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

6.5 Final Selection Phase

Preferred BMP options at this stage of the selection process may include incorporating structural BMPs, retrofits to an existing structural BMP, or the use of nonstructural measures or the modification of an existing nonstructural BMP program. It is possible that some combination of these may be the preferred method of achieving a particular objective. In the final selection process the preferred BMPs are evaluated based on cost-effectiveness and their ability to gain management and community support. This evaluation will result in an alternative that will reflect the unique features of a particular site.

6.5.1 Cost-Effectiveness

In the case of a structural BMP, its relative cost-effectiveness can be established based on published reports concerning its construction costs, annual operation and maintenance expenses, and effective life (how soon the BMP may need to be replaced). This cost information and the use of reported removal efficiencies for a structural BMP would complete a cost/benefit analysis for constituent removal. In making a final decision, additional less quantified management objectives (e.g., control of downstream stream stability, public acceptance) and other considerations (e.g., level of maintenance required) might also be evaluated.

The evaluation of cost-effectiveness of nonstructural BMPs is more problematic, particularly with respect to measuring their ability to meet specific water quality objectives. Nonstructural BMPs that focus, for example, on litter and debris management are likely to have widespread community support for aesthetic reasons. Their effectiveness in reducing loadings of targeted constituents cannot be established, however, without the implementation of targeted monitoring programs. Where there is a strongly identified need for a nonstructural program or where nonstructural BMPs can reduce the maintenance requirements of existing structural BMPs, cost-effectiveness can be established by reviewing existing programs and assigning a value to the performance of the nonstructural BMP.

The evaluation of alternatives that incorporate structural and nonstructural BMPs carries the limitations outlined earlier. The water quality benefits associated with the interaction of the two alternatives are difficult to evaluate and would likely require that implementation be accompanied by a monitoring program. In addition to the need for water quality monitoring, records of the before and after costs of maintaining the structural BMP would be required. The example on the following page illustrates an approach to evaluating the cost-effectiveness of management alternatives.

6.5.2 Management and Public Acceptance

To increase the level of both management and public acceptance for BMPs targeted primarily at water quantity/quality control, selection of cost-effective choices is essential. Management and public acceptance will rely heavily on issues such as aesthetics, public safety, recreational and educational value, and/or local resource protection (e.g., wetlands, stream stability). The preferred BMPs can then be ranked according to their ability to provide these additional benefits. Potential ranking factors for this analysis are given in Table 56; these ranking factors are provided as an example of how they can be determined. Specific requirements and objectives for a specific area will dictate the appropriate ranking factors.

Assigning a value for meeting the additional objectives is difficult and often requires professional judgment as well as consensus. For example, determining the aesthetic value of a particular BMP such as a constructed wetland is arbitrary and depends on the values and judgment of the evaluator. One way to address this concern is to weight the ranking factors equally.

The extended detention pond in the previous example was identified as the highest rated BMP. It will have an effective life of approximately 20 to 50 years but could potentially pose a high risk to public safety. The constructed wetland identified for alternative S poses a lower public safety risk. The nonstructural streetsweeping BMP identified as alternative 2 poses minimal risk but has an effective life of only four to eight years. This effective life will contribute to an overall increase in relative costs when compared on cost per year of effectiveness. Table 57 can be used to evaluate these considerations.

The final result of the evaluation process is a prioritized list of preferred BMPs (or BMP combinations). The following example summarizes and illustrates the BMP selection process from scoping to evaluation to final selection.

Example #1:

Site Description: Ultra-urban area with a drainage area of 2 ha (5 ac) with 80 percent impervious surfaces. Available space for constructing structural controls is approximately 2-3 percent of the total drainage area. However, it is highly desired to have any BMPs located below-grade if possible. Low permeability soils limit the use of infiltration-type BMPs.

Objective: Reduce oil and grease loadings to nearby river.

Criteria: Reduce oil and grease loading by 85 percent.

Activity/Runoff Characteristics: Two uncovered fueling stations with large paved parking area for transport trucks; high levels of suspended solids and oil and grease in dissolved and particulate form.

Existing BMPs: None, however a subsurface storm drain system does exist.

Table 58 illustrates the outcome of the BMP evaluation for the site. Based on the site description and stormwater management objectives, it is possible to quickly identify five candidate structural BMPs, and three potentially beneficial nonstructural BMPs. A description of the process used to make the selection is included below.

First, any structural BMPs considered should fit below-grade such that the land area over the BMP can be used. Given the active use of the surface area, BMPs with a modular design and the ability to fit in a small footprint are desirable.

Also, infiltration-based BMPs are not applicable because existing soils have low infiltration. In addition, there is an existing storm drain system that will limit the hydraulic drop available in the proposed BMP. This means any candidate BMPs must have the flexibility to operate under a wide range of head. Of the BMPs listed in Table 58, five BMPs could operate as stand-alone units and provide the required design features (numbers 9,11,14,15,16).

The constituent of concern factors strongly in the selection of candidate structural BMPs. The constituent of concern (oil and grease) will be attached to suspended sediment and will be floating on top of the stormwater. Of the most promising structural BMPs, organic media filters provide the most consistent and highest removal of oil and grease for all physical phases. As a result, an underground organic media filter is the preferred structural BMP assuming all design and constructability issues can be addressed.

Selected nonstructural BMPs can greatly enhance the performance of the proposed underground organic media filter. In particular there are three nonstructural BMPs that provide additional stormwater management benefit: 1) street and parking lot sweeping, 2) education and training, and 3) containment and diversion.

The first nonstructural BMP will help minimize the large diameter material (e.g., sand and trash) reaching the underground filter. The next two nonstructural BMPs will help limit the amount of spilled fuel, and improve the cleanup of spills (large and small) before they come into contact with stormwater. However, the expense and compatibility of proposed nonstructural BMPs must be evaluated in light of existing funding, resources, and compatibility with existing nonstructural programs.

Prior to designing the preferred structural BMP it is important to evaluate its comparability with existing BMP 0&M programs and program funding limits. Below-grade filters require periodic cleaning and replacement of the media. If the funding does not exist for this type of maintenance, then other less expensive structural BMPs should be considered even if they provide less effective stormwater management.

Example #2:

Site Description: The drainage area includes a 305 m (1000 ft) length of highway through a downtown metropolitan area. The area for stormwater BMPs is limited to the roadway median and shoulder drainage system and small open areas in the interchange. The highway is adjacent to a high visibility pedestrian area. The site has very flat terrain and shallow depth to watertable.

Objective: The receiving water is a major river system where flooding is not a concern. Reducing phosphorus loadings to the river is of primary concern.

Implementation of a stormwater control strategy was needed for a segment of highway in an ultra-urban setting. The broad scoping phase identified that flood and channel protection were not required, that the nutrient and sediment loading to the river were of primary concern, and that the ultra-urban highway was considered a "hotspot."

Table 59 illustrates the outcome of the BMP evaluation for the site. The structural BMPs that were eliminated included the ponds, wetlands, vegetated filter strip, underground storage tank, and infiltration practices. The remaining options consist of the dry swale, bioretention, sand filters, organic media filters, oil/grit separator, catch basin inserts, and some of the manufactured systems. The nonstructural BMPs identified in the broad scoping phase included weekly streetsweeping and an "adopt-a-road" litter control program.

In the final selection phase, the BMP alternatives were evaluated using a combination of elimination and addition. Table 59 illustrates the components that went into this decision process.

The selected structural BMP options include the dry swale in the median of the highway and the underground sand filter along the shoulders. The nonstructural options include weekly streetsweeping and litter control through an "adopt-a-road" program. The dry swale can only accommodate a relatively small portion of the highway drainage, but has high nutrient and sediment removal capability coupled with a more aesthetically acceptable practice in the vicinity of the high visibility pedestrian area. The underground sand filter fits into the linear nature of the roadway and can be designed for very shallow, low head conditions. Pollutant removal capability is good to excellent.

Although the nonstructural streetsweeping practice could help capture the coarsest sediments to help improve pollutant removal efficiencies and prolong the design life of the structural BMPs, it was not selected due to safety considerations. The high speed roadway limits the practical use of streetsweeping. The "adopt-a-road" program is designed to minimize litter in the vicinity of the high visibility pedestrian area.

Example #3:

Site Description: A 1 ha (2.5 ac) transportation department maintenance yard is located in a densely urban area. Vehicle maintenance and equipment and material storage activities occupy all of the site's impervious area. The impervious coverage of the site is 90 percent. The site drains to an urban stream that is highly impacted from hydrologic alterations (accelerated channel erosion). The steam channel is deeply incised, consequently, flooding is not a problem. The channel drains to an urban river that is phosphorus limited. Low permeability soils limit infiltration practices.

Objective: Avoid additional disruptions to receiving channel and reduce pollutant loads for oil and grease, sediment, and phosphorus to receiving waters.

Criteria: Provide stormwater management to mitigate for accelerated channel incision and reduce loadings of key pollutants by the following: oil and grease (85 percent), sediment (80 percent), and phosphorus (60 percent).

Activity/Runoff Characteristics: The site is characterized by several ongoing road maintenance activities including vehicle maintenance and refueling, vehicle wash facilities, sand and salt storage (a northern climate), storage of miscellaneous highway maintenance equipment, and stockpiled construction debris. Stormwater runoff from the site exhibits high sediment levels, highly elevated chloride concentrations, and oil and grease.

Existing BMPs: Catch basins with a 0.6 m sump. The catch basins drain to a subsurface storm drainage system that discharges directly to the urban stream.

Table 60 lists the results of the BMP selection analysis. Based on the scenario, the ponds, wetlands, and all infiltration practices are removed from consideration in the scoping phase. Infiltration is not practical given the soils. The additional element of potentially receiving toxic "hotspot" runoff is a primary concern. A "treatment train" approach is recommended to meet the multiple objectives of the management scenario. First, given the land use and activity, nearly all structural BMPs should fit below grade such that the land above can be utilized. Additionally, as is often the case in the ultra-urban environment, existing drainage and utility constraints will require a BMP that operates over a wide range of head conditions.

A structural BMP is required to provide storage for the "channel protection" volume. In the ultra-urban setting, about the only BMP to meet the storage and space limitations of the site is the underground storage tank.

The objective of reducing pollutant loading requires a more complicated solution. Sediment, oil and grease, and phosphorus are all generally captured by the same pollutant pathways. But due to the high salinity, particulate settlement may be partially compromised in traditional settling chambers.

The structural BMPs selected include:

• Bioretention as a surface facility sized to about four percent of the drainage area. This practice has a good to excellent removal capability for sediment, total phosphorus, and metals (see Table 51). Since almost all the other feasible BMPs rely on settling as the primary treatment, bioretention will help augment pollutant removal.

• An underground sand filter, utilizing the perimeter technique (also called the Delaware sand filter). This practice has a proven performance record. Given the nature of the site as among the highest pollutant sources in the urban landscape, the multiple BMP approach is warranted.

• The existing catch basins on the site are to be cleaned to provide additional capture of the coarsest sediments. Sand storage areas appear to have a disproportionately high amount of this size particle in the stormwater, making this practice more important.

The nonstructural BMPs selected include:

• Streetsweeping, particularly in the winter months (for this northern climate), once a week to remove street particles.

• Maintenance of catch basins (discussed above).

• Education and training of employees on the proper use and disposal of materials.

• Vegetative plantings combined with the bioretention system.

• Covering and handling road sands and salts. One of the primary sources of pollutants at many road and highway maintenance departments is the exposure of sand and salts to surface runoff. Most NPDES permits now require the covering of these materials. Yet, one the biggest sources remains these loading and transfer areas. Extra covered area may be warranted depending on the extent of the activities.

The selected BMPs, both structural and nonstructural, were chosen based on an array of criteria. The bioretention provides potential removal for high salinity runoff while improving the aesthetics of the maintenance yard (people do work in these areas). The underground sand filter is a flexible, easy to access, BMP that lends itself to these types of land uses. The system can be designed for a wide array of structural loading conditions. The underground storage tank can be designed in conjunction with the sand filter to capture the channel protection storage volume as well as act as a potential spill containment facility.

The selected nonstructural BMPs can enhance the performance and design life of the structural facilities. The five nonstructural BMPs listed above can each play a role in the stormwater management effort. The final practices implemented may depend as much on staff and financial resources as on physical feasibility. The expense and compatibility of proposed nonstructural BMPs must be evaluated in light of existing funding, resources, and program capability.

Prior to designing the proposed structural BMPs, it is important to evaluate the capability of the owner's or agency's 0&M program and program funding limits. All BMPs require maintenance, but a multiple BMP treatment train system requires routine and major maintenance at proscribed intervals. If the funding does not exist to maintain this type of program, then other less expensive structural and nonstructural BMPs should be considered even if they are less effective.

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