This article is the first in a series that will provide background on stormwater pollution and mitigation. If you are new to stormwater, I hope these articles will provide some background that can be of use.
The sheer act of increasing the impervious area in a watershed causes increased runoff volume, increased peak flow and reduced travel time. These impervious surfaces include roads, sidewalks, parking lots, and buildings. Natural flow paths in the watershed are replaced by impervious structures typically associated with development; these include: paved gutters, storm sewers, lined and unlined drainage ditches, and artificial creek and river beds. Precipitation amount, surface characteristics, such as soil type, vegetative cover and impervious fraction are the primary variables determining the volume of runoff. Travel time is determined by surface roughness, surface slope and the distance of flow. Peak discharge is determined by the interactions between precipitation and travel time. A hypothetical one square mile watershed can be used in conjunction with a hydrographic modeling system (TR-55) to conceptualize the effects of urbanization on flow rate, volume and timing. The figure below shows the rate of runoff versus time for the pre-developed and developed conditions using . Not only is there increased volume of runoff. But it comes faster and earlier than the native state. The increased volume and velocity hydraulically alter any downstream undeveloped portions of the watershed by causing erosion, depositing sediments and altering watershed morphology.
The effects of course, do not stop there. Erosion, sediments and changes to watershed morphology degrade habitats. This phenomenon has been studied in detail. In western Washington, an approximate 10 percent change in impervious area in a watershed yields observable loss of aquatic system function. Lower levels of development also cause significant impairment of more sensitive water bodies. Urbanization can be mitigated by retaining runoff from impervious surfaces and either infiltrate them or discharge them at a rate and more like the pre-urbanized state. This is the function of new development standards that include requirements reduce the effects of urbanization on local hydrology.