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<br />where <br /> <br /> <br />Kv = vegetation-susceptibility index, III <br />foot-pounds; <br />= vegetation-flexibility factor, III <br />foot-pounds; <br />vegetation-blocking coefficient; <br />= vegetation-distribution coefficient <br />and <br />flow-depth coefficient. <br /> <br />'flex <br /> <br />Cblock/ng <br />Cd/st <br /> <br />C depth = <br /> <br />Information obtained from the sites was <br />considered and utilized in the selection of values <br />for Cblocking, Cd/st, and Cdepth' Values assigned to <br />the three coefficients generally were determined <br />for preflow conditions according to engineering <br />experience gained during the course of this <br />investigation. <br /> <br />Vegetation-Flexibility Factor <br /> <br />The vegetation-flexibility factor (V/lex; see <br />equation 3) is the most significant factor in <br />determining whether vegetation will bend or <br />remain in a generally upright position when <br />subjected to the power of flow. The unique <br /> <br />physical properties of many types of vegetation <br />enable them to bend to extreme angles when force <br />is applied. The flexural strength or stiffness of <br />different species of vegetation is not constant, and <br />the degree of bending varies for a given applied <br />force. The force required to bend or layover <br />vegetation, therefore, was quantified to obtain the <br />flexural strength of different vegetation types. For <br />the purposes of this report, laid over is defined as a <br />condition in which vegetation is bent more than 450 <br />from vertical. <br />Dynamometers, which are mechanical devices <br />that measure magnitude of tension in cables, were <br />used to determine the force required to layover <br />four types of vegetation. The vegetation included <br />saltcedar, willow, mesquite, and palo verde, and <br />ranged in height from 3 to 18 ft. Bending moments <br />were determined by computing the product of the <br />moment arm (distance from the base or pivot point <br />to the location where force was applied) and the <br />force required to bend the vegetation to 450 from <br />the vertical (fig. 2; table 13, see the section entitled <br />"Basic Data" at the end of the report). <br />Attempts were made to place the dynamometer <br />at a vertical distance from the base of the vegeta- <br />tion of about 0.4 times the height of the vegetation <br />(table 13, see the section entitled "Basic Data" at <br /> <br /> <br />Figure 2. Dynamometers used to determine vegetation bending-moment values. <br /> <br />Data Collection and Analysis 9 <br />