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<br />CHAPTER IV <br /> <br />HYDROLOGIC AND HYDRAULIC DETERMINATION <br /> <br />Flood Characteristics <br /> <br />Rainfall in the Boulder County a-rea is :f nfluenced by the orographic <br /> <br />effects of the Rocky Mountains, the topography of the high plains and the <br /> <br />semi.-arid climate of the region. <br /> <br />Rai.nstonl1fl can often have <'lfl "upslope" <br /> <br />character where easterly flow of moisture t::e:ttles against: the mountains. <br /> <br />These types of storms have durations that can exceed si.x nours and produce <br /> <br />large volumes of runoff. <br /> <br />Though this t)'p" of storm does not generally <br /> <br />promote flooding, when comhined with snOWlTll;: :_1: t severe flooding can occur. <br /> <br />Very intense rainfalJ in the Boulder area rE:sul ts from convection storms <br /> <br />or frontal stimulated convective storms. TheBe types of 13t:onns are often <br /> <br />less than two hours in duration and can produce br.ief periods of high <br /> <br />intensity rainfall. <br /> <br />This hfgh intensity, ::;f1ort duration storm, commonly <br /> <br />referred to as a cloudburst, predominate.::: <:lnlOng the llnon..upslopelT storms <br /> <br />in this area. Peak flooding will usually O<~['11r in Lyons 1ust a few hnurs <br /> <br />after the single rainfall event of this type. <br /> <br />Hydrologic Analysis <br /> <br />The hydrologic analysis for the entire St. \!tain (;reek basin was completed <br /> <br />by I:h" u. S. Department of the Army, Omaba lJistri d, Corp of Engineers <br /> <br />(Reference S). Discharges for the 10.., SO,." 100-, and SOO.'year frequencv <br /> <br />13 <br />