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<br />of one foot would correspond to the deposition or removal of a large volume of <br />material. In reality, scour and deposition does not occur uniformly over the <br />entire cross section, so there may be local areas where the bed change is much <br />greater than the average bed change for the cross section. <br />Under natural existing channel conditions, the river bed appears to be <br />reasonably stable for the three single-event floods evaluated. The highest <br />computed average channel veloci ty during the SPF simulation was 6.6 feet per <br />second and occurred at section 1199.3. This happens because the present <br />channel is restricted at that location by an old levee structure. During the <br />simulated SPF, the maximum end of flood scour (-1.3 feet) occurs at cross <br />section 1199.7 (see Table 5.3). Maximum deposits of 1.0 foot (at cross section <br />1197.9) and 1.3 feet (at cross section 1201.0) were present at the end of the <br />SPF event. Therefore, under present channel conditions, approximately one <br />foot of deposition occurs just downstream from the King Arroyo. Table 5.1 <br />through 5.3 also list the conputed water depths at the peak flow of each of <br />the three flood events. For present channel conditions the deepest flows <br />occurring near La Junta are 10.6 feet deep occurring at cross section 1199.2 <br /> <br />5.5.2 Proiect Conditions <br /> <br />Project #3 includes the presence of levees 1 and 3 as shown in Figure <br />5.1. Project #4 includes levees 2 and 3. The anticipated results for <br />Project #3 conditions are shown on pages E-3, E-4, E-9, E-lO, E-15 and E-16 in <br />Appendix E. Pages E-5, E-6, E-ll, E-12, E-17 and E-18 show results for <br />project #4. <br /> <br />General trends indicated slight scouring between river miles 1199.5 and <br />1200.5 (in the upstream portion of the project reach). Deposition occurred at <br />river mile 1199.2 at the beginning of the south levee (levee 3). Deposition <br />occurred in this location due to a rapid decrease in the conveyance area <br />creating a backwater effect due to the constriction of the levees. Uniform <br />scour on the order of two feet occurred from river mile 1198.9 to 1196.9, due <br />to the high flow velocities through the constriction created by the levees. A <br />large amount of depostion occurred near river mile 1196.7 on the downstream <br />end of the project. This deposition (on the order of 2 to 3 feet) was caused <br />by a sudden expansion of the flow resulting in a rapid decrease in velocity <br />and thus a decrease in transport capacity. Data from Appendix E are presented <br />in Tables 5.1, 5.2 and 5.3 for the 50-year, 100-year and SPF flood events, <br />respectively. Also changes in bed elevation, the maximum velocity and water <br />depth computed during a particular flood are given for each cross section. <br /> <br />The figures in Appendix F show the depths of deposit or scour that might <br />occur over time during the standard flood event. This information is <br />important because it shows the simulated change in bed elevation at the time <br />of the peak flow (Time " 2 days) as well as the final bed profile after the <br />event has passed (Time"" 5 days). Most of the scour in the project reach <br />occurs just prior to the peak flow so there may be a small factor of safety <br />associated with levee heights designed using a stationary bed model such as <br />HEC-2. In areas where deposition occurs, however, design levee heights <br />may be insufficient. <br /> <br />The maximum computed scour depths within the proposed project reach (for <br />the project #4 configuration) were on the order of approximately 1.7 feet at <br />cross section 1199.5. Scour depths of only 0.6 feet were computed at the same <br />cross section for the project #3 configuration. <br /> <br />57 <br />