Structural Stormwater Best Management Practices for Small Commercial and Residential Applications Timothy Bruno Watershed Manager PA Department of Environmental Protection
What are STRUCTURAL BMPs? Physical features or measures constructed to manage the volume, rate, and water quality of stormwater runoff.
What is considered a small development? Based on Act 167 Stormwater Management municipal ordinances in Northwest Pennsylvania counties, a small development proposing from 2500 to 5000 square feet of impervious surface will be required to implement structural volume control BMPs but will not be required to complete full engineering. Most developments in this range will consist of small individual commercial and residential properties. The average size single family home ranges from 2000-4000 square feet of impervious surface. Although convenient for small developments, the following BMPs can be used for any size area of impervious surface when designed properly.
What are raingardens? Raingardens are shallow depressions in the ground that contain amended soils that are planted with vegetation. They receive stormwater from impervious surfaces and mitigate it through evapotranspiration and and infiltration into the ground.
Raingarden Design Considerations The type of soils onsite will determine how quickly stormwater will infiltrate into the underlying soils. Many times the native site soils are amended to increase porosity and hydrologic conductivity (ability to infiltrate). The seasonal groundwater level may affect how you design and build a raingarden. Raingardens should be placed more than 10 feet from the foundation of any house or structure unless approved by a design professional. Instead of building one large raingarden for the entire site, consider building 2 or 3 smaller gardens to handle stormwater from areas of the site. Vegetation should be low-maintenance and tolerant of saturated and dry soil conditions. Native species are preferred.
Designing Raingardens Step 1: Measure the square footage of impervious surface area that will contribute stormwater to the garden. Step 2: Divide the impervious square footage by 5. This will give the total required surface area of the raingarden at a generally-accepted 5:1 stormwater loading ratio. For the above calculations to be valid, the raingarden must have a shallow ponding depth of at least 6 inchs, and a 2 to 2.5 feet depth of amended soils. Infiltration should be approximately 1 inch per hour.
Constructing a Raingarden Excavate the area to approximately 1.5 to 2 feet deep being careful not to compact the native underlying soils. Cover area with 2 inches of compost and 2 inches of sand. Rototill the compost and sand into the native soil layer. Repeat the gradual addition of compost/sand and rototilling until the soil level is at approximately 6 inches from the surface. Plant the vegetation. Top dress with a layer of mulch/compost. Be sure to water plants immediately, then on a regular basis until the vegetation is self-maintaining. The raingarden will need regular weeding, maintenance, and watering during the establishment period. Maintenance will significantly decrease over time (2 years).
Raingardens in Poor Soil Conditions Gravel w/geotex Overflow Structure Raingardens can be designed to function in many types of soil conditions. Underdrain C & D type soils may require underdraining. High shallow groundwater depths require a shallow raingarden bottom as well as underdraining. Original Ground Overflow Structure Slide Courtesy HRG, Inc. Outflow These types still provide water quality improvement and runoff rate reduction. 8-12 soil depth 12 min (200 sf raingarden) Slide Courtesy HRG, Inc.
Pictures of Raingardens
What are drywells and seepage beds? Drywells and seepage beds direct stormwater from roof leaders and downspouts into perforated, pre-cast tanks, or rock-filled holes that infiltrate the water into the ground.
Drywell and Seepage Bed Design Considerations Drywells provide infiltration of a limited roof area, while seepage beds have the potential to accommodate large impervious areas. Design should have an intermediate sump box that allows sediments and solids to settle out prior to entering the drywell and allow cleanout. Also a removable filter with a screened bottom should be placed prior to the sump to filter out leaves and other debris. Drywells should be placed no closer than 10 feet from the foundation of a structure, unless approved by a professional. Frost depth should be considered to prevent frost heave. Gutter guards should be considered when using a drywell. Drywell should be designed to draw down between 24 and 72 hours, and should be sized in the same manner of infiltration basins. In less-than-straighforward applications, a design professional should be consulted.
Constructing a Simple Drywell Excavate the area to below the frost line to avoid frost heave when using pre-cast container or sump. Size hole corresponding to size of contributing impervious area. Use straight drainage lines for improved flow. Line infiltration hole with heavy pervious geotextile fabric. Fill hole with 1 to 3 stone or utilize pre-cast or preconstructed plastic tank. An overflow relief line or several drywells in a series will help to avoid surcharging of the system in soils with lower infiltration.
What is a seepage bed? A seepage bed is a gravel bed that is fed stormwater through distribution lines connected to a cistern. They share much in common with infiltration trenches and can be used successfully in tandem with Water Reuse strategies. Due to their relative complexity, they are more than likely to be designed and installed by professionals.
What is an Infiltration Trench? A linear stormwater BMP consisting of a continuously perforated pipe at a minimum slope in a geotextile-lined, stone-filled trench which allows infiltration to occur.
Infiltration Trench Design Considerations The type of soils onsite will determine how quickly stormwater will infiltrate into the underlying soils and the seasonal groundwater level will affect whether an infiltration trench is appropriate. Slope can be no greater than 1%, but can be stepped to accommodate sloped sites. Trenches work well when combined with linear features such as driveways and parking areas that can sheet flow into the BMP. Trenches should be placed more than 10 feet from the foundation of any house or structure unless approved by a design professional. Trenches can have vegetated or gravel surfaces, and can even be bricked over to provide useable patio space. Evenly spaced cleanouts should be installed and an appropriate overflow method established, although pipe-less applications are appropriate for small development.
Designing Simple Infiltration Trenches Step 1: Measure the square footage of impervious surface area that will contribute stormwater to the trench. Step 2: Divide the impervious square footage by 4. This will give the total required surface area of the raingarden at a generallyaccepted 4:1 stormwater loading ratio. For the above calculations to be valid, the trench must have a 2.5 feet depth of aggregate/gravel. Infiltration should be approximately 1 inch per hour.
Constructing a Simple Infiltration Trench Excavate the required surface area of the trench to 2.5 feet depth. Line trench with heavy pervious geotextile fabric and position observation pipe. Fill halfway with 1 to 3 aggregate, level, then install perforated pipe if using. If no pipe, fill entire trench to surface. Direct downspouts to trench through lines or surface flow. If connecting directly to downspouts through underground pipe, a sump box with cleanou and a leaf filter are appropriate.
What are Green (Vegetative) Roofs? Green roofs consist of a veneer of vegetation on a building that provides roughly the same hydrologic response as the native ground. In most cases, green roofs must be designed into a structure and have a high initial cost.
Green Roof Design Considerations The type of Green Roof generally designed for residential and small commercial applications is an extensive design which requires little maintenance except yearly weeding. They can substantially reduce interior heating and cooling costs. Not a do-it-yourself type of project, although some enterprising home owners have adopted tray system green roofs with success. An integrated green roof will need to have much greater structural strength to support the additional weight of the roof with growing medium and vegetation, in addition to saturated conditions and snow load. Due to the increased costs to the structure, green roofs are not usually a viable alternative and tend to be either a stormwater management last resort or conscious living decision.
What is Stormwater Harvesting and Reuse? Stormwater Harvesting includes directing stormwater to storage vessels and reusing the water for flushing toilets and landscape irrigation.
Stormwater Harvesting and Reuse Design Considerations Most appropriate for developments that may have insufficient water access (wells go dry often) or is in an area very high public water rates. They can substantially reduce toilet flushing and irrigation costs. Not a do-it-yourself type of project. Requires a full plumbing design in new construction, and a full redesign if retrofitting. Presents a great opportunity to incorporate harvesting as a component in a treatment train (with infiltration trenches, seepage beds, etc.).
So how much does it all cost?
Structural Best Management Practice Construction Costs Estimated Costs Level Spreader Vegetated Filter Strip Dry Well $5 to $20/foot $0.30 /ft 2 for seed $0.70 /ft 2 for sod $4-$9/ft 3 of storage Rain Garden $3-$5/ft 2 Vegetated Swales $0.30-$0.70/ ft 2 Structural Best Management Practice Porous Pavement with Infiltration Bed Construction Costs Year Source $2K-$2.5K/parking space 2005 PA BMP Manual Infiltration Basin $2.5K-$3.5K/ac 2005 PA BMP Manual (not including excavation or piping) Subsurface Infiltration Bed $5.70/ft 2 2005 PA BMP Manual Infiltration Trench $4-$9/ft 3 of storage 1991-1997 SWRPC, 1991; Brown and Schueler, 1997 Rain Garden / Bioretention $5-$7/ft 3 of storage 2005 PA BMP Manual Dry Well / Seepage Pit $4-$9/ft 3 of storage 1991-1997 SWRPC, 1991; Brown and Schueler Constructed Filter Varies Vegetated Swale $8.5-$50/linear foot 1991 SEWRPC, 1991 Vegetated Filter Strip Varies Infiltration Berm and Retentive Grading Varies Vegetated Roof $8-$15/ft2 2004 PA BMP Manual Rooftop Runoff - Capture and Reuse $1.25/gallon of storage 2005 PA BMP Manual Constructed Wetland $30K-$65K/ac 2004 EPA, 1999 Wetland Fact Sheet Wet Pond / Retention Basin $25K-$50K/ac-ft of storage 2004 EPA, 1999 Wet Detention Pond Fact Sheet Dry Extended Detention Basin 12.4[Volume for 10 year-storm]0.760 1997 Brown and Schueler, 1997 (includes permitting) Water Quality Filter Varies Riparian Buffer Restoration Varies Landscape Restoration >$3K/ac depending on facility 2005 PA BMP Manual Soils Amendment and Restoration $0.8K - $1K/ac 2005 PA BMP Manual (for either tilling or composting)
Questions?