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<br />5 <br /> <br /> <br />Flood Season and Flood Characteristics <br />Flooding along the South Platte River in Weld County normally <br />occurs from May through August with the most frequent flooding <br />occurring during June. Serious floods, however, have occurred <br />from February through December. Floods in Weld County normally <br />occur under non-ice affected conditions. Ice jams, however, are <br />possible even though their historical frequency of occurrence is <br />rare. <br /> <br />The flood characteristics of the basin also vary with the <br />physiographic features of the regions. In the mountain regions, <br />floods result from rapid snowmelt or sudden intense rainfall. The <br />rainstorm floods are characterized by rapid concentrations of run- <br />off which result in sudden rises in stream stages and produce the <br />high velocity flash floods which are particularly hazardous to life <br />and property. The foothil Is regions have flash floods similar to <br />the mountain regions, but generally reflect a greater hazard <br />because dry washes become raging torrents with high velocities <br />that erode the normal boundaries of streamflow. As the foothil Is <br />streams converge to form the main tributaries and tributary streams <br />converge on the main stream, the valleys and flood plains become <br />broader. The broader valleys and flatter stream slopes reduce the <br />discharge rate of the flood by combinations of temporary overbank <br />storage and lower energy gradients. The lower velocities Increase <br />the duration of flooding and permit the sediment load and debris <br />load carried from upstream areas to be deposited on the more pro- <br />ductive lands of the lower valleys. Consequently, the damaging <br />effects In the lower valleys are a combination of destruction by <br />erosion and inundation with reduction in the productive quality of <br />valley lands by flood debris and sediment. The damaging effects <br />of floods are also governed significantly by the nature of storm <br />patterns and tributary runoff. Rainfal I concentrated over one <br />tributary watershed generally limits the destructiveness of floods <br />to the tributary stream affected. However, the runoff from larger <br />