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<br />Removal of vegetation, sediment deposits, debris, and rocks from the floodways of Bear <br />Creek, Mount Vernon Creek, and Bear Creek Tributary No.7 would improve flood control, <br />but the problem of the inadequate capacity of the State Highway 8 box culvert would remain. <br /> <br />There are no flood protection works in the City of Westminster on the streams in the areas <br />that were studied. However, two large reservoirs, Standley Lake and Great Western <br />Reservoir, exist at the upstream limits of the study area and provide incidental protection even <br />though they were not designed for flood control storage. <br /> <br />In addition, Ketner and Jackson Lake reservoirs exist within the study area but are primarily <br />used for irrigation purposes and therefore, were not considered as flood protection structures. <br />Westminster has passed zoning ordinances to prohibit construction within floodplains to <br />lessen potential flood damage. The Ketner Reservoir has been renovated to increase its flood <br />storage capacity, thus reducing the downstream flows and floodplains. <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied by detailed methods in the community, standard hydrologic and <br />hydraulic study methods were used to determine the flood hazard data required for this study. Flood <br />events of a magnitude that are expected to be equaled or exceeded once on the average during any <br />10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as having special <br />significance for floodplain management and for flood insurance rates. These events, commonly <br />termed the 10-, 50-, 100-, and 500-year floods, have a 10-, 2-, 1-, and O.2-percent chance, respectively, <br />of being equaled or exceeded during any year. Although the recurrence interval represents the <br />long-term, average period between floods of a specific magnitude, rare floods could occur at short <br />intervals or even within the same year. The risk of experiencing a rare flood increases when periods <br />greater than 1 year are considered. For example, the risk of having a flood that equals or exceeds the <br />IOO-year flood (I-percent chance of annual exceedence) in any 50-year period is approximately 40 <br />percent (4 in 10); for any 90-year period, the risk increases to approximately <br />60 percent (6 in 10). The analyses reported herein reflect flooding potentials based on conditions <br />existing in the community at the time of completion of this study. Maps and flood elevations will be <br />amended periodically to reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish peak discharge-frequency relationships for <br />each flooding source studied by detailed methods affecting the community. <br /> <br />Jefferson County (Unincorporated Areas) <br /> <br />Detailed discussions of the hydrologic analyses for the streams studied were presented in <br />previous hydrology reports. The hydrologic analyses for Cold Spring, Kerr, Swede, and <br />Switzer Gulches, and the SleD 6200 North Tributary were discussed in a 1982 hydrology <br />report (Reference 39). The remainder of the streams was discussed in a four-part technical <br />addendum prepared in 1979 (Reference 40). <br /> <br />24 <br />