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<br />FREQlIENCY <br /> <br />Frequency analyses for the Animas Rlvor and Lightner Creek were based <br />on streamflow records of rain and snowmelt flOOdS and on data from hIstoric <br />storms.. Frequency an.'llyses for Junction Creek were based on streamflow <br />records and on d~ta developed from similar stream basins In the region. <br />FI~ frequency curves ~ere not developed for Ory Gulch Creek because <br />historic flow and precipitation data were not available. Since floodfloW5 <br />developed for Dry Gulch Creek were not supported by hIstorical data, they <br />should be regarded as approximate and are to be used with cautIon In <br />connection with any flood plaIn use. It Is the Judgment of hydrologists <br />famll iar with the region that the October 1911 Animas River flood had a <br />frequency of about once in 65 years on the average. Floods on LIghtner <br />and Junction Croeks In October lry72 had an estImated frequency of once In <br />40 to 50 years, on the average. <br /> <br />As previously IndIcated, an Intermediate Regional Flood has a frequency <br />of occurrence of about once In 100 years, on the average, end a Standard <br />Project FlOOd would occur less frequently than an Intenmodlate Regional Flood. <br />Although the Standard Project flood would be a rare event, It can reasonably <br />be expected to occur In the future. Floods larger than the Standard Project <br />FlOOd are possible, but their occurrence would be cxtremely rare. <br /> <br />HAZARDS OF LARGE FLOOOS <br /> <br />The amount ",,<,I ~xtent of d"""'ge, and the attendant flOOd hazard, frot:'l <br />any flood dcpends upon the depth and duration of flooding, velocity of flow, <br />rate of rise, and tho develop/ll(lnts In the flood plain. An occurrence of an <br />InTormedlate RegIonal or Standard Project Flood on STreams In the study area, <br />under present conditions of development, would result In e><tenslve Inundation <br />of agricultural larlds, public faci Iltles, and commercIal and Nlsldential <br />ereas. Travel along U. S. Highways 150 and 550, the Denver and Rio Grande <br />Western Railroad, and city and county roadS would ~e Interrupted for extended <br />perl ods. <br /> <br />31 <br /> <br /> <br />Floodwater flowing at high velocity arid carrying floating debris <br />would create conditions hazardous to persons attempting to cross flooded <br />areas On foot or In vehicles. In general, floodwater:5 or mo~ feet deep <br />and flo~;ng at a velocity of :5 or more feet per second could easily sweep <br />a person off his feet, thus creating defInite danger of Injury or drown- <br />ing. Rapidly rising and s~lftly flowing floo4water may trap persons In <br />homes that are ultImately Isolated or In vehicles that could be stranded. <br />Sewage, garbage, and other organic materialS carried or deposited by flood- <br />water could create health hazards, and Isolation of areas by floodwater <br />could create medical, fire, or law enforcement emergencies. Water lines <br />can be ruptured by deposits of debris or the force of flood~ater, thus <br />creating the possIbility of contaminated domestic water supply. <br /> <br />, <br />, <br />," <br /> <br />FLOODED AREAS AND FLOOD DAMAGES <br /> <br />The appro><lmate limits of Inundation that would result from a Shndard <br />Project Flood on the streams under study are shown on Plate 6, which Is <br />also an Index to more detailed maps showing the areas that would be Inundated <br />by the Intermediate RegIonal and Stendard Project Floods (Plates 7-9l. As <br />may be seen from those plates, the Irltenmedlate Regional and Standard Project <br />Floods WOUld Inundate extensive areas and there would be some IntermIngling <br />(a) <br />ot floodwaters from various SOl.lrces. <br /> <br />Ovorflow from the Animas RIver abovo Durango would Inundate broi5d <br />expans~s of agricultural land In the Anl~s Valley. As the Animas River <br />anters Durengo, the character of tM flood plain changes dramatically. <br />upstream from tho city the flood plain Is br~d (approximately 3,000 feet <br />wldel, and velocItIes of tlow are generally low. Through the city, the <br />flood plilin closes to narrow bands along the river and velocities of flow <br />are hIgh. Floodwaters obstructed by the Main Avenue Bridge flow around <br />the south end ot the structure and cross M4ln Avenue bet~een \4th and 15th <br />$tre<lh. After crossing Milln Avonue, the floodwaters rejoin the rlv<lr In <br /> <br />(a) Ie uhoutd be rlI)ted that aH of the flooded areao shoWII on. Plau!I .6-9 <br />LlCutd 1lQt be inundated at the 1IQ1l0 point in timo becauuo f~ooa pl'Oduc1."(I <br />ew,.."ltoool'tiw. uarioU8dzoail'Wl]ebMil18 "",,,tdn.otnell'eeear1.tyoll'Cl<l'con- <br />Cl.<1'l'ently. <br /> <br />32 <br /> <br />