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-2- V. Sediment Ponds 1. The Division has;;ilpntified a number of problems with the design calculations for~lte Pit 4 sediment pond. The applicant used incorrect 1D-year, 24-hour,ainfall-for the site (1.8" vs. 1.6") and did not account for the ytershed shape factor as found 1n Appendix E of "Peak Flows in Coloradd`_ This resulted to an overestimation of peak inflow during the designevent. Therefore, the hole size needed to dewater the pond in 24 hours~as overestimated. The applicant's calculations yielded a 5` radtt; or 10" diameter hole. As a result of the incorrect assumptions used,aecessary storage volume 1s also Incorrect and the primary spillway:elevation should be recalculated. The following aspects of the sAiment pond design must be recalculated: a) The staage volume needed for a 10-year, 24-hour event. b) Princi~l spillway elevation. c) Dewateiing aperture diameter needed to dewater the 10-year event infia,in 24 hours. d) Dutflowveloclties resulting from dewatering. e) Deslgn.~ an adequate non-clogging dewatering device over the dewatering~perture. The downs lope ecalve potential of outflow from the primary and emergency spills has not been addressed by the operator. The outslope of PondfF4 1s steep and has been mapped by the operator's consultant as l,adsllde debris. As such, the outslope will be subject to erosion and;~slble movement if sufficient measures are not taken by the operator to cannel any outflow from the pond down the slope. No calculations areprovided to show that the 6-inch pipe proposed is sufftciefrt to Cary discharge from both spillways. Therefore, the applicant must ~hmit to the division calculations and design for the following: a) Indlvibal outflow volume from the primary and emergency spillwaysiliring a 25-year, 24-hour storm event. b) Providtthe design whereby discharge from the emergency spillway.~11 be routed into discharge from the primary spillway with adegatte energy dissipators, drop structures, or other means to preveifterosion at outflow point. c) Provi~ a design to carry combined outflow of both spillways dounslopel;n a channelized flow to Grassy Creek.