Grading, Drainage, and Stormwater Management
Grading, Drainage, and Stormwater Management
Grading shapes the land to direct water, provide buildable pads, and ensure stable slopes. Candidates must understand cut and fill, balanced earthwork, slope ratios (e.g., 2:1, 3:1), terracing, drainage patterns (sheet flow, concentrated flow), and stormwater management principles: peak flow attenuation, volume control, water quality treatment, and infiltration. Key regulations include NPDES Phase II, local grading ordinances, and stormwater manuals that require post-development peak discharge to not exceed pre-development for specified design storms.
Traps include: confusing slope ratio (horizontal:vertical) with percent slope (rise/run × 100). For example, a 2:1 slope is 50% (1/2 × 100). Another trap is design storm selection: often 2-year, 10-year, 25-year, and 100-year storms are used for different purposes (e.g., 2-year for channel protection, 100-year for floodplain). Also, time of concentration affects peak flow; a common error is to assume it remains constant after development, but urbanization typically decreases it, increasing peak flow. Infiltration facilities require soil testing and may be prohibited in fill or contaminated soils.
Worked example: A 5-acre site has a pre-development time of concentration of 30 minutes and a post-development time of 15 minutes. Using the Rational Method (Q = CiA), if the pre-development C is 0.2 and post is 0.7, and the 10-year rainfall intensity for 30-min is 2 in/hr and for 15-min is 3 in/hr, compare peak flows. Pre: Q = 0.2 × 2 × 5 = 2 cfs. Post: Q = 0.7 × 3 × 5 = 10.5 cfs. This illustrates how both increased C and decreased tc raise peak flow. To attenuate, a detention basin must store the difference in runoff volumes and release at pre-development rates.
Rule of thumb: For small sites, a 10% slope is often the maximum for mowed areas; steeper slopes require vegetation or structural measures. For swales, minimum longitudinal slope is 1% to avoid ponding.