Environmental Hydraulics and Hydrology: Rainfall, Runoff and Groundwater
Hydrology questions test rainfall-runoff estimation, frequency analysis and open-channel or groundwater flow. Hydraulics questions test Manning, Darcy and well equations. Expect formula application with careful unit handling.
Core rules and concepts
- Return period: the T-year event has annual exceedance probability 1/T; a 25-year flood has a 4% chance each year, not a fixed 25-year gap.
- Rational method: Q = CiA; with i in mm/h and A in hectares, Q (m3/s) = CiA/360. Valid for small urban catchments, with storm duration equal to time of concentration.
- Infiltration: Horton and Green-Ampt models; SCS curve number CN relates rainfall to runoff and depends on soil group, land use and antecedent moisture.
- Unit hydrograph: linear superposition and proportionality; baseflow separation before deriving it.
- Open channel: Manning Q = (1/n) A R^(2/3) S^(1/2).
- Groundwater: Darcy Q = KiA; Thiem equation for steady confined flow to a well; drawdown increases logarithmically with distance.
Worked example
An urban catchment of 100 ha has runoff coefficient 0.4 and design intensity 60 mm/h. Q = 0.4 x 60 x 100/360 = 6.67 m3/s. If the same rainfall falls on a catchment with C = 0.75 after paving, Q rises to 12.5 m3/s. The reasoning step is the unit constant 360, which converts mm/h over hectares to m3/s; using 3.6 or forgetting the hectare conversion is the usual error.
Traps that cost marks
Treating the rational method as valid for large catchments; using rainfall intensity for a duration other than the time of concentration; confusing return period with recurrence interval guarantee; and mixing confined (Thiem) with unconfined (Dupuit) well equations.