Natural ground absorbs most rainfall. Pave it over, and that same rainfall has nowhere to soak in — it runs off instead, which is exactly why development dramatically increases stormwater runoff volume.
Hydrology studies how water moves through the environment — rainfall, infiltration into soil, evaporation, and surface runoff. The runoff coefficient, a key input to the widely used Rational Method for estimating peak stormwater flow, captures a simple but consequential fact: the fraction of rainfall that becomes surface runoff instead of soaking into the ground depends heavily on land cover, and impervious surfaces (pavement, rooftops) dramatically change that fraction.
Natural vegetated or unpaved ground allows rainfall to infiltrate into the soil, be taken up by vegetation, and recharge groundwater — only a modest fraction of rainfall on natural ground typically becomes immediate surface runoff. Impervious surfaces like pavement and rooftops block infiltration entirely, forcing nearly all rainfall on those surfaces to become immediate surface runoff instead.
As a site is developed and impervious coverage increases (buildings, parking lots, roads), the runoff coefficient rises correspondingly — the same rainfall event now produces significantly more total runoff volume, and that runoff also tends to arrive faster (since impervious surfaces don't slow and store water the way vegetated soil does), producing a higher, sharper peak flow. Both effects increase flood risk and stress downstream drainage infrastructure.
Because increased impervious coverage measurably increases runoff, most jurisdictions require new development to manage its increased stormwater runoff — through detention ponds, infiltration basins, permeable pavement, or other stormwater controls — specifically to avoid overloading downstream drainage systems and increasing flood risk to adjacent properties. Runoff coefficient calculations, as demonstrated above, are the quantitative basis for sizing that required stormwater management infrastructure.
Because impervious surfaces convert a much larger fraction of the same rainfall into immediate surface runoff, rather than allowing it to infiltrate and be released slowly over time — this produces a higher, faster-arriving peak flow that downstream drainage systems and waterways, sized for the site's prior natural (lower-runoff) condition, may not have capacity to handle.
In practice, a runoff coefficient of exactly 1.0 (100% of rainfall becoming runoff) is essentially a theoretical upper limit, since even very smooth impervious surfaces retain a small amount of water (surface wetting, minor depression storage) and some evaporation occurs — but well-drained, fully impervious surfaces can have runoff coefficients quite close to 1.0 for practical design purposes.
Detention ponds temporarily store increased runoff volume and release it more slowly over time, mimicking the slower, more gradual release that the site's natural (pre-development) ground cover would have provided — this specifically addresses the increased peak flow rate problem caused by added impervious surface, protecting downstream drainage capacity.
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