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<br />C. OrRanization of Report
<br />
<br />Additional field sUrveys were perfor~ed by Waeer Resources Consuleanes,
<br />Inc. ta obtain bridge and culvert opening si.es and to verify the computed
<br />liwits of flooding.
<br />
<br />This report 1S divided into two maln parts. Part 1 provides information
<br />concerning flood hazard areaS based on existing basin developwent conditions.
<br />Part 2 COVers alternative plans for flood contral and drainage improvements
<br />for the study reach of St. Vrain Creek. The details of the recommended plan
<br />are inclUded and are intended to Serve as a master plan for future improve-
<br />ments.
<br />
<br />E. Related Flood Studies
<br />
<br />In June 1972, the U.S. Army Corps of Engineers, Omaha Oistrict, published
<br />a Floodplain Information report for Lower St. Vrain Creek. The reach stUdied
<br />by the Corps extended from the Boulder/Weld County tine upstream about 8.2 wiles
<br />to Sopris Drive. The hydrology was based on a statistical analysis of gaging
<br />records and flood boundaries were dctermined witb fi~ld surveyed cros, Sections.
<br />
<br />D. Mapping and Surveyin~
<br />
<br />In 1973, the City of Longmont published a "Major StOttll Drainage" manual
<br />prepared by Bruns, Inc. In this manual, flOOd hazard area Wer€ delin€ated
<br />On U. ~. G~ological Survey quadrangle map", original scale I" .. 200', for all
<br />major drainageways in the City. The information for Lower St. Vrain Creek
<br />was based on the 1972 Corps report.
<br />
<br />Contour mapping for the study was provided by the Colorado Water Con-
<br />servation Board. This mapping was prepared by Parker and Associates under
<br />a separate contract.
<br />
<br />Specific topographic data provided to the consultant included:
<br />
<br />In January J977. the Federal InsuraoccAdministrationpubliahed a "Flood
<br />Insurance Study" prepared by Gingery A'"oci~t~s, In~. The floodphin, ,-,pre
<br />ddineated in an area defined by the "prilll<' urbanized area boundary" and inctu-
<br />d~d a r.."ch On St. Vrain Cr..ek extendi"8 fro", I 19th Street ~pstTCa", "pproxio\~tely
<br />4.8 miles to the Airport Road (BJthStteet). Their study reachhcgan..b""t I.()
<br />
<br />1. T",enly-four reproducible rnyLr pl':lnl.""tric - topographic ""'p' shp~ts
<br />wirh a COntout int"rval of 2 reet ~"d " ~~"l~ of I" . 200'.
<br />
<br />",il~ uil~tr~.1m f,oOl tl',c B,,~lde,/..'~ld C'H,only lin~. n,~ i1ydrvlo~y was [h~ sa"'"
<br />as ~n the Corps 1972 report and utilized derail..d crOS5 8enion data d~velop~d
<br />by tI1~ Corps. supplimented with additional rro.s sections taken tram a 1" . 400',
<br />two foot-contour int~rval map also prepared by the Corps in 1972. The IOO-year
<br />water surfac~ elevations computed in the Flood Ins~rance Study ar~ generally
<br />within 0.5 foor of thosc published in the Corps Floodploiia Ioformatinn Report.
<br />
<br />2. One set of Xl and GR catds (HEC-2 Co~putcr Program) fQr 49
<br />digitized cross sections of ~he St. Vrain Cree~ valley.
<br />
<br />3. OIle set of 9"X9" black and white photog,aphic contact prints with "
<br />d"te of photogr'lphy ofM.Ht'1, 21,,1979.
<br />
<br />The items listed above are available for review at th~ Colorado WAt~r
<br />
<br />,h~ :.r",it~ <or St. V,..l" CI~~~ "Luciied in <Ilis floodplain Inl'orfll<l~ion.
<br />FlOOd Control, and Floodplain M"na8~"wnt Pl"n are the s,,",o .s in the FlOOd
<br />lnsar.nee Study. However, the disd,,,rges abov,' Lefthand Cr~ek are slightly
<br />lower having be~n rpviMd by the Corps using.. m<'Idern r~infall lIJodcli"g t~cl1niq"(',
<br />Th~ di"charges b~I.~' the confluence ~ith Leftl1~~d Creek h"v~ increased ov~r thos,
<br />u;eLl i.n th~ 1972 study. The ",apping and cross spnicns for this report ~rc
<br />based On a "e~> <e( 'If I" . 200' and t"'o f<lot "on~,,,,, In'erv~ 1 ropog,raph io
<br />maps prepared in :1..rch, 1979. This ~tudy include. the l"~eH hydrologic l~io,--
<br />lllation <lvailable hom the Corps and the ,"o,t Ie~en~ mdpping of the area.
<br />
<br />Conservation Board.
<br />
<br />The City of Longmant conducted field survey. and provided information
<br />related to the first flood elevations of all improvements lo~at~d in the
<br />fIoodplain. Further, information On 1.he ~yp~ of buildin~." .wd t"ei,
<br />location ~u provided by the City of Long""'''t. This informacion "'as used
<br />in as"~"Sln& damages from dit!e~ent !reque~cy fl~od,.
<br />
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