Surface Water: Hydrographs, Routing, and Flow Measurement
The organizing idea
Surface-water questions test whether you can read a hydrograph and connect it to watershed characteristics and channel hydraulics. The hydrograph is the central object: its peak, timing, and volume each respond differently to storm and basin properties.
Concepts to master
- Hydrograph anatomy: rising limb, crest, recession, baseflow. A larger, more intense storm raises the peak and volume; a larger or more impervious basin shifts timing.
- Time of concentration and lag: longer travel paths delay and flatten the peak; urbanization shortens time of concentration and steepens the rising limb.
- Unit hydrograph: response to one unit of effective rainfall over a fixed duration; superposition and proportionality apply to linear unit hydrograph theory. Changing storm duration changes the unit hydrograph.
- Routing: storage and attenuation; a flood wave moving downstream peaks later and lower. Kinematic versus storage routing concepts.
- Flow measurement: discharge Q = velocity × cross-sectional area; stage-discharge rating curves; Manning's equation for open-channel velocity, where velocity rises with hydraulic radius and slope and falls with roughness.
Traps
Assuming peak discharge scales linearly with total rainfall regardless of duration, confusing lag with time of concentration, and forgetting that routing attenuates peaks rather than amplifying them.
Worked rule of thumb
Manning check: for a rectangular channel, compute area A and wetted perimeter, get R = A/P, then V = (1/n)R^(2/3)S^(1/2), and Q = VA. Keep units consistent (SI: metres and seconds). A quick dimensional sanity check catches most unit errors before you pick an answer.