Laserfiche WebLink
• <br />CALCULATION NOTES <br />Subject By <br />Checked <br />__ Acct <br />19 _ <br />General Data (Continued) Sheet No. 2 of 4 Sheets <br />Rainfall (Continued) <br />10 year 24 hour storm = 1.90" <br />25 year 24 hour storm = 2.30" to 2.35" <br /> <br />Per peak flows in Colorado, Figures 5-6, sheet 4 of 5 (10 yr. 24 hr.) pg. 80, <br />and sheet 3 of 5 (25 yr. 24 hr.) pg. 79, Isopluval neaps. <br />~ ANTECEDENT MOISTURE CONDITION (AMC) <br />The Somerset Coal Mine located in east Delta County and West Gunnison County, on <br />the North Fork of the Gunnison river is Semi-arid to arid. The average annual rainfall <br />recorded at the lJilcox Ranch Station (38° 55', 107° 31' W), over a 12 to 13 year period <br />' prior to 1954, is 16.94 inches. Precipitation patterns are not obvious for the area. <br />~ See copy of average monthly precipitation in attachments. <br />~ Precipitation values for the Pa onia Station are more complete than the Wilcox <br />~ Ranch, which shows an average annual precipitation of 15.75 inches fora period of <br />~ 51-53 years of data prior to 1957 and an average annual precipitation of 15.27 inches <br />~ for a 15-16 ear eriod revious to and includin 1972. No clear <br />y p p g pattern of pre- <br />cipitation distribution emerges for this station either. <br />~ AMC-I precipitation condition is normal in this area. <br />~ STORM RUNOFF COMPUTATIONS <br />i <br />~ Storm runoff computations are based on terminology, formulas and information <br />contained in the S.C.S., National Engineering Handbook, Hydrology: Section 4, <br />August 1972, particularly Chapters 4, 7, 8, 9 and 10. Tables, figures and graphs used <br />to determine curve numbers, flow volume and etc. are attached. <br />Formulas used are: Used with programmable calculator. <br />Quantity of runoff: <br />Q = (P-0.25)2 - inches of runoff <br />P + 0.85 <br />Q = Actual runoff- inches <br />P = Potential maximum runoff = 1.90" (10 Yr.-24 Hr.) <br />S = Potential maximum retention (obtained from Table 10.1) <br />CN = runoff curve number or hydrologic soil-cover <br />complex number <br />.• <br />CN = 1000 or S = 1000 - 10 <br />S + 10 CN <br />B-3 <br />