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<br />Table 11. Frequency, probability, and discharge of <br />instantaneous peak flow for the example case <br /> <br />I' <br /> <br />I <br />I. <br />I <br /> <br />Frequency <br /> <br />2 year <br />10 year <br />50 year <br />100 year <br /> <br />Probability 01 <br />occurrence <br />In a given year, <br />in percent <br /> <br />50 <br />10 <br />2 <br />1 <br /> <br />Discharge, <br />in cubic feet <br />per second <br /> <br />1,000 <br />5,000 <br />13,000 <br />24,000 <br /> <br />The following steps will be used for the example <br />case to illustrate use of the information presented <br />in this report. These steps should also be followed <br />by all users ofthis method. <br /> <br />STEP 1. Survey the channel to obtain parameters <br />necessary for standard-step computations. For <br />step I a, in the estimation of roughness coefficients, <br />fully weight the preflow-vegetation conditions for <br />the selected discharge. For subsequent steps, use <br />engineering judgment for estimating roughness <br />coefficients. <br /> <br />I <br />f <br /> <br />SleD 1 a <br /> <br />The 2-year flow is initially selected. According <br />to standard guidelines (Thomsen and <br />Hjalmarson, 1991; and Phillips and Ingersoll, <br />1998) a composite n value of 0.070 (nb = 0.030 <br />and n4 = 0.040) is determined (table 12). Go to <br />STEP 2. <br /> <br />SleD 1 b <br /> <br />Although n may vary with depth, a composite n <br />value of 0.070 is selected for the 10-year flow. <br />Go to STEP 2 (Step 2b). <br /> <br />~ <br />r <br />I <br /> <br />SleD 1 C <br /> <br />Because it was determined that the brush will <br />be laid over as a result of the 10-year flow, n4 <br />for the brush will be considered negligible for <br />step I c. The flow-retarding effects associated <br />with willow, however, are still included for <br />step I c. Because the values for willow plot near <br />the vegetation-susceptibility threshold (fig. II), <br />the vegetation component for willow will need <br /> <br />adjusting (decreased) for possible streamlining <br />of the vegetation. In other words, the effect of <br />the lO-year flow on willow may not be <br />significant but it may cause minor branches to <br />bend in the direction of flow. Consequently, on <br />the basis of engineering judgement, for the <br />50-year flow, a composite n value of 0.050 <br />(nb = 0.030 and n4 = 0.020) is determined. The <br />bed-material component, n b, may decrease as <br />flow depth increases. For the purposes of this <br />example, however, nb will remain constant. Go <br />to STEP 2 (Step 2c). <br /> <br />STEP 2. Run standard-step computations using the <br />surveyed channel parameters and selected <br />roughness coefficients. From the computations, <br />obtain hydraulic radius, average velocity, and <br />water-surface slope for the selected cross section. <br /> <br />SleD 2a <br /> <br />For the 2-year flow, these values are 2.0 ft, <br />2.6 ftls, and 0.0055 ft/ft for hydraulic radius, <br />average velocity, and water-surface slope, <br />respectively (table 12). Go to STEP 3. <br /> <br />SleD 2b <br /> <br />For the 10-year flow, the values for hydraulic <br />radius, average velocity, and water-surface <br />slope are 5.5 ft, 5. I ft/s, and 0.0066 ft/ft, <br />respectively (table 12). Go to STEP 3 <br />(Step 3b). <br /> <br />SleD 2c <br /> <br />For the 50-year flow, the values for hydraulic <br />radius, average velocity, and water-surface <br />slope are 8.0 ft, 8.9 ft/s, and 0.0063 ft/ft, <br />respectively (table 12). Go to STEP 3 <br />(Step 3c). <br /> <br />STEP 3. Using the type and average height of <br />vegetation in the selected cross section, determine <br />the vegetation-flexibility factor, Vjlex, for each <br />vegetation type using equations found in table 3. <br /> <br />Effects 01 Flow.lnduced Vegetation Changes on Channel Conveyances 23 <br />