Water Resources and Environmental Engineering
Water Resources and Environmental Engineering tests hydrology, open-channel and pressure-pipe flow, pumps, water treatment, wastewater treatment, and environmental regulations. The exam expects unit-consistent calculations and correct selection of empirical methods.
Hydrology and open-channel flow
The rational method is Q = CiA, where C is a runoff coefficient, i is rainfall intensity in in/hr, and A is area in acres; Q results in cfs because 1 cfs equals 1 acre-inch per hour. Time of concentration controls the intensity used. Manning's equation for open channels is Q = (1.486/n) A R^(2/3) S^(1/2) in U.S. customary units, where R is hydraulic radius and S is slope.
Treatment and environmental systems
Water treatment commonly includes coagulation, flocculation, sedimentation, filtration, and disinfection. Wastewater treatment uses preliminary, primary, secondary, and tertiary processes, with BOD and TSS as key measures. Environmental calculations often involve loading rates, detention time, and mass balance.
Worked example
A 20-acre site has a runoff coefficient of 0.35 and a 10-year rainfall intensity of 3.0 in/hr. The rational-method peak flow is Q = 0.35(3.0)(20) = 21 cfs. The units are consistent because the rainfall intensity is in inches per hour and the area is in acres. If a composite catchment has different surfaces, use an area-weighted C rather than averaging intensities.
Traps
Use 1.486 in Manning's equation for U.S. customary units and 1.0 in SI. Do not use a rainfall intensity that does not correspond to the time of concentration. For composite catchments, weight the runoff coefficient by area. Check whether detention time is based on average or peak flow, and keep BOD and TSS units consistent.
Key points
- The rational method is Q = CiA with i in in/hr and A in acres.
- Use 1.486 in Manning's equation for U.S. customary units.
- Time of concentration determines the design rainfall intensity.
- Composite runoff coefficients are area-weighted.
- Treatment trains follow unit processes in series, not arbitrary order.