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<br />relief and fan entrenchment are i9'nored. In FEMA I S method, the <br />channel avulsion is based on an ayulsion coefficient that has not <br />been not been confirmed by field or laboratory testing. The method <br />does not consider the loss of an identifiable channel or shallow <br />overland flooding. <br /> <br />* FEMA I S method does not account: for fan <br />development or flood-control measures which <br />effect the flow path and the magnitude of <br />the peak. <br /> <br />The method assumes the flow is unobstructed or unconstrained <br />by levees, entrenched channels, walh; or buildings. The assessment <br />of flood hazards are generally aSElO'ciated with development where <br />flow paths have been altered by walls, levees or debris basins. <br />The existence of levees and structcLres can invalidate the assessed <br />flow probability on the fan. "If the, flow tends to follow previous <br />flows or is guided by structures in t.he flood path, the probability <br />of an event on the fan is diffenmt from that assumed in the <br />standard method" (Dawdy et. al., 1989). Although a method to <br />predict flood hazard boundaries fo:t" reduced fan flow surfaces has <br />been suggested (Mifflin, 1989), the empirical equations for <br />predicting velocities and depths are not suitable for altered flow <br />paths. <br /> <br />Confining the fan arc width to alluvial lolashes, levees or to <br />other physical boundaries has the effect of extending the depth and <br />velocity boundaries further down the fan than would be indicated <br />for an unconfined fan. An example of this confinement is shown in <br />Figs. 3 and 4 where the walls of the wa.sh confine the fan arc width <br />to a distance a few widths larger Ulan the assumed channel width. <br /> <br />* The FEMA method pr'=clicts alluvial fan <br />hydraulics with empirical, channel geometry <br />relationships for eph,=meral streams. <br /> <br />The exponents of the regime equations were based on the <br />hydraulic geometry of ephemeral channels in the semia.rid United <br />States (Dawdy, 1979). The equations for velocity V and depth d as <br />a function of discharge Q are: <br /> <br />V = 1. S QO.2 <br /> <br />c1 = 0.07 QO.4 <br /> <br />The coefficients were derived by assuming that the <br />stabilizes at a point where the change in depth is related <br />change in channel width W accordinq to the equation: <br /> <br />channel <br />to the <br /> <br />ddjdW = --0. OOS <br /> <br />The validity of these empirical equ''lt.ions for alluvial fan channels <br />has not been verified by field observations. 'l'he regime <br />relationships predict channel geo:ml~try for more mildly sloped, <br />semiarid streams and does not refll~ct the geometry of incising or <br /> <br />13 <br />