FLOOD10015 - Laserfiche WebLink

Home Browse Search

V. URBAN STORM DRAINAGE A. Objectives The section of this report onFloodpldinDelineationdescrilles the r.lethod enlployed for eval uat in~ crossi n~ capacity. Provision of adequate dra indge f~cil1t ies Ilithi n an urbdn area enhances tile gener~l healtl1, welfare, econorllic '.Iell bei ng, and qual ity of 1 ife in the cO<llTIunity. The purpose of eXdrnining urbarl dr"in<1~e, then, is to allow pl anni ng and ..val uat i on of dra i nage systellls so tlla ~ the dalllage. haldrd, and nui sance aspects of the stor", runoff I'<l~ tern are rni nil~i zed. C. fxisting Storm Drainage Systern Figure V~l ill ustrdtes the existiflg storm sewer Sjst""1 :.1ontrose. As can be seen, the st reet SjS tel~ is re 1 i ed upon draina~e conv~ance. within forl-'ril"ary The j.Jurpose of this section of the study is to define the ]oc~l- izedstormrunofftributarytothenumerousdesignpointsdesigndted Ulrough~out the city and environs as indic~ted on Figure IV~8. No inter~subb~sin flo'l rout ing was performed, as future stornl sewering (or other flow managelnent techn i que) in tile areas will I~ai ntd i n the isol a~ t ion of the flows during the 5~year frequency stor"" The data deve- loped for this section will serve as the basis fora future study on cOlfllJrel1ensi ve storr.1 runoff 'fIilna~ement for the ellt i re urb~n study area. A furtherllurpose of this section lias to identify dlld propose sol ut ions for ex i st i ng 1 oca 1 i zed drd i nage proiJ 1 e!~ "reas. The 5-year frequency initial design storm was selected for use in this analysis in a policy decision by tile City of Montrose and Montrose County. While a storm sewer syste'~ is des i Ul1ed to cont ro 1 the runoff frolll the init ial (5.year) stonll, ~ less frequent storul (i .e., lOO-year recurrence interval) mdY result insorne inundation, particular lyin areas of exlsting developlllent. The existing stornl sewer system WdS not the resll1t of long rdnge, ar~a Ilide p]~nning. It w~s inst~lled one section at a ti,llc to handl~ local ized drainag~ problems. The storm and sanitary sew~r systeJ~s of Montrose are separate and were never combined. O. Conveyance Capacities 1. Streets B. Methods The street curb and gutter systeln functions as theprilllary drainage system for the urbanized part of Montros~. The streets are not consistant in con~ey~nce capacity, due to varying cross sections, crown slope and presence (or lack) of curb ~nd uutter. Additionally, success i ve aspha 1 t overl aj prograllls over the years ha ~e further 1 imi ted the streets' ability to convey runoff. The allowable flow depth of runoff i n ~ile st reets is 1 illlited by the el evat i on di ffereflce between the top of curo and l10use fini,h floor elevation, or bdsement wind OIl we 11 e1 e~at i on, IIhi chever is lowest. TM i s di fference is s:llal1, par- ticularly in the older sections of ~ontrose. All these factors contri- buteta the need for a larger storm sewer system than otherwise wO uld be necessary were the street section reason~bly uniform and houses plJced well above the top of the curb. Table V~l presents the conveyance capabilities for the collector and arterial streets within Montrose (as defined in the City of 110ntrose 11ajor Thoroughf~re Plan). The locat ion of these streets is sholm on Fi gure V-3. ~ lOll capacity is indic~ted for both 6" depth of flow and 1.5' depth, both measured from the gutter flowline. The street systelllis generally inadequate to contain tlie5-yearst arm runoff at a flow depth less than the curb height (6"),which is the standard basi, for design. The urban drain~ge analys is assumed that the three study stream flows remained separated from the urban storm runoff. For the less frequent storr.Is. this assulllptionmq not be valid. O"overflowfrol'I the study streams, overf1owin~ water may travel SOlfIC distance away from the streJlfl by the street syste'n, rni ngl ing with the urban ~enerdted runoff. The urban st udy areJ was d i v i de\J i n t 0 ~pprox i I,~a te 1 y se venty sub. basins for the analysis. Therdtional forlllula was used to calculate r1lfloff for thl' v~ri OilS sllbbas i ns for both the 5 a nd 1O[)-ye~r fre<jlJt'nc.v storPl1S. Fully de_eloped conditions Here ~ssunled, based on City deflned 1 and use (see Fi ~ure IV-9). Actual use of part of the land defi ned as 'open' is presentlj changing. Therefore, runoff 'luantities may increase over tllat derived frOlll land use as initiJllydefine d. 2. Storm Sewer Capacity Where storm sewerin~ is provi<.led in ~ontrose (see Figure V-l), freyuently runoff has a~assed to the point where the provided inlets are inc~p~b]e of intercepti ng the drai nage. In addition, many of the inlet drain 1 ine; ~re 8" diar.leter pipes which easily clOG with debris carrjed by the storrn runoff. Num..rous secti ons of ~ile storr" sewer system are too sinal I to carry si~nificant flow. Virtually none of the existingn'.djarstormscwerin9provided'lithintheCity is adequate to intercept Jnd conve,y tile runoff genCl'ated by tile 5.year fr~yuer.cy stornl. Where specific drainage prOblem JredS were identified, s~lutions t~ the local ;led dr~i n~ge SitUdt ions were defined. These sol uti on, are site specific and "'.dY, or n1<ly ~ot. be COinPdt i ble wit~ a COlnprehen5ive draina<)escl1e:r.e. -22- -23-