Introduction of Structural BMPs

Structural BMPs are physical devices designed and constructed to reduce runoff velocities or to reduce pollutant load in stormwater via filtration or biological uptake. Structural BMPs for stormwater can reduce flooding, prevent soil erosion, conserve water, and provide habitat, food, and shelter for area wildlife. Necessary factors to be considered for structural BMPs selection are as follows:

• Drainage area

• Land constraint

• Average storm rainfall measurement, influent pollutant concentrations, and rainfall intensity

• Runoff volumes and flow rates

• Soil types

• Site slopes

• Geology/topography

• Availability of land

• Future development/land use in watershed

• Depth to groundwater table

• Availability of supplemental water to support vegetative BMPs

• Susceptibility to freezing

• Safety and community acceptance

• Maintenance accessibility

• Short and long-term maintenance needs

Structural BMPs are categorized into the following groups:

• Infiltration Systems capture a volume of runoff and infiltrate it into the ground.
  

• Detention/Retain Systems capture a volume of runoff and retain that volume for subsequent release. Detention systems do not retain a significant permanent pool of water between runoff events. Retention systems, however, include a significant permanent pool volume of water between runoff events.
  

• Stormwater Wetland Systems are similar to retention/detention systems except the major surface and/or bottom of the systems contain wetland vegetations.
  

• Filtering Systems consist of various filtration media such as sand, soil, mulch layers, carbon, or membrane to remove pollutants from runoff.
  

• Biofiltration Systems are design to convey and remove pollutants from runoff via vegetated swales and strips.

• Bioretention Systems utilize soils to slow the runoff’s velocity and utilizes trees and shrubs to remove pollutants from stormwater.

• Hydrodynamic Separators are flow-through structures with a settling or separation unit to remove sediments and other pollutants.
  

• Manufactured BMP Systems are proprietary control measures that are specifically designed and sized by the manufacturer to intercept stormwater runoff and prevent the transfer of pollutants down stream. These are flow-through structures in that the designed rate of flow into the structure is regulated by the inflow pipe or structure hydraulics as opposed to traditional BMPs designed to store the entire water quality volume.

Copyright @ January,2005 Prepared by Transportation Environmental Resource Center (TERC)