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Flow-Depth Coefficient Flows in vegetated channels do not always result in total submergence of the vegetation. The effect of flow on vegetation depends on the depth of flow in relation to vegetation height (fig. 7). The force required to layover vegetation is inversely related to the length of the moment ann, or, in the case of flow effects on the vegetation, the depth of flow (fig. 8). This relation reinforces the assumption that as flow depth increases, the ability of vegetation to resist the effects of flow will decrease (figs. 7-8). Dimensionless flow-depth coefficients (Cdeplh; see equation 3) were detennined for five different categories that are defined by the ratio of hydraulic radius to vegetation height (table 6). The hydraulic radius is assumed to approximate mean-flow depth as well as the approximate depth of flow at the immediate location of the vegetation. Table 6. Flow-depth coefficients [<, less than; >, greater than] Ratio of hydraulic radius to average vegetation height <0.4 0.4 to 0.6 0.7 to 0.9 1.0 to 1.5 >1.5 Flow-depth coefficient, dimensionless 60 20 5 3 1 Relation Between Stream Power and Vegetation-Susceptibility Index The vegetation-susceptibility indices (eq. 3) were calculated, and the vegetation-susceptibility index was plotted with the stream power for each studied flow (table 7; fig. 9). If the vegetation- susceptibility index is high and computed stream power is low, the vegetation is not substantially affected (fig. 9). As stream power increases, however, the ability of the flow to layover vegetation increases. A vegetation-susceptibility threshold is represented by the line shown on figure 9. The line is defined by the equation SP = 2.054K0.231 v In general, for flows that plot above this line, the vegetation can be expected to layover assuming the characteristics of the vegetation and stream power of the flow in question are within values studied for this report. Vegetation removal does not necessarily depend on initial proneness of the vegetation. Although the power of flow may not be of sufficient magnitude to substantially layover the vegetation, the flow may still be large enough to degrade the channel substrate to the point that the vegetation's root system is exposed and may result in total removal of the vegetation. Data from flows that removed the majority of the channel vegetation, therefore, were not used for the relation in figure 9. Vegetation Removal (4) Vegetation removal or its complete destruction primarily is dictated by the degree of channel-bed and boundary degradation, which in turn is influenced by the magnitude of the shearing forces of flow. Depending on the size of bed material and the size and type of vegetation, two distinct mechanisms for vegetation removal were observed in this investigation. The first mechanism (vegetation scour) is the exposure of root systems and removal of vegetation from transport of sand- and gravel-sized bed material. The second mechanism (vegetation obliteration) is the movement of cobble- and boulder-sized material onto the vegetation that results in the destruction of brush and small trees (fig. 10). Although obliteration occurred at several sites, degradation of channel boundaries and actual scour of root systems is considered the primary mechanism that causes vegetation removal. The primary components that require quantification to evaluate the degree of vegetation removal include (I) the mean size of the substrate material, (2) the flow duration, and (3) the estimated size and density of the vegetation-root system. Mean size of the substrate material for alluvial channels studied generally was detennined by sieve analysis. For substrate material that was too large to sieve, mean diameter was detennined by measuring the intennediate axis of a representative number of particles or estimated on 14 Method to Estimate Effects of Flow-Induced Vegetation Changes on Channel Conveyances of Streems In Central Arizona