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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 result from Intense localized rllI nhllI 01" g9nel";'J1 rein,>, either of which ~y be &ugmented by sno~lt. The Intense rainstorm floods resultfromlntenserelnfi'Jllovererellsotrel"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 at 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 at the floodwater. T~ general rainstorm floods result from rainfall over areas of large areal extent. R&lnfall from this type at storm may occur over severel d~ys. Because of thIs time duration, the affected streams rIse relatively slow; sometimes over a period of several days. The duratIon of the flood periOd also Is sometimes of considerable length. 8ecauseotthelargearaalextentofralntall,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 eftects Can be considerable In the event of any flood regardless at Its orIgin. Floods resulting trom 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 beceuse of the long duration typ!cal of these floods. Intense rainstorm floods as ...ell as geMral rainstorm floods have occurred On the South Platte RIver. Ex~mples of the Infense storms ~re the )965 events on Plum Creek ~nd Bijou Creek ~nd the 1976 event on the 61g Thompson River. Examples of the larger more generall~ed storms are the 1969 ~"d 197} events producing flooding on the South Platte RIver. Past FlOOds Sevo..~1 floodS have occurred along the South Platte RIver in Weld County. T~bles} through 6 Indicate yo~rly peak discharges experienced at the four streem gaging stetlons operated In the past by the U, S, Geological Survey In Weld County. 5 Flood Descriptions The following flood eccounts ere typic~1 of flood events ...hich have oocurre~ along the South PI~tte River in ~nd adjacent fo Weld County, but lire, by no me~ns, a complete history of such. They are presented here to give some Insight ~s to the source and magnitude of flood...aters from the South Platte River ~ffectlng Weld County, 1844 - Floodwaters purportedly covered the bottom lands In Denver vicinity from bluff to bluff. 1864 - Three separate floods caused by he~vy rains over snow occurred during M~y ~nd June on the South Piette River. A resi- dentofPlattevllleleterdescrlbedthefl<X><loftnid-+llryatth..t 10<:"- tiOft eS the largest flood he h..d observed prior to 1925. Flooding occurred In late May and again In early to mid-June. The South Platte River flood In Weld County w~s augmented do...n~tre&m from Denver by tributary Inflow from both the mountain end plelns tributaries. ,