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Where the channel vegetation is well established, covers most of the channel, and controls channel roughness, guidelines for overbank flow resist- ance given in the section "Overbank Flow Resistance" needs to be used to assign n values. Degree of Meandering, ill The increase in channel roughness due to small curves and bends generally is considered to be insignificant. The effects of sharp bends may extend for some distance downstream. Streams containing sharp bends need to be divided into a typical reach as previously discussed. The degree of meandering is computed as the ratio of the straight length of the reach under consideration (L ) divided by the meander length (L ) of the channel in the reach. The mo~ified value for meandering is obtaTned by multiplying the total additive effects of the other factors for this reach by L /L . s ill When floods in meandering channels are out of the banks and flow down- valley across the meanders, the n value is larger. Wormleaton and others (1982) indicate that the traditional methods of calculating the discharge in compound channels (main channel and subdivided overbank flow) do not fully account for energy losses, and that channel discharge capacity is considerably overestimated. Their study indicates that the tendency is further exaggerated when overbank roughness is greater than main channel roughness. No guidelines are presently available for determining the effects of varying roughness in compound channels nor these effects on n values, although most floods occupy the overbanks. CHANNEL FLOW RESISTANCE Natural Channels Median particle size generally is used to.classify natural stream chan- nels as either stable or movable (sand) bed. A sand-bed channel is comprised of an unlimited supply of particles less than about 0.079 in. (2 millimeters) in diameter. For sand channels, bed-form roughness associated with movable or sand-bed channels is important and needs to be considered. Channels composed of coarse material having a median particle size greater than 0.079 in. (2 millimeters) generally are relatively stable; how- ever, even stable channels may be subject to significant bank and bed erosion and act as a movable bed stream at high discharges. If channel erosion has occurred in the past or is anticipated during high flows, it needs to be accounted for in subsequent hydraulic analysis. Sand-Bed Channels Resistance to flow in sand-bed (movable) streams varies greatly, depend- ing on the velocity of flow, grain size, shear, and other variables, because the moving bed material takes on different bed forms. The flows that produce 19