PCWA-L 467.pdf - PCWA Middle Fork American River Project ...

132 MEASUREMENT OF STAGE AND DISCHARGE
1. Vertical-velocity curve.
2. Two-point.
3. Six-tenths-depth.
4. Three-point.
5. Two-tenths depth.
6. Subsurface-velocity.
7. Surface-velocity.
8. Integration.
Less commonly used are the following multipoint methods of determining
mean vertical velocity:
9. Five-point.
10. Six-point.
VERTICAL-VELOCITY CURVE METHOD
In the vertical-velocity curve method a series of velocity observations
at points well distributed between the water surface and the
streambed are made at each of the verticals. If there is considerable
curvature in the lower part of the vertical-velocity curve, it is advisable
to space the observations more closely in that part of the depth.
Normally, the observations are taken at O.l-depth increments between
0.1 and 0.9 ofthe depth. Observations are always taken at 0.2,
0.6, and 0.8 of the depth so that the results obtained by the verticalvelocity
curve method may be compared with those obtained by the
more commonly used methods of velocity observation, Observations
are made at least 0.5 ft (O.15m) below the water surface and above the
streambed when the Price AA meter or the vane meter is used and
are made at least 0.3 ft (0.09 m) from those boundaries when the Price
pygmy meter is used; those meters underregister velocity when
placed closer to the water surface or streambed.
The vertical-velocity curve for each vertical is based on observed
velocities plotted against deptb (fig. 88). In order that verticalvelocity
curves at different verticals may be readily compared, it is
customary to plot depths as proportional parts of the total depth. The
mean velocity in the vertical is obtained by measuring the area between
the curve and the ordinate axis with a planimeter, or by other
means, and dividing tbe area by the length of tbe ordinate axis.
The vertical-velocity curve method is valuable in determining coefficients
for application to the results obtained by other metbods but
is not generally adapted to routine discharge measurements because
of tbe extra time required to collect field data and to compute the
mean velocity.
Intensive investigation of vertical-velocity curves by Hulsing,
Smith, and Cobb (1966) resulted in table 2 wbicb shows average
ordinates of the vertlcal-velocity curve.