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<br />continues until nearly uniform sheet flow conditions exist <br />and the flow is very wide ~nd quite shallow. The braiding <br />process is often confined by local topography in the middle <br />section of the fan. In some cases. local coalescence of <br />braided paths into deeper channels may occur if topographic <br />constrictions are substantial. Three hydraulic regions can <br />be identified based on this flood flow behavior, as shown in <br />Figure 4.2. The channelized flow zone extends from the apex <br />down-fan to the point where braiding begins. The onset of <br />braiding is associated with the intersection of the channel <br />bottom (invert) and the fan surface. Below this point no <br />lateral restriction on channel geometry exists and the flow <br />path begins to meander. A partially confined braided zone <br />may exist in the middle portion of the fan; the length of <br />this zone depends on the topographic variation across the <br />fan and may extend nearly to the toe or may be quite short. <br />A she,t flow zone u~ually exists near the toe of the. fan, <br />where little variation in topography across the fan is <br />present. Due to the major differences in the hydraulic <br />behavior of these three regions. flood hazards will be <br />substantially different between them. The selection of <br />flood plain management tools must consider this variation. <br /> <br />Laboratory experiments using idealized physical models of <br />typical fans (see Part II, Section 3 of this report) <br />revealed that, while the above three-zone pattern is common, <br />the existence and length of the channelized zone depends on <br />fan slope. watershed sediment production. and other <br />characteristics. Models of low slope fans exhibited a <br />complete lack of entrenchment. as did models with very high <br />sediment production. Experiments using the physical model <br />of the Rancho Mirage fan (see Part II. Section 4) clearly <br />illustrate the strong effects of local fan topography on <br />flow direction and spreading rate. <br /> <br />The <br /> <br />results of <br /> <br />field <br /> <br />investigations and <br /> <br />laboratory <br /> <br />42 <br /> <br />