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444 Chapter 12 Urban Runoff and Combined Sewer Overflow Management
5. Emphasize pollution prevention rather than treatment.
6. Are likely to address future problems.
7. Concentrate on addressing the priority problems.
The assessment of individual BMPs results in alternatives based on implementing each
BMP throughout the study area. The comparison of alternatives is then in effect a comparison
of different BMPs. This approach yields useful data on system-wide implementation of partic-
ular BMPs. Although one type of BMP might not address the range of urban runoff problems
or goals in a study area, an urban runoff problem might exist that a particular BMP is well
suited to control. In this case, implementation of that BMP on a regional basis, with the BMPs
strategically located by the municipality, can be more effective and more easily controlled than
requiring each developer to implement that BMP for individual developments.
An example of this method of alternative development is the Henrico County,
Virginia, regional stormwater detention program. Early in the process of developing a
stormwater management plan, it was decided that, given the conditions existing in the wa-
tershed, regional detention basins would be used to control runoff pollution. Regional de-
tention basins were chosen because they provide both flood and pollution control, have
fewer site restrictions than other pollution control structures, and can be designed to ac-
commodate expected new developments. Therefore, the major remaining decision in the
program was the number, location, and size of the detention basins.
The above method leads to the development of alternative plans to address the urban
runoff problems of concern. Although the actual contents of each alternative plan are site
specific and depend on the type of alternative evaluation to be conducted, some general
guidelines for presenting the alternative plans can help in assessing them. Preliminary
sketches, rough cost estimates, expected pollutant removals, and environmental effects can
be included for each alternative so comparisons can be made.

12.7.2 BMP Selection Process

After the alternatives have been developed, they are compared using a decision
process (Fig. 12.21) that evaluates the relative merits of each plan. Because of the
complexity of urban runoff control problems, a number of factors must be considered

Analysis tools
• Watershed models Source flows/loads Inputs
• Water resource models Receiving water conc.
• Ranking models Priority problems
Decision analysis
Design conditions • Holistic
• Wet-weather events More qualitative • Cost-benefit Easier to implement
• PS/NPS flow/quality • Matrix Recommended
• Receiving water flow/quality • Decision factor BMP
Decision factors • Optimization plan
• Cost (capital, O&M) More qualitative Harder to implement
• Program goals
• Environmental effects
• Public acceptance
• Others Inputs

Figure 12.21 Conceptual Diagram of a BMP Selection Method
(Source: U.S. Environmental Protection Agency)