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 At the selected assessment site, identify the bankfull stage and measure a cross-section of a riffle where the stream is free to adjust its boundaries to the frequent flows. Measure the bankfull width, bankfull mean depth and bankfull cross-sectional area. Obtain the drainage area (Step 2) of your reach, and then check the bankfull cross-sectional area to the regional curve for your hydro-physiographic region. Estimate bankfull discharge by field determination of bankfull velocity multiplied times cross-sectional area.
Various methods are appropriate for estimating velocity, as long as the assumptions of the computational method are met. RIVERMorph  is a program that allows a variety of velocity equations to be applied to the prediction. Calibrating several velocity equations at gage stations, where velocity is measured for a wide range of flows, will provide for familiarity and confidence in selecting the appropriate formula to a similar stream type at an ungaged site. For gravel and cobble-bed streams the approach using a resistance (friction) factor and relative roughness (Figure 103) (PDF, 27 kb, 1 p.) is recommended.
The relation (Equation V-1) is presented as:
| u/u* | = 2.83 + 5.7 Log R/D84 |
| Where: u | = mean velocity in ft./sec. |
| u* | = shear velocity ( ) |
| g | = gravitational acceleration (32.2 ft./sec2) |
| R | = hydraulic radius |
| S | = water surface slope |
| D84 | = particle size of bed material of the 84th percentile |
 Another option also uses the Darcy Weisbach friction factor (f), where u/u * is approximately equal to ( ). The use of friction factor to predict a manning's "n" roughness factor
Where f = 
is shown in Figure 104 (PDF, 20 kb, 1 p.).
R/D84 values are obtained from Figure 103, then u/u* values read directly and transferred to Figure 104 to read Manning's "n" values.  The equation for velocity calculation using Manning's procedure is shown in Figure 104 and in Worksheet 11.
Worksheet 11 (PDF, 174 kb, 1 p.) is a form for computing various approaches to velocity.
The use of hydraulic geometry relations are also recommended for application. Hydraulic geometry relations are associated with plotting of actual measured velocity, width, and depth against discharge. This data is obtained from USGS gage stations or other sources, where current meter measurements have been obtained.
An example of hydraulic geometry is shown in Figure 105 (PDF, 169 kb, 1 p.) (Leopold, 1994).
Examples of hydraulic geometry relations by stream type are shown in Figure 23, below.
It generally is a good practice to obtain the drainage area (step 2), obtain the bankfull cross-sectional area from the regional curve. Check this value against the field determined cross-sectional area. Divide the field determined cross-sectional area into the bankfull discharge from the regional curve to see if the mean velocity is reasonable compared to the velocity predictions (Worksheet 11).
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| Figure 23. Downstream width hydraulic geometry for North American gravel bed rivers,
W=3.68 Qb0.5, and U.K. gravel bed rivers, W=2.99 Qb0.5 (Copeland et al, 2001). |
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