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<br />average height of vegetation was determined for <br />conditions at each site, and a corresponding <br />flexural-strength value was computed using the <br />equations in table 3. <br /> <br />In determining flexural strength, it was <br />assumed that different types of vegetation behave <br />in a similar manner when subjected to force. The <br />vegetation types that were studied, however, differ <br />somewhat in appearance and physical <br />characteristics. For example, mesquite and palo <br />verde are characterized by many major branches <br />radiating out from the base; whereas, willow, <br />commonly referred to as a pole tree (Stromberg <br />and others, 1993), is characterized by a main trunk <br />with minor branches from top to bottom. <br />Differences in response to forces from streamflow <br />might not be sufficiently accounted for by <br />bending-moment values alone and thus form an <br />inherent uncertainty associated with this study. <br /> <br />Trunk diameter could be a more applicable and <br />physically correct component than vegetation <br />height in determining flexural strength. Vegetation <br />height, however, is a much more practical <br />component to obtain in the field. In addition, <br />determining a representative diameter for certain <br />types of vegetation is difficult. The strong <br />dependence of vegetation stiffness to vegetation <br />height allows for a straightforward approach to <br />estimating vegetation components in the field. <br /> <br />A separate analysis of the flexural strength of <br />arrowweed and other types of brush was not made. <br />Flexural strength of brush studied during this <br />investigation is assumed to be similar to that of <br />willow and was determined from bending-moment <br />values obtained for willow (table 3). <br /> <br />Vegetation-Blocking Coefficient <br /> <br />The flexural strength of vegetation was <br />obtained by only considering the force required to <br />layover the main stem of the vegetation. <br />Consequently, the actual percentage of the flow <br />area that is blocked by vegetation was measured to <br />account for the combined resistant force associated <br />with the vegetation. The Cblocking value was <br />determined for each site by assigning a weighted <br />value to the estimated percentage of the <br /> <br />cross-sectional area of flow that was blocked by <br />vegetation for preflow conditions (table 4). <br /> <br />Table 4. Vegetation-blocking coefficients for selected <br />categories <br /> <br />[<, less than; >, greater than] <br /> <br />Amount of flow <br />blocked by vegetation, <br />In percent <br /> <br /><30 <br />30 to 70 <br />>70 <br /> <br />Vegetation-density <br />coefficient, <br />dimensionless <br /> <br />1.0 <br />4.0 <br />9.0 <br /> <br />Vegetation-Distribution Coefficient <br /> <br />Collected data for this report suggest that the <br />spatial distribution of riparian vegetation in natural <br />and constructed channels could substantially <br />influence the effects of flow on the vegetation. <br />Vegetation aligned parallel to the direction of flow <br />that generally is the result of consistent base flow <br />(fig. 6A) can result in the redistribution of <br />velocities across the channel section because of the <br />combined resistant effect of the vegetation. The <br />combined resistance causes a decrease in the <br />velocities at the immediate location of the <br />vegetation and mitigates the effects of flow on <br />vegetation conditions. Past experiments in <br />controlled laboratory environments resulted in <br />similar conclusions (Li and Shen, 1973). For <br />vegetation conditions categorized as randomly <br />distributed (fig. 6B), velocity profiles are assumed <br />to remain fairly constant across the channel. <br />Dimensionless vegetation-distribution coefficients <br />(Cdist; see equation 3), therefore, were determined <br />for two categories--vegetation aligned parallel to <br />the flow and vegetation distributed randomly <br />throughout the main channel (table 5). <br />Determination of the Cdis' value (parallel or <br />random) is subjective and requires a certain <br />amount of experience and engineering judgment. <br /> <br />Table 5. Vegetation-distribution coefficients for vegeta- <br />tion orientation <br /> <br />Orientation <br />tollow <br /> <br />Parallel <br />Random <br /> <br />Vegetation-distribution <br />coefficient, dimensionless <br />3.0 <br />1.0 <br /> <br />12 Method to Estimate Effects of Flow-Induced Vegetation Changes on Channel Conveyances of Streams in Central Arizona <br />