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PREFACE . ThU Ile.pOlt.t deJJcJUbeJJ the 6.lood chM.a.cteJLi.6.ti.C6 06 VIly CJr.eek .in ~teIut Bouldell. Coun-ty and houthweJJ.teJtn Weld Coun-ty, Co.lolULdo. The. htudy Il.ea.c.h ex.te.ncU, 61l.Om Bouldell. ReJJeJlvo.ur. .loca.ted abou;t 3 mi1.eJJ noJL:th.eal,.t 06 BouldeJl, CololULdo doWlt6:tJr.emn to :the c.on6tuenc.e 06 Vll.y Cll.eek and St. VIUti.n Cll.eek .loca.ted about 3 mi1.eJJ ~t 06 Longmont, Co.loJta.do. Land u.6e a:tong t:h.U h:tJr.emn .u. pIWnaJr.il.y aglLi.c.ul.twu:tt. ThU Il.epoltt Iltth p1l.epMed 6011. the gtUda.nce 06 loc.at 066-<-- c..i.a.f.t. .in ptann.i.ng :the u.\e and ll.egul.a.t.i.on06 :the 6tood p.ea.in. FoWL poten.ti.a.t 6.lo0cU well.e Med to ll.epll.eJJent degll.eeJJ 06 majoll. 6.loocU.ng .that may occ.ulr. .in :the 6u.tuJte. The.6e 6.lo0cU, :the 10-ye.M, 50-ye.M, 100-yeaJt, and 500-yeaJt, Me de6.ined .in :the GtohhMy and hhould be g.iven app1l.OpJUa.te c.onl,.(.deJul.ti.on .in pta.nn.i.ng 6011. .6a6e.ty 06 devel.op- ment .in :the 6lood plLUn. The 100-yeM and 500-ye.M 6.loocU Me 6WLthell. de6.ined by 6lo0ded Mea. map" .that .6how :the app1l.OUmaU M~ .that would be .inwula.ted. F.lood p1l.06.il.eJJ hhow .the wa..:teIL depth4 06 :the 10-yeaJt, 50-yeaJt, 100-yeaJt, and 500-ye.M 6.loocU Il.el.a.:tive to :the htltemnbed and ~ell. hWL6ace el.eva..ti.o/1.6 06 :theJJe 6.lo0cU acJLOh.6 :the UlUth 06 :the va.Uey. CltOhh hec.tlo/1.6 Me p1l.eJJented to .i.n.cUca.te g1l.Ound level. acJLOhh :the va.Uey and :the. oveJtty.ing 6.lood deptM 06 :the. 100-yeaJt and 500-yeaJt 6lo0cU. The. 6lo0d p1l.06-UeJJ and 6looded Mea. da.ta. pll.eJJ ented Me bt16 ed 0 n :the. ex..u..ti.ng co n.d.i..ti.o /1.6 0 6 :the bt16.in, .6t7lemn, and va.Uey when :the Il.epoltt WM p1l.epMed. POhh.i.ble 6u:tu1r.e .unp1l.Ovemena to c.ontltOl (,toocU Me no.t a CO/l.6.i.dVUl..ti.on 06 t:h.U Il.epoltt. The .in601Ulla..ti.on .in t:h.U Il.epoltt doeJJ not i.mpty any FedeJWt .i.n.teJleJJt 01l. a.u.tholli.tJj to zone 011. Il.egul.a..te u.6e 06 :the (,tood pta.i.n; t:h.U ..u. a .loc.at ll.eJJpOn.6.i.b..i.Uty. The Il.epoltt p1l.Ov.i.deJJ a huU- able bt16..u. 601l. :the. adop.ti.on 06 land u.6e contlwU to guide 6.lood pta.i.n development wUh c.on.6.i.dCWttion 601l. env.iJwnmentat a.t:tJt.i.buteJJ, :thell.eby pll.eve.n:ti.ng ..i.n..te.n.6.i6.i.c.a..ti.on 06 lOM p1l.Oblemh. Beca.uhe:the Il.epoltt .i.de.n:ti.6-<-eJJ 6lo0d p1l.Obtemh, U w.U1. h.timula..te. :the development 06 o:thell. (,tood damage Il.educ..ti.on tec.h.nI.queJJ .6uch t16 6lood contJt.Ol, Il.emovat 06 obhtJulc..ti.on.6, and 6.lood p1l.006.ing; :theJJe .techn.i.queJJ m.i.gh;t be Med .in an oveJLa.U F.lood PWn Management (FPM) pIl.ogJtam. A.t :the ll.eqUeJJt 06 Bouldell. County and We1.d Coun-ty, .th1l.ough. :the Co.loJta.do Wa.:teJr. COn.6eJl.va..ti.on BoCVLd, t:h.U Il.epoltt Wt16 pIl.epMed by :the. Omaha. V.u..tILic.t, COll.ph 06 Eng.ineeM, Wldell..the cont.i.nu..i.ng a.u:tholl.- .i..ty p1l.Ov.ided .in Sec..ti.on 206 06 :the 1960 Flood ContltOl Act (Public. Law 86-645), t16 amended. The. c.oopell.a..ti.on 06 loc.at 066.<.cia.tt, and public. and plli.vate .i.n.tell.eJJU .in p1l.Ov.i.cUng t16h.u.tanc.e and .in601Ulla.ti.on Wt16 mOht hel.p6ul. V.u.:tJr.i.bu:Uon 06 :the Il.epoltt to 066-<-c..i.a.f.t., agenueJJ, and blc:U.V.idlln.t6 conceJtned wUh ptann.i.ng .in .the. Mea. coveJled w.U1. be made by Bouldell. Coun-ty and by We1.d County. The C01l.ph 06 Eng.ineeM w.U1. p1l.Ov.i.de .i.n.tell.p1l.e.ta.ti.on and .tec.Jmi.c.at t16h..u.tanc.e, .i6 ll.equeJJte.d, .in :the appUc.a.ti.on 06 :the Il.epoltt da.ta.. O:theJl guideUneJJ ava.il.a.ble 61tOm .the COll.p.6 06 Eng.ineeM Me a pamphlet, "GuideUneJJ 6011. Reduc.ing F.lood VamageJJ," and a booklet, "lnt1tOduc.ti.on to F.lood P1l.006.i.ng." .. .i CONTENTS Pa~e Preface .i ...................................................... .. Background I nformat T on .............................................................................. 1 5ett I ement ............................................................................................ 1 The Stream and I ts Va II ey ............................................. 1 Development on the Flood Plain .............................. 3. flood Situation.......................................................... 10 Sources of Data and Records ............................... 10 Flood Season and Flood Characteristics .................. 10 Factors Affecting Flooding and Its Impact ............... 10 Obstructions to floodflows ......................... 10 Flood damage reduction measures .................... 11 Other factors and their Impacts .................... 12 Flood warning and forecasting ...................... 12 Flood fighting and emergency evacuation plans ....., 13 Material storage on the flood plain ................ 13 Past Floods ............................................................ 14 Summary of Historical Floods ............................ 14 Flood Records......................................................... 14 Flood Descriptions........................................... 14 May-June 1894 ...................................................... 14 June 1921 ................................................... 14 August-September 1938 .............................. 15 August 1951 ................................................ 15 May 1 969 .............................................. 15 May 1973 ........................................... 18 Future Floods............................................................ 19 Frequency ........................................................... 20 Hazards of Large Floods ........................................... 21 Flooded areas and flood damages .................... 21