Grading, Drainage, and Stormwater Management
What to know
Grading, Drainage, and Stormwater Management tests contours, spot elevations, slopes, earthwork, swales, inlets, detention and retention, and stormwater calculations. Know how to read contour interval and direction, compute gradient, and route water away from structures. Understand the rational method, runoff coefficients, and BMPs. Know accessibility grades and typical minimum grades for drainage.
Traps
Contour lines form V shapes pointing upstream in a valley. Do not confuse percent slope with ratio: 2% = 2 ft per 100 ft = 1:50. Watch cross-slope limits on accessible routes (1:48 maximum, about 2.08%) and running slope (1:20 maximum for an accessible route unless it is a ramp). Common minimum grades are 1% for paved areas, 2% for turf, and 1% for swales, but verify project criteria. Avoid flat spots and ponding. Do not use the rational method with area in square feet.
Worked example
A 1-acre parking lot has C = 0.90 and rainfall intensity i = 2 in/hr. Q = CiA = 0.90 x 2 x 1 = 1.8 cfs. If the same area were given as 43,560 sf, convert to acres first. For a swale carrying 1.8 cfs at 2% slope, check capacity and lining; do not assume a shallow swale works.
Rule of thumb: 1% slope = 1 ft per 100 ft; 2% = 2 ft per 100 ft; 4% = 4 ft per 100 ft. Keep paved drainage at 1% minimum and avoid ponding.