Johnson Count, Kansas, Stormwater Management Program
One of nine consulting teams involved in creating cross-jurisdictional watershed plans for the 480-square-mile, variably urbanized Johnson County, Kansas, Water Resources Solutions was instrumental in assessing risk from erosion patterns and fluvial morphologic changes, as well as hydromodification patterns in two of the county’s 20 watersheds draining more than 1,800 miles of delineated streams.
As part of the $1.5 million study, WRS first modeled and mapped erosion and sedimentation volumes along the two urban streams using historic lidar data. WRS then mapped the stream boundaries to create an “erosion hazard zone,” in which intrastructure might be at risk from erosion currently or in the near future. It then assigned relative risk rankings to the infrastructure in or near that zone, using an adaption of the Water Environment Research Foundation’s Business Risk Exposure method’s balance of likelihood of failure and consequence of failure to identify and prioritize high-risk failure events.
The WRS team then developed concept level management strategies for the prioritized geomorphic failure events.
In conjunction with the same study, WRS engineers also conducted a hydromodification study of the two streams’ watersheds. WRS faced the unique obstacle of being charged with developing flow duration curves using historical USGS streamflow data to quantify the current degree of hydromodification in one if its streams — without the benefit of having any USGS streamgages present within that stream.
In order to accomplish the modeling, based on knowledge of and experience in the region, WRS knew a 35-year-old series of municipal-managed stream gages were present in the stream. The 108 sensors collected real-time rainfall and stage data, available to the public via the Internet. WRS data engineers collected Stormwatch data both for the Turkey creek watershed and the nearby Brush Creek, which did have available USGS gage data. After a painstaking process of extracting and cleaning the stormwatch data, they then used the data to create hydrographs of the two streams, compare them to the hydrograph of a reference stream, and arrive at flow-reduction targets necessary in order to approach the reference stream. Lastly, the team conducted a GIS-based geographic analysis of available sites to assign prioritized target areas for distributed detention through the watersheds to reduce hydrmodification.
The final results were combined with the results of a concurrent flooding study and water quality assessment to prioritize 10 key target areas within each watershed. Potential concept level solutions were proposed and evaluated for feasibility to give the county a priority list to target projects with the best potential for solving not flood issues, but also geomorphic risk, water quality issues, and hydromodification mitigation simultaneously.