Water Resources and Environmental: Hydrology, Hydraulics, and Treatment
This domain blends hydrology, open-channel and pipe hydraulics, and water/wastewater treatment. Core concepts: Rational Method (Q = CiA), Manning's equation (Q = (1.49/n) A R^(2/3) S^(1/2)), continuity and energy equations, pump curves and system curves, and treatment fundamentals (coagulation/flocculation/sedimentation, activated sludge, disinfection CT). Know detention time (t = V/Q), overflow rate (Q/A), and hydraulic loading.
Traps
- Rational Method units: Q in cfs requires i in in/hr and A in acres; the constant is 1.0, not 1.49.
- Manning's constant 1.49 applies only in US customary units; SI uses 1.0.
- Confusing hydraulic detention time with solids retention time in activated sludge.
- Using pipe diameter where radius is required in the hydraulic radius R = A/P.
Worked example
A rectangular channel 6 ft wide, flow depth 2 ft, n = 0.015, S = 0.001. A = 12 ft²; P = 6 + 2 + 2 = 10 ft; R = 1.2 ft. Q = (1.49/0.015)(12)(1.2)^(2/3)(0.001)^(1/2) ≈ 99.3 × 12 × 1.13 × 0.0316 ≈ 42.5 cfs. Rule of thumb: for a rectangular channel, P = b + 2y and R = by/(b + 2y) - compute wetted perimeter before hydraulic radius, every time.