(Answer specific to the City of Austin)
Simply put, a Rain Garden is a shallow depressed landscape feature designed to capture and hold storm water runoff with the end goal of providing both filtration of pollutants and decreased runoff.
Rain gardens work by using the natural filtration properties of native plants and biofiltration media to remove pollutants. Microbial action is provided by bacteria and microorganisms in specially selected soil media.
Uptake, absorption, and transpiration are provided by native plants. Soil particles and roots systems inhibit the movement of pollutants, keeping them from reaching our creeks and rivers. And since storm water does not immediately enter a conventional hard-piped storm system, sudden surges of runoff are limited which is associated with flooding and erosion. Generally, the combined efforts of plants and media in providing water quality measures is referred to as Bioretention.
The City of Austin uses three different types of Rain Gardens. Design and construction requirements can be found in the City of Austin Environmental Criteria Manual, Section 1.6.7.H.
- full infiltration (no underdrain);
- partial infiltration (filtration system with raised outlet or partial underdrain); and
- filtration system with no infiltration.
Detailed sizing and calculation requirements can be found in appendix R-11, of the City of Austin Environmental Criteria Manual.