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Walker to Berry <br /> March 30, 1999 <br /> Page 2 of 5 pages <br /> Grades range between 7 and 15% in the lower reaches of the drainage way. Concrete drop <br /> structures have been installed, but intermittent grades are too steep, and this reach is being <br /> eroded. Enclosure 6 and 7 are of this reach. Note on these enclosures that the channel appears to <br /> be in a state of equilibrium for approximately one-half(or less) the distance between the drop <br /> structures. Field measurements (tape and inclinometer) indicated that nearly exactly half the <br /> required (to arrest erosion) number of structures were installed. For these soil conditions, each <br /> pair of drop structures, the top of a downstream structure should be at or slightly above the <br /> bottom of the upstream structure. As installed, there is approximately a height of structure <br /> separation of the two. To reach a state of equilibrium (near a 1% grade), the upper reaches of the <br /> channel between two structures are eroding. The lack of(or loss of) an energy dissipating <br /> structure at the base of the drop structures results in a depression (circular basin) at the base, in <br /> some cases approximately 4 feet deep. These basins are cutting back under the drop structures, <br /> and underflows of the structures are just a matter of time. The channel is in a state of erosion <br /> below such basins to the lower reach that is in equilibrium. <br /> Drop st ture <br /> Active erosion equilibrium <br /> undercu� <br /> Drop ructure <br /> The drop structures appear to have been constructed without a discharge weir. Shallow weirs <br /> were subsequently chipped out. Low flows appear to be the rule, as the shallow weirs have been <br /> sufficient to keep most of the undercutting localized to the center of the channel. <br /> Road crossings act as drop structures. Areas below the crossings indicate.conditions similar to <br /> the drop structures. <br /> Note areas of green on the left side,just below the center, of the picture at Enclosure 2. This is <br /> an area of small ponderosa pine trees. <br /> Suggestions: <br /> High Priority <br /> 1. Add intermittent drop structures in the lower reaches of the drainage channel. Five or six are <br /> required. The bottom and top of the new ones would overlap the tops and bottoms of existing <br /> structures. The purpose of this action is to arrest erosion in the steep lower portion of the main <br /> 5 <br />