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Last modified
1/26/2010 10:09:16 AM
Creation date
10/5/2006 4:19:36 AM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
53
County
Larimer
Community
Unincorporated Larimer County
Stream Name
Cache La Poudre River
Basin
South Platte
Title
Floodplain Information Report - Cache La Poudre River, Colorado, Volume III
Date
10/1/1975
Designation Date
3/1/1976
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />Obstructions to floodflows - Several roadways varyinn fro~ county <br /> <br />roads to hiqhways to railroads cross the Cache la Poudre River flood olain <br /> <br /> <br />within the confines of the study reach. The physical characteri5tic5 of <br /> <br /> <br />these r0adways, coupled with thA assumption in the hydraulic co~outations <br /> <br /> <br />that I ittle debris was lodqed in the hridqe openings, created conditions <br /> <br /> <br />in the study which resulted in an increase in water surfacA elevations <br /> <br /> <br />i~mediately uDstream from these hridqes ranoinn from 0 to'4 feet for al I <br /> <br /> <br />floods studied. This chanoe in water surface elevation could increase <br /> <br /> <br />drastically especially at those roadways havino hioh emhan~~ents if sub- <br /> <br /> <br />stantial amounts of debris WAre to lodoe in the hridne opening, thus <br /> <br /> <br />reducing effective floodflow conveyinq area. If this were to occur durinq <br /> <br /> <br />a large flood such as the IRF or SPF, the increased bridqe velocities <br /> <br /> <br />would cause added erosion around hridoe Diers and ahutments with possible <br /> <br /> <br />damaqe. Upstream water surface elevations would also increase because <br /> <br /> <br />of thA debris possibly floodinn more area than shown in this report. <br /> <br />t'I't . 'ns the main floodflow by <br />along this southeasterly path un I I reJo, . <br />t q' h ay 25 in the vicinity of reference oOlnts <br />overtopping Intersta e r 19 W <br /> <br />')9-102. <br /> <br />Floodflow obstructions are also present in the for~ of irriqa- <br /> <br />st'ructures. These structures, by removing floodflow <br />tior> diversion <br />f the r.ache la Poudre River channel, not only create <br />conveying area rom ' <br />localized increases in water surface elevation upstrea~ from the ~truc- <br /> <br />ture, but force more floodwater to flow on the adjacent flood plain, <br /> <br />thereby creating higher flood plain velocities. <br /> <br />were not taken into account in the hydraul ic analysis <br />Ice effects <br />. . d to the fact that the occurrence and maanitude <br />of this report. ThiS IS ue - <br />indetermlnant factors and to the fact that no records of <br /> <br />of ice jams are <br />oroblems with ice were found. <br /> <br />AI I roadways within the study reach are overtooped hy the IRF <br />and SPF. '1ost of these roadwflYs are county roads havinn relatively low <br />roadway embankments. This explains the relativelv low increase in water <br />surface upstream from these roadways as evidenced by the Profile. <br /> <br />of 2 feet <br />the study reach for the IRF and <br />second, respectively. Althouqh <br />hazardous, localized velocities <br />the averago velocities. Travel <br /> <br />Velocities of flow - Veloc,lties of 3 feet per second \>lith deoths <br />or more are considered dannerous. Overbank flow v~locities in <br />SPF averane about 1.5 and 2.1 feet oer <br />these avera~e velocities do not appear <br />are often two or three times nreater than <br />through the floodflows should he avoided <br /> <br />The ro I orado and Southern Ra i I road (refflrence nO i nt fl7), I nter- <br />state Hi qhway 25 (rflference nO i nt 92), r.r,f orado State Hi nhwav 392 (rflferencA <br />point 115), a road at thfl Larimer-Wflld rounty line (referflncA point 1/9), <br />and thfl r,rflat Western Railroad (rflferflnce noint 135) are such that the COM- <br />bination of roadway embankment heioht and hridne oOflninq slzfl create staqe <br />incrflases in excess of one foot for thfl IRF. One of thesfl roadways, Inter- <br />state Hi(jhway 25, creates a sionificant floodwater diversion due to the <br />skewed angle at which it crOSSflS the flood olain. Floodwater on the rioht <br />bank which does not pass throuoh the Interstate Hi~hway 25 bridoe at <br />reference point 92 continues to flow downstrflam in a southeasterly direc- <br />tion. This floodwatflr flows at an elevation different from that flood- <br />water followinq thfl adjacent Cache 111 Poudre River chllnnel. It continues <br /> <br />OXCflpt for extremfl flmeroencics. <br /> <br />ratfl of rise find duration of floodino - Flood Deaks in the. study <br /> <br />intense rainfall or a combination of raInfall <br />,each can result from either <br />Storm co~binations would oroduce two distinct flood <br />and snowmelt. Certain <br />Standard Project Flood under tho douhle neak conditions, the <br /> <br />for the first oeak would be about q hours <br /> <br />SMa I ler peak resultino from mountain runoff. <br /> <br />could last about 7 days. <br /> <br />peaks. For a <br />rate of rise or peakinq time <br />fol lowed 2 1/2 days later by a <br />The total durlltion of floodinn <br /> <br />32 <br /> <br />31 <br />
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