Hydraulics and Hydrology (Breadth)
Concepts
Open-channel flow (Manning), pressure pipe flow (Darcy-Weisbach, Hazen-Williams), weirs and orifices, pumps and system curves, rational method, time of concentration, runoff coefficients, culvert inlet/outlet control, erosion and sediment control, and stormwater detention. Construction applications include dewatering, bypass pumping, cofferdams, and temporary drainage.
Traps
Units in Manning: the 1.486 factor is for feet and seconds, 1.0 for metric. Hydraulic radius R = A/P, not depth and not wetted perimeter. Rational method Q = CiA requires i for the time of concentration and consistent units. Weir flow varies with head to the 3/2 power, not linearly. Culverts can be inlet- or outlet-controlled; check both.
Worked example
A rectangular concrete channel is 6 ft wide, flow depth 3 ft, slope 0.001, n = 0.013. A = 18 ft², P = 6 + 2(3) = 12 ft, R = 1.5 ft. Q = (1.486/0.013)(18)(1.5)^(2/3)(0.001)^(1/2) ≈ (114.3)(18)(1.310)(0.03162) ≈ 85 cfs. Velocity = Q/A ≈ 4.7 fps; check freeboard and scour.
Rule of thumb
For rough planning, Q ≈ CiA; a 10-acre site with C = 0.6 and i = 2 in/hr gives about 12 cfs.