The Development and Evaluation of Alternative Erosion Control and Flood Control Projects to Support the Calumet-Sag Detailed Watershed Plan

This paper summarizes the development and evaluation of erosion control and flood control projects to support the Calumet-Sag Detailed Watershed Plan (DWP) for the Metropolitan Water Reclamation District of Greater Chicago (the District). The 151 square-mile Calumet-Sag Watershed, located in the southwestern part of Cook County, is one of six major watersheds within the county. In 2004, the District was granted regional stormwater management authority through State Legislation. The six DWPs developed by the District are intended to identify projects to address regional stormwater problems. The objective of the DWP was to supply the District with information on existing conditions, stormwater problems, alternative improvements considered to address stormwater problems, and other information necessary to prioritize alternative improvement projects. An integrated watershed management approach, combining extensive community involvement, as well as hydrologic and hydraulic (H&H) modeling, inundation mapping, and damage assessment was applied to develop and analyze potential projects. To evaluate stormwater problems and potential projects, H&H models were developed and calibrated to USGS and observed data. The fully dynamic unsteady flow modeling approach was used to account for significant in-channel storage due to a relatively flat watershed. H&H models were then used to assess the risk of flooding for various recurrence interval storms. Property damages due to flooding were estimated using a methodology consistent with the U.S. Army Corps of Engineers Flood Damage Assessment program. Estimated flood damage was considered in combination with the probability of event occurrence. A Python script was developed to automate the batch geoprocessing of inundation areas with GeoRAS water surface elevation TINS to efficiently support damage estimation. Erosion damages were assessed for structures or infrastructure at risk of loss due to actively eroding stream banks. Stormwater improvement alternatives were developed at a conceptual level to address regional stormwater problems. Benefits were calculated for each project as the difference between existing and alternative conditions damages. Conceptual level cost estimates were produced and included design, construction, and maintenance costs. An innovative web-based database was developed to facilitate consistent calculation methodology and data management of damages, benefits, and cost estimates for alternatives evaluated for all six DWPs. A benefit-to-cost (B/C) ratio was developed for each alternative. In addition to the B/C ratio, features such as water-quality impact, number of structures protected, and the impact on wetland and riparian area were noted for each alternative. Alternatives were recommended based upon consideration of their ability to reduce regional stormwater damages. The District's Board of Commissioners will prioritize the implementation of recommended stormwater improvement projects.