The science of stormwater runoff has evolved quickly over the last 20 years. Initially, engineering for storm runoff was based on collecting and transporting rainfall from a site as quickly and economically as feasible.
Runoff from developed areas discharge more suddenly than the native forested condition, and eventually causes scouring and undercutting of streams that receive the runoff. This disturbs the habitat function of the stream, especially in urban environments with intense development.
Storm mitigation provides manmade features that specifically attempt to discharge runoff from a development in a way that mimics the natural condition.
The earliest methods of storm mitigation measures were quantity control (detention) systems: collecting the runoff in a tank or pond, and slowly 'metering' it off the site, to more closely resemble the discharges from an undeveloped condition.
The next adaption included quality control (treatment) since runoff from vehicle usage areas tends to carry higher pollutant loads.
Every new era of regulations, generally required larger detention and treatment systems to meet criteria. As these systems grew in size, it was more economical to construct a single centralized system for a site, instead of multiple smaller systems.
Low Impact Development (LID) practices have been developed to assist in decentralizing a project's storm mitigation facilities.
Currently, storm mitigation technology is split into three categories:
- Flow-control: peak discharges of runoff from a development must mimic the discharge of a forested condition.
- Storm Management: runoff from the developed site must return a similar volume to the ground, as in the forested condition
- Quality control: treatment of the runoff before it is discharge from the site, or to groundwater, to reduce pollutants as much as possible.
The design process for storm mitigation requires selecting appropriate facilities for the soils and the terrain, integrating the facilities with the site design, sizing the facilities appropriately, and providing a design that is affordable and maintainable.