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

5. DISCHARGE-GURRENT METER METHOD 147
be appreciably heavier than that indicated by the above rule. The
rule is not rigid, but it does provide a starting point for deciding on
the size of weight required at various stages.
The Price type AA current meter is generally used in the U.S.A.
when making discharge measurements from a cableway. The depth is
measured by use of a sounding reel, and the velocity is measured by
setting the meter at the proper position in the vertical. (See table 4.)
Table 4 is designed so that no velocity observations will be made with
the meter closer tban 0.5 ft \0.15 m) to the water surface. In the zone
from the water surface to a depth of0.5 ft, the current meter is known
to give erroneous results.
Some sounding reels are equipped with a computing depth indicator.
To use the computing spiral, the indicator is set at zero when
the center of the current-meter fotor is at the water surface. The
sounding weight and meter are then lowered until the weight touches
the streambed. If, for example, a 30 C .5 (see table 4) suspension is
used and if the indicator reads 18.5 ft when the sounding weight
touches the bottom, the depth would be 19.0 ft (18.5 ft + 0.5 ft). To
move the meter to the 0.8-depth position (0.8 X 19.0 ft), the weight and
meter are raised until the hand on the indicator is over the 19-ft mark
on the graduated spiral (fig. 60); the hand will then be pointing to
15.2 on the main dial. 'To set the meter at the 0.2-depth position
W.2x 19.0 fl.l, the weight and meter are raised until the hand on the
indicator is pointing to 3.8 ft on the main dial.
One problem found in observing velocities from a cableway is that
movement of the cable car from one station to the next causes the car
to oscillate for a short time after coming to a stop. The hydrographer
should wait until this oscillation has been dampened to the extent
that it is negligible before counting meter revolutions.
Tags can be placed on the sounding line at known distances above
the center of the meter cups as an aid in determining depths. Furthermore,
the use of tags allows the meter to be kept submerged
throughout the discharge measurement to prevent freezing in cold air
during the winter measurements. The tags, which are usually
streamers of diffel'ent colored binding tape, are fastened to the sounding
line by solder beads or by small cable clips. Tags are used for
determining depth in either of two ways.
1. In the procedure that is usually preferred, a tag is set at the
water surface, after which the depth indicator is set at the distance
between that particular tag and the center of the meter cups. This is
equivalent to setting the indicator at zero when the center of the
meter rotor is at the water surface, and the hydrographer then proceeds
with his depth settings as described in a preceding paragraph. If
debris or ice is flowing, this method prevents damage to the meter,