Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring
6. Selection of Best Management Practices
Agencies dealing with intense urban development and a high percentage of impervious areas are faced with new challenges in selecting cost-effective stormwater management alternatives to assist in implementing environmental protection and water quality goals and objectives. In many cases, controlling stormwater discharges in ultra-urban areas addresses multiple objectives and concerns. These concerns may include protection from flooding generated by highly impervious surfaces, protection of sensitive downstream conditions such as stream physical stability, or maintenance of instream water quality. To address these concerns comprehensively through the development of effective stormwater management alternatives, both structural and nonstructural practices may be considered.
Structural BMPs control runoff and improve water quality through storage, flow attenuation, infiltration, filtration, and biological degradation processes. Their use in the ultra-urban environment, however, generally requires deviations from standard designs to meet space limitations and other site restrictions. As these design modifications become significant, evaluation of the performance of the BMP in controlling stormwater runoff becomes increasingly subjective; available information on the performance of the BMPs might not be applicable. A BMP selection process must therefore weigh these and other concerns before making a final and often complex decision.
Nonstructural BMPs include planning, education, management, and operational procedures to prevent or mitigate impacts from stormwater runoff at the source. Areas with insufficient space, cost-prohibitive conditions, or in situations where the targeted water quality constituents are not effectively controlled using structural stormwater BMPs, may be appropriate applications for nonstructural BMPs.
In addition to weighing the options that structural and nonstructural BMPs offer, issues regarding the selection of stormwater management practices for retrofitting existing ultra-urban areas may include (1) the lack of space to construct on-site controls, (2) limited incentives due to a lack of specific regulatory requirements for retrofit situations, (3) difficulty in characterizing problem sources and their corresponding constituent loadings, (4) inconsistency in the reported performance of BMPs, (5) limited funding, and (6) limited experience with source control and prevention programs. Another challenging issue is the selection of compatible and complimentary combinations of BMPs consisting of both nonstructural and structural measures that cost-effectively maximize overall constituent load reduction.
In this chapter, a three-step decision-making process employing both quantitative and qualitative criteria for sequentially screening BMP alternatives is described. A preferred management alternative (a single BMP or a combination of BMPs) suited to site-specific conditions is the result of this process. This process builds on the knowledge and information summarized in previous chapters of this report and on other BMP selection processes reported in the literature (City of Portland, 1995; Claytor and Schueler, 1996; Driscoll and Mangarella, 1990; NVPDC, 1992; Schueler, 1987; Young et al., 1996).
The proposed BMP selection process is designed as a sequential approach that incorporates a series of checks and balances at each stage, integrates management objectives and site conditions, and relies on current knowledge of stormwater BMP technology and demonstrated experience and case studies. The process is designed to allow decisions to progress from a preliminary screening level to a more detailed evaluation and selection of candidate best management alternatives. The three steps of this selection process are illustrated in Figure 67 and include:
- a scoping phase,
- an evaluation phase, and
- a final selection phase.
A brief description of the selection process within each phase is provided below.
The key processes used during the scoping phase consist of sequential elimination of nonapplicable structural and nonstructural options based on a predefined set of criteria. These criteria are derived from an in-depth understanding of the management objectives and the anticipated functional role of the BMP in preventing or controlling stormwater discharges. An understanding of site conditions, predominant sources or causes of the constituent release, and key processes governing the removal of constituents from stormwater is also essential to this process. Analysis in this phase results in a set of feasible BMPs that could potentially be used to completely or partially achieve the program objectives.
The evaluation phase consists of three types of analyses. First, the list of potential structural BMPs is further narrowed down using criteria derived from the physical characteristics of the site. Examples of physical site characteristics are drainage area, soil type and infiltration capacity, depth to groundwater, and site topography. Second, BMP effectiveness information is used to identify and rank BMPs with demonstrated performance in controlling targeted constituents in stormwater runoff. Design modification of BMPs for adaptation to the ultra-urban environment should be evaluated and the potential impact on performance considered. Third, combinations of the remaining nonstructural and structural BMPs should be evaluated for their compatibility and complimentary performance. The maintenance burden (e.g., frequency of cleanouts) should also be considered. Analyses performed in the evaluation phase result in management alternatives composed of either a single BMP, a combination of structural and nonstructural BMPs, or a combination of BMPs (multiple-BMP treatment train) to treat identified water quality problems.
The final selection phase consists of additional analysis of the management alternatives to refine the list of BMPs and BMP combinations developed in the evaluation phase. These analyses consist of
- evaluation of cost and expected benefits associated with each management alternative;
- evaluation of any additional benefits including aesthetics, recreational value, and habitat expansion; and
- consideration of overall public acceptance and support.
These analyses will result in the final selection of the preferred management alternative. For a successful BMP selection process, several supporting data collection activities are critical, as illustrated in Figure 67. Data collected include
- available information on BMPs and their use in similar ultra-urban settings;
- drainage area characteristics and qualitative evaluation of the sources and magnitude of constituents;
- physical constraints at the site;
- local cost elements including land acquisition, construction and maintenance cost; and
- public acceptance and any additional benefits provided (e.g., aesthetics, recreational value).