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Mr. Michael Cunningham 4 <br />CDRMS <br />April 22, 2014 <br />Response: <br />An additional sediment basin has not been added to the southwest corner of the TSF as <br />requested by the Division. UMC will revegetate the outside slope of the TSF as <br />discussed in Response 1 above. While the plant growth is being established, silt fence <br />will be installed at the outer toe of the TSF embankment at the southeast corner. <br />10. It appears from available information on the drawing, combined with hydrologic analyses <br />presented in Appendix A that the contact channel(s) on the east and south sides of the TSF <br />have insufficient capacity to pass the 100 year, 24 -hour peak flow should it discharge over <br />the TSF emergency spillway. Please address this apparent discrepancy. <br />Response: <br />The South Contact channel on the south and east side of the TSF is a distinct channel <br />from the East Diversion channel that is used to convey surface water around the TSF. <br />The emergency spillway will discharge to a channel that is connected to the East <br />Diversion channel, and not the South Contact channel. The East Diversion channel has <br />adequate capacity to handle the flows from the emergency spillway. A typical cross <br />section was added to Drawing DR -2 showing the configuration of the East Diversion and <br />South Contact channels and TSF. <br />Drawing DR -2. <br />11. Open Channel Section No. 3 shows a "top of sediment trap" crest 0.25 feet below the top of <br />the channel section. This means there is less than the required 0.5 feet of freeboard at the <br />sediment trap. Please revise the design to provide the requisite minimum freeboard and <br />provide analyses to demonstrate as much. <br />Response: <br />The detail in question, Open Channel No. 3, indicates a 2 -ft deep channel with a 1.25 -ft <br />high sediment trap. There is 0.75 foot difference between the top of channel and top of <br />sediment trap. The Weir Report for the sediment trap shows a depth of flow of 0.16 feet <br />at 100 -year peak flows. When the weir flow depth is taken into account and Open <br />Channel No. 3 is used at the South Contact channel, the freeboard is 0.59 feet. <br />12. Geotextile for riprap underlayment <br />a. Only the TSF spillway shows a geotextile underlayment below proposed riprap. <br />All riprap designs will need either a geotextile or granular filter under the riprap <br />to prevent piping of the finer soils under the riprap. Please revise the details <br />accordingly, (note the Division engineering staff recommends granular filters be <br />used in channels steeper than JO to 14percent). <br />b. No anchor trench is shown for the geotextile underlayment. Anchor trenches are <br />necessary to prevent the geotextile from being pulled to the channel bottom <br />during construction. Please add geotextile anchor trenches to the detail(s). <br />