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DRAINAGE CRITERIA MANUAL MAJOR DRAINAGE . 5.4.4 Channel Bends The potential for erosion increases along the outside bank of a channel bend due to the acceleration of flow velocities on the outside part of the bend. Thus, it is often necessary to provide erosion protection in channels which otherwise would not need protection. In erosion resistant soils, no extra protection is required by these criteria along bends where the radius is greater than 2 times the top width (as measured for the major flows) but in no case less than 100 feet. . For bank protection requirements in sandy soils, consult the specific criteria for channels on sandy soils. However, for channels in erosion resistant soils not requiring riprap protection along straight sections, channel bends with radii smaller than stated above require Type L or SM9 riprap protection. As stipulated earlier, such riprap needs to be covered with native soil and revegetated in accord- ance with Section 5.1.1. The minimum allowable radius for a riprap lined bend is 1.2 times the top width of the design flow water surface and in no case less than 50 feet. The riprap protection should be placed along the outside of the bank and should extend downstream from the bend a distance equal to the length of the bend. Where the mean channel velocity exceeds the allowable non-eroding velocity so that riprap protection is required for straight channel sections, increase the rock size by one category (e.g., Type L to Type M) around bends. The minimuM allowable radius for a riprap lined bend in this case is also 1.2 times the top width of the design flow water surface. 5.4.5 Transitions Scour potential is amplified by turbulent eddies in the vicinity of rapid changes in channel geometry such at transitions and bridges. Table 5-5 may be used for selecting riprap protection for subcritical transitions (Froude numbers 0.8 or less) by increasing the channel velocity by twenty (20%) percent. . 5-1-84