2011-06-30_REVISION - M1981302 (12)

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Flood Control Levee August 24, 1999 Page 1 The h drograph for the 100-year flood at US Highway 36 is shown on Figure 7, and Table summarizes the peak flow and flow on the overbank for the 10-7 --, 100 -,, and Soo -year floods. The 10- - 100- and 500-year flood profiles are shown on Figure 2 of the 1 986 G&O report (Appendix . Flood Plain Lfnit The 100-year base flood plain is defined by the 1986 Go flood -plain study. It is shown on Drawings 9A through 12 of 31 of the 1986 G&O report (Appendix . It is also shown on the current FENIA FIRM (Appendix , and on the attached Figure 3 prepared by the Urban Drainage and Flood Control District. Duration One of the most important factors in determining the effectiveness and safety /stability of flood control levees is the amount of time that a flood would be against the levee. It affects both the potential for erosion of the levee, and the potential for seepage through it, Figure 7 is the hydrograpi reportedly used by the USAGE and by G &o in the 1986 study. It is believed to have been generated by the USA E, and it was obtained from Taggart. It has a double peak; the first and highest peal results from runoff below Gross Reservoir, oir, and the second and lower peak is from the basin above Gross Reservoir, ervoir, which 1s attenuated as it passes through the reservoir. In the 1973 R.W. Beck study, the 1 00-year flood hydrograph had only one peals because the flood above Gross Reservoir was assumed to be completely stored in the reservoir, The flood rises and falls steeply. The attached Table 3 and Figure 8 summarize the time %n hours between flogs of various magnitude, ranging from no flow up to 5,900 efs. Based on the information presented M the G&O report, it appears that the l 0-year flood is contained within the South Boulder Creek channel. The I 0-year flood is about 1,750 efs* It is therefore probable that flows must be greater than about 1,75 o efs before any significant flow would occur against the levee. It is estimated that there would be some flow against the levee for a duration of about 1 I to 19 hours . during the loo -year base flood. This includes tin for the flood to rise up the face of the levee and LEONARD RACE CONSULTING WATER TER ENGI EER , INC.