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FREOUe:NCY Floodlngcllnoccu.....lth..ellltlvelyllttledlschll..geln theeventofchanneJ blockagecllused by debris 0.. lce. Inthese situations, water su..face olevlltions..lse until relief Isachioved by flows ove.. the flood plain. Hlstorlcal data, howllver, Indicate thet obst..uctlons due to Ice lire not likely to occur In the study ..each. The fo~tlon of obst..uctlons at b..ldges In the study re&eh 15, howeve.., quite possible - especially due to debris IIccumullltlon. Tho SOO-yoa.. flood Is not the III..gest flood thllt clln occur, but the probllbllity of la"gerfloodslsrOl1'lOte. As can be seen from the 91l9in9 ..ecords fo.. tho Cache la Pbudro River, dlschll..gos smllller thlln olthe.. the 100-year 0.. SOO-yellr floods lI..e much more common. La..ge floods, howeve.., can heppen; this wllS clee..ly demonst,,"ted by tho Denver erell floods of 1965, the June 1972 flood 111' Rapid City, South Dakota, and the July 1976 flood In the 6ig Thompson canyon. North ForkCa<:he I<lPoudreRive.. Llvermo..e,Colorado 20,000 HAZARDSOFLARGEFLOOO$ The extent of damage cllused by IIny flood depends on the topog..aphy of the area flooded, depth IInd durlltl~n of flooding, velocity of flow, rate of ..Ise, developments on the flood plein, IInd amount of debris In the floodwllter. A loo-YOllr flood or a SOO-year flood on the Cacho la Pbud..e River and Its trlbutllrles In the study rellch would result in the Inundetlon of, lInd sub- sequent damage to residentlel p..operties and aS50Clllted develop- mentS<lswoll as public utilItIes and publlc rO<ldweys. Deep floodwllter flowIng et high velocity and cllrrylng floating debris woul~ create conditions h~zardous to persons and vehicles atTempting to cross flooded lI"eas. In generlll, floodwllter :3 or .."re feet deep IInd flowing 111' a velocity of :3 or more feot per second could eeslly 5WOOP ~duit per5Qns off their feet, Thus creating definite danger or Injury or drownIng. Rapidly risIng IInd swiftly flowing floo~- ~lIter m<lY trap persons In homes that lire ultimately destroyed or In vehicles th"t"re ultlmlltely suoo.ergedor flO>lted. Isolation of IIreas by floodwaters could creoto hazllrds In terms of medIcal, fire, orlllwenforcemonternorgencies. The Cliche 111 PoudreRlver In the PoudrePllrkll..ee, liS evidenced by tllbles 2 IInd 3, has not experienced II flood of magnl- tude slmll11rto the 100-Y8llr flood formllnyyellrs. I-low""var,strea", beslns In the gonerlll vlclnlty of The Cliche I'll Paudre River basin have experienced such floods in recent yellrs. The following tllble Indlcetes the magnitude of these discharges IInd the dr"lnllge area upstrellm from the point of discharge measurement. For comparison purposes, the CaChe la Poudre Rive.. at Poudre Park has 11 drainage area of 415 squII..e mi les upstreem from Fells GulCh <lnd a 100-yellr flOOd dIscharge 111' thllt locetlon of 16,300 c.f.s. T~ble 5 PeakDlschargeslnST~",81Islns in the Vicinity of the Cache I'll Poudro Rlv~r $trellm lOClltlon ~ ""V 1904 Drll I nllQO Area (sq. mi.) '" Dlschal"lJe (c.f.s.l Rapid Creek l! R<lpld City, SouthDllkota Above Drake, Colorado June 1972 July 1976 S2y 31,200 EllgTnompson River 180 31,200 FIOO<Jed areas and flood dam"ges - Water surfllce proflles for the floods studied In this repOrt were developed using the back.."ter cot:1i'uter program HCC-2, "water Surface I'roflles", develope<l by the U.S. Army Corps ot Engineers. The compufatlons were bllsed 1/ Located upstream from Canyon Lllke Dam. Z! Uncontrolled drainage "rea downstream trom Pactolll Dam. 11 11