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<br />Available information indicates that no floods of consequence have <br />occurred since 1965. <br /> <br />2.4 Flood Protection Measures <br /> <br />Available information indicates that no structural or nonstruc- <br />tural flood protection measures exist for the TOwn of Larkspur. <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied by detailed methods in the community, <br />standard hydrologic and hydraulic study methods were used to determine <br />the flood hazard data required for this study. Flood events of a magni- <br />tude which are expected to be equaled or exceeded once on the average <br />during any 10-, 50-, 100-, or SOO-year period (recurrence interval) have <br />been selected as having special significance for flood plain management <br />and for flood insurance rates. These events, commonly termed the 10-, <br />50-, 100-, and SOO-year floods, have a 10, 2, 1, and 0.2 percent chance, <br />respectively, of being equaled or exceeded during any year. Although <br />the recurrence interval represents the long-term averaqe period between <br />floods of a specific magnitude, rare floods could occur at short intervals <br />or even within the same year. The risk of experiencing a rare flood <br />increases when periods greater than 1 year are considered. For example, <br />the risk of having a flood which equals or exceeds the 100-year flood (1 <br />percent chance of annual exceedence) in any 50-year period is approxi- <br />mately 40 percent (4 in 10), and, for any 90-year period, the risk <br />increases to approximately 60 percent (6 in 10). The analyses reported <br />herein reflect flooding potentials based on conditions existing in the <br />community at the time of completion of this study. Maps and flood eleva- <br />tions will be amended periodically to reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak discharge- <br />frequency relationships for each flooding source studied by detailed <br />methods affecting the community. <br /> <br />For East Plum Creek, the synthetic hydrograph method was used to <br />obtain the peak (rates of runoff). The analysis was based on a <br />storm duration of 24 hours and a Type II A distribution, as <br />described in the SCS National Enqineerinq Hand~ok (Reference 6). <br />The amount of rainfall was obtained from the National Oceanic and <br />Atmospheric Administration Precipitation-Frequency Atlas for <br />Colorado (Reference 7), and an areal adjustment was applied to <br />convert the point precipitation values to average precipitation <br />over the watershed area. Hydrologic soil cover complexes and <br />associated Runoff Curve Numbers were obtained from field investi- <br />gations, a soil survey of Castle Rock (Reference 8), an unpu- <br />blished SCS study of the area, and land use and natural plant cover <br /> <br />6 <br />