Water Resources and Environmental: Runoff, Hydraulics, and Treatment
Hydrology and hydraulics
Water resources questions combine rainfall-runoff, open-channel flow, pipe hydraulics, and treatment. For small urban catchments, the rational method gives Q = CiA, with Q in cfs when i is in inches per hour and A is in acres; the 1.008 conversion is usually taken as 1.0. Use the time of concentration to select rainfall intensity, and remember that C varies with land use and return period. A frequent trap is using a 24-hour rainfall depth as an intensity.
Open channel and pipes
Manning's equation, Q = (1.486/n) A R^(2/3) S^(1/2), is in US customary units. Hydraulic radius R = A/P, not depth. For circular pipes, partial-flow conditions can produce higher velocity than full flow, so do not assume full-pipe capacity is maximum. Energy and hydraulic grade lines help track head loss; minor losses use K V²/2g.
Worked example
A 10-acre catchment has C = 0.5 and a 25-year intensity of 2.0 in/hr. Q = 0.5 × 2.0 × 10 = 10 cfs. If the intensity were given as 2.0 inches in 24 hours, the result would be wrong by orders of magnitude. Keep rainfall intensity and depth distinct.
Treatment
For water treatment, know coagulation, flocculation, sedimentation, filtration, and disinfection, plus CT concepts. For wastewater, understand BOD, suspended solids, nitrification, and solids retention time. A trap is confusing influent and effluent concentrations when computing removal efficiency. Efficiency = (in − out)/in × 100.