Creating Affordable Multi-Benefit Flood Solutions within a CSO Basin Requires Bigger Thinking

The Westport Regional Business League (WRBL) in conjunction with the City of Kansas City, Missouri (City) are finally addressing frequent and severe flooding within the Westport district of the Brush Creek CSO basin. This project is applying a multi-objective approach to design a more affordable and valuable solution. The overall objective was to significantly reduce flood depths at a more affordable cost within a combined sewer basin. The project team achieved this objective by first developing a detailed 2D stormwater model that accurately simulated historical flood depths and then used that model to inform the city and key stakeholders on the flood reduction benefit for a range of solutions. Overlaying the 2D model results over parcel and street information served as a clear communication tool to demonstrate the flood reduction benefit. Unlike typical stormwater designs the goal was not to eliminate flooding, but to reduce the flooding to a point that property was not damaged, and public roads could be passable. An example of the before and after model results graphics are provided as Figure 1. Westport's flooding problems have gone through numerous studies in the past. These efforts have ultimately resulted in solutions that were held to a traditional stormwater design standard that was not affordable or constructible within this highly developed and utilized area of the city. The City and WRBL are now interested in pursuing stormwater solutions that can improve their frequent flooding problems, outside of a traditional stormwater design standard. The project team approached this effort using available rainfall data for two recent rain events and targeted stormwater improvement solutions that would alleviate flooding for these historical events using a Infoworks ICM 2-Dimensional Model. This tool was developed to accurately reflect observed flooding conditions for the recent rain events to understand how much reduction in flood depth the proposed improvements could provide. While 'solving' flooding problems in Westport for all storm events is not a feasible goal, the project seeks to significantly improve the flooding conditions for the more frequent rainfall events. Reducing flood depths from over 4 feet to less than 6 inches can transform what has historically been expensive flood damage to buildings and vehicles to short-term ponding contained mostly within the street right-of-way. As part of the federal consent decree compliance for CSO control, the City implemented the Smart Sewer Program (SSP) and developed a systemwide hydrologic and hydraulic model of its sanitary and combined system. The SSP model was developed in Innovyze InfoWorks ICM Version 7.5 and is a one-dimensional (1D) model calibrated to simulate the underground combined sewer system and CSO volume. The Brush Creek basin model was used as the starting point to develop the Westport stormwater 2D flood model. The 2D model was invaluable during this project. Because 1D models are limited to showing flood depths only at modeled manholes and inlets, but cannot show how the overland flow travels above ground and the flood depths and extents. The 2D model also better simulates the duration those flood depths and extents are experienced. The 2D model provided a much better understanding of how floodwater traveled and accumulated in the problem sump areas causing flooding of cars and businesses. The model results were confirmed by personal accounts, photos, and videos of simulated historical flood events. The 2D flood model was developed using a digital elevation model (DEM) that was created from the City's latest LiDAR-derived DEM data and updated with the field topographic survey collected as part of the project. A 2D mesh zone, a series of contiguous triangles, was then created in the model. Rainfall data is then directly applied to the 2D mesh zone, and the model routes the simulated runoff to the closest inlet point or lowest depressed area based on the DEM. This more accurate approach for stormwater runoff in Westport, which has a very high amount of directly connected impervious area, proved to be critical in not only defining the flood depths but the significant surface routing through the area. Figure 2 provides an example of the surface routing in the model. The other benefit of using a highly detailed 2D surface mesh was that green infrastructure collection solutions were simulated as depressions in the 2D mesh representing proposed conditions. This approach better represented the stormwater benefit of green infrastructure intercepting surface runoff, storing stormwater runoff, and then metering the storage volume back to the pipe system. The 2D watershed model was then utilized to evaluate both green and gray stormwater improvement solutions. Gray stormwater improvement solutions included curb and grate inlets, collection piping to underground storage, and conveyance piping. Green stormwater improvement solutions included pervious pavements, bioretention, vegetated swales, and tree planters. A combination of green and gray stormwater improvement alternatives were evaluated with the 2D stormwater model for a range of rainfall events. Stormwater improvements were sized and located to align with the Westport District plan which included parking improvements, streetscape enhancements, increased pedestrian safety and connectivity while maintaining the historical significance of the area. Using this multiple objective integrated approach, the design team was able to develop a preferred improvement alternative that provided a flood reduction benefit, addressed the Westport District plan requirements, and was cost-effective. The preferred stormwater alternative is a system solution that collects, stores, and conveys stormwater at multiple locations. Two large underground storage units form the upstream and downstream control points while green infrastructure collection in the form of pervious pavements along with traditional curb inlets provide additional collection and storage. A large stormwater pipe system connects the upstream and downstream storage creating a complete system solution to maximize the stormwater function. At the very downstream point the captured stormwater must be metered back into the combined sewer system. The stormwater solutions were located in areas that also required park amenities, street improvements, and defined parking which maximized the overall restoration benefit. (See Figure 3 for graphic of proposed improvements). The preferred improvement solution is tens of millions of dollars less than previous solutions, which makes this project fundable by the city. The city plans to move this project forward to design and construction to finally solve this frequent and severe flooding in Westport. In addition, this integrated flood reduction project will have a significant impact on reducing the CSO volume discharged downstream to Brush Creek. During the design phase other project performance improvements will be evaluated including real-time controls of the storage volume, stormwater re-use opportunities, and integration of this project with another green infrastructure project downstream to increase the performance and maximize the benefit of both projects. Thinking bigger than just solving the flooding problems has resulted in a project that a broad range of stakeholders can support at a more affordable price.