Grading, Drainage, and Stormwater Management
Grading, Drainage, and Stormwater Management
This domain tests terrain modification and stormwater control: contour interpretation, spot elevations, slope calculation, swales, inlets, culverts, detention, infiltration, erosion control, and permit triggers.
Core rules
- Contours connect equal elevations. Closely spaced contours mean steep slope; evenly spaced contours mean uniform grade.
- Flow crosses contours at right angles. Swales follow contour low points; ridges follow high points.
- Minimum grades keep water moving: pavement and turf need positive drainage, and swales need longitudinal slope. Flat grades invite ponding and mosquito problems.
- The rational method estimates peak flow: Q = CiA, where Q is cubic feet per second, C is runoff coefficient, i is rainfall intensity in inches per hour, and A is area in acres.
- Stormwater design follows a hierarchy: infiltrate, evapotranspire, detain, and only then discharge. Include erosion and sediment control during construction.
Worked example
A 1.5-acre parking area has a runoff coefficient of 0.6 and a design rainfall intensity of 4 inches per hour. Q = 0.6 x 4 x 1.5 = 3.6 cubic feet per second. The reasoning: multiply the dimensionless coefficient by intensity and area in the correct units. If the calculated flow exceeds the swale capacity, adjust the swale geometry, add an inlet, or provide detention. Check units before selecting a pipe or channel.
Traps
- Reading contour interval as elevation rather than vertical spacing.
- Forgetting offsite drainage that crosses the property.
- Designing a swale with insufficient longitudinal slope.
- Treating detention as the first option instead of reducing runoff at the source.
- Missing construction-phase erosion control.