Sensitivity of Stormwater Conveyance System Cost to Design Recurrence Interval

by David Kreeger,
Stuart Walesh,
Zuhdi Al-Jobeh,

Document Type: Proceeding Paper

Part of: WRPMD'99: Preparing for the 21st Century


(No paper) The cost of a piped and/or swaled stormwater system is determined principally by the storm recurrence interval (e.g., 5-year design storm) used to design it. That is, the present worth or the annualized cost of the storm drains for a new residential development increase as the severity of the design storm increases. The owner should know how sensitive cost is to recurrence interval. With this information, the optimum system is more likely to be selected and constructed. Presented in this paper is a methodology for determining drainage system costs as the function of design storm recurrence interval. The Manning equation and sewer and swale cost data were combined to yield a relationship giving the cost per unit length of sewer or swale as a function of design flow. A typical residential development was created and a stormwater modeling computer program (Rational Method and Manning equation) along with derived cost functions were repeatedly used to design and cost the drainage system for a range of recurrence intervals. The process was applied using rainfall intensity-duration-frequency data for three different locations in the United States. One finding is that the cost of conveyance systems is relatively insensitive to recurrence interval. That is, a large degree of additional flood protection can be obtained for little additional cost. For example, the cost of the 50-year recurrence interval storm sewer systems was only about 20 percent more than the cost of the 5-year recurrence interval systems. This paper presents a practical process that can be used by a drainage designer to determine cost as a function of recurrence interval. The authors hope this will improve the design of stormwater systems.

Subject Headings: Benefit cost ratios | Stormwater management | Storms | Drainage systems | Sewers | Hydraulic design | Residential construction | United States

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