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City of Mission Hills, Kansas

The City of Mission Hills, Kansas, needed a better understanding of Brush Creek’s flow through a proposed low-water crossing replacement than it could get with standard numerical modeling. Water Resources Solutions built a physical model of the stream reach, discovering in the process two locations where a hydraulic jump occurs, potentially imperiling the design solutions being proposed. Its findings made the final solution more reliable at less cost.

This purpose of this hydraulic model study is to address bank instability issues for a reach of Brush Creek from Belinder Road to East Mission Drive. One particular location of concern is located on the right bank of Brush Creek along the property located at 5550 Mission Drive. Significant erosion as unearthed a sanitary sewer owned and operated by the City of Kansas City, Missouri, and has caused significant settlement and loss of soil in the backyard of 5550 Mission Drive. Historically, gabion baskets were constructed to protect the bank and, presumably, the sanitary sewer encasement. Within the past few years, the gabion baskets have started to fail and the sanitary sewer encasement has become completely exposed. In addition, soil from the 5550 Mission Drive backyard has fallen through the voids in the sanitary sewer encasement and has resulted.

Both one-dimensional and two-dimensional numerical models were used for the entire reach from Belinder to Mission Drive. A threedimensional physical model was used to analyze the creek through the reach near 5550 Mission Drive. The purpose of the three-dimensional model was to illustrate flow patterns and provide physical measurements of hydraulic parameters that would be used to provide more realistic boundary conditions for the two-dimensional model.

In addition to taking velocity measurement, the model was visually studied to determine the flow patterns. Dye was injected into the water and also small floating particles were placed in the flow to aid in visualizing the flow. The erosion effects that the flow had on the banks around the sewer main and retaining wall were also carefully studied. The water first flowed into the stream through the head box, and then continued through a straight section. When making the first bend, the water ran into the first retaining wall located on the left bank at the edge of the golf course. As the water flowed against the wall, standing waves, which occur in supercritical flow when the flow is near critical, were observed. The flow was observed to be fastest along the wall and decreased further away from the wall toward the right bank. The high velocity near the wall caused a large amount of erosion to occur at the toe of the retaining wall. During the physical model experiments, it was observed that a standing wave developed along the left bank of the creek along the Mission Hills Country Club wall. A standing wave occurs when two waves of the same frequency merge. This standing wave was also observed in Brush Creek during a rain flow event in May 2016. A standing wave forms during supercritical flow as the flows approaches critical depth. Supercritical flow occurs when the Froude Number is greater than 1. Critical depth occurs when the Froude Number is 1. The importance of this standing wave is that as the standing wave oscillates it can be very destructive. The standing wave also disrupts the flow around it, making the flow more difficult to control.

After flowing along the retaining wall, the stream makes another bend near where the sewer main is located. The bend, however, is too sharp to allow the water to make the turn, and the water runs into the hillside containing the sewer main. This is not much of a problem at low flowrates, but with increasing flowrates more damage will occur. With increased flowrates, however, more damage was observed to the gabions and streambank. Running the model at a significantly higher flowrate, it could be seen that the two gabions furthest downstream had both failed and that a significant amount of erosion had occurred on the hillside behind it. In real life, the furthest donwnstream gabion was the first one to fail, which matched the model results. After observing the flow, the next step was to determine the best concept design solutions alternatives for preventing these results from occurring.