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I I I I I I I I I I I I I I I I I I I 100-year discharge at Boulder from approximately 7,400 cfs to approximately 12,000 cfs. Table 1 summarizes the discharge values for the study reach, TARLE 1 BOULDER CREEK DISCHARGE VALUES THROUGH STUDY AREA Flooding Source: BouldAr CreAk Peak Discharge (cfs): 10-vr SO-yr 100-yr SOO-vr Sec. no, 13 3,000 8,200 11,800 21,200 Sec. no. 14 - 16.1 2,200 7,800 11,150 19,800 Sec. no. 16.2 - 19.1 2,200 7,800 9,650 19,800 Sec. no. 19.2 2,200 7,800 9,350 19,800 Sec. no. 20 - 20,1 2,200 7,800 9,050 19,800 Sec. no. 21 - 22 2,200 7,800 8,900 19,800 Note: These values represent basin runoff assuming future development conditions. Information for the Arapahoe Avenue Overflow is available only for the 100 Year flood. 3.2 Hvdraulic AnalvRis 3,2,1 General The spatial variation of flows in Boulder Creek through the study reach has been extensively analyzed for the 100 - Year flood in order to determine the extent of protection provided by the completed grading project. Flood profile elevations were computed using the U.S. Army Corps of Engineers HEC-2 water surface profiles computer program (Ref. 5). The City of Boulder exists upon an alluvial floodplain composed of moderately erodible strata. Earlier studies have shown that hydraulic modeling in the area must take into account upstream conditions as well as downstream conditions in the preparation of backwater curves. As flows enter either overbank of Boulder Creek they sometimes become hydraulically disconnected as they spill from the main channel. Such a condition exists upstream of the study area approximately one half mile to the west where floodwaters from the creek overtop Arapahoe Avenue on the north and are split off from the main body of water during the 100 - Year event (Arapahoe Avenue Overflow, sheet 5 to 8 of 19, Appendix 1, These flows travel down Arapahoe to the Love & Associates, Inc. Page 4