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2.3 Principal Flood Problems The streams that flow into Standley Lake and Great Western Reservoir, along with Big Dry and Little Dry Creeks, are generally well defined with relatively narrow channels. The tributaries to Big Dry and Little Dry Creeks at time are not very well defined and are generally shallow with low banks. Industrial and residential developments already exist along portions of the streams, especially below the reservoirs. Generally, the streams are intermittent, having little or no flow most of the time. Flooding in the Westminster area is caused by heavy local rainstorms. Flooding occurred in May 1973, especially in the vicinity of Big Dry Creek at Huron Street. No other record of flooding could be identified during the reconnaissance and library searches. 2.4 Flood Protection Measures There are no flood protection works on the streams in the areas that were studied. However, two large reservoi rs, Standley Lake and Great Western Reservoir, exist at the upstream limits of the study area and provide incidental protection even though they were not designed for flood control storage. In addition, Ketner and Jackson Lake reservoirs exist within the study area but are primarily used for irrigation purposes and were, therefore, not considered as flood protection structures. Westminster in recent years, has passed zoning ordinances to prohibit construction within floodplains to lessen potential flood damage. Ketner reservoir has been renovated to increase its flood storage capacity, thus reducing the downstream flows and floodplains. 3.0 ENGINEERING METHODS For the flooding sources studied by detailed methods in the community, standard bydrologic and hydraulic 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 sOO-year period (recurrence interval) have been selected a shaving special significance for floodplain management and for flood insurance rates. These event s, commonly termed the 10-, 50-, 100-, or sOO-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 average period between floods of a specific magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood which equal s or exceeds the 100-year flood (1 percent chance of annual exceedence) in any 50-year period is approximately 40 percent (4 in 10), and, for any 90-year period, the risk 5