title: Riverton Road
title: flood mitigation modeling
titleType: Subtitle
Creators: name: Young, Kaleb
Publisher: name: University of Iowa
Year: 2022
Subjects: public abstract

description: Riverton Road is just west of the town of Riverton, Iowa. It lies within the East and West Nishnabotna Rivers in Fremont County and has experienced a high number of floods due to its topography and proximity to the two rivers. Within the last decade Riverton Road has overtopped on 22 different occasions, requiring the road to be closed. One of these flood events, in March of 2019, resulted in a death when a Nebraska man attempted to drive across the closed roadway and was swept downstream by the high floodwaters. Road closures due to the floods are a serious threat to health if somebody attempts to cross them, but also because the detour is fifteen miles and can delay emergency medical vehicles.
description: This project was funded through the Iowa Watershed Approach (IWA) and focused on mitigation options to prevent the flooding of Riverton Road. The IWA is a collaborative program funded through Housing and Urban Development that focuses on factors that lead to flooding and nutrient flows in nine watersheds throughout the state of Iowa. Riverton Road lies within the East and West Nishnabotna River Watersheds which are a part of the IWA program. Due to its consistent flooding and the risk involved with these floods, Riverton Road was an obvious candidate for work through the IWA.
description: My work began by studying the hydrological assessments that had already been developed for the watersheds by the Iowa Flood Center. JEO Consulting Group Inc. was a partner throughout the project and had developed a hydraulic model of the study area as they had already completed a 100-year case study of the road. JEO’s case study had looked at the 100-year flow on both rivers and several mitigation options to prevent flooding. All the options for flood prevention that they studied were deemed unfeasible and it was decided that looking at the more frequent flows, such as the 1.25-year through 10-year, would give a more realistic view of mitigation options. My project examined these more frequent flows and built on the work of JEO by finding levee alternatives to prevent overtopping events during these flows.
description: Work began by using an improved version of JEO’s model of the existing topography to determine at what magnitude of flow the roadway is overtopped and then at what level the water is above the road during those overtoppings. As well as the existing conditions model, flow data at USGS was pulled from the dates of known flood events. It was found that flows between the 1.25-year event and 5-year event were causing the majority of the overtoppings within the last decade at Riverton Road. Once model runs had been completed on the existing conditions model and the previous flow data had been analyzed, I was able to develop targeted levee improvements and additions to prevent flood events. Different alternatives were developed throughout the project, and each was tested within HEC-RAS to determine its feasibility. Along with the flood prevention performance, each alternative had a benefit-cost-ratio (BCR) calculated by JEO. Flow combinations of the 1.25-year, 2-year, 5-year and 10-year events were run on each river for each alternative that was developed. Flows pulled from historic flow data of overtopping events was also ran through the model to determine if the previous floods could have been prevented.
description: Following the model simulations, overtopping results and BCRs for each alternative were compared. Also, talks with the DNR who manages a wildlife management area within the study area occurred. Because the DNR is a major stakeholder in this project, considerations of how the levee alternatives affect their operations are important. The results of the testing combined with the opinions of the DNR allowed for several favorable alternatives to arise. These alternatives were taken through further analysis to determine the water surface elevation differences in the alternatives during flood events compared to those of existing conditions. This information is important to the DNR who has experienced a high number of levee blowouts during the recent flood events.
description: Following simulation runs, future work for this project was investigated. Some of the options that were developed involve a further analysis of alternatives that provide the most protection to the road, while meeting the needs of those landowners whose property are affected. The future work will require a more thorough BCR, discovering options for grants to pay for the project, and a more analyzed hydraulic model of the alternative. A watershed-scale approach to preventing the overtoppings would also provide benefits beyond those just seen at Riverton Road. By installing floodwater storage practices throughout the watershed, downstream areas besides just Riverton will experience lower peak flows during storms. These practices will also help will nutrient reduction, which was beyond the scope of this project.