|
<br />the overall floodplainanalJsi5,whilethesecondportiondescribesthe
<br />more 10cdlized hydrologic approdcll used for the urban arlalysi s.
<br />The fifth section of this report involves the ~rban drdinage ana-
<br />lysis. Tile method of analysis is presented, asw"ll asthecvcrdll
<br />dr'~fn~5e ,ituation. The ~nalys is ~l so presents dn inventory of tbe
<br />existl ng system dnd fa,i 1 it ies. The hyaril~l i, 1 illlitilt ions of t),~
<br />exist ins facilities are discussed, ana recolmlendiltions for preventative
<br />and correcti ve rrocaSJ.;res for the urbclrl clrea arc ~rescnted.
<br />
<br />5.YearEvent
<br />
<br />TheS-JeueventwclS sele,tedas the "initial storrll" for the urban
<br />analysis. This "ilS 0 policy decision made by CHy of v.ontrose and
<br />Mont rose County staff, presented to cl joi nt meet i ng of ~he City Coun, i 1
<br />clnd County COlTlf;]issioners on June 16, 1978. The initial stonll is that
<br />event uti 1 ized ~s the desi gn Standdrd for the "ini t ial <Ira inage system"
<br />andtyp1callywill haveafre<.;wencyrangingfroJllonceintwDyearsto
<br />once i n ten yeu,. The 5 -yea r i n it i a 1 s t 0 r'~ frequency was se 1 eeted as
<br />best meeting the drainage needs of the City.
<br />
<br />The sixth section of the report, "Floodplain Oelineation" presents
<br />the hydraulic analysis for the three study streams, lildch results in
<br />definition of the floodplain boundary as presented on the plates at the
<br />~nd of this report. Basic data is presented, as are rcCOIIllICrldE'<i pre-
<br />ventative and corre,tivelr.easures as related to thc three study
<br />streams.
<br />
<br />I n urban i zed areas, there are two stor'~ dra i rlage syster'lS: the
<br />i ni ~ i a 1 system and the fIl<lj or system. The in i t ia 1 storlll drai nage syste'~
<br />dra 1 ns the runoff frOl~ the i 0 it i a 1 storm, I/ith the purpose of reduci 09
<br />street fIl<lintenance costs and minimizing recurring da,;]agefromr r.orefre.
<br />quentstol"1llrunoffaswel1 clS inconvenien,e tQ the urban residents.
<br />Inlets and stormsewering are typically a part of the 1nitial drain age
<br />system. The Irlajor storm drainage system corresponds to the system
<br />whiCh drains runoff for storm frequencies up to tbe lOO-year event,
<br />withthepurpaseofmini.nizingmajorpropertydamageandrisktolife
<br />Ilithintheurbanarea. The two drainage systerns should be integrated.
<br />Senefit.cost comparisons are thebclsis for seleetion of the init icll
<br />storm frequency. Storrn seweringand other initial stornrfacilities
<br />should be des igf1ed to handle the r~noff resulting frorn tbe init icll
<br />storfllwithin theurbcln area,
<br />
<br />The glossary of tertliS and abbreviaticns. and thebibliograplly
<br />follow the sixth se,tion. The plates presenting the floodplain data
<br />for the three study streams follows this information.
<br />Interpretation clnd Use
<br />
<br />The proYision of adequate storm drdind~e is necessary to ~reserve
<br />and promote the ~eneral health, wel fare, and econ01,lic \Iell being of the
<br />area. As drai nage is a feature affecti ng all parcel s of ~ro~erty, a
<br />consistent, integrated drainage clnalysis ane plan is essential :0
<br />ilSSIJreorderlygrowth,con;petentresolutionofexistingdrdinage
<br />probleM, ancpreservationofthequalityoflifccurrentl,yenjoyed In
<br />Montrose,
<br />
<br />IO-Year clnd 50-Year Flood Events
<br />
<br />Se ve ra 1 s t or~1 f requ e nc i es ha y e been ana 1 yze<l for th i s re porot .
<br />Each "'ay ~ utilf~~d ill the plclrmin~ process, <15 desc,'itted below.
<br />
<br />1 nfor:'tati on regarding the lower frequency floods (lO-andSQMY ear
<br />eyents) is especially useful for future engineerin9 studies and land
<br />us~ pl anning purpo~e~ reI ated to minor road systems, minor channel
<br />im;Jrovements, the location of parks and rccreational facilities, agr1-
<br />cllltural lands, Jnd Jppurtenilnt str~ctures. For structures an~ ~ses. of
<br />tr.~s ty;;e or, the s'~aller tribu:Jr:es ar.j ir. JrNS ',/here the r,~gher r~sk
<br />ofstru,tural failure is econorr,;,allyjustifiable and the hazard to
<br />life and property nonexistent, the use of the lo,ler frequency f10 ods as
<br />the design standard lIldy be considered.
<br />
<br />The 10-, 50-, 100-, and SOO-yea,' flOOd events were ~sed as thc
<br />floodfrequenciesforthef1oodj.ilainclnalysis. :bus, the data de vel-
<br />oped in this report will be 'ompatible rKlt only for rugulation pur-
<br />;Jcses and Coloradc H.B. 1041 deslgnHions, but also for Federal
<br />I nsuran,e .\dministrat ion flood insurance rate s:~cies.
<br />
<br />lOa-Year FlOOd Event
<br />
<br />The S-ar.d lOa-year flood events \.ere
<br />cicsfortheurbaoarcadraillageallalysis.
<br />discussed bel 011.
<br />
<br />~sed as the flood frequen-
<br />Selectiuncriteria is
<br />
<br />The LOOMyear flood event ';'.o.y be used for en~ineeriol~ des i ~n pur-
<br />~oscs where J lower risk of fa flure th"n llle 10- or 50-year flood is
<br />desfred. However,thelllost important use of the laO-year flood event
<br />1 i~s in floodj:olain d",i gnat ion ~n<1 land ~,e regulation a, set forti1 in
<br />tile Colorado State Statute~. TMe State of Colorado cons iders the 100-
<br />year frequency flood as tbe flood or "mcljOr ston;I': event to be u~ed in
<br />designing and protecting structllres clnd Gl-iellings for human oc cupation.
<br />Therefore. all floodplain regulations are ~sed upon the lOa-year
<br />flood. I\lso, the area whidl would be inundat",d by Ule lOO-yNr flood
<br />"lay be del i neated as a~ dre3 of stdte interest ';$ set forth in Coloraec
<br />H.B. 10q, the Stolte's ldnd loSt' bfll.
<br />
<br />These various fl ood el'ents haye an average occ~rrcnce of once in
<br />the rlLll<lber of years indicated. Forexar.v1e,thelOO-yearflaodocCcrs,
<br />on the averil~e, once; n a 100 year reri od, Jf'd has a one percent chance
<br />(,f being equal I'd or exceedee in any ~iven year.
<br />
<br />Tile part lc.uldr ~ses for the 'ladoLo, flood c'icnts in "del: i or-, t.c
<br />:hose stated above ~rL' as follo"s:
<br />
<br />.6.
<br />
<br />,
<br />','
<br />
|