After an Extreme Storm: Madison WI's Approach to Flood Mitigation Planning

In response to flooding that occurred in August 2018, the City of Madison responded by initiating a comprehensive watershed planning program. Prior to this a number of large rainfall events had occurred in the City and caused flooding in various area. The tipping point for the City was reach on August 20, 2018, when an extreme storm event that was in excess of the 1,000-year storm event occurred and resulted in flash flooding and lake-level flooding within the City. A USGS rain gage recorded in excess of 10-inches of rain in under 12-hours. Precipitation amounts of up to 15-inches were estimated. The Atlas 14, 1000-year, 24-hour rainfall event for the area is 8.92-inches. The purpose of this project is to describe how one community moved forward from an extreme event and developed a watershed study program to become more resilient to future events. The Waite Circle area of Madison was particularly hard-hit by flooding, and as a result, was studied as part of the Wingra West Watershed Study as one of four initial projects undertaken. A detailed one-dimensional and two-dimensional hydrologic and hydraulic model was developed for the watershed to evaluate the existing stormwater conveyance system. The model was calibrated to monitored storm events and validated compared to historical observations from the August 20, 2018 event (see Figure 1). The model output was used to prepare flood inundation mapping that could be shared with the public and various stakeholders to help understand flood risks throughout the watershed under various storm events. The study considered various flood mitigation solutions including dispersed green infrastructure at varying levels of implementation (see Figure 2). Gray infrastructure that was considered included local storm sewer improvements, new relief sewers, new detention basins, improvements to existing detention basins, and culvert improvements. Ultimately a suite of measures, including improvements to five existing detention basins, enlargement of a culvert, construction of two relief sewers, and numerous local storm sewer improvements was selected to meet the City's flood mitigation goals for 10-, 25-, and 100-year storm events (see Figure 3). The project included three public information meetings and more than ten focus group meetings. The study began in 2019 and was ultimately completed in 2022. This presentation will provide information regarding the severity of flooding and the development of the City's watershed study program which included a hydrologic and hydraulic modeling guidance document and establishment of a stormwater system monitoring program. The Wingra West Watershed Study will be presented as a case study to show how model development and calibration occurred, how flood mitigation solutions were evaluated and selected, and how public participation was incorporated into the study process. The presentation will also include how the study phase was then transitioned into an implementation phase. One result of the flood response was a fast-track emergency culvert improvement project that was designed and constructed in less than 9 months to mitigate flooding in the Waite Circle area. This project was undertaken during the development of the overall watershed study and added a layer of complexity to the study. Project design began in May 2019 and construction was completed in December 2019. This compressed project time-frame required an expedited material procurement process and complex construction scheduling to achieve the required project completion date. The original culvert had failed, and an initial stop-gap repair had reduced the culvert capacity and contributed to the severity of flooding. A solution to this problem was needed prior than the schedule for the full study allowed. The improvement project enlarged the culvert to restore the original capacity, as well as allow for flow to be increased through the culvert in the future to further mitigate flooding. This was accomplished by designing the culvert to fully pass the 100-year storm event. Restriction plates were then installed to reduce flows until the downstream system capacity is improved such that higher flows can be safely handled. Additional complexities of the project included a small project site with numerous utility conflicts, limited site access, steep slopes, and challenging site restoration. In particular, a sanitary sewer interceptor sewer needed to be relocated from being located directly under the culvert. The relocation allowed for future maintenance and reduction of inflow and infiltration (see Figure 4). The presentation will conclude with a summary of the current status of the City's watershed study program and next steps for the program. It will also provide lessons learned and how the program has evolved to better serve the City. The Wingra West watershed study has been described by City Staff as one of the 'guinea pigs' of the program. Sharing the lessons learned from this study will allow others to improve their own stormwater programs and become more resilient to large rainfall events. By attending this session, participants will be better able to: 1.Describe challenges associated with flooding and flood mitigation in an urban watershed. 2.Recognize the importance of having a strong public outreach and communication program with various stakeholders for planning and implementation. 3.Understand the benefits and limitations of both gray and green infrastructure to mitigate flooding from large storm events.