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

5. DISCHARGE-CURRENT.METER METHOD 149
CURRENT·METER MEASUREMENTS FROM BRIllGES
Bridges are often used for making discharge measurements of
streams that cannot be waded. Measurement cross sections under
bridges are often satisfactory for current-meter measurements, but
cableway sections are usually superior.
No set rule can be given for choosing between the upstream or
downstream side of the bridge for making a discharge measurement.
The advant.ages of using the upstream side of the bridge are:
1. Hydraulic characteristics at the upstream side of bridge openings
usually are more favorable.
2. Approaching drift can be seen and thus can be more easily
avoided.
3. The streambed at the upstream side ofthe bridge is not likely t.o
be scoured as badly as the downstream side.
The advantages of using the downstream side of the bridge are:
1. Vertical angles are more easily measured because the sounding
line will move away from the bridge.
2. The flow lines of the stream may be straightened by passing
through a bridge opening with piers.
Whether to use the upstream side or the downstream side of a
bridge for a current-meter measurement should be decided individually
for each bridge after considering the above factors. Other pertinent
factors relate to physical conditions at the bridge, such as location
of the walkway, traffic hazards, and accumulation of trash on
pilings and piers.
In making the discharge measurement either a handline, or a
sounding reel supported by a bridge board or by a pertable crane, is
used to suspend the current meter and sounding weight from the
bridge. The velocity is measured by setting the meter at positions in
the vertical as indicated in table 4. If velocities are high, the equipment
is used no closer than several feet from piers and abutments. In
that situation depths and velocities at the pier or abutments are
estimated on the basis ofobservations in the vertical nearest the pier.
(See p. 82.)
Where piers are in the measuring section, it is usually necessary to
use more than 25-30 subsections to obtain results as reliable as those
obtained with a similar measuring section that has no piers. Piers not
only affect the horizontal distribution of velocities, but they
frequently affect the direction of the current, causing horizontal angles
that must be carefully measured.
Whether or not to exclude the area ofa bridge pier from the area of
the measurement cross section depends primarily on the relative locations
of the measurement section and the end of the pier. If meas-