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MLRD - 2 - ~ July 10, 1989 <br />The Hydrologic Supplemerto the Amendment Application prepared <br />by Obering, Wurth ~ Associates prop s specific hydraulic <br />recommendationSand engineex~ti>_stru es to control stcrm water <br />runoff and contained sediment, include: <br />Silt fences with design per' ers of Q10 <br />A holding pond with desig ~ ameters of Q25 <br />A spillway structure with esign parameters of Q100 <br />Are these structures adequate to contain or control the flood <br />waters and sediment load? <br />The probability of a Q100` t occurring during 25 years of quarry <br />activity is 1/4. This the ~1 probability is not reassuring. <br />According to the earlier-men ~ d Gazette-Telegraph article, <br />significant flooding occurred n Manitou springs in 1882,1894,1902, <br />1921, 1940, 1947 abd 1964. Time interva}.yk between these floods range <br />from 7 to 19 years, with the average be~3' i3.6 years. This recurrence <br />interval is likewise not reassuring, Ga~1'the liklihood of a major <br />storm event occurring during the lifet~te of the quarry, what con- <br />sideration has been given to the mechanical integrity of the silt <br />fences and holding pond in the event of a Q100 storm ? Would the <br />fences and pond, and the accumulated sediments give way, and <br />catastrophically release the debris into Williams Canyon? What kind <br />of maintainence would the water and sediment control structures <br />require after quarrying has ceased? Have adequate provisions been <br />made for post-closu~ monitoring of the site? <br />Given the vulnerability of Manitou Springs and Colorado Springs to <br />flash flood damage and hazards, the proximity of Snyder Quarry to <br />Manitou Springs, and the greatly-increased rate of storm water <br />runoff and sediment transport associated with quarrying activities, <br />I recommend that a more thorough analysis of possible e:Efects of <br />Q100 storm damage be made before approving a permit to mine the <br />Amendment area. <br />Sincerely, <br />~.~,~ <br />Harold J. Prostka PhD <br />President, Ute Pass Preservation Society <br />Enclosures <br />