City of Leavenworth, Kansas
2018-2020
WRS first conducted a streambank stabilization alternatives feasibility study for the City of Leavenworth, investigating contributors behind and possible solutions to a streambank failure on an unnamed tributary to Fivemile Creek adjacent to homes along Independence Court.
The feasibility study recommended the most appropriate solution to specific threatened sites, and also reported the results of a tributary-wide investigation to evaluate the condition of the tributary and to aid stakeholders in prioritizing possible projects in the future, in order to prevent similar streambank failures that may be expected in other portions of the tributary. The analysis of the stream indicated systemic instability at from 60% to 80% of the tributary that shows bed lowering, bank widening, and mass wasting.
This stream stability study included a field assessment, a description of each reach and a score based on the American Public Works Association’s channel condition scoring matrix, opinions of probable construction costs to stabilize all reaches of the tributary, concept-level design recommendations to stabilize the specific bank failure along the backyards of Independence Court, opinions of probable construction costs for the proposed alternatives, and recommendations.
Site-specific alternative concepts to prevent further bank failure included longitudinal peaked stone toe protection along two bends to armor and restore the failing banks, vegetative plantings along the banks and stream to stabilize the channel, and installation of a series of grade controls in the stream to reduce channel slope and water velocity in the project area and rebuild the scoured banks.
The second suggested option would have re-aligned the stream throughout the reach to an ideal natural meander, to keep the stream in equilibrium and reduce further erosion similar to that which is threatening the at-risk properties. It would provide a complete solution to protect the private and public property currently at risk in the project area.
A concept level one-dimensional hydraulic model was created for the stream reach considered to be most at risk, to determine the range of hydraulic forces accruing on the banks and to inform the choice of alternative solutions. The hydraulic model was created using the U.S. Army Corps of Engineers Hydrologic Engineering Center River Analysis System (HEC-RAS) software to provide information on the velocity and shear stress that is affecting the channel degradation occurring around the threatened homes and infrastructure. The flow of the hydraulic model was estimated utilizing TR-55, and information from the Environmental Protection Agency was used to determine the sub-basin area information. WRS also conducted an assessment of permit requirements.
The City selected WRS's recommended option, and retained WRS to take it to final design. The option to specifically address the bank failure installed longitudinal peaked stone toe protection along two bends, to armor and restore the failing banks, and included vegetative plantings along the banks and stream to stabilize the channel. It also installed a series of grade controls in the stream to reduce channel slope and water velocity in the project area and rebuild the scoured banks. The complete solution to protect the private and public property at risk in the project area was completed in late 2020.
