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Walker to Berr}• <br />March 30, 1999 <br />Page 2 of 5 pages <br />Grades range between 7 and 1 ~% 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 strictures are eroding. The lack of (or loss of) an energy dissipating <br />structure at the base of the drop structures resultsin 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 tore <br />Active erosion equilibrium <br />undercu= --_____,~ <br />Drop ructure <br />The drop stntctures appear to have been constructed without a dischazge weir. Shallow weirs <br />were subsequently chipped out. Low flows appear to be the nde, 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 />Sue~estions: <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 [o arrest erosion in the steep )ower portion of the main <br />