Water Resources and Hydraulic Engineering: Fluid Mechanics, Hydraulics, Hydrology, Irrigation
Questions are formula-driven: Bernoulli and momentum applications, open-channel flow, hydrologic abstractions and unit hydrographs, plus irrigation duty and canal design. Expect two or three calculation items and one conceptual item on dimensional analysis or similitude.
Core rules
- Manning: V = (1/n) R^(2/3) S^(1/2); for a wide rectangular channel R ≈ y.
- Specific energy E = y + Q²/(2gA²); critical flow occurs at Fr = 1, with yc = (q²/g)^(1/3) for a rectangular channel.
- A hydraulic jump dissipates energy; conjugate depths satisfy y₂/y₁ = 0.5(√(1 + 8F₁²) − 1).
- Unit hydrograph theory is linear: an n cm storm gives n times the 1 cm hydrograph, and successive storms are added with a lag.
- Rational method: Q = C i A/3.6, with i in mm/h and A in km².
- Duty and delta: D (ha per cumec) = 8.64 B/Δ, with base period B in days and delta Δ in metres.
Worked example
A crop has a base period of 120 days and a delta of 0.5 m. Duty D = 8.64 × 120/0.5 = 2073.6 ha per cumec. A command area of 6000 ha therefore needs a continuous flow of 6000/2073.6 = 2.89 cumec. If the channel runs for only 80% of the period, the required discharge rises in inverse proportion to about 3.6 cumec.
Traps
Feeding delta in centimetres into the duty formula, a factor of 100 error; dropping the 3.6 when intensity is in mm/h; using a 6-hour unit hydrograph as if it were a 1-hour one; and applying an SI Manning's n to a problem stated in fps units.