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I I I I I I I I I I I I I I J I I I I The following section briefly describes some of the major storm events that have caused flooding Dn South BDulder Creek (Reference 2). Due to the relatively undevelop~d nature of the South Boulder Creek basin, information and observations related to major flood events is somewhat lacking in comparison to the adjacent, more heavily developed Boulder Creek floDdplain. crop losses were suffered on lands outside the floodplain which were dependent on irrigation diversions from Boulder Creek. 3l August - 4 September, 1938 FLOOD of 29 May - 2 June, 1894 This storm produced general rains over all of eastern ColoradD. The largest amounts of precipitation occurred in the mountains where more than 6 inches was reported west of Eldorado Springs. Eldorado Springs recorded 4.42 inches of rainfall. Approximately 80 percent of the total precipitation falling in the South Boulder Creek basin fell in the late afternoon and evening of September 2nd. The resulting flood, with a peak discharge of 7,390 cfs passed through Eldorado Springs at lO:OD p.m. on September 2nd. The resort community of Eldorado Springs suffered heavy damage and numerous buildings were destroyed when flood waters eroded their foundations. The valley from Eldorado Springs to Boulder Creek and down Boulder Creek to St. Vrain Creek was in shambles. This flood is the highest recorded flood on South Boulder Creek. Heavy ra i ns fe 11 over the mounta i ns extend i ng from the Co 1 orado-Wyomi ng border southward into the Republican and Arkansas River basins. Rainfall over the Boulder and South Boulder Creek basins was particularly heavy. Rainfall records for a 96-hour period ending at 3 a.m. on 2 June, 1894 indicate that the mountain drainage area received from 4.5 to 6.0 inches of precipitation. Rainfall amounts over the high plains gradually decreased from west to east and varied from 5 inches at Boulder to approximately 2.5 inches at the mDuth of Boulder Creek. The mountain rainfall combined with the snowmelt runoff produced the greatest flDOd known at Boulder, inundating the valley during the night of 30 May, 1894. Buildings, bridges, roads, and railroads were washed away. Computations made l8 years later produced estimates of the peak discharge on Boulder Creek in Boulder that ranged from 9,000 cubic feet per second to 13,600 cubic feet per second. The EldDradD Springs stream gaging station was not functioning during this flood event. 4-8 May 1969 In Boulder, floodwaters covered the entire area between Water Street (Canyon Boulevard) and University Hill to depths as great as ei9ht feet. Every bridge in Boulder and a number of residences were swept away. Other types of damage included commercial establishments, pUblic utilities, railroad property, roads and streets, and irrigation structures. Many people were trapped in their homes and had to be rescued. Only one life was lost; this was due, in part, to the flood's slow onset. Flooding in May of 1969 resulted frDm a long duration general storm. Runoff resulted from a combination of rainfall and snowmelt, with the heaviest preci- pitation reported in the mDuntains. Rainfall continued at a moderate rate for nearly four days. Total precipitation for the storm amounted to 7.60 inches at Boulder and 9.34 inches at the Boulder Hydroelectric Plant on Boulder Creek located about 3 miles up the canyon from Boulder. Precipitation amounts totalled 8.11 inches at Eldorado Springs and 10.05 inches at Gross Reservoir on South Boulder Creek. Peak flooding occurred on the 7th of May at Boulder and Eldorado Springs. A peak discharge of l,690 cfs occurred on South Boulder Creek at Eldorado Springs. Large portions of the floodplain were inundated belDw the confluence of South Boulder and Boulder Creeks. In the valley, downstream from BDulder, the floodplain was reported to have been inundated to an average width of approximately one mile for several days. Agricultural damages included loss of livestock, crops, pastures, fences, roads and deposition of sand and silt on floodplain lands. In addition, considerable Analysis of gaging recDrds show that floods the size of the May 1969 flood OCcur on an average of about once every seven years on South Boulder Creek (Reference 2) . 5