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Floodplain Considerations Impact Roadway Design


National Flood Hazard Layer of the project area.

To provide access to Microsoft’s new data center and open additional area for development in southwest West Des Moines, city leaders contracted with Snyder and Associates and our structural engineering subsidiary, Shuck-Britson, to design the roadway extension of Grand Avenue, west of Grand Prairie Parkway, and across Johnson Creek. The project also included implementing a conveyance improvements plan along Johnson Creek to lower flood elevations upstream of Raccoon River Drive.

Much of the land north of Raccoon River Drive is in the floodplain of the Raccoon River, necessitating the Johnson Creek improvements. When the property is developed within the floodplain, buildings will be elevated one foot above the one percent annual flood chance.  However, flood elevations in Johnson Creek were higher than those of the Raccoon River under certain future conditions, putting developments in the corridor at risk of flood events even though they were raised above river flood events. Additionally, restrictions caused by narrow bridges under the Iowa Interstate Railroad and Raccoon River Drive were improved as part of the project.

Multiple Bridge Projects Highlight Expansion Efforts

Custom-designed monuments for the City of West Des Moines

Custom-designed monuments for the City of West Des Moines.

Our team created a 2D hydraulic model to evaluate the designs of new bridges over Johnson Creek and obtained floodplain permits for several projects along the creek. We relocated a portion of the creek to allow for the construction of a new overpass of the Iowa Interstate Railroad on Grand Prairie Parkway. We designed two new bridges at Raccoon River Drive and the newly constructed Grand Avenue. Our team also worked with Iowa Interstate Railroad to size and permit a new bridge over Johnson Creek at its track location.

The project included 4,800 feet of new roadway along Grand Avenue and a 204’-long bridge over Johnson Creek. The bridge was designed with a 10’-wide trail on the north side and a 5’-wide sidewalk on the south side with custom anodized aluminum railings. Our team also designed four decorative monuments at each corner of the bridge. These monuments match the design our team created for the City of West Des Moines to be included at all its creek crossings in the southwest part of the city. All project work is scheduled to wrap up in the spring of 2024.

Spearheaded by the Snyder and Associates team, the Grand Avenue Extension project in West Des Moines will improve traffic flow in response to the increased demands of this rapidly expanding area of the City. The incorporation of a conveyance improvements plan along Johnson Creek not only addressed floodplain considerations but also enhanced the overall infrastructure resilience. With the completion of multiple bridge projects, including a new overpass and a 204’-long bridge over Johnson Creek, the city now boasts 4,800 feet of new roadway, contributing to the city’s ongoing development efforts and ensuring a more efficient and secure transportation network for its residents.