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SleD 7a The power associated with the 2-year flow is not substantial enough to significantly affect either the brush or the willow (fig. I I). Go back to STEP 1 using the 10-yearflow (step Ib). SleD 7b Plotting position for the brush now indicates that it probably will be laid over as a result of the 10-year flow. The willow, however, plots just below the vegetation-susceptibility threshold, and the effect of flow on it is questionable (fig. II). Consequently, com- putations are made again but now only the effect of flow on the willow is evaluated. Go back to STEP 1 using the 50-year flow (step Ic). SleD 7c As indicated by the plotted position of willow, there is a high probability that it will be laid over as a consequence of the 50-year flow (fig. Il). At this discharge, the flow-retarding effects associated with the brush and willow will diminish greatly, and depending on the amount of proneness, the retarding effects associated with the vegetation may become negligible. No further steps are required. Because it was determined that a 50-year flow is. adequate to layover vegetation growing in this channel, it is assumed that the vegetation will be laid over on the rising limb of the hydrograph for the 100-year flow. The resultant effect would be a dynamic decrease of flow resistance and increase in channel conveyance compared to conveyance computations made on the basis of preflow- vegetation conditions. For determination of the water-surface elevations of the 100-year flow in this example case, selection of Manning's n should only account for the flow-retarding effects associated with the bed material and any residual effects of laid-over vegetation. Because the residual effects were considered minor, for this example, a Manning's n of 0.035 would be recommended for design- discharge computations for the 100-year flow. The result is a substantial reduction in n compared to estimates made on the basis of preflow-vegetation conditions using current guidelines and techniques. By comparing computed water-surface elevations for n values equal to 0.035 and 0.070, the potential error in water-surface elevation computations resulting from incorrect assessment of peak-flow vegetation conditions for this example case for the I OO-year flow is 3.58 ft. As discussed previously in this report, a sufficient amount of information was not available in this study to develop a substantive relation for the removal of vegetation and, therefore, the removal of the vegetation for the example case was not considered. ASSUMPTIONS AND LIMITATIONS Although not documented in this study, the possibility of laid-over vegetation springing back to an upright position when the power and forces of flow subside is a concern. An assumption required for analysis of data gathered for this report demands that the flow-affected vegetation remain in a position following flow similar to that during peak flow. For example, if vegetation was laid over during peak flow, it is assumed to remain in this position when flow subsides. Additionally, caution should be taken when transferring the results from this study to other sites especially if the channel and vegetation conditions of the other sites are substantially different from the channel and vegetation conditions used to derive the relation in this report. For the simple semiempirical relation presented in this report (fig. 9), stream power was used to describe the flow-induced changes to vegetation; however, some important inherent uncertainties exist. For example, peak-flow velocity was required for use in the stream-power equation, and barring a direct current-meter measurement, peak-flow velocities were primarily obtained by use of Manning's equation. Estimates of peak-flow n values made after the flow subsided were based on the assumption that any noticeable change in roughness elements (vegetation for example) occurred before peak flow, and postflow- vegetation conditions were assumed identical to those at peak flow. This assumption may be . , 26 Method to EstImate Ellects of Flow-Induced Vegetation Chenges on Channel Conveyances of Streams In Central Arizona