WATER RESOURCES BULLETIN VOL. 26, NO.1 AMERICAN WATER RESOURCES ASSOCIATION FEBRUARY 1990 ECONOMIC FRAMEWORK FOR FLOOD AND SEDIMENT CONTROL WiTH DETENTION BASINS' Glenn E. Moglen and Richard H. McCuen2 ABSTRACT: A framework for combining economic factors and the hydrolo of detention basins is provided. The general development of economic production functions for water quality (sediment) and flood control is discussed. Example production functions are generated to compare water quality (sediment control only) and flood control. For the given example, the design of a detention basin for downstream sediment control is economically unwarranted. When compared to on- site detention facilities, regional detention structures appear to be more practical from an economic standpoint for water quality control. Since sediment was the only water quality parameter assessed, it is entirely possible that the design of a detention basin for water quality control would be justified if the effects of all pollutants of concern could be quantified. Policy aspects of detention facilities that relate to the economics of water quality control are also discussed. (KEY TERMS: detention basins; economics; water quality; flood con- trol; production function; stormwater management; regional control.) INTRODUCTION Land development and the accompanying flood, ero- sion, and water quality problems have fostered a vari- ety of stormwater management practices. The detention basin is the most widely used of these practices. In gen- eral, stormwater management policies have required the design of detention basins to control after-develop- ment peak discharge rates to those that existed before the development took place. At best, these policies achieved mixed results in their effectiveness. The peak discharge rates were controlled but, due to the larger runoff volumes and longer durations of near-peak flows that accompany land development, the rate of erosion in the channel section below the detention basin outlet was often exacerbated by the presence of the detention basin (McCuen and Moglen, 1988). Numerous studies have clearly demonstrated that non-point sources of pollution in urban areas contribute a much larger fraction of the total pollutant load than previously believed. The cost of the pollution associated with increases in both erosion and nonpoint source pol- lutants must be recognized. Cost figures provided by ASCE (1969) demonstrate the magnitude of the prob- lem. The cost manifests itself in ways as diverse as eco- logical damage to our environment, as a serious and growing concern to the policy makers because of the need to use public resources for clean-up rather than for other social services, and as an economic liability to the taxpayers. While originally designed to control downstream flood and erosion problems, detention basins are gener- ally believed to improve water quality characteristics. Because detention basins retain water it is believed that they allow sediment, as well as other pollutants, to settle out before the water is discharged from the basin. The trap efficiency, which characterizes the effective- ness of a basin as a pollution control practice, indicates the proportion of an inflowing impurity that settles or is trapped by the basin. While several studies have evaluated the trap efficiencies of particular existing de- tention basins (e.g., Davis et al., 1978), these studies have not made an attempt to translate trap efficiency into an economic criterion. Others such as Nix et al. (1983) have presented frameworks for determining the most cost-effective designs. Heaney et al. (1978) pro- vided a methodology for estimating the cost of pollution control in urban areas. While the technical details of detention basin design are reasonably well established, there are many unan- swered questions related to the economics of detention basin design. Can we expect the economic benefits of water quality control using detention basins to effect the benefit-cost ratio that is currently dominated by flood control benefits? To answer such a question, what types of information will be needed to make the economic analyses? What elements of stormwater detention policies will make water quality control a practical economic criterion? To approach these prob- lems, especially the first two questions, a framework 1Paper No. 89113 of the Water Resources Bulletin. Discussions are open until October 1, 1990. 2Respectively, Graduate Research Assistant and Professor, Dept. of Civil Engrg., Univ. of Maryland, College Park, Maryland 20742. 145 WATER RESOURCES BULLETIN