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Ken Nelson <br />December 20, 2011 <br />Page 12 <br />either upon filling the irrigation system, or under normal flow through the exposed soil matrix, a <br />new closed conduit may develop complications. This is especially critical if the new conduit size is <br />selected based on ideal characteristics of new pipe rather than aged pipe. Air vent(s) Should be <br />considered as a contingency if necessary, in the event the flow capacity is diminished, or if the <br />hydraulic grade line at tunnel inlet caUSCS Surcharging or overflow upstr.eani from the tunnel after <br />the tunnel liner conduit is completed. <br />Other considerations for tunnel rehabilitation construction include adequate fresh air ventilation for <br />workers inside the tunnel, and additional shoring along the tunnel length as necessary. For long- <br />term rnanagernent, access manholes such as a vertical tee fit( ing, with vented cover, should be <br />installed outside each the tunnel bulkhead for unforeseen future need to access the tunnel liner <br />conduit. <br />C_einalDitch. Alternatives for lining dirt ditches for the primary purpose of preventing vegetative <br />growth and preventing overflow and erosion are limited to closed pipe. Concrete, shot-crete, gunitc, <br />or other impervious material may be effective, in controlling salinity, but may not effectively <br />eliminate growth of aquatic plants, and do not protect against batik overflows. <br />ADS low-head irrigation pipe has been suggested as a cost, effective alternative to the open ditch <br />system. It is available in 42", 48" and 60" diameter. It is a dual walled pipe with corrugated outer <br />shell for structural strength and smooth interior wall for improve(] flow characteristics. ADS pipe is <br />intended for gravity flow applications having less than 10.5 psi internal pressure, and requires <br />nominal backfill where it is riot subject to vehicular traffic loadings. At roadway crossings, traffic <br />rated box. culvert Structures could be used, Or the pipe Would need to be backfilled in a concrete <br />encasement for Structural support to prevent collapse. <br />The existing hydraulic grade line from the tunnel outlet to the inverted siphon, and from tunnel inlet <br />to 2100 Road is very limited at 0,05% with siphon and tunnel inlets surcharged to historic limits, <br />and open channel canal gravity flow to 2 100 Road. The largest ADS pipe available (60") could <br />convey the full 50 CfS, but will flow about 60% to 90% full depending on the pipe roughness factor <br />after years of service. <br />The NDIC may also want to determine whether there is U Substantial increase in TDS or selenium <br />between the Trap Club and inverted siphon to warrant installing a pipe in the ditch system as a part <br />of a long-range capital improvement plan. This Would be in addition to collecting additional <br />information to characterize evaporative and seepage losses as necessary, if the tunnel rehabilitation <br />conduit uses the smaller atternative 54" O.D. HDPE as future flow capacity degrades, <br />Summary of Recommended Improvements: <br />In summary, the recommended capital improvements plan generally consists of three components of <br />the canal SYSICM: 1) replace the inverted siphon in its existing alignment and salvage existing inlet <br />and outlet structures, 2) clear debris front the tunnel and reinforce the tunnel with HDPE pipe with <br />