Green Technology Best Management Practice Approach for Stormwater Management

Good stormwater management used to mean moving the water from where it fell to streams that could move it away from developed areas as quickly and efficiently as possible.  Unfortunately, this approach leads to high pollutant loading of our waterways, and needs to be changed for the health of our watersheds.

In recent years a "green technology" approach has been designed to intercept runoff from rooftops, parking lots, roads, and other impervious surfaces as close as possible to its source.  It was then directed it into vegetative recharge/filtration facilities incorporated into the overall site design and runoff conveyance system.  The green technology system includes a quantitative estimation of pollutant loads, and projects how well a particular design will remove such pollutants.  It's a technology capable of providing realistic estimates of pollutant loading and removal while addressing the hydrologic and hydraulic parameters involved in urban site design.  The detailed design principles based on this technology are incorporated into the Delaware Urban Runoff Management Model (DURMM).

Delaware's green technology best management practices (BMPs) include conservation site design, impervious area disconnection, conveyance of runoff through swales and biofiltration swales, filtration through filter strips, terraces, bioretention facilities, and recharge through infiltration facilities.  These practices are incorporated into green technology at the site engineering level.  Unlike structural BMPs, such as detention ponds, green technology BMPs don't appear highly complex.  Their proper design and implementation rely on the same hydrologic and hydraulic principles as their structural cousins, and result in a "greener" appearance. 

Centralized treatment and/or storage facilities located at the “end of pipe” discharge from developed sites are classified as structural best management practices.  While structural BMPs such as stormwater ponds and wetlands can be effective in controlling peak flows from the site, regulatory requirements for these structures do not address storms that erode stream banks, and do little or nothing to promote recharge.  Structural systems can contribute to downstream flooding when discharges from multiple on-site structural BMPs overlap. Although they can be effective in pollutant removal, structural systems are usually not designed for groundwater recharge, consume space, and require extensive maintenance.  Wet detention structural BMPs often contribute to elevated stream temperatures and discharge algae laden effluent, which can substantially degrade the bottom-dwelling organisms in the receiving stream.

Green Technology Best Management Practices

Conservation Site Design — These are site design methods to reduce the extent of impervious surfaces on-site and increase the extent of wooded areas, as expressed in the “Conservation Design Manual for Stormwater Management”, produced by DNREC and the Brandywine Conservancy (1997).  This manual addresses many of the background issues in urban runoff and discusses specific methods in detail.   DURMM provides a quantitative approach to define the benefits of conservation design.

Source Area Disconnection — Disconnection is the process of directing runoff from impervious surfaces over adjacent vegetated surfaces, providing infiltration and pollutant removal.  DURMM quantifies the runoff reductions by disconnecting flow from impervious surfaces as it discharges onto adjacent permeable areas.

Filter Strips — Filter strips spread runoff uniformly over a vegetated surface, providing infiltration and pollutant removal.  Filter strips can provide substantial treatment if not overloaded by sediment and runoff.  DURMM quantifies the runoff reductions and pollutant removal of filter strips.

Biofiltration Swales/Grassed Swales — A swale is an elongated depression in land that is at least seasonally wet or marshy, is usually heavily vegetated, and is normally without flowing water.  Biofiltration swales convey runoff at shallow flow depths.  They can be very effective in removing Total Suspended Solids (TSS) and adsorbed metals, although less effective in terms of nutrients.   While swales are not thought to be capable of quantity management, designs incorporating check dams can provide substantial attenuation of peak flows.  DURMM quantifies the runoff reductions and pollutant removal of overland conveyance through properly designed swales.

Terraces — Terraces are swales extending across gentle slopes designed to intercept runoff and increase the potential for infiltration.  In terms of pollutant removal, terraces operate as filter strips, as runoff flows into them from upslope and are similar to swales in terms of runoff response.  DURMM quantifies the runoff reductions and pollutant removal of overland conveyance through properly designed terraces.

Bioretention Structures — These are landscaped pocket depressions designed to infiltrate runoff through an engineered soil media.  Incorporated into the urban landscape, they can provide substantial filtering and nutrient transformations before runoff is discharged into the conveyance system.  Research suggests that bioretention structures can be designed to provide substantial soluble phosphorus removal capabilities, unlike most other BMPs.  DURMM quantifies the runoff reductions and pollutant removal of overland conveyance through properly bioretention structures.

Infiltration Practices — Most green technology BMPs incorporate infiltration as part of the treatment process.  Specific infiltration facilities include trenches. Infiltration trenches located in swales provide additional wetted surface area and storage volume, and often they can be designed to penetrate shallow impermeable soil profiles to recharge deeper soil horizons.  DURMM quantifies the runoff reductions of infiltration trenches.

Complementing these engineered BMPs, riparian buffer systems (RBS) and stream bank restoration (SBR) BMPs are other important green technology systems that can enhance receiving waters.  They provide substantial improvements in stream habitat and stability, as well as reducing pollutants from urban runoff.  RBSs bring benefits to streams through shading, bank stabilization and litterfall.  RBSs can also provide substantial runoff filtering and pollutant removal when conditions are favorable.   Since RBSs are sensitive to concentrated flows, design procedures to ensure sheet flow through level spreaders, filter strips and parallel swales can be incorporated into their design.  A companion document specifically focused on RBS design is being prepared by DNREC.

Stream bank restoration differs from other BMPs in that it provides no direct hydrological controls, nor does it remove pollutants from upland runoff.  However, by stabilizing eroding stream banks, it may be the most effective mitigation measure for unstable streams stressed by urban runoff.

For additional information on green technology BMPs” and/or the Delaware Urban Runoff Management Model (DURMM), please contact the Division of Soil & Water Conservation at (302) 739-4411.