Laserfiche WebLink
Ken Nelson <br />Deeeniber 20, 2011 <br />Page 7 <br />No action requires future operation. of two independent conveyanee systems similar to the 2011 <br />irrigation season. The western portion of the canal system downstream from the inver -ted siphon <br />would need to be supplied through negotiated purchase of water shares if/when water is available <br />each irrigation season. This not only adds to normal annual budget expenses, but also does not <br />guarantee water will be available to the majority of headgates Oil the system. <br />No ruction also creates safety and liability concerns with respect to tAing the inverted siplion and <br />tunnel out of service. If these were abandoned in-place, an effort to close off all openings should be <br />considered, 10 prevent unauthorized an([ unsafe entry into pipes, structures and the turtnel portals. <br />The inverted siphon and tunnel may also need to be filled to eliniinate the existing below ground <br />voids to further prevent potential future collapse or catastrophic failure. <br />Recommended Capital Iniprovernerit Plan, The second attached 24" x 36" Plan sheet, Figure 2, <br />generally shows the recommended mininiuni capital improvement plan for the study reach of the <br />canal system. Improvements are needed to provide Icing -term irrigation service for the inverted <br />siphon, tunnel and open ditch components of the system are further discussed below. Improvements, <br />identified describe MininlUrn requirements. The NDIC could increase dimensions, upgrade <br />structure strength, or extend the limits of improvements, at their discretion. <br />Iii,i,ertedSip tact! Rel,)Iaceiiietit, Two alternative alignments were developed for replacing the <br />inverted siphon. The first follows (fie existing alignment, in order to inaintain existing hydraulics, <br />and to utilize existing casements and structures, The existing pipe/conduit would be removed in <br />conjunction with installing the new pipe, and could better facilitate permit approvals that would be <br />categorized as maintenance and repairs of'existing facilities. The second alternative alignment <br />represents a new pipe from the tunnel's outlet portal in a straight alignment beneath Highway 65, <br />along an existing improved driveway and fence lines to a new outlet structure about 900 feet <br />downstream from the existing outlet. Although the second aligninent, is about 100 feet longer than <br />the existing, it would eliminate about 500 feet of open ditch, However, the second alignment does <br />not gain sufficient elevation differential to allow for a smaller pipe diameter. It also would require <br />removing or filling the abandoned siphon to reclairn the former easement, and obtaining a new <br />easement. Relocation of one headgate and service pipe would also be required. It may also be more <br />difficult to obtain permits needed for construction, Preliminary assessment of alternative <br />alignments indicates that iniplenienta(ion costs, and ad ni in istrative and permitting issues would <br />likely make the existing alignment most feasible. <br />New pipe for the inverted siphon will need a pressure rating greater than 22 psi at the Tongue Creek <br />crossing. Large diameter PVC pipe would be Suitable, but is limited to 48-inch diameter and is <br />typically more costly than other pipe inaterials. High density polyethylene (HDPE) pipe is available <br />in increments of 48". 54", 63" and 65", based on outside dimensions (O.D.). Inside dimensions are <br />reduced by pipe wall thickness. HDPE pipe strength is specified as a dimension ratio (DR) that is <br />the outside diameter divided by wall thickness, ie., lower DR ratios indicate thicker wall and <br />stronger pipe. Large diameter HDPE is available in DR] 7 (125 psi) through DR32.5 (65 psi). <br />DR32.5 would be suitable pipe selection for [lie inverted siplion, with a safety factor of 10. <br />