**STREAMKEEPERS OF
****CLALLAM**** ****COUNTY**

**PROPOSAL FOR IMPROVING WADE-ACROSS FLOW
MEASUREMENTS**

For the Swoffer current meters that we
use for wade-across flow measurements, we currently calibrate each rotor/prop
unit individually twice a year.
When we calibrate, we test each rotor/prop at both a fast and a slow
speed (2 and 0.5 ft/sec), then average the two for the calibration number used
in flow calculations.

We devised this approach after
realizing that the unit turns about 10% less efficiently at 0.5 ft/sec than at 2
ft/sec, dropping off significantly below about 1 ft/sec; and even in the winter
and spring, half of our wade-across flow cells have velocities < 1
ft/sec. 2 ft/sec seemed like a good
midpoint for the upper range of speeds, and 0.5 ft/sec for the lower
range.

This “average” calibration number is
more accurate on the whole than using only “fast” or “slow” trials to calibrate,
but it becomes less accurate as the velocities in a given reach become more
extreme toward either the fast or slow end.

In order to improve the accuracy of our
flow measurements, we’d like to suggest the following:

- Test each unit at both a “fast” and a
“slow” speed (perhaps 1.5 & 0.5 ft/sec to best bisect the fast- and
slow-speed ranges encountered in our streams and the Swoffer’s accuracy
ranges).
- When calculating flow in the
database, use the “fast” calibration number for flow cells with velocities
>= 1 ft/sec. Use the “slow”
calibration number for velocities < 1 ft/sec.
- (Optional:)
For an even more accurate reading, use the floating-object method instead of
the flow meter for cells with velocities < 0.5 ft/sec. This might be a bit ungainly to do in
actual conditions, but we could try coming up with a procedure that would be
easy to implement.
- (Alternative, simpler option:) Calibrate the units at only 1.5 ft/sec and use
that calibration number for all readings above 0.75 ft/sec. At slower speeds,
use a floating object.

We’d appreciate your
feedback!