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I , Flood Se~son ~nd Flood Ch~r~cterlstlcs Flooding ;'Jlong the South Platte River In Weld County normally occurs from ~ey through ~ugust with the most frequent flooding occurring during June. Serious floods, however, have occurred from Febrvi'Jry through December. FlOOdS In Weld CounTy nannlll Iy occur under non~lce affected conditIons. ICB )i'lms, however, ;'Jre possIble even though their hlstorlclll frequency of occurrence Is .."re. Floodwi'lters on the South Platte River In Weld County Ci'ln resul1' from Intense localized rllI nhllI 01" g9nel";'J1 rein,>, either of which ~y be &ugmented by sno~lt. The Intense rainstorm floods resultfromlntenserelnfi'Jlloverel"ellsotrel"tlveIYsm!lllllrea' e~tent. These storms cen produce extremely rapId concentri'ltlons of floodweters nod ensu~nt rapid rls~s In stre~m lavels. Beceuse of lIttle warning time prior to streams ~ttalnlng flood stag~s, fhls type of storm can produce floods whiCh are extremely ha2ardous, This type of flood produces hIgh peak discharge on trlbutarl~s In the general vicinity of the rainstorm, Th~ discharge, however, att~nuetes rapidly on the South Platte River downstream from th~ contributing tributaries due to valley storage Of the floodwater. T~ general rainstorm floods result from rainfall over areas of large areal extent. R&lnfall from this type of storm may occur over severel d~ys. Because of thIs time duration, the affected str8~S rIse relatively slow; sometimes over a period of several days. The duratIon of the flood periOd also Is sometimes of considerable length. 8e<;auseot the largearMI extent of ralnfall,lIO;)retrlbutar/escon- tribute to the peak dIscharge on the South Platte River. This results In Increasing discharge progressively downstream over a considerable length of the South Platte River, However, downstream from the con- trIbuting tributaries, the effects of valley storage CauSe flood dls- charqe attenuation, Erosive effects Can be considerable In the event of any flood regardless of Its orIgin. Floods resulting from Intense rainstorms typically create severe erosion damages in a short perIod of time due to the relatIvely large peak discharge and high velocity of the floodwater, ErosIon damages from snowmelt or general raInsTorm floods usually result from prolonged exposure of the floodwater because of the long duration typ!cal of these floods. Intense rainstorm floods as ...ell as geMral rainstorm floods heve occurred On the South Platte RIver. Examples of the Intense storms ere the )965 events on Plum Creek and Bijou Creek and the 1976 event on the Big Thompson River. Examples of the larger more generall~ed storms are the 1969 and 197} events producing flooding on the South Platte RIver, Past FlOOds Several floods haveoccurrad alOftg the South Platte RIver in Weld County. Tables 3 through 6 Indicate yearly peak discharges experienced at the four stream gaging stations operated In the past by the U, S, Geological Survey In Weld County. 5 Flood Descriptions The following flood accounts are typical of flood events ...hich have occurre~ along the South Platte River in ~nd adjacent to Weld County, but are, by no means, a complete history of such. They are presented here to give some Insight as to the source and ~gnltude of flood...aters from the South Platte River affectIng Weld County, 1844 - Floodwaters purportedly covered the bottom lends In Denver vicinity from bluff to bluff, 1864 - Three separate floods caused by heavy rains over snow occurred during May and June On the South PI~tte River. A resi- dentofPlattevlllelaterdescrlbedthefl<X><lofmid-+layafth..t loc..- tiOft eS the largest flood he h..d observed prior to 1925, Flooding occurred In late May end again In early to mid-Juno. The South Platte River flood In Weld County was augment~d do...nstre&m from Denver by tributary Inflow fr~ both the mountain and plains tributaries. ,