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I I il I I I I I I I I I I I I I I I I this pressure flow condition which causes overtopping of the Boulder Creek bank into the Research Park, To better model overtopping of the Lower Arapahoe Bridge, a bridge weir was described with BT cards at section 18, This weir extends across the bridge to join with the top of the right bank where the split flow description begins, As some of the water which overtops the bridge will be lost down Arapahoe to the east (towards section 16,1), a normal outflow consisting of a street section across Arapahoe was described between section 17 and 18, resulting in a loss of about 400 cfs. In addition, the data used to describe the bridge was modified to reflect a 450 skew across the channel at this point (Xl,8, X2,9) Sections 19 through 22, desCribe the remainder of the study reach upstream from the Lower Arapahoe Bridge. Sections 19, 20, 20.1, 21, and 22 were laid out on mapping created as0 a part of this study so that they are aligned as closely as possible with the sections of the original FHAD study. This was done to insure the greatest degree of consistency between this and the earlier study. Data points were carefully digitized to reflect the terrain, while ignoring local depressions and trash piles. These sections were then encroached on the right to the top of the bank to confine flows to the creek (Fig. 6). The left overbank is considerably higher than the right, and is hydraulically connected to the channel flows. Sections were added between 19 and 20 (19.1, 19.2) to increase the accuracy of the model. Flows were varied along this reach to reflect flows returning from the Arapahoe Avenue Overflow. This was done by reducing the available flow with the QT card upstream of the returning flows, For example, 300 cfs is returned to the creek above section 19.1, so the QT card in front of section 19.2 is reduced from 9,650 to 9,350 cfs (Appendix 2a). Total available flows were varied in accordance with the published FHAD, A split flow weir along the right bank was described between the Arapahoe Avenue Bridge (section 18) and Potts Field (section 21) to reflect the loss of water spilling into the Research Park from the main channel (Figures 4a and b). The split flow line generally follows the top of the bike path (Fig. 5c), and weir coefficients were varied according to the degree of submergence encountered (Ref. 7). Results of this analysis showed that over 2,500 cfs is lost from the main channel and flows across the UCBRP along the study reach during the 100 - Year flood (Table 3) . Love & Associates, Inc. Page 8