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Mr. Miguel Hamarat <br />March 9, 2012 <br />Page 3 <br />• A boulder retaining wall was installed on the left inner bank of the canal from about <br />Station 100 +50 to 103 +80 to mitigate sloughing on the cut slope above the canal. <br />The wall is about four to six feet tall and constructed with random -sized boulders up <br />to about 24 inches in diameter. The boulder wall ties into the existing gabion <br />retaining wall at Station 103 +80. <br />• 60- inch - diameter corrugated metal pipe (CMP) was installed from Station 175 +13 to <br />175 +73 and from Station 217 +60 to 218 +00 to replace the existing CHDPE pipe <br />culverts at haul road crossings. The existing CHDPE pipes were reportedly <br />deforming under heavy haul road traffic. <br />• The canal invert and inner banks were lined with shotcrete from Station 203 +30 to <br />204 +00 to mitigate erosion. <br />• 60 -inch- diameter CHDPE pipe was installed to connect the new CMP at Station <br />218 +00 to the start of the existing CHDPE drop culvert at Station 221 +72 to mitigate <br />sloughing on the inner banks, to mitigate erosion of the canal invert, and to prevent <br />intrusion of runoff from nearby topsoil stockpiles. <br />• 60- inch - diameter CHDPE pipe was installed from Station 234 +20 to Station 241 +00 <br />to lengthen the existing canal culvert, to mitigate sloughing on the inner banks, and <br />to prevent excessive canal leakage to the pervious soils underlying the canal invert. <br />The 2010 as -built survey data along with observations and measurements made during the <br />2011 canal inspection were incorporated into an updated hydraulics model using the U.S. <br />Army Corps of Engineers HEC -RAS computer program. <br />The modifications to the canal system that took place after the record drawings were issued <br />had a minor impact on the hydraulic capacity of the canal at certain locations. The HEC- <br />RAS model indicates that the canal system is still capable of passing the 200 -year snowmelt <br />flood without overtopping the outer bank of the canal. However, some water may be <br />released into the TSF decant pond through the spillway structure at about Station 93 +00. <br />The addition of the 60- inch - diameter CHDPE culvert at Station 95 +46 creates a backwater <br />effect, which leads to water spilling out the spillway structure at Station 93 +00. The volume <br />of this inflow into the decant pond is not expected to be significant and can be managed <br />within the process water system. <br />