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, , . " '. . ' areas east of the foothills receive more precipitation (a few tenths of an inch per event) than the areas adjacent to the foothills. The occurrence of precipitation varies; however, most of the rainfall is concentrated in May. Thunderstorms also occur irregularly throughout the summer months. The temperature extremes at Boulder are evidenced by mean maximums ranging from 46.30 in January to 88.30 in July, and by mean minimums ranging from 21.60 in January to 61.00 in July. 2.3 Principal Flood Problems Flooding in Boulder County is due mainly to snowmelt combined with heavy rainfall, although heavy rainfall, especially in the form of cloudbursts, are alone capable of causing flooding. Floods caused by rainstorms can peak within only a few hours after the rain, leaving little time for evacuation. Much of the floodplain is used for agriculture, thus the flood damage is mainly to crops, i rrigat ion equipment, roads and bridges. Damage to resident ial and industrial areas occurs in and around several communities which are excluded from the study area. However, mllny of these communities are experiencing rapid growth, and thus are increasing encroachment on the adjacent floodplains. There are also undefined, very localized zones of velocity hazard throughout the studied reaches of Lefthand Creek. Generally, these zones are in the channel and near bridges. Significant floods occurred in the South Boulder and Boulder watersheds in 1894, 1914, and 1938; and a somewhat smaller occurred in 1938. Flooding occurred in the Lefthand watershed in 1864, 1876, 1894, 1921, 1938, 1949, and 1951. Creek fl ood Creek 2.4 Flood Protection Measures There were no structural flood protection measures at the time of the initial study in Boulder County. Presently, Barker Reservoir on Boulder Creek and Cross Reservoir on South Boulder Creek are far enough upstream from the areas studied by detai led methods that their flood protection effects are negl igible. Since 1971, Boulder County has undertaken an active program of floodplain management. 3.0 ENGINEERING METHODS For the flooding sources studied by detailed methods in the community, standard hydrologic and hydraul ic study methods were used to determine the flood hazard data required for this study. Flood events of a magnitude which are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as having special significance for floodplain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively, of being equaled or exceeded during any year. Although the recurrence interval represents the long-term 5