Urban Hydrology

Impact of Urbanization

Urbanization drastically alters the hydrologic response of a catchment.

1. Increased Imperviousness: Roads, roofs, and parking lots replace soil and vegetation.

   • $\downarrow$ Infiltration
   • $\uparrow$ Surface Runoff Volume
   • $\downarrow$ Groundwater Recharge

2. Hydraulic Efficiency: Gutters, storm sewers, and paved channels convey water faster than natural channels.

   • $\downarrow$ Time of Concentration ($t_c$)
   • $\uparrow$ Peak Discharge ($Q_p$)

Result: Urban floods rise more rapidly (flashy), have higher peaks, and recede faster than rural floods.

Stormwater Management

The goal is to mitigate the adverse effects of urbanization, often aiming to maintain post-development peak flows at pre-development levels.

Structural Control Measures (BMPs - Best Management Practices):

1. Detention Ponds: Temporarily store runoff and release it at a controlled rate (throttle). Reduces peak flow but not volume.
2. Retention Ponds: Permanently store water (wet ponds). Allow some infiltration and evaporation.
3. Infiltration Trenches/Basins: Promote groundwater recharge.
4. Permeable Pavement: Allows water to pass through the surface.
5. Green Roofs: Vegetated roof systems that absorb rainwater.

Design of Storm Sewers

Typically designed using the Rational Method ($Q = CiA$).

Steps:

1. Delineate Sub-catchments: Identify area draining to each inlet.
2. Determine $t_c$: For the first inlet, $t_c$ is the inlet time (overland flow). For subsequent pipes, $t_c$ is the cumulative travel time.
3. Select Return Period: Typically 2-10 years for residential, higher for commercial.
4. Calculate $Q$: Using IDF curves for the location.
5. Size Pipe: Using Manning's Equation to carry $Q$ flowing full (or partially full).

$$Q = \frac{1}{n} A R^{2/3} S^{1/2}$$