<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
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