Leawood, Kansas
Client:
City of Leawood
The City of Leawood was experiencing flooding and erosion problems along the Dykes Branch tributary to Indian Creek. The stream alignment is located mostly in backyard areas of affluent residences in “Old Leawood” with two separate roadway crossings within the project limits. This stream floods nearly 20 homes during large flood events. Since the stream is part of the backyard landscape of numerous residences, the main design challenge is to develop flood control solutions that can obtain the necessary environmental permits and will be acceptable to the impacted homeowners. Terra Technologies was included on the project team by Black & Veatch, the lead consultant on the project, to evaluate and design biotechnical options that would be acceptable to both the permitting agencies and the affected property owners.
In order to reduce flooding, the conveyance capacity of the stream had to be dramatically increased. Traditional structural lining options were considered unacceptable because of the resulting difficulties in obtaining the necessary permits and aesthetic impacts. The existence of a privately owned pond immediately adjacent to the stream limited stream-widening options in some locations. The resulting design provides a wide “flood bench” area adjacent to the existing stream to increase stream conveyance capacity. To minimize impacts to adjacent property, Terra Technologies designed reinforced slopes at the flood bench limits to allow construction of steeper slopes, up to a maximum of 1 horizontal to 2 vertical. The reinforced slopes will be covered with stabilizing vegetation providing the appearance of a naturally occurring stream bank. The existing channel alignment will remain unaffected by this project but allowances are being made to account for future stream transitions that are likely to occur. The resulting project will create an intensively engineered channel that will appear to have occurred naturally.
At the roadway crossings, transitions from existing vertical walls to graded slopes are being accommodated with biogabions. These features can be readily vegetated, hiding the existence of the underlying structure. The channel bottom immediately downstream of the roadway crossings is to be reinforced with articulated block systems that allow the establishment of vegetation while providing structural resistance to the hydraulic scour forces that typically occur downstream of concrete structures. The vegetation established within the articulated blocks absorbs a majority of the excess hydraulic energy caused by the upstream restriction. Once the excess hydraulic energy has been dissipated, the natural channel bottom will have sufficient resistance to hydraulic shear to prevent the future occurrence of a large scour hole immediately downstream of the roadway crossings.
Native grasses utilized were selected for their stabilization capacity and ability to create a dominant landscape. The end result will provide an aesthetically pleasing project that can withstand the hydraulic shear forces that occur during large flood events.
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