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

5. DlSCHARGE-CURRENT-METER METHOD 151
Another, less widely used, method of determining depth requires
that the meter cups be set at the water surface, after which the sounding
weight is lowered to the streambed while measuring, with tape or
rule, the length of line that is let out. This measured distance, plus
the distance from the bottom of the sounding weight to the meter
cups, is the depth of water.
When using a handline, enough cable is unwound from the handline
reel to keep the reel out ofwater when the sounding weight is on
the streambed at the deepest part of the measuring section. That
prevents submergence of the reel and thick rubber-covered cable and
attendant drag on the equipment in high-velocity flow, and unless the
bridge is relatively close to the water surface, it still permits the
hydrographer to raise and lower the meter by means of the rubbercovered
cable rather than the bare-steel cable. When the meter is set
for a velocity observation, the hydrographer stands on the rubbercovered
cable or ties it to the handrail to hold the meter in place. By
doing so his hands are free to operate the stopwatch and record the
data.
If the bridge has vertical truss members in the plane of the measurement
cross section, the handline can be disconnected from the
headphone wire and passed around the truss member with the sounding
weight on the bottom. This eliminates the need for raising the
weight and meter to the bridge each time a move is made from one
vertical to another and is the principal advantage of a handline,
CURRENT·METER MEASUREMENTS FROM ICE..cOVER
Discharge measurements under ice cover (fig. 93) are usually made
under conditions that range from uncomfortable to severe, but it is
extremely important that they be made, because the reliability of a
large part of the computed discharge record for a winter period may
depend on one such measurement.
Cross sections for possible use for measuring under ice cover should
be selected during the open-water season when channel conditions
can be observed and evaluated. Commonly the most desirable measurement
section will be just upstream from a riffle because slush ice
that collects under the ice cover is usually tbickest at the upstream
end of the pools created by riffles. The equipment used for cutting or
drilling holes in the ice was described on pages 124-129.
The danger of working on ice-covered streams should never be underestimated.
When crossing the stream, the hydrographer should
test the strength of the ice with solid blows using a sharp ice chiseL
Ice thickness may be irregular, especially late in the season when a
thick snow cover may act as an insulator. Water just above freezing
can slowly melt the underside of the ice, creating thin spots. Ice