City of Mission Hills, Kansas
6/2017 to 2/2018
The stormwater system in Mission Hills, Kansas, consists of about 750 storm drainage structures and 11 miles of storm drainage pipe. More than 2,000 feet of those pipes are still corrugated metal, most of which is approaching or has exceeded its usable life, and more than 10,000 feet are made of PVC, much or which is undersized for the needed capacity.
The city selected Water Resources Solutions to design improvements to one of those pipe systems lying beneath Oakwood, from 59th Street to Mission Drive, based on a 2016 pipe system masterplan update WRS conducted for the city. WRS’s masterplan update revised the previous hydrologic model using new precipitation data to generate updated hydrographs for each storm inlet across the city. The updated hydrographs were next entered into previous hydraulic models WRS built for the city, to compute the hydraulic parameters of the pipe systems and determine whether each pipe system’s capacity was up to the job of conveying the runoff from the 10-year storm. The results? WRS’s systematic analysis showed 197 pipes do not have adequate capacity to convey the runoff from the 10-year storm event.
The system at Oakwood, from 59th Street to Mission Drive Street, was targeted as an early candidate for improvement. WRS’s role in the project included design not only for the replacement of the undersized and aging stormwater system, but also for the street, gutter and sidewalk improvements above the replaced system.
The stormwater conveyance system design for the existing system, in which eight of the inlets flooded during the 10−year rainfall event, consisted of multiple curb inlets and conduits to collect and convey the stormwater along Oakwood Road and Overhill Road. The system outfalls directly into the stream channel that parallels Mission Drive. To solve the flooding issues, WRS used HEC-HMS to calculate the runoff to each of the inlets, and used EPA SWMM to evaluate the hydraulics of the pipe system, in order to finally recommend that eleven pipes in the system be replaced or upsized, ranging by from 20 percent to as much as 71 percent.
The proposed improvements included 20 new storm structures, over 1,600 linear feet of reinforced concrete pipe, 5,600 feet of new curb and gutter, and over 7,100 square yards of asphalt mill and overlay. Street improvements included new curb and gutters, asphalt mill and overlay, and new driveway approaches.
Several complications posed a challenge when laying out the proposed system, including multiple sanitary sewer crossings throughout the system alignment. The designers were also faced with taking into account concurrent improvements being made to the Sunken Garden historical sight at the intersection of Oakwood Road and Mission Drive, including coordinating with the consultant designing those improvements to ensure full integration with the project — including tying in the site’s outfall with the new system. WRS’s constant coordination with utility providers and the separate subconsultant responsible for the Sunken Garden improvements helped ensure minimal disruption to this established and mature neighborhood.
The project design and construction plans were completed in Winter 2017. Construction totaling an estimated $1.1 million was completed in mid 2018.