Bioretention – Rain Garden – Recommendations

Bioretention is a depressed landscape feature which stores, filters, and infiltrates stormwater runoff.

Bioretention is an attractive Best Management Practice (BMP) on many developments because it can be tucked into greenspace such as curb and cul-de-sac islands, streetscape, and planter boxes. Bioretention elements provide volume reduction, water quality treatment, channel protection storage and Peak Flood Detention Storage.

Basic components important to most St. Louis area bioretention “cells” are vegetation; organic soil that will drain well and provide growing media for plants; a graded filter of sands and gravels below the soil; a perforated underdrain pipe beneath the graded filter to ensure the bioretention will drain; and an overflow structure to pass storms larger than the bioretention design storm.

Considerations should include:

  • Trail under 5% slope where area can be graded to form a shallow depression
  • Linear bioretention facilities along the trail are appropriate with slopes less than 3%. Higher slopes could require internal depth controls such as weirs or check dams to maintain storage volume, especially in long, continuous bioretention facilities.
  • Trail over 5% slope if area can be graded to form a shallow depression
  • Greater slopes are more difficult to work with and sometimes impossible to maintain maximum 3:1 slopes approaching bioretention facilities.
  • Bioretention facilities can be “terraced” and hydraulically connected in areas with steep slopes.
  • Adjacent to and hydraulically connected to permeable pavers. Bioretention facilities can act as an inlet to the storage beneath surface pavers and act as a backup connection to the gravel storage if the pavers become clogged.