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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />The gaging records show that floods the size of the May 1969 flood <br />occur on an average of about once every five years on Boulder Creek <br />and about once every seven years on South Boulder Creek." <br /> <br />The 1986 G&O study reports the 1969 event flow as 1690 cfs, and states that the <br />recurrence interval is approximately once every seven years. We obtained <br />calibration records for the Eldorado Gage. There were notes indicating that the <br />boulders used as a control sill in the bed of the river moved. <br /> <br />Ms. Smith reports extensively regarding this event. Numerous streams, including <br />Bear Canyon Creek, flooded. A photograph of South Boulder Creek by the Boulder <br />Daily Camera shows the wide floodplain of South Boulder Creek. Regardless of <br />location, flooding of this width would correlate to spills at Highway 93 and 36, <br />based upon hydraulic analysis by others and TEA. <br /> <br />In the Wright McLaughlin report Urban Storm Drainaoe: Maior Drainaoewav <br />Plannino, South Boulder, September 1970, it was reported that Highway 36 was <br />overtopped and traffic halted for more than a day" . . . in the Lower Viele Basin." <br />The study denotes the need for improved drainage facilities to take care of flow <br />from Viele and Anderson Ditch, and future areas of urbanization. <br /> <br />In our further research, we identified the CDOT project manager, Ken Morrow, who <br />observed the construction of the barrier. He was an eyewitness at the site during <br />the flood. He indicated that the barrier was overtopped by flood waters, without <br />damage, in the same area identified by Love and this study. Overtopping occurred <br />for a brief period, followed by gradual draining through the openings in the barrier <br />and the drainage network. Mr. Morrow reported that he was criticized by highway <br />safety critics for allowing the openings. <br /> <br />We believe it is highly likely that a combination of local drainage from the Anderson <br />Ditch tributary area, Viele, other local tributary areas, and spillage from South <br />Boulder Creek led to this overtopping. We doubt that local flooding alone could <br />lead to a shutdown in excess of a day. <br /> <br />GEOMORPHOLOGY <br /> <br />Figure 11-1 is a compilation of 1950 and 1942 USGS topographic maps of South <br />Boulder Creek for the reach of immediate interest. The topography of the valley <br />bottom illustrates alluvial fan tendencies as evidenced by the fan shaped contours <br />and numerous shallow stream patterns across the valley. We have delineated a <br />zone where it is physically possible for flood waters in the valley bottom <br />mainstream just above Highway 93 to disperse laterally across the valley bottom. <br />Note that the South Boulder Creek main channel is on the right or east side of the <br /> <br />11- 13 <br />