Why Stormwater Needs Managing
When land is developed, natural ground that once absorbed rainfall is replaced by roofs, roads, and parking lots. These impervious surfaces drastically increase the volume and speed of runoff, producing higher and faster flood peaks, channel erosion, and pollutant washoff. Stormwater management is the engineering response: controlling both the quantity (peak flow and volume) and the quality of runoff so development does not overwhelm downstream systems or degrade receiving waters.
The Rational Method
For small drainage areas, the time-honored tool for estimating peak runoff is the rational method:
Q = C · i · A
- Q — peak runoff rate (cfs, when i is in in/hr and A in acres, the units conveniently cancel).
- C — runoff coefficient, the fraction of rainfall that becomes runoff (0 to 1).
- i — rainfall intensity for a storm of duration equal to the time of concentration.
- A — drainage area.
The method rests on a key assumption: peak flow occurs when the entire watershed is contributing simultaneously, which happens once the storm has lasted at least one time of concentration. It is intended for small, relatively uniform catchments — generally under a few hundred acres.
The Runoff Coefficient
| Surface | Runoff coefficient C |
|---|---|
| Asphalt / concrete pavement | 0.70 – 0.95 |
| Roofs | 0.75 – 0.95 |
| Lawns, sandy soil, flat | 0.05 – 0.15 |
| Lawns, heavy soil, steep | 0.25 – 0.35 |
| Forest / open space | 0.10 – 0.30 |
For mixed land uses, a composite C is computed as an area-weighted average.
Time of Concentration and IDF Curves
The time of concentration (Tc) is the travel time of runoff from the most hydraulically distant point to the outlet. It determines which rainfall intensity to use, because shorter, more intense bursts produce the peak on small catchments while longer storms govern large ones. Once Tc is known, the design rainfall intensity is read from an Intensity-Duration-Frequency (IDF) curve for the chosen design storm (for example, the 10-year or 25-year event). Shorter durations yield higher intensities, so Tc strongly influences the computed peak.
Detention and Retention
Because development raises peak flows, regulations typically require that the post-development peak not exceed the pre-development peak. The classic solution is storage:
- Detention basins temporarily hold runoff and release it slowly through a restricted outlet, shaving the peak. They are dry between storms.
- Retention ponds maintain a permanent pool, releasing only by infiltration and evaporation. The standing water also settles out sediments and nutrients, providing water-quality benefits.
Designing these facilities is a storage routing exercise: the inflow hydrograph is routed through the basin, and the outlet structure (orifices and weirs) is sized so the peak outflow meets the allowable release rate while the storage volume contains the excess.
Low-Impact Development and Green Infrastructure
Modern practice increasingly favors low-impact development (LID), also called green infrastructure, which manages runoff where it falls using natural processes rather than piping it away. Common practices include:
- Bioretention cells / rain gardens: shallow planted depressions that filter and infiltrate runoff.
- Permeable pavement: surfaces that let water soak through rather than run off.
- Green roofs: vegetated rooftops that store and evapotranspire rainfall.
- Vegetated swales and filter strips: grassed channels that slow, filter, and infiltrate flow.
LID reduces runoff volume, recharges groundwater, removes pollutants, and helps restore the natural water balance — often at lower cost and with greater amenity value than conventional pipe-and-pond systems.
Regulation: The NPDES Program
In the United States, stormwater discharges are regulated under the Clean Water Act through the National Pollutant Discharge Elimination System (NPDES). Municipalities (through MS4 permits), construction sites over one acre, and many industrial facilities must obtain permits, control pollutant discharges, and implement best management practices (BMPs) — including erosion control, sediment basins, and the green-infrastructure measures above. Stormwater design today is therefore as much a regulatory and water-quality exercise as a hydraulic one.