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<br />PREPARED IN COOPERATION WITH
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<br />Topogrllph,cmap lurnlshedby
<br />C,ty 01 Boulder,Colorado
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<br />Topograph,c map lurnlshed by
<br />C,tyol Boulder Colorado
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<br />FIGURE 6 -ESTIMATED AREA INUNDATED AND WATER.SURFACE CONTOUR LINES FOR HYPOTHETICAL 100,YEAR FLOOD ON BOULDER CREEK AT BOULDER COLORADO
<br />
<br />FIGURE 4 -HISTORICAL FLOOD DATA AND ESTIMATED AREA INUNDATED BY HYPOTHETICAL 25,YEAR FLOOD ON BOULDER CREEK AT BOULDER, COLORADO
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<br />FREQUENCY AND EXTENT OF FLOOD
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<br />INUNDATION ON BOULDER CREEK
<br />
<br />AT BOULDER, COLORADO
<br />
<br />IntrodlLctwn -ThiS atlas summarizes the results of an
<br />investigation of the flood f10\\ charactenstlcs of Boulder
<br />Creek and Its tnbutarJes 10 a selected area at Boulder,
<br />Colorado HIstoncal data for past floods are compiled. and
<br />hazards of future floodmg are estimated ComputatIons of
<br />C100d elevations, boundanes of Inundated area. and velocltle~
<br />of flow are based on the conditIOn of the creek valley and Its
<br />constnctlOns eXisting m the fall of 1959 All elevatIOns are
<br />referred to the datum. 4,2480 feet above mean sea level,
<br />used by the city of Boulder The Sixth Street crest.stage
<br />gage IS located on the left (north) bank of the Boulder Creek
<br />channel, 100 feet upstream from thecenterhne of SIxth Street
<br />Flood H,story of Boulder Creek -Outstandmg floods
<br />occurred on Boulder Creek In May-June 1844, June 1864, May
<br />1876, May 31-June I, 1894, June 2, 1914, and June 6, 1921
<br />The flood In 1894 probably was the greatest. although the
<br />flood In 1864 was comparable The only data concermng
<br />elevatIOns reached by past floods on Boulder Creek In the
<br />stud,} area are those shown in figure 4 for the flood of 1894
<br />Two floods m the penod 1889-1959. those of 1894 and 1914,
<br />caused damage m Boulder The flood of 1921. mIght have
<br />caused damage If present channel constnctlons had eXIsted at
<br />that time FIgure 1 shows the estimated elevatiOns the floods
<br />of 1894, 1914. and 1921 would have reached at the Sixth Street
<br />crest-stage gage under present conditions of flood plam
<br />de\elopment SIX outstanding floods occurred durmg the
<br />period 1844-1959, an average of one flood every 19 yearn
<br />Hov. ever, durmg the penod 1894-1959, three floods occurred
<br />at mtervals of 20,7, and 38 years, respectively (fig. 1) The
<br />erratic occurrence of floods 18 eVident
<br />Eletatwn andfrequency offloods -Frequency of floods wa.~
<br />computed from the combIned record of flood peaks at seven
<br />gagmg stattons on Bou\deT CTeeK and on foul' otneT I.'.trcaml.'.
<br />In the area All fIVe streams havf' SImilar ("hardcteTlstla;.
<br />their sources are at the Contmental DIVide, and thelr dramage
<br />basinS are subject to Blmtlar meteorological phenomena The
<br />gagmg stations are located near the base of the foothills of
<br />the Rocky Mountams The penod of record of annual peaks
<br />at the gagtng statIOns ranges from 62 to 70 years, WIth
<br />certam hlstoncal mformatlOn available for floods as early as
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<br />Boundary of estimated area mundated by hypothetical floods.
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<br />TopographiC map furnished by
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<br />FIGURE 5 -ESTIMATED AREA INUNDATED AND WATER.5URFACE CONTOUR LINES FOR HYPOTHETICAL 50. YEAR FLOOD ON BOULDER CREEK AT BOULDER COLORADO
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<br />1844 LocatIOns of the statIOns are shown In figure 2 The
<br />relatIOn between flood elevatIOn and recurrence Interval at
<br />the Sixth Street cre8t-sta~e gage site under present conditions
<br />of channel constrictIOn IS shown In figure 3 ItlS emphasized
<br />that recurrence Intervals are average figures-the average
<br />mterval wlthm which floods of a gwen ma~"fiItude "'0111 be
<br />equaled or exceeded once Another way of companng flood
<br />frequencies IS 10 terms of theIr probablhtws of occurrence
<br />(the reciprocals of their recurrence mtervals) Thus. a flood
<br />"-Ith a recurrence Interval of 50 }eal'S has a 2 percent chance.
<br />and a flood With a recurrence Interval of 100 years has a 1
<br />percent chance, of bemg equaled or exceeded In any given
<br />year. The recurrence Interval of the 1894 flood IS about 95
<br />years; of the 1914 flood, about 40 years, and of the 1921 flood,
<br />about 23 years The probablhtIes of these floods bemg
<br />-equaled or exceeded In any given year are. respectl\ely.about
<br />1 0, 25, and 4 3 percent The recurrence mtervals of the
<br />h)'pothetlcal floods whose estimated areas of inundation are
<br />shown 10 figures 4-6 are 2.5. 50. and 100 years
<br />Flood depths -Depth of flooding can be estimated for the
<br />hypothetIcal 50.year flood from figure 5 and for the h)'"l)()-
<br />thetIcal l00-year flood from figure 6 by computmg the dif-
<br />ference between the elevatIOn of the water surface, shown
<br />by the heavy contour hnes, and the elevation of the ground,
<br />shown by the light contour hnes Depths In the shaded areas
<br />shown on fIgure 5 and 6 will be WIthin the ranges mdlcated
<br />Plood t'floclt,f8 -VelOCities probably Will be In excess of
<br />20 feet per second m the center of the main channel during
<br />both the 50- and 100-year hypothetical floods, on the other
<br />nand, there WIll be points on the overflov. plam where the
<br />velOCity may be zero .Mean velOCities on the o\erflow plam
<br />to the left of the maIO channel near 6th and 9th Streets wdl
<br />be about 5 feet per second for the 50-year flood and about 6
<br />feet per second for the l00-year flood In the VICInity of the
<br />IClty Hall, these velOCities are estimated as 7 and 10 feet per
<br />lsecond respectively Mean velOCitIes In the overflow plam
<br />will decrea.~e gradually downstream from the City Hall to 1es8
<br />than 1 foot ver second fOT both the 50- and l00-year floods m
<br />the area Just upstream from 28th Street. It IS emphaSized
<br />that some JX>lnt velOCIties probably Will exceed mean velOCIties
<br />substantially It IS not pOSSible to accurately predIct what
<br />the velOCity will be for a given flood at a gl\.en pomt 10 the
<br />channel
<br />Tnbutary streams - Floods on Sunshine Gulch and Gregory
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<br />There are insuffICient data to form a basIS for dependable
<br />analYSIS of floods on small streams draining the foothills of
<br />the Rocky Mountams, but It IS known that at least one flood,
<br />that of July 8. 1906. on Sunshme Gulch caused appreCiable
<br />damage An estimate of the area Inundated by that flood IS
<br />shown by the shaded portIOn of figure 4 The delineatIOn of
<br />the area IS the result of a very rough field estimate. and
<br />should not be conSidered fmal No data are avaIlable con-
<br />cerning floods on Gregor".} Creek. although field inspectIOn
<br />mdlcates that areas contIguous to the stream WIll be mun-
<br />dated by large floods
<br />fJlect of alteratIOns -Changes In the regtmen of Boulder
<br />Creek, either by substantial artIfiCial regulatIOn of flood peaks
<br />by storage and dlVernlOns or by alteratIOns to the flood plam
<br />and maIO channel. \I, III modify the results shown In the past.
<br />regulatIOn of flood peaks has been neghglble, but there has
<br />been mcreasmg encroachment on the flood plam Areas of m.
<br />undatlOn. flood depths, and velOCities In the overflow sectIOn
<br />Will be mcreased If bridges become plugged with debrIS, If
<br />additIOnal encroachment takes place on the C100d plam. or If
<br />additIOnal growth of trees and underbrush occurs along the
<br />mam channel but will be reduced If the main channel is Im-
<br />proved hydrauhcally. and there IS no additional encroachment
<br />on the flood plam
<br />Addttwnal data -Other informatIOn concern 109 floods on
<br />Boulder Creek can be obtained at the office of the U S
<br />Geologtcal Survey, Room 1227, Building 25, Denver Federal
<br />Center, Denver. Colorado. and from the followmg report
<br />U S Geological Survey. 1948, Floods In Colorado, U S
<br />Geol Survey Water-Supply Paper 997
<br />Cooperatwn and acknowledgments - The preparation of
<br />thIS atlas IS a part of an investIgatIon fmanced through a
<br />speCial cooperatl\e agreement between the Geological Survey
<br />and Boulder County, Colorado
<br />The topographiC base map used 10 fIgures 4-6 was furnIshed
<br />by the city of Boulder Mr Theadore MIkesell. Planmng
<br />Director of the city of Boulder, furnished prevIous engineer-
<br />Ing reports concerning floods In Boulder
<br />The atlas was prepared by Chfford T Jenkms. under the
<br />directIOn of Wallace T Miller. Dlstnct Engmeer, Geological
<br />Survey Mr Enc Me}'er aSSisted the author In the field tn~
<br />\'estlgatlOns and computatIOns
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<br />FIGURE 1 -fLOODS ABOVE ELEVATION 1118 FEET
<br />1889-1959 AT SIXTH STREET CREST STAGE GAGE
<br />SITE ON BOULDER CREEK AT BOULDER COLORADO
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<br />I Cache la Poudre RlVerat mouth of canyon. near Fort eonlfUI
<br />2 St. Vram Creek at Lyon,
<br />3 Boulder Cl"e('k near Boulder
<br />4 Boulder Creek near Orodell
<br />5 South Boulder Creek near Eldorado Spnnp
<br />6 Cle.ar Creek near Golden
<br />7 Clear Creek at Forb Creek
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<br />20 25 35 SO
<br />RECURRENCE INTERVAL IN 'fEARS
<br />
<br />FIGURE 3 -FREOUENCY OF FLOODS ABOVE ELEVA
<br />TION 1118 FEET AT SIXTH STREET CREST STAGE
<br />GAGING SITE ON BOULDER CREEK AT BOULDER
<br />COLORADO
<br />
<br />'00
<br />
<br />FIGURE Z -LOCATION OF GAGING STATIONS USED
<br />TO DETERMINE FREOUENCY OF FLOODS ON BOUL
<br />DER CREEK AT BOULDER COLORADO
<br />
<br />FLOODS AT BOULDER, COLORADO
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<br />CONTOUR INTERVAL 5 FEET
<br />ELEVATIONS SHOWN "RE REFER REO 10 BOULDER City O"rUM
<br />TO CONVERT TO ELEV"TlONS "eOVE ME"N SE" LEVEL ADD 42480 FEET
<br />
<br />1961
<br />
<br />'NTEIl OR_GEgLOG'CM. "UIl\lE'" W_S"'NGTCN " e_'O~tt
<br />
<br />For sale by U S Geolog,cal Survey Price 50 ~nts
<br />
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