C153833 Feasibility Study - Laserfiche WebLink

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I I il I I I I I I I I I II I I I I II I ALTERNATIVES EVALUATED NPIC #6 OUTLET DITCH The NPIC #6 Outlet Ditch is the conveyance ditch for all waters leaving NPIC Reservoirs #5 and #6. The dams of both NPIC Reservoir #6 and NPIC Reservoir #5 were renovated in 1995 at a cost of approximately 1.6 million dollars, The two reservoirs have a combined storage capacity of 18,000 acre-feet of water, The NPIC #6 Outlet Ditch originates at the outlet to NPIC Reservoir #6 and flows approximately 150 feet south before passing beneath State Highway #1 in a six-foot diameter corrugated metal pipe, The 1300 feet of ditch immediately downstream of Highway I is a deep open channel restricted in width by irrigated cropland on both sides and an access road paralleling the ditch on the west side, The depth of the channel is determined by the Highway I culvert upstream and concrete structures further downstream, Embankment seepage is causing sloughing on both ditch embankments and is beginning to encroach on the adjacent cropland and road, The width restrictions prevent cutting the embankments to a shallower slope while maintaining the current invert of the channel. Reshaping the embankments to fit the existing geometry would produce slopes that are unstable given the seepage and soil conditions. Seepage is not only occurring in the embankments, but through the channel floor itself, The bottom of the channel is very soft and will require stabilization before equipment can be supported on it. Problems associated with the slope stability, seepage, and mucky bottom were considered when developing alternative design and construction options, Initial consideration was given to replacing the ditch with a large diameter pipeline capable of handling emergency flows from NPIC Reservoir #6 but the diameter of pipe required to handle these flows was immediately determined to be cost prohibitive. Using a pipeline to carry only normal irrigation flows was much more economical and NPIC already owned approximately 700 feet of used 54-inch diameter reinforced concrete pipe (RCP), A 54-inch RCP placed in the bottom of the ditch with approximately two feet of cover reduces the depth of the channel by nearly seven feet. The reduced depth allows the embankments to be flattened to a stable slope, Emergency flows can be handled in an open channel above the pipeline, Embankment seepage for the buried portion of the embankment can be captured in the granular Rep bedding and conveyed through the bedding to the downstream open channel. The existing muck bottom will need to be stabilized in order to support the new pipeline and associated backfill, NPIC was able to purchase approximately 2000 cubic yards of ungraded riprap, for the cost of hauling, during the summer of 1998. This material will be used to stabilize the area below the pipeline and will allow seepage to drain downstream. The final design problem that needed to be solved for the project is the embankment seepage, A piezometer near Highway 1 showed the water table to be at approximately seven feet below the surface at a point less than 100 feet away from the 22-foot deep outlet ditch. Initially, a synthetic geotextile drain was designed for both embankments. The textile was designed to collect seepage from the embankment and drain into a 6-inch diameter pipeline running parallel to the RCP. Figure 2 shows this drain design and a cost estimate for this option is shown in Table 1. Page 4