveyance uses Manning’s equation; track crown, flowline, and invert. Provide freeboard above the 100-year water surface and detention for peak-rate control.
Worked example
A 2.0-acre site has C = 0.35 and the 10-year intensity for its time of concentration is 3.0 in/hr. Q = 0.35 × 3.0 × 2.0 = 2.1 cfs. A pipe run of 180 ft must drop at least 0.9 ft to achieve 0.5% minimum slope. A swale carrying the same flow would typically need 1% or steeper and a check for erosive velocity.
Traps
- Using 2% as 0.2 instead of 0.02 in the slope formula.
- Averaging runoff coefficients instead of area-weighting them.
- Drawing flow lines along contours rather than perpendicular to them.