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

182
MEASUREMENT OF STAGE AND DISCHARGE
4. The depth measurements are correct. and the velocity and depth
vary linearly with distance between verticals.
Assumption 1 was tested by comparing the ratings obtained for
Price current meters when rated in flumes of different sizes. It was
found that the ratings can be repeated within a fraction of 1 percent.
When a Price meter was tested in a wind tunnel under differing
degrees of turbulence, its performance was not affected by increased
turbulence. The similarity ofresults using the Price, Ott, and Neyrpic
current meters has already been discussed on page 89. It was
therefore assumed that the standard deviation (SR,) of the error ratio
between measurement results obtained with different current meters
equals 1 percent.
Assumption 2 was tested in 23 different rivers where velocities for
consecutive timeperiodsof15, 30, 45, 90, 120, and 240 s wereobservedfor
a 1-hr period at points corresponding to 0.2, 0.4, 0.6, and 0.8 depth.
The measurement verticals ranged in depth from 2.4 to 26.7 ft (0.7 to
8.1 m), and velocities ranged from 0.43 to 7.9 ft/s (0.13 to 2.4 m/s).
Statistical analysis showed that velocity fluctuations were randomly
distributed in time and space and that if45-8 observations were taken
at the 0.2- and O.S-depth positions in 30 verticals, the standard deviation
(SR t ) of the error ratio between observed and true point velocity
was 0.8 percent.
Assumption 3 was tested using more than 100 stream sites. The
standard deviation (SR,) of the error ratio between the mean velocity
obtained from 0.2- and O.S-depth observations and the true vertical
velocity, using 30 verticals for the discharge measurement, was 1.15
percent.
Assumption 4 was tested using discharge measurements made at
127 stream sites, in which more than 100 verticals were measured in
each cross section. The discharge for each site was again computed
using the data for 1/2, 114, 1/5, 117, and 1/10 of the total number of
verticals in each cross section. Error ratios between those computed
discharges and the discharges computed using all observation verticals
were determined. When 30 observation verticals were used, the
standard deviation (SN) of the error ratios was 1.6 percent.
The standard error ofa discharge measurement (ST) was computed
from the equation
(16)
For a measurement using velocity observations of45 s at the 0.2- and
O.S-depth positions in each of 30 verticals, ST equaled 2.2 percent.
That means that if single discharge measurements were made at a
number of gaging sites using the standard method recommended in