|
<br />the overall floodplainanalJsi5,whilethesecondportiondescribesthe
<br />more 10cdlized hydrologic approocll 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,,11 asthecvcrall
<br />dr'~fna5e ,ituation. The analys is al so presents dn inventory of tbe
<br />existl ng system dnd fa,i 1 it ies. The hyarilul i, 1 illlitilt ions of t),~
<br />existins facilities are discussed, ana recolmlendiltions for preventiltive
<br />and correcti Ye rrocaSJ.;res for the urban area arc ~rescnted.
<br />
<br />5.YearEvent
<br />
<br />TheS-Jeareventwas sele,tedos the "initial storrll" for the urban
<br />analysis. This "ilS 0 policy decision mode by CHy of v.ontrose and
<br />Mont rose County staff, presented to a joi nt meet i ng of ~he City Coun, i 1
<br />and 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 id1 <Ira inage system"
<br />andtyp1callywill haveafre<.;wencyrangingfroJllonceintwDyearsto
<br />once i n ten yea r,. The 5 - y ca 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, "FloodplainOelineation" presen ts
<br />the hydra~l ic 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 ddta is presente<l, as are reCOlllllcndE'<i pre-
<br />ventative and correctivelr.easures as related to thc three study
<br />streams.
<br />
<br />I n urban i zed areas, there are two stor'~ dra i nage syster'lS: the
<br />i ni ~ i d 1 system and the fIl(Ij or system. The in i t ia 1 storlll drai nage syste'~
<br />dra 1 ns the runoff frOl~ the in it i a 1 storm, Ilith the purpose of reduci n9
<br />street fIl(Iintenance costs and minimizing recurring da';]agefromr r.orefre.
<br />quentstQl"1Ilrunoffaswel1 as in,onvenien,e to the urban residents.
<br />Inlets and stormsewering are typically a part of the initial drain age
<br />system. The Irlajor storm drdinage system corresponds to the system
<br />whiCh drains runoff for storm frequencies up to tbe 100-ye~r event,
<br />withthepurposeofmini.nizingmajorpropertydamageandrisktolife
<br />Ilithintheurbanarea. The two drainage systerns should be integrated.
<br />Senefit.cost comparisons are the basis for seleetion of the init ial
<br />storm frequency. Storrn seweringond other initial stornrfacilities
<br />should be desigf1ed to handle the runoff resultingfrOlntbe initial
<br />storfllwithin the urban area,
<br />
<br />The glossary of tertliS and abbreviaticns and thcbibliograplly
<br />follow the sixth section. The plates presenting the floodplain data
<br />for the three study streams follows this information.
<br />Interpretation and Use
<br />
<br />The proYision of adequate storm drdina~e is necessdry to preserve
<br />and promote the ~eneral hedlth, wel fare, and econ01,lic \Iell being of the
<br />area. As drai nage is a feature affecti ng all parcel s of ~roper'ty, a
<br />consistent, integrated drainage analysis ane plan is essentidl :0
<br />aSSl,reorderly growth,con;petent resolution of existing drain age
<br />probleM, ancpreservationofthequalityoflHccurrcntl}enjoycd In
<br />Montrose.
<br />
<br />IO-Year and 50-Year Flood Events
<br />
<br />5e ve ra 1 s t or~1 f requ e nc i es ha y e been d na 1 yzed for th i s re pon .
<br />Each "'oy ~ util f~ed ill the plarmin~ process, d5 desc,'itted below.
<br />
<br />1 nfor:'tati on regarding the lower fr~quency floods (10- and SQMyear
<br />eYl:'nts) is espe,ially useful for future enginel:'rin9 studies and land
<br />us~ pl anning purpo~e~ reI ated to lTlinor road systems, minor channel
<br />i rn;Jrovements, the 10,at ion of parkS and recreat i()n~l faci lities, agrl-
<br />cultur~l lands, ~nd Jppurtenilnt str~ctures. For structures ~n~ ~ses of
<br />tr.~s ty;;c cr, the s'~illler tribu:~r:es ar.j ir, JrNS ',Inere the r,~gher r~sk
<br />of structural fai I ure is econorr.i,ally justi fi~ble and the hazard to
<br />life and property nonexistent, the use of the 10,ler frequency flo ods as
<br />the design standard lIldy be considered.
<br />
<br />The 10-, 50-, 100-, and SOO-yea,' flOOd events were ~sed as the
<br />floodfrequenciesforthefloodj.ilainanalysis. :bus, the data de vel-
<br />oped in this report will be compatible rKlt only for regulation pur-
<br />;Jcses and Coloradc H.B. 1041 deslgnHions, but also for Federal
<br />I nsuran,e ,\dministrat ion flood insurance rate St~cies.
<br />
<br />lOa-Year' FlOOd Event
<br />
<br />The 5- ar.d 100-ye~r flood events \.ere
<br />cies for the urban ~r~~ drai!l~ge all~lysis.
<br />discussed bel 011.
<br />
<br />~sed as the flood frequen-
<br />Selectiuncriteria is
<br />
<br />The lOO~year flood event ';'.o.y be used for en~ineeriol~ des i gn pur-
<br />poses where J lower risk of faflure tha!l llle10-or50-yoJrflood is
<br />desfred. However,thelllost important USC ofthe:OO-YCarfloodevent
<br />1 i~s in flvodj:olain d~si gnat ion ~n<1 land u,e regulation a, set forti1 in
<br />tile Colorado State Statute~. TMe State of Colorado cons tders the 100-
<br />year frequency flood as tbe flood or "major ston;I': event to be used in
<br />designing andprote'ting stru'tllres and GYiellings far human oc cupation.
<br />Therefore. all flOOdplain reguldtions are ~scd upon the laO-year
<br />flood. I\lso, the area which would be inundat",ct by Ule lOO-yNr flood
<br />"lay be del i neated as a~ area of state interest a$ set forth in Coloraac
<br />H.B. 10.:!, the State's land loSt' bfll.
<br />
<br />TheSe various fl ood CI'cnt$ haye an average occ~rrcncc of once in
<br />the rll.ll<lbcr of years indicated. For exar.v1e, the lOa-year flood occcrs,
<br />on the averil~e, once in a 100 year reriod, Jf'd Iwo J one percent chance
<br />(,f being equal ed or exceedec in any ~iven year.
<br />
<br />Tile pdrt leuldr ~s~s for the 'larioLo, flood C'ients in "del: i or-, to
<br />:hos~ stated above drL' as follo"s:
<br />
<br />.6.
<br />
<br />,
<br />-,'
<br />
|