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<br />past flooding could not be identified during the reconnai~sance and <br />library searches. It is likely that severe flooding has occ~rred <br />on the study streams which was not recorded since much of the <br />development on the streams did not occur until recently. <br /> <br />2.4 Flood Protection Measures <br /> <br />Flood protection measures on the study streams consist primarily of <br />channelization projects and improvements to a number of important <br />bridges. Future protection measures on the streams have been <br />designed, as outlined in the UDFCD plans entitled Maior Drainagewav <br />Planning Study for Lower Ralston/Van Bibber and Levden Creeks, <br />(Reference 1), hereafter referred to as the Major Drainageway <br />Planning Report in this study. These future measures will consist <br />of additional channelization and the use of detention ponds. <br /> <br />A number of reservoirs exist on the study streams which provide <br />incidental flood protection, even though they were not specifically <br />designed as flood control structures. Arvada Reservoir, which is <br />owned and maintained by the City of Arvada, and Ralston Reservoir, <br />which is owned and maintained by the Denver Water Board are located <br />on Ralston Creek. Lake Arbor, which is located on Tributary C of <br />Little Dry Creek, and the Pomona Lakes are located on Little Dry <br />and are designated as recreation facilities that are owned and <br />operated by the ci ty of Arvada. Leyden Lake, located on Leyden <br />Creek, is currently a water storage facility owned and operated by <br />the Farmers High Line Canal Company. All of the above reservoirs, <br />with the exception of Leyden Lake and Arvada Reservoir, are <br />expected to attentuate the 100-year frequency event. In addition, <br />the City of Arvada has provided assurances to FEMA that these <br />reservoirs, with the exception of Leyden Lake, would continue to be <br />maintained and operated. <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied in detail in the community, standard <br />hydrologic and hydraulic study methods were used to determine the flood <br />hazard data required for this study. Flood events of a magnitude which <br />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 <br />selected as having special significance for floodplain management and <br />for flood insurance premium rates. These events, commonly termed the <br />10-, 50-, 100-, and SOO-year floods, have a 10, 2, 1, and 0.2 percent <br />chance, respectively, of being equaled or exceeded during any year. <br />Although the recurrence interval represents the long-term average period <br />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 <br />flood increases when periods greater than 1 year are considered. For <br />example, the risk of having a flood which equals or exceeds the 100-year <br />flood (I-percent chance of annual exceedence) in any SO-year period is <br />approximately 40 percent (4 in 10), and for any gO-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 /> <br />5 <br />