Hydrology and Water Resources Engineering
Hydrology and Water Resources Engineering covers the hydrologic cycle, rainfall analysis, infiltration, hydrographs, flood frequency, reservoir routing, irrigation, and water-balance calculations. The exam favours formula-based numericals and definitional clarity.
Core rules
- A unit hydrograph is the direct runoff hydrograph from 1 cm of rainfall excess in a specified duration; it assumes linearity and time invariance.
- The φ-index is the constant infiltration rate that makes rainfall excess equal observed runoff.
- Return period T = 1/P for annual exceedance probability P.
- Duty and delta are related by delta = 8.64B/D, with delta in metres, B in days, and D in hectares per cumec.
- Reservoir storage is derived from the mass curve or from inflow-outflow routing.
Worked example
A storm of 5 cm occurs in 6 hours and produces 2 cm of direct runoff. The total abstraction is 5 − 2 = 3 cm, so the φ-index is 3/6 = 0.5 cm/h. If the same 6-hour unit hydrograph has a peak of 20 m³/s, a rainfall excess of 3 cm produces an approximate peak of 3 × 20 = 60 m³/s under the unit-hydrograph assumption. State the linearity assumption explicitly.
Traps that cost marks
- Forgetting that a unit hydrograph represents 1 cm of rainfall excess, not total rainfall.
- Confusing return period with probability; T = 1/P, so a 100-year flood has P = 0.01.
- Not separating base flow from direct runoff.
- Mixing units of rainfall depth, runoff volume, and discharge.
Exam focus
Be able to compute φ-index, unit-hydrograph ordinates, return period, and duty-delta conversions, and to interpret mass curves.