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<br />This distribution clearly shows the brunt of the storm occurs within the first 15 to 20 minutes of <br />the event. The maximum duration of the storms reported by Livingston and Minges was <br />approximately 55 minutes. This helps to justify using a short, intense storm pattern. <br /> <br />The average distribution is only 45 minutes in duration. For the analysis to remain consistent, <br />the 100-year, I-hour depth should be adjusted slightly to match to duration of the distribution. <br />The NOAA Atlas 2 provides reduction factors to the I-hour depth to estimate depths with <br />duration less than one hour. Interpolating between the suggested values (Figure 4), the I-hour <br />depth should be reduced by a factor of 0.91 to be compatible with the 45-minute storm <br />distribution. <br /> <br />1.00 <br /> <br />0.90 J ~-----__--__--m_--___m__---__m________m____m_______m___m__m___m___, <br />" , <br /> <br />1: <br />0.80 : <br />: <br />, <br />, <br /> <br /> <br /> <br /> 0.70 <br />~ <br />0 0.60 : <br />- <br />0 <br />co <br />u. <br />r:: 0.50 <br />0 <br />:; <br />0 <br />::l 0.40 <br />'C <br />., <br />0:: <br /> 0.30: <br /> 0.20 - <br /> 0.10 .' <br /> 0.00 <br /> 0 <br /> <br /> <br />, <br /> <br />I <br />i <br />, <br />: <br />: <br />[ <br />, <br />-- .-~~-- <br /> <br />10 <br /> <br />20 <br /> <br />30 <br />Storm Duration (mln) <br /> <br />40 <br /> <br />50 <br /> <br />60 <br /> <br />Figure 4 -- Reduction factors for the I-hour depth for durations less than one hour. (Adapted from Miller, 1973) <br /> <br />3,2 Paleoflood analysis <br /> <br />Floods transport vast amounts of sediment and debris. As the floodwaters recede from the peak, <br />the mobilized sediment begins to fall from suspension to form slack-water deposits of sand and <br />silt and bouldery flood bars. The flood damage and these flood deposits (referred to as <br />paleostage indicators [PSI)) can be used to estimate the maximum stage of past floods within <br />:t5 percent. (Jarrett, et ai, 1998). In semi-arid regions, woody debris and damage to plant life can <br />be preserved for decades. Non-inundation surfaces, where there is little or no evidence of <br />alluvial deposits, are important in estimating the probable upper limit of past flooding (Jarrett <br /> <br />6 <br /> <br />/ <br />