A proposed method for assessing and classifying current construction and postconstruction structural best management practices.

By David Woelkers and Marc S. Theisen

With the onset of the National Pollutant Discharge Elimination System Phase II stormwater rule requirements for construction and postconstruction best management practices (BMPs), professionals, contractors, and end users are struggling to develop a systematic and logical method for selecting the appropriate BMPs to be integrated into the various construction phases of their projects. Currently, widespread confusion exists regarding selection of the best options for a particular site. As a result, many projects end up with inappropriate BMPs for the applications and issues that they are attempting to deal with. This article offers planners a proposed method for classifying BMPs and a process for selecting the most appropriate and cost-effective BMPs for their particular projects.

Most regulators have two primary concerns that underlie stormwater requirements in the site-plan approval processes. These are the control of water quantity and treatment for quality both during and after the active construction phase. Water-quantity outputs from sites generally are limited to predevelopment levels. Water-quality issues focus on reducing contaminants from runoff prior to their discharge from the site. Sediment is the most prevalent constituent of concern for receiving waters. Other problematic constituents typically include nutrients, metals, hydrocarbons and other organic compounds, and bacteria. As a result, each site must be analyzed to determine specific application needs. Understanding what types of structural BMPs are available and how they interact with one another will help provide guidance in selecting the right mix for a specific site.

According to EPA's Preliminary Data Summary of Urban Stormwater Best Management Practices, an urban stormwater BMP is a "technique, measure or structural control that is used for a given set of conditions to manage the quantity and improve the quality of storm water runoff in the most cost-effective manner." With ongoing research, new BMPs are constantly emerging. In fact, the term "best management practices" would be more accurately phrased as "better management practices" because what is "best" varies with each application.

To ensure maximum benefit is achieved, planners should assess and evaluate various BMPs for the preconstruction, active construction, and postconstruction phases to make sure their plans are approved in a timely and cost-effective manner and that they include the most appropriate BMPs for a specific application.

Phases of Construction

Preconstruction

The preconstruction phase requires a careful assessment of the specific site. The first step is to gain a clear understanding of what stormwater controls will be required by relevant stormwater regulations, local ordinances, and site-plan approval processes. Nearly all regulations will require controls during the active construction phase to control sediment and limit runoff from the site to ensure minimum impacts on downstream receiving waters. The primary construction concern is sediment control, and a wide range of both temporary and permanent BMPs will be needed. Each application must be examined to determine site-specific needs for laying out the sequence of selecting both temporary and permanent BMPs. This sequence is commonly referred to as the "treatment train," and a clear understanding of all available options is critical for a successful site plan.

Several factors must be considered in devising an effective classification of BMPs to assist planners and end users in the assessment and selection process. First is the proposed land use of a project. Possible uses include industrial, commercial, residential, and streets and highways. For each use, the specific site application needs must be determined. Consideration should be given to whether the project is new or redevelopment and how much land will be available for BMP installations. A detailed review of receiving-water concerns, along with an analysis of the site's potential to generate pollutants of concern both during and after construction, also must be completed prior to BMP selection.

Once a review of the land use and receiving-water concerns is completed, an assessment of the appropriate BMP options can be evaluated. The wide range of BMP options can be organized into several classifications by determining what each BMP can accomplish. Many are designed to control erosion and contain sediment transport. These are particularly important in the active construction phase where site stabilization has not yet occurred. Some BMPs can and should be installed before construction. Sediment containment devices, such as silt fences, continuous berms, and turbidity barriers, often are mandated and installed before construction commences. Other BMPs deal with controlling the quantity of runoff that will occur as a result of construction activities and postconstruction changes in flow that will result from increased imperviousness on the completed site. Finally, many different types of BMPs focus on water quality by treating the runoff to reduce other pollutants that are generated during the construction and postconstruction phases.

Another consideration is how maintenance will be performed over the long run. Planners need to think of BMP selection as a revolving process of installation, inspection, maintenance, and enforcement (I²ME). Although this article focuses on the selection aspects, decision-makers need to consider I²ME to ensure long-term performance of BMPs. Many techniques and technologies involve lower up-front costs, but installation and maintenance costs over time must be factored into the equation.

Many quality and quantity issues can be resolved through efficient site designs that incorporate practices that prevent the transport of water and pollutants from increasing as a result of development. These preventive measures can greatly reduce the need for reactive designs and technologies that are needed to contain water and remove pollutants of concern. This article focuses on the organization and classification of structural BMPs and related stormwater treatment devices (SWTDs), which are structural or nonstructural BMPs that positively impact stormwater quality before, during, or after construction or construction-related land-disturbing activities. SWTDs might be temporary or permanent, depending on their desired application or function. SWTDs might be "proactive" or "reactive" in their approach or application. Examples of proactive SWTDs include erosion control practices, green roofs, vegetative filter strips, and rain barrels. Reactive techniques might employ sediment control practices, inline treatment devices, sedimentation ponds, and detention/retention systems.

Active Construction

Sediment Containment Systems

The role of sediment control systems is to create conditions for sedimentation, allowing soil particles that are held in suspension to settle. When soil-particle transport mechanisms flow at slow rates, particles can settle out of suspension. How deposition occurs depends on several parameters.

Sediment control systems generally are hydraulic controls that function by modifying the storm-runoff hydrograph and slowing water velocities, allowing for the deposition of suspended particles by gravity. Some of the more common names for these structures are sediment basins, sediment ponds, and sediment traps. When designed correctly, sediment containment systems should provide sufficient containment storage volume to handle incoming waters, create uniform flow zones within the containment storage volume for the deposition of suspended particles, and discharge water at a controlled rate.

When all runoff waters are captured, the efficiency of the containment system is nearly 100%. Retaining all runoff waters from a construction site usually is impossible, however, because large containment areas and volumes are required. In addition, evaporation and infiltration might not be sufficient to drain the system before the next storm event occurs, which might cause flooding problems.Finally, retained waters might hamper maintenance of the system because removing captured sediments becomes more complicated with the presence of water.

Because of these concerns, rather than attempting to retain all runoff waters, a containment system should provide sufficient volume for capturing suspended particles while allowing discharge to occur. This provides the advantage of detaining incoming runoff to control the discharge of suspended particles while not requiring large areas to store runoff waters. Flooding problems from sequential storm events are reduced because contained waters usually will be drained from the system between events. Finally, frequent maintenance is facilitated because the sediments do not remain saturated with water.

If detaining runoff from construction sites is to be effective in removing suspended particles, contained waters must remain long enough for the deposition of suspended particles within the system. Because outflow from the system will occur, 100% reduction of all incoming suspended particles will not be possible. However, high efficiencies can occur for sediment containment systems developed for design-size particles and are described in detail by Jerald S. Fifield in Designing for Effective Sediment and Erosion Control on Construction Sites (2001) and Field Manual on Sediment and Erosion Control Best Management Practices for Contractors and Inspectors (2002).

Documentation on the effectiveness of containment systems for trapping suspended solids is limited, and there are conflicting opinions on their actual effectiveness. If properly designed, constructed, inspected, and maintained, however, containment systems are effective in trapping sediment.

This discussion focuses on selected manmade, nonstructural sediment-containment systems that act as barriers or filters. A barrier is any structure that obstructs or prevents the passage of water. If runoff cannot pass through a barrier, then water will either be contained or flow over the structure. Commonly used manmade barrier devices include silt fences, continuous geotextile-wrapped berms, wattles, turbidity barriers, and geosynthetic silt dikes. Because their effectiveness is minimal for large runoff events, these devices must be carefully installed, and their usefulness generally is limited to low-volume flows from smaller storm events. As such, these systems are typically only used and installed during the preconstruction and active-construction phases of a project.

Appropriate places to use sediment control barriers include:

• along sections of a site perimeter
• below disturbed areas subject to sheet and rill erosion,
• below the toe of exposed and erodible slopes,
• along the toe of stream and channel banks,
• low-flow swales and ditches,
• around area drains or inlets located in a sump.

Turbidity barriers are used in low-flow streams, tidal areas, or lakes.

Inappropriate places to use sediment control barriers include:

• parallel to a contour when installed on a hillside;
• in channels where concentrated flows occur, unless properly reinforced;
• upstream or downstream of culverts where concentrated flows occur;
• in front of or around inlets where concentrated flows occur and sump conditions do not exist;
• in continuously flowing streams or ephemeral channels.

Filtration Devices

Other SWTDs used during active construction are designed to provide sediment containment and/or filtration. These might include geotextile catch basin inserts, geosynthetic drainage and curb inlet filters, geotextile tubes, and geotextile filter bags. These materials allow water to flow through them while filtering or capturing sediment. Selecting the correct geotextile or fiber consistency will reduce the possibility of blinding or clogging the device with excessive sediment.

Appropriate places to use geosynthetic filters include in front of or around gutters and drain inlets where sump conditions exist and in areas of dewatering of detention/retention ponds or dredging of construction and/or industrial spoils.

Inappropriate places to use geosynthetic filters include in front of or around inlets where concentrated flows occur and sump conditions do not exist, in channels where concentrated flows occur, and in continuously flowing streams or ephemeral channels.

Manmade geosynthetic SWTDs and filters have numerous advantages over traditional sediment control practices derived from natural materials. They usually are easier to transport, install, and maintain as compared to straw and hay bales or soil and rock structures. Manufacturing and fabrication consistencies enable the performance of geosynthetic devices to be more predictable and generally superior to that of natural materials. In some cases these devices may be washed and reused.

Postconstruction

Postconstruction structural BMPs are techniques that can be used to address flow quantity control of and treatment for water quality through pollutant removal in wet-weather runoff. These BMPs can include site-specific engineered designs as well as proprietary systems. The challenge with any attempt to organize or classify BMPs by type or function is that many fit into multiple categories. In the interest of clarity, however, structural BMPs can be grouped into several classifications by function, including the following:

• Infiltration systems
• Detention systems
• Retention systems
• Vegetated systems
• Filtration systems
• Hydrodynamic separation systems

These structural BMPs can be further organized into three primary types: vegetative techniques and open-space designs, designed structures, and manufactured technologies. Table 1 shows a proposed classification of BMPs based on type and function and provides examples of each type. Remember that many BMPs serve multiple functions.

For a broad overview of the types and functions of structural and nonstructural BMP types, see "Preparing for the Storm: BMP Selection for Phase II Compliance" (Stormwater Buyers Guide 2002, www.forester.net/sw_0105_preparing.html). A clear understanding of the postconstruction BMP options will help clarify the assessment and selection process for meeting active construction and postconstruction requirements. The balance of this article presents a proposed matrix system for selecting appropriate manufactured stormwater treatment devices for specific site application needs during all phases of the construction process.

Functions of Manufactured SWTDs

Basic functions of manufactured SWTDs can be grouped into five major categories. These are sediment containment, filtration, separation, infiltration, and underground detention. Although it is beyond the scope of this article to describe and classify all the types of BMPs that might be used to fulfill these functions, various manufactured SWTDs may be grouped by primary function, as shown in Table 2.

Once the function required of an SWTD has been determined, it is time to consider when and where it should be employed. Failure to properly install an SWTD in the correct location or sequence of a land-disturbing activity might result in failure or compromised performance.

After the appropriate application or function of the required stormwater treatments has been determined, these parameters may be coupled to facilitate selection of the most appropriate SWTD. Table 3 presents a matrix that combines function with construction phases for identifying potential SWTDs for selection consideration.

Table 3. Function and Typical Construction Phase(s) for Applying Manufactured Stormwater Treatment Devices

Finally, where to use an SWTD must be considered. Although it is beyond the scope of this article to present specific site locations for the vast potential variances of SWTD applications, Table 4 presents a matrix coupling site location with the various construction phases. Combining Tables 3 and 4 might help end users make informed decisions when considering SWTDs for various functions, construction phases, and site locations.

Table 4. Site Location and Typical Phase(s) of Construction for Applying Manufactured Stormwater Treatment Devices

Conclusion

To ensure that regulators, planners, engineers, and contractors have a clear picture of what techniques and measures can be used in the various construction phases for proper BMP management, a solid understanding of the options is essential. By classifying the various sediment controls and postconstruction BMPs into proper applications, stormwater professionals are far more likely to develop efficient yet cost-effective stormwater plans for specific projects. A thorough understanding of the installation, inspection, maintenance, and enforcement requirements also will result in a more comprehensive and realistic cost analysis of the project. The result will be cleaner water and a more satisfied general public.

David Woelkers is president of Hydro Compliance Management Inc. in Whitmore Lake, MI. Marc S. Theisen, M.S., CPESC, is director of business development with SI Geosolutions in Chattanooga, TN. Substantial input for this article was received from Jerald S. Fifield, Ph.D., CPESC.

SW - July/August 2003

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