Laserfiche WebLink
Flood Control Levee August 24, 1999 Page 1 <br />time for it to fall back down again. There are only about 10. 8 hours when the 1 -year hydrogaph <br />is above 2,000 efs, 7.3 hours when the flow is above 3,000 efs, and only about two hours when the <br />flog is above 42000 cfs. The entire 100 -year flood, from Mart to wish (from zero back to zero cfs) <br />lasts only 2.5 days hours). <br />The short duration of flooding is a significant factor in the stability and safety of the levee. For <br />example, CTL /Thompson, Inc. estimated the extent of seepage through and under the levee using <br />a transient fm te element computer model. CTL estimates that during the 100-year flood, <br />t 't 5 <br />... seepage progresses approximately one to two feet into the levee." <br />FREEBOARD <br />In addition to the two official flood studies RW Beek and G &, two independent flood profile <br />WC-2) studies were completed by others in this reach of South Boulder Creep in 1 996. The Love <br />study was done for the University prior to its purchase of the land from entities affiliated with the <br />Flatiron Sand and Gravel Company, and the Taggart study was done for the l roan Drainage and <br />Flood Control District after are error in delineation of the G &o flood plain at US Highway <br />(downstream of the levee) was identified by Love. <br />Neither ofthese studies 'indicated that the levee would be overtopped in the upper or middle reaches. <br />Both of them, however, indicated that there were a few locations along the levee where the <br />0 rrr�Vrnurn three feet of freeboard was not then available. Freeboard is the difference in height <br />between the base flood elevation and the top of levee elevation at a given location. The main reason <br />why these to studies produces less freeboard than indicated by the G&O study is the alignment of <br />the cross sections drawn by the engineers. In any case, the Flatiron Sand and Gravel Company <br />desired that there be adequate freeboard no matter which 1 -year flood profile is referenced or <br />becomes the base flood profile in the future). Therefore, the Flatiron Sand and Gravel Company <br />increased the height of the levee where necessary between SCA stations 105+00 and 150+70 to <br />achieve a rninl of four feet of freeboard over the highest of two of the three 1 -year flood <br />profiles which presently exist* <br />the 1986 Greenhorne and "Mara profile (same as the present FED base -flood profile); <br />and <br />the 1996 Taggart Associates, Inc. flood profile. <br />The freeboard for these two flood profiles is shown on Table 5 and on Figure 6. Since the profile <br />from the Love study 'is believed to have produced a flood profile similar to the Taggart study, we <br />'Page 10 of CTL /Thompson, Inc.'s August 6, 1 999 report, "Levee at Dee P e Farm Pit, <br />Boulder,, Colorado' (Appendix . <br />y LEONARD RICE CONSULTING WATER ENGINEERS, INC. <br />