The City of Austin requires landscaping for development. Additional non-required vegetation, especially trees, can help reduce stormwater runoff and enhance groundwater recharge by breaking the impact of raindrops and improving soil structure. A tree's effectiveness in this capacity is correlated with the size of the crown and root zone area.
There are numerous environmental and stormwater benefits to additional vegetation. Non-required vegetation can act as a natural stormwater management area by filtering particulate matter, including pollutants, some nutrients, sediments, and pesticides, and by absorbing water. A study done by the U.S. Department of Agriculture's Center for Urban Forest Research found that a medium-sized tree can intercept 2,380 gallons of rain per year (Center for Urban Forest Research 2002).
Non-required vegetation is eligible for water quality credit, in terms of pervious area (impervious area reduction), if it meets the criteria described in Section 1.6.7.G of the Environmental Criteria Manual. However, it is not eligible for credit in the Barton Springs Zone (BSZ)watersheds.
Unlike conventional centralized stormwater management systems, rain gardens may employ multiple controls dispersed across a development, and may be incorporated into the landscape to provide aesthetic as well as ecological benefits.
Rain gardens can be designed to provide a level of treatment equivalent to sedimentation/filtration, and also provide extended detention that enhances baseflow and reduces stream erosion.