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

170
MEASUREMENT OF STAGE AND DISCHARGE
CASE D. ;vtETfR CANNOT BE SUBMERGED
If it is impossible to keep tbe meter and weight in the water because
ofhigh velocities and (or) floating drift, use the following procedure:
L Obtain depths at the measurement verticals by the method explained
for case B if a standard cross section is avail able, or by
the method explained above for case C if no standard cross section
is available.
2. l\:leasure surface velocities with an optical current meter, as explained
on pages 91-93,137-138.
3. Compute the measurement in the normal manner using the surface
velocities as though they were the mean velocities in the
vertical, and using the depths from step L
4. Apply the appropriate velocity coefficient to the discharge computed
in step 3; use a coefficient of0.86 for a natural channel and
0.90 for an artificial channeL
If an optical current meter is not available, time floating drift over
a measured course. (See p. 261-262.)
It should be noted here that the amount of floating drift or ice is
usually greatly reducedjust after the crest ofa rise in stage. It may be
possible at that time to obtain velocity observations with a standard
current meter.
COMPUTATION OF MEAN GAGE HEIGHT OF A DISCHARGE MEASUREMENT
The mean gage height of a discharge measurement represents the
mean stage of the stream during the measurement period. Because
the mean gage height for a discharge measurement is one of the
coordinates used in plotting the measurements to establish the
stage-discharge relation, an accurate detennination of the mean gage
height is as important as an accurate measurement of the discharge.
The computation of the mean gage height presents no problem when
the change in stage is uniform and no greater than about 0.15 ft (0.05
m), for then the mean !!lay be obtained by averaging the stage at the
beginning and end of the measurement. However, measurements
must often be made during periods when the change of stage is
neither uniform nor slight.
As a prerequisite for obtaining an accurate mean gage height, the
clock time at the beginning and end of the measurement should be
recorded on the measurement notes, and additional readings of the
clock time should be recorded on the notes at intervals of15 to 20 min
during the measurement. After the discharge measurement has been
completed, the recorder cbart should be read, and breaks in tbe slope
of the gage-height graph that occurred during the measurement
should be noted The breaks In slope are useful In themselves and are