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<br /> <br />2. Determine point of bifurcation into multiple channel region through use of <br />Figure B-1 and the calculation of near fanhead canyon slope to fan slope ratio. <br /> <br />3. If the point of bifurcation is downfan from the lower boundary of the one-foot <br />depth zone as computed by the standard single channel method, the standard single <br />channel method will be used for the determination of all flood boundaries on the <br />fan. <br /> <br />4. If the point of bifurcation is upfan from the upper boundary of the one-foot <br />dcpth zone as computed by the standard single channel method, the one-foot depth <br />zone boundaries will be changed to that computed for the multiple channel method. <br />Substitute those boundaries for the boundaries computed by the standard single <br />channel method. <br /> <br />5. If the point of bifurcation is upfan from the one-foot depth zone boundaries as <br />computed by the standard single channel method, compute the depth and velocity <br />at the point of bifurcation by the standard single channel method. Compute the <br />velocity and depth boundaries for velocities and depth less than those determined <br />for the point of bifurcation by use of the multiple channel method. Substitute <br />those boundaries for the boundaries computed by the standard single channel <br />method. <br /> <br />5. FIRM <br /> <br />Whcn thc fan arc widths that form flood hazard zone boundaries have been com- <br />puted, these distances should be scaled onto topographic base maps, taking care to make <br />the boundarics parallel to contours. <br /> <br />The sketch map shown in Figure B-1 depicts the typical distribution of flood in- <br />surance rate zones on an active alluvial fan, as determined by the methodology outlined <br />in this Appendix. <br /> <br />6. REFERENCES <br /> <br />I. David R. Dawdy, Flood Frequency Estimates on Alluvial Fans, Journal of the Hy- <br />draulics Division, ASCE, Proceedings. Vol. 105, No. HYII, pp.1407-l413, 1979. <br /> <br />2. DMA Consulting Engineers for FEMA, Alluvial Fan Flooding Methodology - An <br />Analysis. August 1985. <br /> <br />3. U.S. Department of the Interior, Interagency Advisory Committee on Water Data, <br />Office of Water Data Coordination, Hydrology Subcommittee, Bulletin No l7B, <br />Guidelines for Determining Flood Flow Frequency, September 1981, revised March <br />1982. <br /> <br />C-17 <br />