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<br />. <br />. <br />I <br />I <br />I <br />. <br />. <br />. <br />. <br /> <br />t. <br /> <br />. <br />". <br />!. <br /> <br />. <br />'". <br />' , <br /> <br /> <br /> <br />I <br />I <br />L <br />. <br />. <br /> <br />Futura Engineering Inc. <br /> <br />Engin99ring Consultants and SUfV9yors <br /> <br />12741 East Caley Avenue, Suite 126 <br />Englewood. Colorado 80111 <br />(303) 649-9292 FAX (303) 649-9499 <br /> <br />August 18, 1994 <br /> <br />Ms. Rebecca Davidson <br />Director of Public Works <br />City of Black Hawk <br />P.O. Box 17 <br />Black Hawk, CO 80422 <br />FAX 642-3550 <br /> <br />RE: Transmittal of Black Hawk Cbonnel Design Information <br />Project No. 83.000.13 <br /> <br />Dear Becky: <br /> <br />Pursuant to our meeting 1ast week, I am submitting the attached design information on the Black <br />Hawk ChunneI. The information is predominantJ.y hydrology end hydraulic data end analyses which <br />ensure the feasibility of the project. We are working on preliminary construction plans on en on-going <br />basis. A few of the went points are swnmarized below. <br /> <br />Hydrology <br /> <br />The attached graph mows a comparison between the regionallltream gage II1l81ysis and the regulatory <br />loa-year estimate from the 1980 Owen ~ers lItudy. The regionallltream gage ena1yais, using 568 <br />etation-ye&rll of data. was cmnpleted by FIIlurrJ for Gilpin County (and previous1y IlUbmitted to FEMA. <br />eopy attached). The Owen II3ers lItudy WIllI completed for Black Hawk end the Colorado Water <br />Conservation Board. <br /> <br />Owen ~ers predicts Q.oo - 2,670 cfs lit the downstream end of the Cbonnel project, below the waste <br />water treatment plant. This value is plotted on the graph Oag-probability paper) for COllIpIIriaon with <br />- the recurrence lnterwl curve from the regional analysis. Two thousand IJix hundred and lIe.cUt, ds <br />exceeds the regional gage enaIysis 1,()()()-year event. <br /> <br />The loa-year, 95% upper confidence limit on the regional gage ana1ysis is 1,050 cfs. This means the <br />chances are 19 out of 20 the 1tlO-,year event is less than 1,050 cfs. The probability of the loa-year <br />event being 340% of the regional gage lIII81ysis prediction ((2670n85)Ol00) end 250% of the upper <br />confidence limit is very very small. <br /> <br />Cost Estimates <br /> <br />Cost estimates for the Chwmel have been generated by FubJru, Hensel Phelps end M.A. Mortenson. <br />The working figure for 4,500 feet of Cbunnel from S.H. 279 to be10w the treatment plant is 'l,OOO/ft. <br /> <br />A 14' x 6' concrete box culvert is needed for Q100 - 2,670 cfs. An 54-inch diameter pipe handles Q100 <br />- 785 efs. The 8IDlI!1er pipe would save 33%. <br />