FLOOD06466

• County: Statewide
• Basin: Statewide
• Title: Techniques of Water-Resources Investigations of the US Geological Survey Discharge Measurments at Gaging Stations
• Date: 1/1/1969
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

DISCHARGE MEASUREMENTS AT GAGING STATIONS

9

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r

of the moving car is determined for each run by
making an independent measurement of the
distance it travels during the time that the
revolutions of the rotor are electrically counted.
A scale graduated in feet and tenths of a foot
is used for this purpose. Eight pairs of runs
are usually made for each current meter. A pair
of runs consists of two traverses of the basin, one
in each direction, at approximately the same
speed. Practical considerations usually limit the
ratings to velocities ranging from 0.1 fps to
about 15 fps, although the rating car can be
operated at lower speeds. Unless a special
request is made for a more extensive rating,
the lowest velocity used in the rating is about
0.2 fps, and the highest is about 8.0 fps.
For convenience in field use, the data from
the current-meter ratings are reproduced in
tables, a sample of which is shown in figure 1I.
The velocities corresponding to a range of 3 to
350 revolutions of the rotor within a period of
40 to 70 seconds are listed in the tables. This
range in revolution and time has been found to
cover general field requirements. To provide the
necessary information for extending a table for
the few instances where extensions are required,
the equations of the rating table are shown
in the spaces provided in the heading. The
equation to the left of the figure in parentheses
(2.28 in fig. 11) is the equation for velocities
less than 2.28 fps and the equation to the right
is for velocities greater than 2.28 fps. The 2.28
fps is the velocity common to both equations.
It should be noted that the equations given
are those of the rating table, and not necessarily
those of the actual rating. If a rating table
already on file matches a rating within toler-
ances, that table is selected in preference to
preparing a new one. Those tolerances are lis ted
below.

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&lIolutiom ofTOtor
per mOM
0.0________________________________
1.0 and above___________~__________

ToleTanu,
inptTcenl
1.0
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Sounding equipment

Sounding (determination of depth) is com-
monly done mechanically, the equipment used
depending on the type of measurement being
made. Depth and position in the vertical are
measured by a rigid rod or by a sounding weight

:128-~;j3 ~m-3

suspended from a cable. The cable is controlled
either by a reel or by a handline. A sonic sounder
is also available, but it is usually used in con-
junction with a reel and a sounding weight.
Sounding eqnipment used by the Geological
Survey is described in the following categories:
wading rods, sounding weights, sounding reels,
handlines, and sonic sounder.

Wading rods
The two types of wading rods commonly used
are the top-setting rod and the round rod. The
top-setting rod is preferred because of the con-
venience in setting the meter at the proper depth
and because the hydrographer can keep his
hands dry. The round rod can be used in making
ice measurements as well as wading measure-
ments, and has the advantage that it can be
taken down to 1-foot lengths for storing and
transporting.
The top-setting wading rod has a Jl-inch
hexagonal main rod for measuring depth and a
%-inch diameter round rod for setting the posi-
tion of the current meter. (See fig. 12.)
The rod is placed in the stream so the base
plate rests on the streambed, and the depth of
water is read on the graduated main rod. When
the setting rod is adjusted to read the depth of
water, the meter is positioned automatically for
the O.6-depth method. (See fig. 13 and p. 32.)
The 0.6-depth setting might also be described
as the OA-depth position up from the stream-
bed. When the depth of water is divided by 2
and this new value is set, the meter would be
at the 0.2-depth position up from the streambed.
When the depth of water is multiplied by 2 and
this value is set, the meter would be at the 0.8-
depth position up from the streambed. These
two positions represent the conventional 0.2-
and 0.8-depth positions in reverse. (See p. 32.)
The round wading rod consists of a base
plate, lower section, three or four intermediate
sections, sliding support, and a rod end (not
essential). (See fig. 14.) The parts are assembled
as shown in figure 15. The meter is mounted
on the sliding support and is set at the desired
position on the rod by sliding the support.
The round rod is also used in making ice
measurements. Intermediate sections of the
round rod are screwed together to make an ice
rod of desired length. (See fig. 16.) The most
convement length for an ice rod is about 3 feet