City of Leavenworth, Kansas
2018
In mid-summer 2018, a homeowner living along a small creek in Leavenworth, Kansas, called the city's public works department to report holes and areas of settling appearing under and near a shed in her backyard.
Upon investigation, Water Resources Solutions concluded the holes were likely "piping failures," a progressive internal soil erosion caused by backward erosion from the stream upward into the bank, until a continuous “pipe” is formed between the upstream area and the stream. Such continuous pipes are associated with failing banks. The homeowner also expressed her belief that the creek had moved closer to her yard in the years since she had first moved into the residence, and that recently the stream seemed to be eroding at a faster pace. WRS further confirmed that the neighboring residence to the west was experiencing similar levels of erosion and bank failure.
During the site investigations, it become obvious the bank instability issues weren't an isolated event for one yard. Instead, the streambank failures appeared to be systemic. Based on historic aerial photography from 1947 to 2018 for the area, the formerly rural pastureland of the basin had undergone significant development from 1975 through 1990, consisting mainly of eighth-acre single-family residential homes. Some of the development areas were built close to the stream. As the stream experienced additional flow volume and higher peak discharges from the upstream development runoff, its planform — the stream geometry as viewed from above — along the backs of the properties along Independence Court changed. Most or all of the tributary most likely is adjusting to changed hydrology in the watershed due to the development. The stream appears to be incising and increasing its meander in order to flatten its slope to minimize energy. WRS's analysis of the stream indicated systemic instability is occurring at from 60% to 80% of this tributary that shows bed lowering, bank widening, and mass wasting.
Because of that need for a systemic evaluation, the City of Leavenworth requested WRS conduct a field assessment of the entire tributary, to evaluate its stability. The report and 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 at Independence Court, opinions of probable construction costs for the proposed alternatives, and recommendations. Based on the CCSM scores, the WRS priority list will allow the city to systematically phase possible future projects along the tributary. The reach ranking conducted by WRS will permit consideration to be given to those reaches where property and infrastructure may be at risk because of stream instability. Additionally, targeted system maintenance opportunities were identified for immediate action to improve the system health.
Reach 6 improvements: A set of options for specifically addressing the bank failure along Independence Court.
Reach 6 option 1
Option 1 to specifically address the Reach 6 bank failure involves three components:
Longitudinal peaked stone toe protection (LPSTP) along two bends to protect and restore the bank. LPSTP is a row of stone placed at the bottom of the slope, running parallel with the stream. It armors the vulnerable toe of the bank against further erosion and supports the bank above it while helping stabilize the slope by allowing sediment to deposit behind the stone and rebuild the upper bank over time. The upper bank above the stone toe protection gradually slumps to a stable slope.
Native vegetative plantings along the banks and stream to stabilize the channel by restoring the root system that helps resist erosion.
A series of four riprap grade controls in the stream to reduce channel slope and water velocity in the project area and rebuild the scoured banks.
Option 2 will re-align the stream throughout Reach 6 to an ideal natural meander, or the normal course of curves and loops a stream takes from side to side as it flows across its floodplain. Realignment will keep the stream in equilibrium and reduce further erosion similar to that which is threatening the at-risk properties.
The planform geometry for the option was determined based on studies and summarized in Sour and Thorne (2001). It has been shown that the dimensions of stable natural channels are proportionally related to the stream discharge, and the dimensions of the channel are inter-related. Using HEC-RAS in combination with indicators such as bar height and lower limit of woody vegetation, the bankfull discharge was determined to be approximately 2 feet. The stream forming flow at that flow depth was determined to be 350 cubic feet per second. Using the stream chart and aerial photos, an improved stream alignment can be proposed from those figures. Table 8 shows the results of the calculation for the ideal width, length, and radius of curvature for the re-aligned stream meander pattern.
Option 2 will also install four rip-rap grade controls to reduce channel slope and velocity in the project area and rebuild the scoured banks. Vegetative plantings along the banks and stream to help stabilize the channel are also recommended.
Tributary improvements prioritization
Table 6 presents a prioritization for future stream reach improvements based on the opinions of probable construction cost and project cost, ranging from a low of $154,688 to a high of $506,250 for reach 2 and reach 4, respectively.
Reach 6 option 1
Option 1 to specifically address the Reach 6 bank failure would install longitudinal peaked stone toe protection (LPSTP) along two bends to armor and restore the failing banks, include vegetative plantings along the banks and stream to stabilize the channel, and install a series of grade controls in the stream to reduce channel slope and water velocity in the project area and rebuild the scoured banks. Option 1 would provide a complete solution to protect the private and public property currently at risk in the project area, at an estimated cost of $567,750.
Reach 6 option 2
Option 2 would re-align the stream throughout Reach 6 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, at an estimated cost of $1,365,000. It may require more involved permitting under an individual 404 permit.
Recommended alternative
Tributary improvements prioritization
Reach 6 options
In Reach 6 the channel degradation needs intervention to protect private property and city infrastructure. In order to protect the at-risk property, option 1 is the recommended alternative. Although option 2 would provide a more holistic approach, option 1 is a more targeted solution that would provide a complete solution to the project problem, at a cost of 41.5 percent of option 2.
WRS's streambank stabilization alternatives feasibility study investigating contributors behind and possible solutions to the failures on this unnamed tributary to Fivemile Creek adjacent to homes along Independence Court recommended the most appropriate solutions based on the City of Leavenworth’s requirements and relative advantages and disadvantages of each proposed solution. In addition, it reported the results of a tributary-wide investigation to evaluate the condition of the tributary and 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.