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Notation <br />U/S Levee <br />Hydrograph <br />(Levee Station <br />137 +75) <br />Middle Levee <br />Hydrograph <br />(Levee Station <br />128 +92) <br />D/S Levee <br />Hydrograph <br />( Levee Station <br />118 +33) <br />Full 100 -Year <br />Flood at US <br />Highway 36 <br />Hours <br />CFS <br />Hours <br />CFS <br />Hours <br />CFS <br />Hours <br />CFS <br />Base of <br />Hydrographs <br />14 <br />0 <br />14 <br />0 <br />14 <br />0 <br />14 <br />- <br />5.9 <br />250 <br />5.8 <br />250 <br />8.3 <br />250 <br />12.9 <br />250 <br />2.2 <br />400 <br />4.1 <br />400 <br />5.8 <br />400 <br />12.2 <br />400 <br />3.5 <br />500 <br />4.9 <br />500 <br />11.9 <br />500 <br />2.5 <br />750 <br />3.3 <br />750 <br />10.8 <br />750 <br />1.7 <br />1,000 <br />2.1 <br />1,000 <br />10.5 <br />1,000 <br />0.9 <br />1,200 <br />0.8 <br />1,200 <br />8.7 <br />1,200 <br />Peaks of <br />Hydrographs <br />0 <br />443 <br />0 <br />1,275 <br />0 <br />1,236 <br />0 <br />7,241 <br />Levee are shown in Figures 27 through 29. We assumed that the three floods adjacent to the <br />Levee also had durations of 14 hours. <br />IX.2. Duration of 100 -Year Flood Flows <br />One of the most important factors in determining the effectiveness and safety /stability of any <br />flood control levee is the amount of time that floods will be against them. It affects both the <br />potential for erosion of the levee, and the potential for seepage through the levee. <br />The main flood and the flood against the Levee rise and fall steeply (see Figures 26 through 29). <br />Steep rising and falling limbs of flood hydrographs make for short durations. The short duration <br />of flooding is a significant factor in the stability and safety of the Levee. For example, in 1999, <br />CTL /Thompson estimated the extent of seepage through and under the Levee using a transient <br />finite element computer model. During the 100 -year flood, ... <br />"... seepage progresses approximately one to two feet into the Levee. " <br />The table below summarizes the duration for various flows during the 100 -year flood at three <br />locations along the Levee, as well as for the main flood. <br />Table 5- Duration of 100 -Year Flood Flows <br />These short durations contribute to the safety of the Levee. <br />© Leonard Rice Engineers, Inc. <br />Report in Support of the Recertification of the <br />University of Colorado 's Flood Control Levee <br />Page 14 <br />1 CTL /Thompson, Inc., "Levee at Deepe Farm Pit, Boulder, Colorado," Sept. 28, 1998, page 10 (Appendix B -9). <br />June 2009 — 207FIP05 <br />