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The turnout structures under study divert flows from the Weber-Davis Canal near Clearfield, Utah. A portion of the canal in this area was realigned as a result of the construction of the Interstate Highway System. The twin turnout structures, used to divert water to the West Branch Irrigation Company and West Layton Irrigation Company, were constructed in conjunction with the realignment of the concrete -lined canal.

A three-dimensional drawing of the twin turnout structures is shown in Fig. 1. Water is diverted from the canal by passing under the discharge diverted through each of the structures is approximately 35 cfs (cubic feet per second, or second-feet). To properly allocate and assess the quantity of flow delivered to each of the irrigation companies, Parshall flumes having a throat width of four feet and a depth of four feet were placed inside each structure. After passing through the Parshall flumes, the water is conveyed by twin corrugated metal arch pipelines, located under the newly constructed freeway, to existing irrigation distribution systems which serve lands west of the highway. A portion of the twin turnout structures are covered with concrete (Fig. 3) to accommodate the service road located on the west side of the canal.

One of the difficulties encountered in the design of the structures was the space available between the canal and the cut bank on the freeway right-of-way. This space limitation resulted in the entrance of the four-foot Parshall flume being located less than nine feet away from the diversion gate. The situation was further aggravated by placing the lip of the gate opening two feet lower than the bottom of the canal. The net result was a high velocity jet passing under the gate with maximum velocities reaching 15 feet per second. The high velocity jet resulted in considerable turbulence and wave action within the structure. The instability of the flow created concern regarding the reliability of the standard calibration for Parshall flumes in predicting the actual flow being diverted to each of the two irrigation companies. In an effort to reduce the height of waves, a metal stilling float was placed in each structure (Fig. 4). The floats did not materially improve the flow conditions.


Publication made available electronically April 25, 2012.