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38 . III. DESIGN CONSIDERATIONS 3.1 General A predominant characteristic of alluvial channels is the change in location, shape, and hydraulics that the channel and cross sections experi- ence with time. These changes are particularly significant during periods when alluvial channels are subjected to comparatively high flows. In most instances, when considering the stability of alluvial channels, it can be shown that approximately 90 percent of all river changes occur during that five to ten percent of the time when large flows occur, if duration is significant. Regardless of the fact that the majority of changes occur during comparatively short time periods, there may also be regions within a river in which some degree of instability is exhibited for all flow conditions. Also, any modification to a channel can significantly alter the system. Urbanization will always alter the natural river system, such as in- creasing peak flow rates, decreasing sediment supply, encroachment into the floodplain, and etc. Design of stable channels and hydraulic structures must be considered and evaluated at the planning and design stages to pro- tect from flood damages including human lives and property. The large number of variables which affect the river channel are interdependent. Major hydraulic variables affecting channel design and sediment transport are velocity, depth, and slope. Major factors which cause instability of channels and hydraulic structures are degradation and aggradation, general scour, local scour, lateral migration, subsurface flow, uplift force, seepage, and freeboard. . . . . . . . . . .