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<br />~ <br /> <br />Required Maintenance <br />Removal of sediment after each flood. <br />Repair of channels and spillway. <br /> <br />Levees and Charrnels <br /> <br />Susceptibi 1 i ty <br />Unarmored levees and channels are extremely <br />vulnerable to scour damage. and potential <br />failure due to velocities exceeding 20 feet per <br />se.cond. Levees on Rancho Mi ra.ge and Palm <br />Desert fans have been breached qy floods. <br />Physical model tests (Part II. Section 4.5) <br />show rapid erosion and undercutting of <br />unarmored levees. <br />~ Riprap lined channels and levees resist <br />moderate floods but may be severely damaged by <br />large events as shown in the physical model <br />studies. <br />Sediment deposition may cause channel blockage <br />and levee overtopping where abrupt decreases in <br />velocity occur. <br />Upstream debris basins or detention dams will <br />substantially reduce risks of overtopping or <br />sediment deposition, but may increase scour <br />problems. <br /> <br />Effectiveness <br />When <br /> <br />properly <br /> <br />designed, <br /> <br />armored, <br /> <br />and <br />flood <br /> <br />maintained, <br /> <br />channels <br /> <br />eliminate all <br /> <br />ha=ards on the fan. <br />Since velocities are extremely high in the <br />channels, safety problems are important. <br />A levee/channel system must be continuous from <br />apex (or debris basin if used) to the toe of <br /> <br />78 <br />