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<br />.2-
<br />
<br />mrOOIU:TION
<br />
<br />nrral housing on !T"IJst of it in the near future. A major ~tor.n, l'1't'/\ with
<br />the five floocll-olte-rretarding sttu<:tures in place, cOlltd cause extensiv e
<br />flooding in this area.
<br />
<br />This report evaluates the flood potential along Boxelder erN'\<: from 11
<br />point about 12 miles northwest of ""e111ngton to its point of conflu(,lln'
<br />with the Cache La Poudre River, southeast of Fort Collins, Larimer
<br />County, Colorado. The evaluation 10'115 made unlet present conditions and
<br />fuwre conditions with the ProlXlsed five floodwater rctar,ling $tructures
<br />and one grade stabilizer installed. The proposed structural program
<br />was developed by the Soil Conservation Service unleT the authority of
<br />Public Law 566 Revised. TI;i~ re?Qrt ;'3 ",;;:::!(> for the C.olorado \\'ater Con-
<br />servation Board to promte the best utilization "f t:'~ land sub;",,;~ to:.
<br />oVerf1C1W.Thisreportisbascdonhistoricalinf~tion.lli1tad('vel.
<br />oped by use of the computer, and data developed manually us.;:;: computer
<br />output and other tedmical data.
<br />This report contains maps, water surface profiles, flood plain ~ross
<br />section locations, and elevations at all cross sections. Flood risks.
<br />may be appraised and the construction of facilities planned to l~JL.IC"
<br />flood damage. Although the flood plain near I1:Irt Collins is not devel-
<br />oped, at this time, it is expected to be urbanized in the near future.
<br />The po~sibili~y.of a 10?-year floc;rl fre9uencr storm rUlloff or greater
<br />OCCUIrlng defInItely e~l~tS, a~ It IS Impo~tant that the probability
<br />of such a flood be antiCIpated In the pIal'\1lmg of flood plain dev"lo~r>t.
<br />
<br />FlmJRE FLOODS
<br />
<br />~loods ~iat oc~r after the project r~s been ins~~l~~ ~ii: VP gT0atlv
<br />reduced. The 100-year frequency storm will h", r"d'.lo.:ed to "'lOut a '<-yel'r
<br />frequency stormbelOl>' structures, on the average. Itmu:;tl)CI<.:lderst<>OJ
<br />that runoff from un-reser\~ired areas can still cause extenji,0 fJ,~ing
<br />in the lower part. Proper ~oning based on extent .u.J ,iepth 0;' fJoo<iing
<br />could prevent damages from OC~:lrring in this lower area.
<br />
<br />A floud b ,,;,'sr:rib~ as an overflo..' 01 ;'..;..; ::~jdo..:ent to a stre,,;r, or river.
<br />not normally cove:'ed by" watfrl. The IOO-year frequency '1:>0<1 is dee,crir<'<l
<br />as a flood having an averag" frequency elf occurrence in the o-rder of
<br />once in 100 years alth"..,;!. the flood IlRY oc:cur in any year. It is haS'.-d
<br />on statistical ana.ly~is "f rainfall and runotf ch..,~rteristics in the
<br />g~heral Lr~;n~ uf :~~"IJcr C~eek ~d~~~hc~.
<br />~
<br />
<br />ARf.A5 FLOODfD ;\.'ID 0;::<:11 OF nroDL'I(;
<br />
<br />$ID.f.WW OF FWJDS
<br />
<br />A :':;~-~e al'C' ,i ;;.,;:o:?].l"'r Creek tlc.c;ll,l:!::.:: """"0:' :c i::'J",l:\te<l .,hould the
<br />IOO-year frequency ~torm occur. Figures 4 through 9 o,.tUnc the extcr.t
<br />of flooding both with and without structures. An index map showing the
<br />general relationship of these areas is found on Figure 1. llfJpths of
<br />nl7W can be estimated from the flood profile; on Figun'$ 2 and 3.
<br />The limits and depth of flooding shown on the figures were J(,'termined
<br />from computer runs on water surface profiles and flood routing. Veloc-
<br />ities an:i depth of flooding vary greatly throughout the flood plain.
<br />Rainfall amow1ts used in the computer run for flood routinp: were takeJI
<br />from Weather Bureau Technical Paper 40 revised maps for ColoraJo.
<br />
<br />The portion of Boxelder Creek flood plain that is considered in this
<br />report extends a~t. ,3 rni17s ahove its lIVuth. Flooding occurs throur.h-
<br />out the flood plaIn In varyll\g degrees. Local residents outlined areas
<br />of flooding that have occurred in the past, and flooding occur~ on an
<br />average of once every two years. There is no official gaging station
<br />in the watershed and no knCMtl flood flows have been measured.
<br />
<br />GIiNERAL CG.'IDITIO:>lS A.'lD PAST FLOOIl.'>
<br />
<br />A map of areas flooded as recalled by local residents is on file with
<br />the Soil Conservation Service, Watershed Planning Staff office. 1h"
<br />map, technical data and statements from 10C.11 people ont1lined Ln the
<br />field support the high frequency of flooding throullhout the flood plain.
<br />'I.he Town of Wellington is suhject to flooding from 11 combination of
<br />flow from Boxelder Creek and Coal Creek, and there is 11 good proh.lbilitv
<br />of a IIIlj()r storm causing ex~ensive flooding in ~'ellin!:ton. 11\i~ ,'Tea on
<br />elther Slde of Interstate /hRhway ZS and south of State hir.h~'~", 14 is
<br />subject to flooding. This area is <l!-:riwltllral 11{\l', I.'''" l~' rl~'lr,,'" br
<br />
<br />\lURKS or I~trROVll<-n.:.\IT
<br />
<br />The Soil Conservation Service under the authority of Publi~ L.!\'i 5M
<br />Amerded has developed a structural program <.:onsistinl( of f;V(, (1.-,.,-.;-
<br />",,!.ter retarding structures and one grade stahili~er. TIa' loc"tinrl~ d
<br />these structures are shown on the index map, Figure I. Thes,' stn1l;tur..~.
<br />wh.m completoo, wi;1 reduce th~ floo..i pe:lks. [n);., all fre<j;lel1;.;:" ,.~",:-:;t.
<br />throughout the watershed. Table Numher 1 rrf'st'1\ts the 1IlO-yc"\T rr''>l''e'''~\"
<br />ilO',;s ~or selected poin~s in t)'.c arca LJ..'"1deT pn>,'c"t ,;o:),!,'i.OI~'.. 'I'~J :Jl~.,
<br />pn'5l.'nts ~.he toO-yem' fr""1ueocv flows that will r..<."I.' "~'" ,-("""l..:;'..','
<br />of the rro\,..,~ed stTlJ<.:tul"al program.
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