New Development Management Measure

Note: This information is provided for reference purposes only. Although the information provided here was accurate and current when first created, it is now outdated.

II. URBAN RUNOFF

A. New Development Management Measure

By design or performance:

After construction has been completed and the site is permanently stabilized, reduce the average annual total suspended solid (TSS) loadings by 80 percent. For the purposes of this measure, an 80 percent TSS reduction is to be determined on an average annual basis, or
Reduce the postdevelopment loadings of TSS so that the average annual TSS loadings are no greater than predevelopment loadings, and

To the extent practicable, maintain postdevelopment peak runoff rate and average volume at levels that are similar to predevelopment levels.
Sound watershed management requires that both structural and nonstructural measures be employed to mitigate the adverse impacts of storm water. Nonstructural Management Measures II.B and II.C can be effectively used in conjunction with Management Measure II.A to reduce both the short- and long-term costs of meeting the treatment goals of this management measure.

1. Applicability

This management measure is intended to be applied by States to control urban runoff and treat associated pollutants generated from new development, redevelopment, and new and relocated roads, highways, and bridges. Under the Coastal Zone Act Reauthorization Amendments of 1990, States are subject to a number of requirements as they develop coastal nonpoint source (NPS) programs in conformity with this management measure and will have flexibility in doing so. The application of management measures by States is described more fully in Coastal Nonpoint Pollution Control Program: Program Development and Approval Guidance, published jointly by the U.S. Environmental Protection Agency (EPA) and the National Oceanic and Atmospheric Administration (NOAA) of the U.S. Department of Commerce.

For design purposes, postdevelopment peak runoff rate and average volume should be based on the 2-year/24-hour storm.

2. Description

This management measure is intended to accomplish the following: (1) decrease the erosive potential of increased runoff volumes and velocities associated with development-induced changes in hydrology; (2) remove suspended solids and associated pollutants entrained in runoff that result from activities occurring during and after development; (3) retain hydrological conditions to closely resemble those of the predisturbance condition; and (4) preserve natural systems including in-stream habitat. For the purposes of this management measure, "similar" is defined as "resembling though not completely identical."

During the development process, both the existing landscape and hydrology can be significantly altered. As development occurs, the following changes to the land may occur (USEPA, 1977):

Soil porosity decreases;
Impermeable surfaces increase;
Channels and conveyances are constructed;
Slopes increase;
Vegetative cover decreases; and
Surface roughness decreases.

These changes result in increased runoff volume and velocities, which may lead to increased erosion of streambanks, steep slopes, and unvegetated areas (Novotny, 1991). In addition, destruction of in-stream and riparian habitat, increases in water temperature (Schueler et al., 1992), streambed scouring, and downstream siltation of streambed substrate, riparian areas, estuarine habitat, and reef systems may occur. An example of predicted effects of increased levels of urbanization on runoff volumes is presented in Table 4-4 (USDA-SCS, 1986). Methods are also available to compute peak runoff rates (USDA-SCS, 1986).

The annual TSS loadings can be calculated by adding the TSS loadings that can be expected to be generated during an average 1-year period from precipitation events less than or equal to the 2-year/24-hour storm. The 80 percent standard can be achieved by reducing, over the course of the year, 80 percent of these loadings. EPA recognizes that 80 percent cannot be achieved for each storm event and understands that TSS removal efficiency will fluctuate above and below 80 percent for individual storms.

Management Measures II.A, II.B, and II.C were selected as a system to be used to prevent and mitigate the problems discussed above. In combination, these three management measures applied on-site and throughout watersheds can be used to provide increased watershed protection and help prevent severe erosion, flooding, and increased pollutant loads generally associated with poorly planned development. Implementation of Management Measures II.B and II.C can help achieve the goals of Management Measure II.A.

Structural practices to control urban runoff rely on three basic mechanisms to treat runoff: infiltration, filtration, and detention. Table 4-5 (53k) lists specific urban runoff control practices that relate to these and includes information on advantages, disadvantages, and costs. Table 4-6 presents site-specific considerations, regional limitations, operation and maintenance burdens, and longevity for these practices.

Infiltration devices, such as infiltration trenches, infiltration basins, filtration basins, and porous and concrete block pavement, rely on absorption of runoff to treat urban runoff discharges. Water is percolated through soils, where filtration and biological action remove pollutants. Systems that rely on soil absorption require deep permeable soils at separation distances of at least 4 feet between the bottom of the structure and seasonal ground water levels. The widespread use of infiltration in a watershed can be useful to maintain or restore predevelopment hydrology, increase dry-weather baseflow, and reduce bankfull flooding frequency. However, infiltration systems may not be appropriate where ground water requires protection. Restrictions may also apply to infiltration systems located above sole source (drinking water) aquifers. Where such designs are selected, they should be incorporated with the recognition that periodic maintenance is necessary for these areas. Long-term effectiveness in most cases will depend on proper operation and maintenance of the entire system.

NOTE: Infiltration systems, some filtration devices, and sand filters should be installed after construction has been completed and the site has been permanently stabilized. The State of Maryland has observed a high failure rate for infiltration systems. Many of these failures can be attributed to clogging due to sediment loadings generated during the construction process and/or the premature use of the device before proper stabilization of the site has occurred. In cases where construction of the infiltration system is necessary before the cessation of land-disturbing activities, diversions, covers, or other means to prevent sediment-laden runoff from entering and clogging the infiltration system should be used (State of Maryland DNR, personal communication, 1991).

Filtration practices such as filter strips, grassed swales, and sand filters treat sheet flow by using vegetation or sand to filter and settle pollutants. In some cases infiltration and treatment in the subsoil may also occur. After passing through the filtration media, the treated water can be routed into streams, drainage channels, or other waterbodies; evaporated; or percolated into ground water. Sand filters are particularly useful for ground-water protection. The influence of climatic factors must be considered in the process of selecting vegetative systems.

Detention practices temporarily impound runoff to control runoff rates, and settle and retain suspended solids and associated pollutants. Extended detention ponds and wet ponds fall within this category. Constructed urban runoff wetlands and multiple-pond systems also remove pollutants by detaining flows that lead to sedimentation (gravitational settling of suspended solids). Properly designed ponds protect downstream channels by controlling discharge velocities, thereby reducing the frequency of bankfull flooding and resultant bank-cutting erosion. If landscaped and planted with appropriate vegetation, these systems can reduce nutrient loads and also provide terrestrial and aquatic wildlife habitat. When considering the use of these devices, potential negative impacts such as downstream warming, reduced baseflow, trophic shifts, bacterial contamination due to waterfowl, hazards to nearby residents, and nuisance factors such as mosquitoes and odor should be considered. Siting development in wetlands and floodplains should be avoided. Where drainage areas are greater than 250 acres and ponds are being considered, inundation of upstream channels may be of concern.

Constructed wetlands and multiple-pond systems also treat runoff through the processes of adsorption, plant uptake, filtration, volatilization, precipitation, and microbial decomposition (Livingston and McCarron, 1992; Schueler et al., 1992). Multiple-pond systems in particular have shown potential to provide much higher levels of treatment (Schueler et al., 1992). In general, the potential concerns and drawbacks applicable to wet ponds apply to these systems. Many of these systems are currently being designed to include vegetated buffers and deep-water areas to provide habitat for wildlife and aesthetic benefits. Where such designs are selected, they should be incorporated with the recognition that periodic maintenance is necessary. Long-term effectiveness in most cases will depend on proper operation and maintenance of the entire system. Refer to Chapter 7 for additional information on constructed wetlands.

Water quality inlets, like ponds, rely on gravity settling to remove pollutants before ponds discharge water to the storm sewer or other collection system. Water quality inlets are designed to trap floatable trash and debris. When inlets are coupled with oil/grit separators, hydrocarbon loadings from areas with high traffic/parking volumes can be reduced. However, experience has shown that these devices have limited pollutant-removal effectiveness and should not be used unless coupled with frequent and effective clean-out methods (Schueler et al., 1992). Although no costs are currently available, proper maintenance of water quality inlets must include proper disposal of trapped coarse-grained sediments and hydrocarbons. The costs of clean-out and disposal may be significant when contaminated sediments require proper disposal.

Inadequate maintenance is often cited as one of the major factors influencing the poor effectiveness of structural practices. The cost of long-term maintenance should be evaluated during the selection process. In addition, responsibility for maintenance should be clearly assigned for the life of the system. Typical maintenance requirements include:

Inspection of basins and ponds after every major storm for the first few months after construction and annually thereafter;
Mowing of grass filter strips and swales at a frequency to prevent woody growth and promote dense vegetation;
Removal of litter and debris from dry ponds, forebays, and water quality inlets;
Revegetation of eroded areas;
Periodic removal and replacement of filter media from infiltration trenches and filtration ponds;
Deep tilling of infiltration basins to maintain infiltrative capability;
Frequent (at least quarterly) vacuuming or jet hosing of porous pavements or concrete grid pavements;
Quarterly clean-outs of water quality inlets;
Periodic removal of floatables and debris from catch basins, water quality inlets, and other collection-type controls; and
Periodic removal and proper disposal of accumulated sediment (applicable to all practices). Sediments in infiltration devices need to be removed frequently enough to prevent premature failure due to clogging.

Operation and Maintenance

Proper operation and maintenance of structural treatment facilities is critical to their effectiveness in mitigating adverse impacts of urban runoff. The proper installation and maintenance of various BMPs often determines their success or failure (Reinalt, 1992).

During a field study of 51 urban runoff treatment facilities, the Ocean County, New Jersey, planning and engineering departments determined that the major source of urban runoff problems was a failure of the responsible party to provide adequate facility maintenance. The causes of this failure are complex and include factors such as lack of funding, manpower, and equipment; uncertain or irresponsible ownership; unassigned maintenance responsibility; and ignorance or disregard of potential consequences of maintenance neglect (Ocean County, 1989). The analysis of the field data collected during the study indicated the following trends:

Bottoms, side slopes, trash racks, and low-flow structures were the primary sources of maintenance problems.
Infiltration facilities seemed to be more prone to maintenance neglect and were generally in the poorest condition overall.
Retention facilities appeared to receive the greatest amount of maintenance and generally were in the best condition overall.
Publicly owned facilities were usually better maintained than those that were privately maintained.
Facilities located at office development sites were better maintained than those at commercial or institutional sites; facilities in residential areas received average maintenance.
Highly visible urban runoff facilities were generally better maintained that those in more remote, less visible locations (Ocean County, 1989).

The following program elements should be considered to ensure the proper design, implementation, and operation and maintenance of runoff treatment and control devices (adapted from The State of New Jersey Ocean County Demonstration Study's Storm Water Management Facilities Maintenance Manual):

Adoption, promulgation, and implementation of planning and design standards that eliminate, reduce, and/or facilitate facility maintenance; coordination with other regulatory authorities with jurisdiction over runoff facilities;
Establishment of a comprehensive design review program, which includes training and education to ensure adequate staff competency and expertise;
Design standards published in a readily understandable format for all permittees and responsible parties including regulatory authorities; the provision of clear requirements to promote the adoption of planning and standards and expedite facility review and approval;
Publication of specific obligations and responsibilities of the runoff facility owner/operator including procedures for the identification of owners/operators who will have long-term responsibility for the facility;
Development of a procedure for addressing maintenance default by negligent owner/operators;
Periodic review and evaluation of the runoff management program to ensure continued program effectiveness and efficiency;
Runoff facility construction inspection program; and
Provisions for public assumption of runoff control facilities.

3. Management Measure Selection

This management measure was selected because of the following factors.

Removal of 80 percent of total suspended solids (TSS) is assumed to control heavy metals, phosphorus, and other pollutants.
A number of coastal States, including Delaware and Florida, and the Lower Colorado River Authority (Texas) require and have implemented a TSS removal treatment standard of at least 80 percent for new development.
Analysis has shown that constructed wetlands, wet ponds, and infiltration basins can remove 80 percent of TSS, provided they are designed and maintained properly. Other practices or combinations of practices can be also used to achieve the goal.
The control of postdevelopment volume and peak runoff rates to reduce or prevent streambank erosion and stream scouring and to maintain predevelopment hydrological conditions can be accomplished using a number of water quality and flood control practices. Many States and local governments have implemented requirements that stipulate that, at a minimum, the 2-year/24-hour storm be controlled.

Management Measure II.A.(1)(b) was selected to provide a descriptive alternative to Management Measure II.A.(1)(a). Where preexisting conditions do not already present a water quality problem, preservation of predevelopment TSS loading levels is intended to promote TSS loading reductions that adequately protect surface waters and are equivalent to or greater than the levels achieved by Management Measure option II.A.(1)(a). In some cases, local conditions (e.g., mountainous areas with arid, steep slopes) may preclude the implementation of Management Measure II.A.(1)(a). Where local conditions do not allow the implementation of BMPs such as grassed swales or detention basins, and preconstruction/predevelopment (existing conditions) TSS loadings from the site are significant, it may not be cost-effective or beneficial to require 80 percent TSS postdevelopment loading reductions. Management Measure option II.A.(1)(b) was provided to allow flexibility where such conditions exist. This flexibility will be especially important in cases where loadings from surrounding undeveloped areas dwarf the TSS loadings generated from the new development. (NOTE: Predevelopment is defined, in the context of Management Measure II.A.(1)(b), as the sediment loadings and runoff volumes/velocities that exist onsite immediately before the planned land disturbance and development occur.)

4. Practices

As discussed more fully at the beginning of this chapter and in Chapter 1, the following practices are described for illustrative purposes only. State programs need not require implementation of these practices. However, as a practical matter, EPA anticipates that the management measure set forth above generally will be implemented by applying one or more management practices appropriate to the source, location, and climate. The practices set forth below have been found by EPA to be representative of the types of practices that can be applied successfully to achieve the management measure described above.

Cost and effectiveness information for these practices is shown in Tables 4-7 (34k) and 4-8. Many of these practices can be used during site development, but the focus of this section is the abatement of postdevelopment impacts.

a. Develop training and education programs and materials for public officials, contractors, and others involved with the design, installation, operation, inspection, and maintenance of urban runoff facilities.

Training programs and educational materials for public officials, contractors, and the public are crucial to implementing effective urban runoff management programs. Contractor certification, inspector training, and competent design review staff are important for program implementation and continuing effectiveness. The State of New Jersey Ocean County Demonstration Study's Storm Water Management Facilities Maintenance Manual addresses many of these issues and provides guidance on programmatic elements necessary for the proper operation and maintenance of urban runoff facilities. Several other States and local governments, including Virginia, Maryland, Washington, Delaware, Northeastern Illinois Planning Commission, and the City of Alexandria, Virginia, have developed manuals and training materials to assist in implementation of urban runoff requirements and regulations.

The State of Delaware passed legislation requiring that "all responsible personnel involved in a construction project will have a certificate of attendance at a Departmental sponsored or approved training course for the control of sediment and storm water before initiation of land disturbing activity." The State provides personnel training and educational opportunities for contractors to meet this requirement and has delegated program elements to conservation districts, counties, and other agencies. The program has been well received and from February 1991 to July 1991, over 1,100 individuals from 300 companies and organizations participated in the program (Shaver and Piorko, 1992).

b. Ensure that all urban runoff facilities are operated and maintained properly.

Once an urban runoff facility is installed, it should receive thorough maintenance in order to function properly and not pose a health or safety threat. Maintenance should occur at regular intervals, be performed by one or more individuals trained in proper inspection and maintenance of urban runoff facilities, and be performed in accordance with the adopted standards of the State or local government (Ocean County, undated). It is more effective and efficient to perform preventative maintenance on a regular basis than to undertake major remedial or corrective action on an as needed basis (Ocean County, undated).

c. Infiltration Basins

Infiltration basins are impoundments in which incoming urban runoff is temporarily stored until it gradually infiltrates into the soil surrounding the basin. Infiltration basins should drain within 72 hours to maintain aerobic conditions,

which favor bacteria that aid in pollutant removal, and to ensure that the basin is ready to receive the next storm (Schueler, 1987). The runoff entering the basin is pretreated to remove coarse sediment that may clog the surface soil pore on the basin floor. Concentrated runoff should flow through a sediment trap, or a vegetated filter strip may be used for sheet flow.

d. Infiltration Trenches

Infiltration trenches are shallow excavated ditches that have been backfilled with stone to form an underground reservoir. Urban runoff diverted into the trench gradually infiltrates from the bottom of the trench into the subsoil and eventually into the ground water. Variations in the design of infiltration trenches include dry wells, pits designed to control small volumes of runoff (such as the runoff from a rooftop), and enhanced infiltration trenches, which are equipped with extensive pretreatment systems to remove sediment and oil. Depending on the quality of the runoff, pretreatment will generally be necessary to lower the failure rate of the trench. More costly than pond systems in terms of cost per unit of runoff treated, infiltration trenches are suited best for drainage areas of less than 5 to 10 acres or where ponds cannot be applied (Schueler et al., 1992).

e. Vegetated Filter Strips

Vegetated filter strips are areas of land with vegetative cover that are designed to accept runoff as overland sheet flow from upstream development. They may closely resemble many natural ecotones, such as grassy meadows or riparian forests. Dense vegetative cover facilitates sediment attenuation and pollutant removal. Vegetated filter strips do not effectively treat high-velocity flows and are therefore generally recommended for use in agriculture and low-density development and other situations where runoff does not tend to be concentrated. Unlike grassed swales, vegetated filter strips are effective only for overland sheet flow and provide little treatment for concentrated flows. Grading and level spreaders can be used to create a uniformly sloping area that distributes the runoff evenly across the filter strip (Dillaha et al., 1987). Vegetated filter strips are often used as pretreatment for other structural practices, such as infiltration basins and infiltration trenches. Refer to Chapter 7 of this guidance for additional information.

Filter strips are less effective on slopes of over 15 percent. Periodic inspection, repair, and regrading are required to prevent channelization (Schueler et al., 1992). Inspection is especially important following major storm events. Excessive use of pesticides, fertilizers, and other chemicals should be avoided. To minimize soil compaction, vehicular traffic and excessive pedestrian traffic should be avoided.

A berm of sediment that must be periodically removed may form at the upper edge of grassed filter strips. Mowing of grassed filter strips at a minimum of two to three times per year will maintain a thicker vegetative cover, providing better sediment retention. To avoid impacts on ground-nesting birds, mowing should be limited to spring or fall (USEPA, undated). Harvesting of mowed vegetation will allow for thicker growth and promotes the retention of nutrients that are released during decomposition (Dillaha et al., 1989).

Forested areas directly adjacent to waterbodies should be left undisturbed except for the removal of trees presenting unusual hazards and the removal of small debris near the stream that may be refloated by high water. Periodic harvesting of some trees not directly adjacent to waterbodies removes sequestered nutrients (Lowrance, Leonard, and Sheridan, 1985) and maintains an efficient filter through vigorous vegetation (USEPA, undated). Exposure of forested filter strip soil to direct radiation should be avoided to keep the temperature of water entering waterbodies low, and moist conditions conducive to microbial activities in filter strip soil should be maintained (Nutter and Gaskin, 1989).

f. Grassed Swales

A grassed swale is an infiltration/filtration method that is usually used to provide pretreatment before runoff is discharged to treatment systems. Grassed swales are typically shallow, vegetated, man-made ditches designed so that the bottom elevation is above the water table to allow runoff to infiltrate into ground water. The vegetation or turf prevents erosion, filters sediment, and provides some nutrient uptake (USDA-SCS, 1988). Grassed swales can also serve as conveyance systems for urban runoff and provide similar benefits.

The swale should be mowed at least twice each year to stimulate vegetative growth, control weeds, and maintain the capacity of the system. It should never be mowed shorter than 3 to 4 inches. The established width should be maintained to ensure the continued effectiveness and capacity of the system (Bassler, undated).

g. Porous Pavement and Permeable Surfaces

Porous pavement, an alternative to conventional pavement, reduces much of the need for urban runoff drainage conveyance and treatment off-site. Instead, runoff is diverted through a porous asphalt layer into an underground stone reservoir. The stored runoff gradually exfiltrates out of the stone reservoir into the subsoil. Many States no longer promote the use of porous pavement because it tends to clog with fine sediments (Washington Department of Ecology, 1991). A vacuum-type street sweeper should be used to maintain porous pavement.

Permeable paving surfaces such as modular pavers, grassed parking areas, and permeable pavements may also be employed to reduce runoff volumes and trap vehicle-generated pollutants (Pitt, 1990; Smith, 1981); however, care should be taken when selecting such alternatives. The potential for ground-water contamination, compaction, or clogging due to sedimentation should be evaluated during the selection process. (NOTE: These practices should be selected only in cases where proper operation and maintenance can be guaranteed due to high failure rates without proper upkeep.)

h. Concrete Grid Pavement

Concrete grid pavement consists of concrete blocks with regularly interdispersed void areas that are filled with pervious materials, such as gravel, sand, or grass. The blocks are typically placed on a sand or gravel base and designed to provide a load-bearing surface that is adequate to support vehicles, while allowing infiltration of surface water into the underlying soil.

i. Water Quality Inlets

Water quality inlets are underground retention systems designed to remove settleable solids. Several designs of water quality inlets exist. In their simplest form, catch basins are single-chambered urban runoff inlets in which the bottom has been lowered to provide 2 to 4 feet of additional space between the outlet pipe and the structure bottom for collection of sediment. Some water quality inlets include a second chamber with a sand filter to provide additional removal of finer suspended solids by filtration. The first chamber provides effective removal of coarse particles and helps prevent premature clogging of the filter media. Other water quality inlets include an oil/grit separator. Typical oil/grit separators consist of three chambers. The first chamber removes coarse material and debris; the second chamber provides separation of oil, grease, and gasoline; and the third chamber provides safety relief should blockage occur (NVPDC, 1980). While water quality inlets have the potential to perform effectively, they are not recommended. Maintenance and disposal of trapped residuals and hydrocarbons must occur regularly for these devices to work. No acceptable clean-out and disposal techniques currently exist (Schueler et al., 1992).

j. Extended Detention Ponds

Extended detention (ED) ponds temporarily detain a portion of urban runoff for up to 24 hours after a storm, using a fixed orifice to regulate outflow at a specified rate, allowing solids and associated pollutants the required time to settle out. The ED ponds are normally "dry" between storm events and do not have any permanent standing water. These basins are typically composed of two stages: an upper stage, which remains dry except for larger storms, and a lower stage, which is designed for typical storms. Enhanced ponds are equipped with plunge pools near the inlet, a micropool at the outlet, and an adjustable reverse-sloped pipe as the ED control device (orifice) (NVPDC, 1980; Schueler et al., 1992). Temporary and most permanent ED ponds use a riser with an antivortex trash rack on top to control trash.

k. Wet Ponds

Wet ponds are basins designed to maintain a permanent pool of water and temporarily store urban runoff until it is released at a controlled rate. Enhanced designs include a forebay to trap incoming sediment where it can easily be removed. A fringe wetland can also be established around the perimeter of the pond.

l. Constructed Wetlands

Constructed wetlands are engineered systems designed to simulate the water quality improvement functions of natural wetlands to treat and contain surface water runoff pollutants and decrease loadings to surface waters. Where site-specific conditions allow, constructed wetlands or sediment retention basins should be located to have a minimal impact on the surrounding areas. (The State of Washington requires that constructed wetlands be located in uplands (Washington Department of Ecology, 1992).) In addition, constructed urban runoff wetlands differ from artificial wetlands created to comply with mitigation requirements in that they do not replicate all of the ecological functions of natural wetlands. Enhanced designs may include a forebay, complex microtopography, and pondscaping with multiple species of wetland trees, shrubs, and plants. Additional information on constructed wetlands is provided in Chapter 7.

m. Filtration Basins and Sand Filters

Filtration basins are impoundments lined with filter media, such as sand or gravel. Urban runoff drains through the filter media and perforated pipes into the subsoil. Detention time is typically 4 to 6 hours. Sediment-trapping structures are typically used to prevent premature clogging of the filter media (NVPDC, 1980; Schueler et al., 1992).

Sand filters are a self-contained bed of sand to which the first flush of runoff water is diverted. The runoff percolates through the sand, where colloidal and particulate materials are strained out by the cake of solids that forms, or is placed, on the surface of the media. Water leaving the filter is collected in underground pipes and returned to the stream or channel. A layer of peat, limestone, and/or topsoil may be added to improve removal efficiency.

n. Educate the public about the importance of runoff management facilities.

"... the value of a comprehensive public information and education program cannot be overemphasized. Such a program must explain the basis, purpose, and details of the proposal and must convince the public and their elected officials that it is both necessary to implement and beneficial to their interests. It must also explain the fundamentals of storm water management facilities, the vital role they play in our lives, and their need for regular maintenance. This information can be presented through flyers, brochures, posters, and other educational aids. Work sessions and field trips can also be conducted. Signs at facility sites can also be erected. Finally, presentations to planning boards, municipal councils and committees, and county freeholders by storm water management experts can also be of great assistance" (New Jersey, undated).

5. Effectiveness and Cost Information

The box and whisker plot in Figure 4-3 summarizes efficiencies for selected structural TSS removal practices, as reported by Schueler et al., 1992. The whiskers of each box represent the range of reported TSS removal efficiencies. The box ends delimit the 25th and 75th percentiles. The horizontal line represents the median, or 50th percentile. Circles represent outliers. Figure 4-3 and Table 4-7 (34k) illustrate the range of removal efficiencies, based on monitoring and modeling studies, for total suspended solids for several of the structural practices. The reviewed literature reported a median TSS removal efficiency above 80 percent for three practices constructed wetlands, wet ponds, and filtration basins. However, it has been reported that the other practices are capable of achieving 80 percent TSS removal efficiency when properly designed, sited, operated, and maintained. More detailed information on the removal efficiencies of the practices and factors influencing the removal efficiencies is presented in Table 4-7 (34k). Costs of the practices are shown in Table 4-8.

In many cases, a systems approach to best management practice (BMP) design and implementation may be more effective. By applying multiple practices, enhanced runoff attenuation, conveyance, pretreatment, and treatment may be attained (Schueler et al., 1992). In addition, regionalization of systems (installing and maintaining a BMP or BMPs for more than one development site) may prove more efficient and cost-effective due to the economies of scale of operating one large system versus several smaller systems.

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