Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring

6.3 The Scoping Phase

The scoping phase provides an initial screening analysis of potential structural and nonstructural BMPs. Structural BMPs are generally designed to remove constituents in stormwater runoff, whereas nonstructural measures focus on the prevention of source-related constituent-generating activities from contaminating stormwater (e.g., covering salt piles) and on the removal of constituents that might contaminate stormwater (e.g., streetsweeping). In the scoping phase, the ability of structural and nonstructural BMP options to meet management objectives is evaluated. The scoping phase may address the following questions:

• Does the BMP address one or more management objectives?
• Does the BMP provide both stormwater quantity and quality control?
• Are data available on BMP effectiveness?
• Is the BMP applicable to site conditions?
• Does the nonstructural BMP help to reduce long-term structural BMP maintenance requirements?
• Is the BMP costly to implement?
• Does the BMP provide auxiliary benefits such as public education?

A number of different factors are used to evaluate BMPs in the scoping phase. Management goals and objectives, the characteristics of the site (in terms of constituent sources and types and general site characteristics), and the characteristics of the BMP provide the framework for evaluating the applicability of both structural and nonstructural BMPs.

6.3.1 Management Goals and Objectives

An effective ultra-urban stormwater management program focuses on meeting well-defined environmental protection goals and public needs in a cost-effective manner. In many cases, an ultra-urban management plan is designed to address multiple environmental and safety concerns at various scales ranging from site-specific to a larger watershed scale. The development and implementation of stormwater management plans are driven by a variety of conditions including public pressure, applicable regulations and policies, downstream impacts on sensitive resources, or a combination of any of these conditions. Within these plans, downstream impacts are usually expressed qualitatively in terms of objective statements such as "control of flooding conditions", "control of stream degradation and associated sediment loadings", "restoration of a water quality impairment", or "protection of aquatic habitat".

Meeting these multiple objectives may require that several potential BMP siting locations be identified and considered. Potential BMPs can be determined for each site location to form a comprehensive management action plan. These selected BMPs, as well as the overall management actions (whether at the site-specific drainage area or at the watershed scale), will contribute to achieving the predefined management objective(s). An in-depth understanding of stormwater management objectives prior to the selection of a BMP or combination of BMPs is essential to the development of a successful management plan. This understanding should facilitate the development of management objectives into measurable indicators or criteria that can be used to screen out nonapplicable BMPs. Table 45 provides a set of potential screening criteria that can be derived for different management objectives.

Typical stormwater management objectives applicable to ultra-urban settings include:

1. Flood protection.
2. Reduction in loadings of nonpoint source constituents.
3. Measures for stabilization of downstream erosion to reduce sediment loading.
4. Aesthetic enhancement.
5. Public safety.
6. Reduced public facilities maintenance costs.
7. Provisions for recreation.

The numbers shown for each objective are included in the summary tables provided later in this chapter. The objectives provided here will be used to help illustrate which of these objectives may be achieved by the BMPs analyzed during the selection process.

6.3.2 Site Characterization

Site characterization includes evaluation of the drainage area to identify runoff and constituent-generating activities and sources, and characterization of the magnitude and the areal extent of each source. Several procedures can be used to assess loadings from ultra-urban areas (USEPA, 1983), including highway runoff in particular (Driscoll et al., 1990), and to define probable impacts on receiving water bodies. Chapters 2 and 4 of this document describe additional site characterization elements that can assist in evaluating the runoff potential and source-specific loading. Characterization of dominant sources and constituents, definition of the constituent fate and transport pathways, and identification of the method and processes by which constituents enter stormwater runoff are key elements supporting the selection of appropriate ultra-urban BMPs.

6.3.3 Screening of Structural BMPs

The screening of structural BMPs is intended to eliminate those BMPs that are obviously impractical, implausible, or ineffective. It is unlikely that any single BMP will be able to completely meet all management objectives; trade-offs between cost and performance almost always occur. Often, more than one BMP will be necessary. The resources and effort required to evaluate these trade-offs make it desirable to remove from consideration any BMPs that do not fulfill -- or do not contribute significantly in combination with other BMPs to fulfilling -- management objectives. The nature and scope of the planned project, water quantity and quality management objectives, and any other limiting management objectives should be used to evaluate the suite of available structural BMPs.

Nature and Scope of Planned Project

The nature of a proposed project in an ultra-urban area often dictates which BMPs are impractical due to size or settings. For example, the placement of an elevated highway over a busy roadway in an ultra-urban setting may present an opportunity to retrofit small stormwater BMPs into an existing drainage system. Developing the list of feasible BMP options for this site would begin with a recognition of its key features, which are (1) limited available surface area, (2) limited airspace between the original roadway and the elevated roadway, and (3) the need to sustain the existing operability of the roadway during BMP installation and maintenance. It would be inappropriate to install area-intensive BMPs like ponds and wetlands at this site, so these can be quickly screened from further consideration. Small BMPs such as infiltration trenches and water quality inlet devices could be considered potentially feasible at this point in the process.

Table 46 indicates which BMPs are most compatible with the ultra-urban setting because of their relatively small footprint, design adaptability, and effectiveness in removing typical constituents from stormwater. It should be noted that BMPs that may not be compatible with ultra-urban settings do have applicability for roadway projects in less urban and nonurban settings where larger land areas are available and greater flexibility in siting BMPs exists.

Table 47 provides the site considerations needed to evaluate the feasibility of various BMPs based on the nature and scope of proposed projects. This includes the percent of the drainage area that must be set aside for BMP installation and a consideration of whether the design is dependent on in situ soils. Both of these elements will impact on the screening of structural BMPs.

Water Quantity and Quality Management Objectives

Local ordinances or state and federal regulations frequently mandate a design level of performance for both water quantity and quality management. For example, a proposed highway project may be required to manage stormwater runoff such that the 25-year peak runoff rate does not exceed the preconstruction condition (Driscoll et al., 1990; Young et al., 1996). The required flood protection and water quantity management for the highway would not be provided by BMPs like oil-grit separators and vegetated filter strips. While these BMPs can facilitate the process of flood protection by providing pretreatment of sediment and other constituents, they alone cannot fulfill the flood protection objectives. BMPs that are obviously unable to fulfill dictated management objectives related to water quantity can be identified using the information in Table 46 as primarily water quality BMPs, and screened out from further consideration. BMPs that can be utilized for pretreatment are supplementary water quality management measures and can be retained for further evaluation.

Water quality control is usually defined by identified pollutants of concern and the desired level of removal expected. Typical constituents of concern associated with management objectives include nitrogen and phosphorus, suspended sediments, and trace toxics such as heavy metals. For highways, an in-depth listing of constituents and their sources can be found in Tables 48 and 49. Due to high levels of imperviousness and road density, stormwater from ultra-urban areas will likely contain similar constituent loadings. Loadings for some constituents, particularly those associated with specialized commercial or industrial land use activities, however, may be significantly different. These specialized land use activities may include the introduction into stormwater of dissolved constituents that are more difficult to remove using structural BMPs.

Most structural BMPs rely on sedimentation and infiltration/filtration processes to remove constituents from stormwater. These practices may have a limited effect when dealing with dissolved and highly mobile constituents. The removal effectiveness of structural BMPs on a wide range of potentially harmful, primarily trace organic constituents is not known. Some structural BMPs such as wetland complexes may achieve the removal of dissolved nutrients through biochemical processes such as the nitrification of nitrate-nitrogen or through temporary storage of nutrients in plant tissue. Typical constituents of stormwater runoff, their primary transport phase and their control mechanisms are provided in Tables 48, 49, and 50. The ability of a structural BMP to meet constituent removal criteria depends on the removal mechanisms inherent in its design. Table 46 categorizes BMPs on their general ability to remove two broad categories of pollutants -- suspended constituents and dissolved constituents. Suspended constituents include suspended solids as well as those constituents that can be removed by physical processes (e.g., sedimentation and filtration) including oil and grease, metals, nutrients, and trace organics associated with suspended solids.

Dissolved constituents that are predominately removed through adsorption and biochemical processes include the soluble phases of metals, nutrients, and trace organic constituents. The actual removal performance of individual BMPs will be highly dependent on their hydraulic design and the hydrologic conditions encountered during stormwater treatment. Screening out BMPs that are obviously ineffective for targeted pollutants facilitates the selection of an effective BMP. Detailed information on the demonstrated removal effectiveness of BMPs for specific constituents is provided in Table 51.

Other Management Objectives

Initial screening of the suite of available BMPs can also be performed based on elements that are not related to the performance of the BMP. For example, fiscal management objectives such as providing stormwater management for a specified dollar amount or a percentage of the total project cost can serve as a means to remove high-cost BMPs from further consideration. Table 52 indicates the relative cost for various BMPs, and the BMP Fact Sheets (see Chapter 3) contain additional cost-estimating data that can be used to generate budgetary cost estimates. A final comparative analysis of costs for recommended BMP alternatives is completed in the final selection phase.

6.3.4 Screening Nonstructural BMPs

Nonstructural BMPs provide a flexible method of protecting water quality and improving water resources. Improper handling, use, and disposal of materials in an ultra-urban environment may generate a range of constituents that can contaminate nearby waterways. Contamination of stormwater runoff can often be prevented through the use of nonstructural BMPs, such as covering deicing materials, employee training, and minimizing the use of hazardous products. This is particularly true for a wide range of potentially harmful trace organic constituents that can be prevented from contaminating stormwater through the implementation of nonstructural BMPs. An added benefit to their use is that maintenance requirements for downstream structural BMPs may be reduced.

Nonstructural measures can include activities ranging from pesticide and fertilizer management to chemical storage practices (Young et al., 1996). The nonstructural BMPs that can be applied in both ultra-urban and highway areas can be grouped into six general categories:

• Litter and debris control.
• Education and training.
• Landscaping and vegetated practices.
• Chemical handling and storage.
• Containment and diversion.
• Pesticide and fertilizer management.

The methods or techniques that fall under these categories can be implemented to address constituents at a variety of spatial scales for a specific site or at community or watershed scales. In addition, nonstructural BMPs have little, if any, space requirements, making their use ideal for ultra-urban areas. Screening procedures for nonstructural BMPs require an analysis of specific constituent-generating activities or practices within a drainage area that may contribute constituents to stormwater and an assessment of their ability to qualitatively meet management objectives if controlled.

Analysis of Constituent-Generating Activities

The analysis of constituent-generating activities in a drainage area focuses on the feasibility of practices that avoid the exposure of any potential constituent-generating activity to stormwater. These practices may reduce the need for any structural treatment BMP by altering, enclosing, covering, or segregating the activity. Many industrial facilities include this analysis in their Storm Water Pollution Prevention Plans required for NPDES permits (see Section 3.9, Other Nonstructural BMPs). A flow chart that identifies constituent pathways, and both existing and potential management measures is a useful tool to help identify and assess potential nonstructural BMPs. Each nonstructural BMP, for example, uses a specific constituent removal process or mechanism. For nonstructural BMPs, the processes or mechanisms used to remove constituents include:

• Prevention (elimination) of an activity (e.g., pesticide use).

• Change in process to minimize the constituent loss (e.g., optimize salt application).

• Segregation of the constituent from the stormwater runoff to prevent its loss (e.g., berms to control spills).

• Education and training about the proper use and disposal of materials (e.g., employee training programs).

Table 53 provides a listing of potential nonstructural BMPs and their associated constituent removal processes.

Ability to Meet Management Objectives

Potential nonstructural BMPs are also screened on their ability to achieve management objectives. This can be done on an individual or combined basis. Table 53 illustrates the nonstructural BMPs that have the potential to address the seven management objectives identified previously (see Section 6.3.1).