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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />delays, the Corps of Engineers changed the design to a grouted hand-placed stone <br />channel. The channel was constructed and completed by 1950. <br /> <br />The typical channel geometry is an eight feet wide bottom with side slopes of 11/2 to 1 as <br />shown in Figure 2. The depth and top width of the channel varies due to seven different <br />vertical slopes along the channel centerline. The channel has three vehicular bridge <br />crossings in the Corps' design and a railroad bridge crossing at the downstream portion. <br />A fourth bridge crossing, State Highway 149 bridge, was in place. The design peak <br />discharge for the channel was 1,800 cubic feet per second (cfs) for an expected flood <br />event. <br /> <br />J <br /> <br /> <br />f c.L. ~hannel <br />C) <br />8'-0" ~ ~ <br />I'- <br /> <br />Stone mason <br />lined channel <br /> <br />:? <br />.... <br /> <br />Figure 2. Typical existing channel cross-section looking downstream <br /> <br />Over the years the channel performed and conveyed adequately the normal annual <br />discharges from the Willow Creek watershed. The channel, however, has not passed the <br />design flood flow according to data from the gauging station at the mouth of the channel. <br />The mortar rock riprap channel has been subjected to numerous maintenance problems. <br />Most problems are associated with erosion and spalling of the lining especially in the <br />areas of the flow lines. Several reasons, as confirmed by the Corps of Engineers in their <br />inspections and a June 18, 1988 report are freeze-thaw action, mortar and hand placed <br />rock eroded by the fast current, sediment impact, an aggregate mortar reaction, and <br />disintegration of the mortar. <br /> <br />NRCS Northern Plains Engineering Team 2 <br />