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<br />Lilley Gulch Is carried under South C~~~ St~eat by a COn- <br />crete box culvert, adequate to p~ss the Intermediate Regional Flood <br />without overflOloling the rood enlbankmant. <br />Coon Creek Is crossed by five ~oads of IoIhlch one, a pri- <br />vate drive, crosses at streambed level. Three others have corrugated <br />metal culverts. South Carr Street has a concrete box structure and <br />Is the only structure on Coon Creek that will pass the Intermediate <br />Regional Flood IoIlth(l<Jt ove~topplng the road embankment. <br /> <br />PAST f'LOOOS <br /> <br />This section presents the flood histories and storm <br />characteristics typical of the Denver area. <br /> <br />Obstructions to Flood Flows <br />Elevated roadways crossIng flood plains Impede flood flows <br />and raIse the ~ter surface to a degree dependent on the height of <br />the roadlol3Y and the sIze of the waterway opening at the culvert or <br />bridge. The effect of roadway obstruction on flood flows Is shown <br />In tables 8 thru II, Flood PlaIn Reference Data. The Increased stages <br />usually occur only a short distance upstream of the crossings because <br />of the steep grade of the streams. <br /> <br />Flood Reco~ds <br />Discharge measurements, hIgh water ~rks, and other <br />f I cod data have not been off Icia Ily reco~ded for the four streams. <br />Developments In the flood plain are recent or non-existent; for <br />these reasons, the problems of past floods were of little concern <br />and were unnotIced. <br /> <br />RlllnfallRecords <br />Table 5 shows the mIIximum dally precIpitation occurring <br />each year at the Denver city gage for the 22 year per 10<:1 , 1949 <br />through 1970. Only the maximum dally precipitation that occur~ed <br />durir.g the year is shown In the table. Itisposslblethlltsev<lral <br />raInfalls almost as large occurred on other days during the year. <br />T~ble 6 shows the maximum short duration precIpitation <br />amounts at the Oenve~ city gage from 1876 to 1935 and at the Denver <br />airport gage after 1935. The 2.2 inches of rainfall produced In <br />I hour on 14 July 1912 and the 1.99 Inches produced in 30 minutes <br />on 25 July 1965 dpproximdto the intensity of tho Intermediate <br />Regional Flood r~lnfall at a spot location. Howover, the ent;~e <br />ar04 of any of the drainage basins under conslde~atlon must receIve <br />similar rainfall amounts to produce an Interma<llateReglon<l1 Flood. <br />Tnelntonsltyofthel912and 1965eventslnanhouror lessdistln- <br />gulshes them from the dally maximums shololn In fable 5. <br /> <br />nlJr~tlon and R~t.. of RI"" <br /> <br />M<ljorfloodsonDutch,Lllley,orCoonCreekscouldbe <br />produced by "cIO\ld~u~s1"" typoeo ~a Ins1"or..s. Tn", ~asln slop"" "r.. 'ita",p <br />and floodwaters ~ould combTne quickly and produc<l flash-type flooding. <br /> <br />20 <br /> <br />" <br />