Hydrology and Water Resources Engineering
Hydrology and Water Resources Engineering covers rainfall analysis, infiltration, evaporation, streamflow, hydrographs, flood frequency, reservoir yield and groundwater. The exam expects unit conversions among mm, ha, cumec and MCM, plus a clear distinction between rainfall depth and runoff rate.
Core rules
- Rational method Q=CiA/360 for i in mm/h, A in ha and Q in m³/s.
- Thiessen polygons weight gauge rainfall by area; isohyets give depth contours.
- A unit hydrograph is direct runoff from 1 cm of effective rainfall in a specified duration; linearity and superposition apply.
- Infiltration follows Horton or Philip models; the φ-index is average abstraction.
- Flood frequency uses Gumbel or log-Pearson III; return period T=1/(1−P).
Worked example
A 20 ha catchment has C=0.6 and design intensity 50 mm/h. Q=0.6×50×20/360=1.667 m³/s. If the time of concentration is 30 min and the storm lasts 30 min, the rational-method peak is reasonable; for a shorter storm the full catchment may not contribute. Convert Q to daily volume only when storm duration and hydrograph shape are known.
Traps
- Using A in km² in the rational formula without converting.
- Treating base flow as direct runoff in unit-hydrograph derivation.
- Taking a 100-year return period as a 100-year safe life.
- Ignoring catchment lag when routing.
- Mixing rainfall depth in mm with runoff rate in cumec.