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<br />I <br />, <br /> <br />Flood Se~son ~nd Flood Ch~r~cterlstlcs <br />Flooding ;'Jlong the South Platte River In Weld County normally <br />occurs from ~ey through ~ugust with the most frequent flooding <br />occurring during June. Serious floods, however, have occurred <br />from Febrvi'Jry through December. FlOOdS In Weld CounTy nannlll Iy <br />occur under non~lce affected conditIons. ICB )i'lms, however, ;'Jre <br />possIble even though their hlstorlclll frequency of occurrence Is <br />.."re. <br />Floodwi'lters on the South Platte River In Weld County Ci'ln <br />result from Intense localized rllI nhllI 01" g9nel";'J1 rein,>, either <br />of which ~y be &ugmented by sno~lt. The Intense rainstorm floods <br />resultfromlntenserelnfi'Jllovererellsotrel"tlveIYsm!lllllrea' <br />e~tent. These storms cen produce extremely rapId concentri'ltlons of <br />floodweters nod ensu~nt rapid rls~s In stre~m lavels. Beceuse of <br />lIttle warning time prior to streams ~ttalnlng flood stag~s, fhls <br />type at storm can produce floods whiCh are extremely ha2ardous, <br />This type of flood produces hIgh peak discharge on trlbutarl~s In <br />the general vicinity of the rainstorm, Th~ discharge, however, <br />att~nuetes rapidly on the South Platte River downstream from th~ <br />contributing tributaries due to valley storage at the floodwater. <br />T~ general rainstorm floods result from rainfall over areas of <br />large areal extent. R&lnfall from this type at storm may occur over <br />severel d~ys. Because of thIs time duration, the affected streams <br />rIse relatively slow; sometimes over a period of several days. The <br />duratIon of the flood periOd also Is sometimes of considerable length. <br />8ecauseotthelargearaalextentofralntall,lIO;)retrlbutar/escon- <br />tribute to the peak dIscharge on the South Platte River. This results <br />In Increasing discharge progressively downstream over a considerable <br />length of the South Platte River. However, downstream from the con- <br />trIbuting tributaries, the effects of valley storage CauSe flood dls- <br />charqe attenuation. <br />Erosive eftects Can be considerable In the event of any flood <br />regardless at Its orIgin. Floods resulting trom Intense rainstorms <br /> <br />typically create severe erosion damages in a short perIod of time <br />due to the relatIvely large peak discharge and high velocity of <br />the floodwater. ErosIon damages from snowmelt or general raInsTorm <br />floods usually result from prolonged exposure of the floodwater <br />beceuse of the long duration typ!cal of these floods. <br />Intense rainstorm floods as ...ell as geMral rainstorm floods <br />have occurred On the South Platte RIver. Ex~mples of the Infense <br />storms ~re the )965 events on Plum Creek ~nd Bijou Creek ~nd the <br />1976 event on the 61g Thompson River. Examples of the larger more <br />generall~ed storms are the 1969 ~"d 197} events producing flooding <br />on the South Platte RIver. <br /> <br />Past FlOOds <br />Sevo..~1 floodS have occurred along the South Platte RIver in <br />Weld County. T~bles} through 6 Indicate yo~rly peak discharges <br />experienced at the four streem gaging stetlons operated In the <br />past by the U, S, Geological Survey In Weld County. <br /> <br />5 <br /> <br />Flood Descriptions <br />The following flood eccounts ere typic~1 of flood events ...hich <br />have oocurre~ along the South PI~tte River in ~nd adjacent fo Weld <br />County, but lire, by no me~ns, a complete history of such. They are <br />presented here to give some Insight ~s to the source and magnitude <br />of flood...aters from the South Platte River ~ffectlng Weld County, <br />1844 - Floodwaters purportedly covered the bottom lands <br />In Denver vicinity from bluff to bluff. <br />1864 - Three separate floods caused by he~vy rains over <br />snow occurred during M~y ~nd June on the South Piette River. A resi- <br />dentofPlattevllleleterdescrlbedthefl<X><loftnid-+llryatth..t 10<:"- <br />tiOft eS the largest flood he h..d observed prior to 1925. Flooding <br />occurred In late May and again In early to mid-June. The South Platte <br />River flood In Weld County w~s augmented do...n~tre&m from Denver by <br />tributary Inflow from both the mountain end plelns tributaries. <br />, <br />