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j III IIIIIIIIIIIIIIII ~r~c. CTe,.e.-.~A <br />.r <br />999 <br />The Corley Company <br />Phone 632-5050 PO Bo. 1821 <br />COLORADO SPRINGS,COLO RADO 80901 <br />Sept. 20, 1993 _-_ _. _ , , - <br />Ms. Erica S. Crosby Srra 2`~~~ <br />Division of Minerals & Geology <br />1313 Sherman St. - _ _.:%^!ony <br />Denver CO 80203 ~ - -- ae0r09~] <br />Dear Ms. Crosby: <br />We are still trying to understand the hydrology estimates for the West <br />Pit. The September 9, 1993, estimates are based on a 100 yr., 24 hr <br />event of 3.8 in. and av. CN of 73. In the past, the estimates were <br />based on different events and amounts and different CNs. L,'hile you have <br />provided us with copies of the NOAA Atlas, we do not have a definitive <br />answer for the size that the Division will accept for a 10 yr., 24 hr. <br />event and for a 100 yr., 24 hr. event. For example, the Division itself <br />has used 2.6 in. and 2.8 in. for a 10 yr., 24 event for the same West <br />Pit area and 3.8 in, and 4.4 in. for a 100 yr. , 24 hr. event; we were <br />forced to use 3.2 in. for a 10 yr. , 24 hr. event for an area a 'Z mi. <br />away. The difference between 2.6 and 3.2 in, is great. Sorry to push <br />for specific answers, but it has major significance in sizing <br />structures. We also note that there has been no consistency in the <br />average CN, the watershed area, elevation difference, time of <br />concentration, or the hydraulic length for the West Pit watershed <br />calculations. We understand that some of these factors such as CN are a <br />little subjective, but others should not be subjective. We do not feel <br />very comfortable with Mr. Berry's comments concerning your survey and <br />that his calculations were "rushed" and that there was an "apparent <br />paucity of information". Finally, the design results in a velocity of <br />about 6 fps. Is that satisfactory? <br />We will need to know the cost figures for the tasks 3A and 3C before we <br />can give our approval for those projects. <br />We previously asked how the cost estimate for the Harrison Western <br />liability was calculated at $18,500 while the accepted bid amount for <br />that work was almost $33,000. How did this under estimate occur? <br />Sincerely, <br />C~! <br />W.D. Corley, <br />President <br />