Modification of Detention Basin Outlet Structures using Calibrated SWMM Models
The U.S. Environmental Protection Agency Storm Water Management Model, SWMM, is a large, relatively complex software package capable of simulating the effects of runoff from sub-catchments through pipe/channel networks and storage facilities, and finally to receiving waters. This chapter describes a study where detailed SWMM models were developed as an integral part of a simulation-analysis mechanism of the outlet structure and associated modifications. By exercising the developed models, different outlet structure configurations can be compared, thereby resulting in an optimal configuration of the structure.
The need is based on changes in the Metropolitan St. Louis Sewer District's (MSD) detention basin design requirements (Akan, 1989). It was previously required that the outlet structure controls the outflow of the 15-year storm to the same level as the pre-development discharge rate. It is now required that the outlet structure control the outflow of both the 2-year and 100-year outflows to the pre-development discharge rate or to the required basin-wide release rate determined by MSD (McEnroe, 1992). Another factor motivating modification of outlet structures is the observation by a number of interested individuals and communities of "little or no storage" of storm water occurring in these basins during storm events with subsequent downstream flooding. Detailed SWMM modeling of eight different basins in the St. Louis, Missouri, metropolitan area showed "little or no" reduction of the 2-year storm outflow, but significant reduction of the post-development 100-year storm outflow. Thus, the objective reached by modifying the existing outlet structures is to reduce the 2-year storm outflow to the pre-development or the required basin wide release rate determined by MSD. Also, if possible, the present 100-year storm outflow rate should be reduced to the pre-development or required release rate. If this was not possible, then the resulting maximum outflow attained by the structure modification should be reduced to the existing structure's maximum outflow, or reduced as much as possible with one foot of freeboard (Whipple, Randall, 1983).
Given these objectives and the detailed SWMM models, optimal modifications of the existing outlet structures were designed for these basins (Walesh, 1989). The detailed SWMM models of six basins were calibrated with 5-minute incremental precipitation and basin depth of water and then used to predict the effects of the modifications. This chapter presents these calibrated models and shows the results of these modifications, including general recommendations for modifying all basins designed under the old detention basin criteria.
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