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Platte County, Missouri

The Clemstone Addition and Red Rock Acres subdivisions, near Platte City, Missouri, experienced flooding due to what appeared to be an inadequately sized storm drainage system. About 75 acres drain through this location, and the area’s watershed is expecting further development, which will only increase runoff through the undersized system.

Flooding issues for this project include flooding at the intersection of Red Rock Drive and Flint Street, at the intersection of Cobblestone Drive and Flint Street, and along Ironstone Circle. Flooding also occured at the culvert under Flint Street, approximately 300 feet east of the intersection of Clemstone Drive and Flint Street. A rain event in summer 2016 resulted in significant flooding in Ironstone Circle, including residences.

Water Resources Solutions performed a hydrologic analysis to determine the peak flowrates to each of 11 inlet drainage basins, including time of concentration for each drainage area. The resulting runoff time-series hydrographs for each drainage area were calculated using the U.S. Army Corps of Engineers Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS).

WRS used EPA’s Storm Water Management Model to evaluate both the proposed and existing pipe systems' drainage basins using field measurements. Hydrographs for the 10-year event were exported from HEC-HMS for each of the existing and proposed inlets, and the resulting runoff flows were routed through the pipe systems. The Army Corps of Engineers River Analysis System (HEC-RAS) program was used to evaluate the open channels and culverts. The existing and proposed culverts were analyzed using the 1-year, 2-year, 10-year, and 100-year storm events.

The results of the hydraulic analysis indicated the existing storm system did not have the capacity to convey the runoff from the 10-year storm event. For instance, the 30-inch CMP draining one basin was found to handle only 2 percent of the necessary flow capacity for the basin, due both to pipe size and too-flat slope of the downstream receiving pipe.

Based on its analysis, WRS recommended building a detention basin behind the homes on the south side of Ironstone Circle to reduce the runoff rate into the stormwater system. It also recommended improvements to the rest of the stormwater system to increase capacity, including replacing the existing 60-inch corrugate metal pipe under Flint Street with a 12-foot by 5-foot reinforced concrete box culvert.

Associated services in the normal course of the designs included topographic surveying to create a planning basemap and to support the hydraulic modeling, geotechnical borings and testing to guide retaining-wall placement, preliminary and final construction plans and technical specifications beyond the county’s Standard Technical Specifications, preliminary and final cost estimates for the project, all required easement documents encompassing at least 11 properties, necessary permitting applications and submittals to the regulatory agencies, including U.S. Army Corps of Engineers Section 404 Permit application and follow-up bidding services, construction services, and construction observation, including at least five site visit, photos, and a summary report of the site visit.

WRS worked with the Platte County Planning Commission to ensure all the bases were covered in this complex project, helping manage some typical project problems that sometimes aren’t anticipated up-front. Regular citizen meetings to solicit feedback, for example, discovered additional drainage problems not at first appreciated. In another instance, careful coordination with all the affected utilities and understanding of all the engineering demands led to a solution that incorporated a sanitary sewer crossing into a stormwater transition, saving the county an estimated $60,000 in the process. WRS also helped the county walk through potential cost over-runs and interruptions at risk by installing a temporary construction road off a state highway, which could cause ramifications with the Missouri Department of Transportation.

The project, estimated at $744,000 total construction cost, was completed in summer 2019.