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Ken Nelson <br />December 20, 201' 1 <br />Page 5 <br />The debris pile at the tunnel's failure has been inspected and is described by the ditch rider and <br />other NDIC representatives, as ceding or roof failure, Descriptions of cobble rock debris and tunnel <br />structure do not indicate horizontal movement, but rather, failure of the tLIMICI roof span. There is <br />no obvious evidence of Subsidence at the ground surface above the tunnel in the vicinity I of the <br />horizontal movement. Ibis is of particular importance, because residential development exists <br />directly above the tunnel. <br />The tunnel should not be placed in service until debris is •emove(], and the tunnel is reinforced to <br />prevent a recurrence of the 2011 irrigation season shut-down. The NDIC has indicated that <br />alternatives for tunnel repair do not need to provide for future physical access into the funnel for <br />routine inspections or maintenance, however, inspection manholes should be considered. <br />Open Ditches. The majority component of the SUI jCCt canal system consists of open ditches, hand <br />excavated into native soils. A combined 7,640 lineal foot length of dirt lined ditch is encompassed <br />within the study reach. This includes a short 460-foot length of ditch between the inlet of the <br />inverted siphon and the outlet portal of the tunnel with one (1) headgate. The remaining length of <br />ditch is approximately 7,180 lineal feet, and located between the tunnel and 2100 Road, It includes <br />two bridges and one culvert for cross vehicular access, an overflow structure, and two (2) headgates. <br />Observations along the segments of ditch that were shut down during the 2011 irrigation season <br />indicate that the flowline slope is inconsistent with low points and/or reverse slope. This is not <br />uncommon for early 1900's construction, since the overall slope is very nominal at 0.04% <br />(4314 inch in 1,000 feet). <br />With few exceptions, the ditch is excavated into an existing hillside with side slopes along the canal <br />north bank, and overburden placed on the southern, down-slope ditch bank that provides a narrow <br />bench for limited maintenance and vehicular access. An approximate 1-mile stretch of the ditch <br />upstream from the tunnel is along very steep embartknient that rises upwards of 100-feet between <br />the river bottom and the plateau above. This segment of ditch was constructed about 20- to 30-feet <br />above the toe of the lower bench of the river bottom. There are several isolated areas of roadway <br />erosion of this segment of the canal that should be repaired, Near the cast tunnel portal, narrow <br />cracks at the road surface are a potential precursor of" a weakness that could fail in the future. <br />Although the NDIC has not experienced ditch overflow leading to a catastrophic breach and <br />substantial erosion along the steep embankment, such an event could cause future seasonal shut <br />down of the canal system, and related emergency reconstruction. <br />The segment of the canal Upstream from the tunnel to the bridge at 2100 Road supports a prolific <br />growth of filamentous algae or moss, Other segments of the ditch system appear to be unaffected. <br />Aquatic vegetation requires several chemical treatments and/or harvesting each season. The <br />chemical herbicide currently used, Hydrothol 191, is reportedly not regulated or considered toxic to <br />aquatic life at low doses, therefore, tailwater can be discharged to the Gunnison River. The <br />herbicide label indicates that the product is highly corrosive, and may not be fully compatible with <br />crop irrigation immediately following application, depending on the dosage. This seasonal <br />maintenance is also relatively costly. If aqLiatic weed control is not conducted or is postponed, <br />