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<br />V. URBAN STORM DRAINAGE <br /> <br />A. <br /> <br />Objectives <br /> <br />The section of this report onFloodpldinDelineationdescrilles <br />the r.lethod enlployed for eval uat in~ crossi n~ capacity. <br /> <br />Provision of adequate dra indge f~cil1t ies Ilithi n an urbdn area <br />enhances tile gener~l healtl1, welfare, econorllic '.Iell bei ng, and qual ity <br />of 1 ife in the cO<llTIunity. The purpose of eXdrnining urbarl dr"in<1~e, <br />then, is to allow pl anni ng and ..val uat i on of dra i nage systellls so tlla ~ <br />the da"lage, haldrd, and nui sance aspects of the stor", runoff I'<l~ tern <br />are rni nil~i zed. <br /> <br />C. fxisting Storm Drainage Systern <br />Figure V~l ill ustrdtes the existiflg storm sewer syst""1 <br />:.1ontrose. As can be seen, the st reet sys tel~ is re 1 i ed upon <br />draina~e conv~ance. <br /> <br />within <br />forl-'ril"ary <br /> <br />The J-lurpose of this section of the study is to define the ]oc~l- <br />izedstormrunofftributarytothenumerousdesignpointsdesigndted <br />Ulrough~out the city and environs as indic~ted on Figure IV~8. No <br />inter~subb~sin flo'l rout ing was performed, as future stornl sewering (or <br />other flow managelnent techn i que) in tile areas will I~ai nta i n the isol a~ <br />t ion of the flows during the 5~year frequency stor"" The data deve- <br />lopedforthissectiofiwill serve as the basis fora future study on <br />cOIIlI-'rel1ensi ve ston.1 runoff 'fIdna~emefit for the ellt i re urb~n study ared. <br />A furtherllurpose oftliis section liaS to identify dlld propose <br />sol ut iOfis for ex i st i ng 1 oca 1 ; zed dra i nage proiJ 1 e!~ "reas. <br />The 5-year frequency initial design storm WdS selected for use in <br />this analysis in d policy decision by tile City of Montrose and Montrose <br />County. While a storm sewer syste'~ is des i Ul1ed to cont ro 1 the runoff <br />froll' the init ial (5.year) stonll, ~ less frequent storul (i.e., lOO-year <br />recurrence interval) miiy result insorne inundation, particular lyin <br />areas of exlsting developlllent. <br /> <br />The existing stornl sewer system WdS not the resll1t of long rang~, <br />ar~a Ilide p]~nning. It w~s inst~lled one section at a tillle to halldl~ <br />local ized drainag~ problems. The storm and sanitary sewer systeJ~s of <br />Montrose are separate and were never combined. <br />O. Conveyance Capacities <br />1. Streets <br /> <br />B. Methods <br /> <br />The street curb and gutter systelnfUllctionsas theprilllary <br />draina~e system for the urbanized part of Montros~. The streets are <br />not consistant in con~ey~nce capacity, due to varying cross sections, <br />crown slope and presence (or lack) of curb ~nd uutter. Addition"l1y, <br />success i ve aspha 1 t overl ay progralns over the years ha ~e further 1 imi ted <br />the streets' ability to convey runoff. Thedllowable flow depth of <br />runoff i n ~ile st reets is 1 i",ited by the el evat i on di fference between <br />the top of curD and lIouse fi ni:ill floor elevat ioo, or bds~."ent wiodoll <br />we 11 e1 e~at i 011, \/hi chever is lowest. Th i s di fference is S:lldl1, p.lr- <br />t icularly in the older sections of ~ontrose. All thes~ fJctor, contri- <br />bute to the ne~d for a larger storm sewer system than otnerwis~ wOlll d <br />be necessary were the street section reasonably uniform and house 'i <br />plJced well "bove th~ top of the curb. <br />Table V~l presents the conveYJnce cd~abilities for the collector <br />andarteriJl streets within Montrose (as defined in the City of <br />110ntros€ 11ajor Thoroughf~re Plan). The 10cat ion of these streets is <br />sholln on Fi gure V-3. ~ low capacity is indic~ted for both 6" depth of <br />flow and 1.5' depth, botll measured from the gutter flowline. The <br />street systelllis generJl1y inadequate to contain the5-yeorsto rm <br />runoff at aflowdeptn less than the curb height (6"),which is the <br />st"ndard UJsis for design. <br /> <br />The urball draill~ge analys is assumed thJt the three study stream <br />flows remain~d separated from the urban storm ruooff. For the less <br />frequent storr.ls. this assulllption<ilq not be valid. Olloverflowfrol,I <br />the study streams, overf1owin~ water may travel ,01fIC distance away frorTI <br />tne streJIII by the street syste'n,rninglingwiththe urban generJted <br />runoff. <br /> <br />The urbJn st udy areJ was d i v i de(j i n t 0 ~pprox i I,~a te 1 y se venty ,ub. <br />basins for the analysis. The rational forlllula was used to c"lculJte <br />r1lnoff for thl' v~ri OilS sllbbas i n, for both the 5 "nd 1O[)-ye~r fre<jlJt'nc.v <br />storPl1S. Fully de,~loped conditions Here ~ssunled, bas~d on City def1ned <br />1 and use (see Fi ~ure IV-9). ActuJl use of ~art of the lMld defi ned dS <br />'open' is presently changing. Therefore, runoff 'luantities may <br />incredse over tllat derived frOlll land use a, initiJllydefine d. <br /> <br />2. Storm Sewer Capacity <br />Where storm sewerin~ is provi<.led in ~ontros~ (see Figure V-l), <br />freyuently runoff has amassed to the point where the ~rovided inlets <br />are inc~p~b]e of intercepti ng the drJi nage. In addition, many of the <br />inlet drain 1 ines ~re 8" diJr.leter pipes which easily clOG with debris <br />carried by the storrn runoff. Num..rous 'iecti ons of ~i1e storr" sewe,. <br />system are too sinal I to carry si~nificant flow. Virtually none of the <br />existingn'.djarstormseweringprovided'lithintheCity is adequate to <br />i nte rcept and con vey tlll~ runo ff gene I'J ted by the 5. yeJ r f r~yu ency <br />stornl. <br /> <br />Where specific drdinage prOblem JredS were identified, s~lutions <br />t~ the local ;zed dr~i n~ge SitUdt ions were defined. These sol uti 00, are <br />site specific and "'.dY, or n1<ly ~ot, be cOinpat i ule with a cOlnprehen,ive <br />draina<Jescl1e:r.e. <br /> <br />-22- <br /> <br />-23- <br />