Best Management Practices as They Relate to the Treatment of Stormwater Runoff in the Minnehaha Creek Watershed District
By James M. Hafner, Jr., Michael Panzer, P.E., and Kane Radel
- Abstract
- Introduction
- BMPs by Category
- I. Ponds
- II. Infiltration
- III. Filtering Systems
- IV. Open Channel
- V. Structural
- VI. Housekeeping
- VII. Planning
- VIII. Temp./Const. Erosion Measures
- Conclusion
- Bibliography
Abstract
As the percent of impervious area increases, the quality of stormwater runoff decreases. In an effort to protect and preserve the quality of surface water, ground water, and overall water quality, it becomes imperative to provide best management practices (BMP's) for the treatment of runoff prior to its entering surface water systems. These BMP's are permanent in nature, unlike temporary measures provided during the course of construction. Their primary function is to reduce velocities and remove sediment loading. The Minnehaha Creek Watershed District (MCWD), in an effort to make their stormwater management rules more practicable and feasible, has included BMP's as an integral component of those rules. Selection of a particular BMP or combination of BMP's is often peculiar to specific site conditions. The choice(s) should be made by considering what will provide the best protection for those conditions.
Introduction
Impervious areas include all the hard or impermeable surfaces in a landscape. As the percent of those areas increases there is a dramatic impact on water quality parameters - runoff coefficient, time of concentration, peak discharge rate, runoff volume, runoff velocity, sediment transport, and nutrient loading - compared to the same amount of pervious area. Lawmakers have created public agencies, such as the Minnehaha Creek Watershed District (MCWD), to administer regulations for control of water quantity and to improve the water quality of surface and ground water resources.
The MCWD initially created a stormwater management rule requiring wet detention ponds, designed to NURP standard, for all increases of imperviousness. Persuaded to review the effectiveness and feasibility of this rule, the MCWD Board of Managers initiated a task force that developed new thresholds for the application of stormwater treatment methods. Wet ponds would now be required only on sites over eight acres. Best management practices (BMP's) became the standard treatment method on sites under eight acres and required on all sites.
BMP's, by definition, include all stormwater treatment methods from wet detention ponds to sweeping parking lots. Just as in advertisement, what works in one area does not necessarily work in another. Some BMP's are more effective than others in specific conditions. A key to maximizing applications of BMP's is to determine the best use of each method and then provide a matrix to be used as a reference by permit applicants. This puts the responsibility of choosing BMP's on the applicant but allows the regulatory agency flexibility to add other BMP's as needed.
This paper will identify a variety of BMP's and group them in general categories; describe each method and its most effective use(s); finally, provide a matrix that can be adopted to a stormwater management rule as a guideline for treatment of runoff. The list is not intended to be all inclusive but rather identify those BMP's which are commonly used, or are recent developments worth considering. Some methods may have been inadvertently overlooked and could be added in the future. Given the importance of treating stormwater runoff, new methods are in development at all times and could also be considered as they become available.
BMPs by Category
I. PONDS:
Wet retention pond: A single cell permanent pool of standing water with no outlet.
Wet detention pond: Consists of a single permanent pool of standing water with a depth between three and five feet. Allows pollutants to settle and usually built with a defined forebay and outlet structure.
Extended detention pond: Stormwater detention basins designed to temporarily hold stormwater for an extended period of time, allowing the physical settling of pollutants. They differ from detention ponds in that they may be normally dry, have a shallow marsh, or have a permanent pool.
Dry pond:
Constructed wetland: Shallow pools that enhance growing conditions for marsh plants to maximize pollutant removal. They do not reproduce the ecological diversity of natural wetlands.
II. INFILTRATION:
Infiltration trenches: Shallow, excavated trenches filled with a coarse aggregate material and covered with a pervious soil layer. The trench serves as an underground reservoir. Runoff diverted into the trench exfiltrates from the bottom through the subsoil to the water table where an underdrain carries it to a stormwater conveyance system. May provide storage for snow melt. Site soils may prohibit the use of this method.
Infiltration basins: Large open depressions that store stormwater runoff while percolation occurs through bottom and sides. Soils, slope, geology and hydrogeology restrict the use of these basins.
Porous pavement: An alternative to conventional pavement, it diverts runoff through a porous asphalt layer into an underground reservoir constructed of crushed stone or gravel. The stored runoff gradually infiltrates into the subsoil and water table. Only used in low traffic areas and requires high maintenance.
Depressed pervious area: Commonly constructed to capture runoff from parking areas, it is lower than the surrounding surfaces, contains permeable soils and may have a built-in filtration system and/or a beehive drain. It can add to the aesthetics of a site through planting of trees or shrubs in the depression.
Infiltration tube: Usually a corrugated polyethylene tube with perforated dual wall, covered by a filter fabric sock and buried vertically in a bed of aggregate. A thin, permeable soil top cover may also be vegetated.
Peat-sand filters: Consists of a bed of peat and sand mixture constructed over a drainage system. The drainage system collects treated runoff and discharges it back to surface waters. While it is still experimental, it has been found to be very effective for removal of suspended solids and associated pollutants, removing up to 70% of phosphorous from runoff as reported in a few studies.
III. FILTERING SYSTEMS:
Buffer strips: Areas adjacent to the aquatic vegetation of streams, wetlands, and lakes serving to facilitate nutrient buffering while providing additional benefits of shading to reduce water temperature, habitat, aesthetics and soil stabilization. Recent engineering studies seem to conclude that buffer widths exceeding 25 - 35 feet do no enhance removal. Widths of 50 feet or more are beneficial for bird habitat and buffering from human intrusion.
Vegetated filter strips: Consist of grass or other close growing vegetation designed to receive overland sheet flow. The vegetation slows runoff and traps particulate pollutants. Effectiveness of these strips is a function of the length and slope of the filter strip, soil permeability, the size of the drainage area, and the type and density of the vegetative cover. Good performance for pollutant removal can be expected from widths of 50 to 75 feet and an additional 4 feet of width for every one percent of slope.
Vegetated swales: Broad shallow channels with a dense stand of vegetation established in them that are designed to promote infiltration and trap pollutants. The combination of low velocities and vegetative cover provides for pollutant settling or treatment by infiltration. This practice can also provide reduced volumes of runoff and peak discharges. Low gradient swales are perform better than steeper grades.
Sand filters: Sand can be used for filtering pollutants by placing it in gutter-like containers at the perimeter of parking lots. Runoff filters through the sand to an underdrain and is then conveyed to a storm sewer system or returned to surface waters. This requires relatively high maintenance.
Rain gardens: Areas of lower elevation adjacent to roads or parking areas, and can be incorporated into a landscaping plan in developments. Underlayment may consist of permeable materials or may resemble a mini-wetland. Hydric plants and shrubs are used to provide an aesthetically pleasing garden that also provides temporary storage of peak flows and infiltration of stormwater runoff.
Kenaf and other natural fibers: Kenaf is a natural fiberous material derived from the Hibiscus plant. Other fibers, including tobacco, have been tested for the propensity to filter metals and nutrients from stormwater. Available studies show Kenaf to have the highest effective removal rate. Generally used in conjunction with an infiltration swale or similar BMP, the fiber is relatively low in cost and requires periodic replacement.
IV OPEN CHANNEL:
Grassed channel: (see vegetated swale)
Dry swale:
V STRUCTURAL:
An in-ground containment structure designed to remove sediment and hydrocarbons from urban runoff. They are normally used close to the source before pollutants are conveyed to storm sewers or other BMP's. Typically used in areas of heavy traffic or high potential of petroleum spills. High stormwater flows will generally cause resuspension of settled materials.
Sump catch basins: Storm sewer catch basin, varying in diameter from 3 to 6 feet, with a storage chamber below the outlet pipe that will vary from 3 to 6 feet in depth. The sump or storage chamber provides residence time for runoff, allowing the physical settling of sediments. High stormwater flows will generally cause resuspension of settled materials.
Floatable skimmers: Devices used to retain floating debris and oil in detention areas. The debris and oil may be skimmed off the surface. Some oils will biodegrade. Materials that sink to the bottom may then be removed through routine maintenance.
Curb cuts: Areas cut out of street curbs that direct runoff into pervious areas or infiltration devices.
Slope impervious toward pervious: As the description suggests, the grade of impervious areas are sloped to direct runoff into pervious areas.
Stormceptor: Man-hole type device with two chambers designed for smaller sites specific to removing sediment, especially large grains. Provides some oil skimming. Not effective in large rain events.
Stormtreat: Combines hydric plants with a contained filtration system. In otherwords, a mini-wetland planted in a container designed to use the vegetation and built-in filters to treat runoff. Will treat up to one acre sites, with discharge to surrounding and
V2B1: Similar to Stormceptor.
Vortechnics: Similar to Stormceptor.
Stormwater management filters: In-line filters that make use of organic materials as the filtering medium. Used in sequence with methods such as Stormceptor, V2B1, etc. it can remove soluble phosphorous which does not get removed in the other methods.
VI HOUSEKEEPING:
Street/parking lot sweeping: The removal of grit, debris, and trash from urban impervious areas such as streets, parking lots and sidewalks. Effectiveness depends on frequency and method. The most effective method is a vacuum sweeper, dry (does not use water), and uses a filter to control dust.
Litter control: Leaves and lawn clippings are major sources of phosphorous. Removal of these as well as trash, pet waste, oil, and general debris before they enter surface waters is highly recommended.
Phosphorous free fertilizers: Involves the control of rate, timing, and method of fertilizer application as well as content (low or no phosphorous).
Catch basin and grit chamber cleaning: Thoroughly cleaned out at least twice per year, spring and fall.
Storage of de-icing materials: Road salts and related de-icing materials should be kept under cover to prevent leaching and runoff being transported to surface and ground waters.
Animal waste collection: Collection and proper disposal of pet wastes; regulating feed lots for control of runoff and handling of manure.
Reporting of BMP maintenance: Accurate reporting and tracking of required maintenance is critical to ensure the maintenance is actually performed as required and ensure BMP devices are functioning at optimum capacity.
VII PLANNING:
Permeable spillover parking: In situations where a commercial concern or institution has a need for overflow parking, increases in impervious areas may be eliminated through this method. A substructure, such as a geogrid, is placed in the ground below grade. The spaces of the grid are filled with a permeable material and vegetation is planted at the surface. The grid provides support for the vehicles, while allowing for infiltration of stormwater. Surface vegetation controls erosion and provides some peak flow control.
Islands in cul de sacs: These structures are used to reduce total impervious area and still allow safe passage of traffic. They can be turned into aesthetic amenities through planting of trees, shrubs, and other vegetation.
Cluster housing: Grouping living units in close proximity to one another can reduce total impervious area through fewer sidewalks, driveways, streets etc. and maximize greenspace areas.
Narrower street widths: Streets widths can, in many cases, be reduced from the standard recommended widths. Consideration of traffic levels, parking, and location may allow for a narrower width and less impervious area.
Fewer paved sidewalks: Impervious areas can be reduced by constructing walkways with vegetation or wood chips for surface covering or by using permeable surfaces.
Shorter driveways: Locate house pads and/or plan landscaping to reduce driveway lengths and imperviousness.
Shared driveways: Coincides with cluster housing or multi-unit housing.
Greenways/corridors: Increasing the area set aside for green spaces and recreation/wildlife corridors will limit the space used for impervious uses.
Fit structure to slope of lot: Keep on-site activities off steeper slopes that are more susceptible to erosion.
Suitable soils: Plan construction and traffic so that highly erodable soils are avoided.
Soil stabilization: Use of mulches and blankets to stabilize soil and reduce erosion due to wind and water.
Re-aerate compacted soils: Soils become compacted and relatively impermeable after the high traffic of construction. Discing or tilling before sodding, seeding or other restabilization efforts will reintroduce permeable conditions.
Minimize clearing: Preserving more trees and other vegetation may reduce the area of soil being disturbed, reducing the incidence of erosion.
Phasing of construction: Plan construction activity to minimize the total area of disturbed soil at any given point in time.
Roof storage: Provides for peak flow control and some water quality benefits.
Recirculate/reuse stormwater: Capture stormwater runoff from roof surfaces and reuse it for heating/cooling or amenities such as fountains, landscaping or educational displays.
VII TEMPORARY/CONSTRUCTION EROSION MEASURES:
Temporary sediment basin: An impoundment that temporarily stores sediment laden runoff releases it at a reduced rate, preventing sediment from being transported off-site.
Temporary sediment trap: A small temporary ponding area formed by constructing an earthen berm with an outlet across a swale. Intended for smaller sites.
Silt fence: A temporary sediment barrier consisting of a filter fabric which is attached to supporting posts and trenched in to the ground, filtering runoff as it passes through the fabric.
Straw bale sediment trap: A row of entrenched and anchored straw bales installed so that they filter sediment laden runoff.
Flotation silt curtain: A silt barrier for use within a lake, pond or stream. The curtain consists of a floating filter fabric weighted at the bottom and attached to afloating device at the top. It is used to isolate silt laden runoff from migrating off a construction site.
Temporary rock construction entrance: This is a stone pad located at points where vehicles leave a construction sight. The purpose is to remove mud from vehicle tires before the vehicle leaves the site.
Diversion: A channel constructed across a slope with a supporting ridge on the lower side. Diversions are used to intercept runoff and divert it to stabilized outlets at non-erosive velocities.
Temporary diversion: A temporary ridge of compacted soil, a channel or a combination of these located across a slope above a disturbed area. They are used to divert runoff away from an unprotected slope to a stable outlet or a sediment trapping structure.
Temporary right-of-way diversion: A ridge of compacted soil, loose rock or gravel placed across a disturbed right-of-way or similar long sloping area.
Storm water conveyance channel: A permanent waterway, shaped and lined with appropriate vegetation or structural material that can carry storm water runoff.
Subsurface drain: A perforated pipe, tubing, or tile installed below the ground surface to intercept and transport water.
Temporary slope drain: A flexible conduit extending from the top to the bottom of a disturbed slope.
Grade stabilization structure: A permanent structure or series of structures designed to drop water to a lower elevation without erosion.
Outlet protection: The use of protective measures to prevent erosion at the outlet of pipes, culverts, or paved channels by breaking up velocities and turbulence of runoff.
Lot benching: The grading of lots within a subdivision so that runoff from each one is directed to a stable outlet rather than to an adjacent lot. This is most practicable on hilly or rolling terrain.
Temporary stream crossing: A temporary road crossing constructed over a stream to transport construction traffic.
Riprap: A permanent, erosion-resistant protective layer of loose stones, normally installed over a geotextile fabric.
Structural streambank protection: The stabilization of streambanks with permanent structural measures including, but not limited to, riprap, modular concrete or gabions.
Temporary seeding: The establishment of temporary vegetative cover on disturbed areas by seeding with suitable fast-growing annual vegetation.
Permanent seeding: The establishment of perennial herbaceous cover on a adisturbed area.
Sodding: The stabilization of a disturbed area with perennial vegetation by laying sod.
Mulching: The application of plant residues or other suitable matrials to the soil surface. Mulch prevents erosion by protecting the soil from raindrop impact and reducing the velocity of overland flow. It will also promote the germination and growth of seedlings by preserving moisture, providing protection for temperature extremes and controlling weeds.
Conclusion
Some BMP's make significant contributions to the treatment of stormwater. However, most of them have not been subjected to enough or any testing to prove quantitative effectiveness. Available documentation shows infiltration to be the most effective type and can be adapted to smaller sites. Soil permeability is a limiting factor. Wet ponds are next in effectiveness but the area of land required to construct a useful pond can be limit this method to larger sites or to regional treatment. Buffer strips and vegetative filters are third in effectiveness. Presently, there appears to be considerable interest in this method and on-going research is expected to develop its function.
Choosing a BMP or combination of methods will be site specific. Tables I and II, included with this report, indicate preferred methods based on effectiveness, removal efficiency, and maintenance, as supported by the literature referenced. Many of these methods may require soils testing to verify that on-site conditions will provide the expected results. This should be taken into consideration as rule changes are considered. Removal rates, in many cases, were derived from small sampling numbers. While representative, they reflect a median range that may vary considerably with site specific conditions. Efficiency of BMP's used in tandem is not cumulative. That is, 25% for one and 13% for another and 20% for a third does not total 58% removal rate. They must be looked at individually.
Finally, just like the MCWD rules, this a living document. Developments in the field of erosion control and water quality frequently provide new results and documentation. These changes should be considered as they become available and rules adjusted as needed to remain current and effective.
Bibliography
Caraco, Deborah and Richard Claytor. 1997. Stormwater BMP Design - Supplement for Cold
Climates. U.S. Environmental Protection Agency. Washington, D.C.
Brown, Whitney and Thomas Schueler. 1997. National Pollutant Removal Performance
Database for Stormwater BMP's. Center for Watershed Protection. Ellicott City. MD
Prey, Jeffrey. The Wisconsin Stormwater Manual - Part One: Overview. Wisconsin Department of Natural Resources. Madison, WI
Minnesota Pollution Control Agency. 1989. Protecting Water Quality in Urban Areas. St. Paul, MN
Rice Creek Watershed District. Stormwater Infiltration Guidance. Arden Hills, MN
Nonpoint Source News-Notes. August/September 1995, Issue #42. Urban Runoff Notes. Prince George County, MD
Natural Resource Conservation Service. 1996 - 1999. Various bulletins on specific BMP's.
Center for Watershed Protection. 1994. The Importance of Imperviousness. Watershed Protection Techniques, Vol. 1, No. 3. Ellicott City, MD
Minnesota Board of Water and Soil Resources. Alternative Stormwater Best Management Practice Guidebook. St. Paul, MN
Water Resources Clip Art. University of Wisconsin- Extension, Environmental Resources Center. Madison, WI