Flows and Transport in Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 90

distribution is then further assumed to be a fully developed turbulent boundary layer whose velocity
proile can be approximated by the log-law-of-the-wall (Chow 1959). For these conditions, the aver-
age velocity occurs at 60% (0.6) of the depth (as measured from the surface).
So, for shallow rivers, the USGS guideline is that a single velocity measurement be taken at 60% of
the depth of each section and used to represent the average velocity. For deeper sections, the average
velocity is based on the average of two measurements taken at 20% and 80% of the depth. Where there
are irregularities in the channel impacting the low, a three-point method is recommended (Table 4.1).
Traditionally, a low measurement would involve the USGS visiting the gauging site and taking direct
measurements of the river depth, width, and velocity, using mechanical instruments such as a sounding
rod or cable, a tagline, and a current meter. From these data, low rates are computed (Figure 4.9).
The most common velocity meter traditionally used is the small Price (AA) current meter, which
has a cup that, when placed in a current, rotates and the speed of the rotation is related to the veloc-
ity of the low (Figure 4.10). The number of rotations in a measurement period may be manually or
electronically measured and then converted to a velocity measurement. The meter could be used
with a sounding rod for wadeable rivers. For deep rivers, the meter would traditionally (as well as
presently) be suspended by a cable, with a sounding weight attached to help keep the cable as verti-
cal as possible, as illustrated in Figure 4.11.

TABLE 4.1
Procedures for Determining Mean Vertical [ U ( y )] Velocities in a Lateral Section
No. of Points
in Vertical

Vertical Depth
of Measure

Application Mean Velocity

1 0.6 D Depth <0.5 m or quick

measurement required

() = ( )

Uy uy ,06D

2 0.2 and 0.8 D Preferable where size of

meter allows (D > 0.5 m)

( ) + ( )

Uy u y

,. 02 D u y ,. 08 D

() =

2

3 0.2, 0.6, and 0.8 D Where irregularities distort

the velocity proile and
depth is suficient

( ) + ( ) + ( )

Uy u y

,. 02 2 6 D u y ,. D u y ,. 0 8 D

() =

4

Source: Gordon, N.D., McMahon, T.A., Finlayson, B.L., Gippel, G.J., and Nathan, R.J., Stream Hydrology—An Introduction
for Ecologists , 2004. Copyright Wiley-VCH Verlag GmbH & Co. KGaA.
Note: y = lateral section; D = vertical distance between the water surface and the streambed, measured downward.

Width

Velocity

Depth

FIGURE 4.9 Cross-section proiling for low measurements.