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

34 MEASUREMENT OF STAGE AND DISCHARGE
If oil (generally kerosene) is to be added to prevent freezing in the
vent pipe, tbe top of the well screen should be a sufficient distance
below the minimum expected stream stage to retain the required
depth of oil (fig. 15). To prevent variations in the depth of oil from
affecting the manometer reading, the bubble orifice should be below
the top of the screen so that the bubbles emerge into the water.
It is emphasized that for satisfactory operation ofthe well point the
streambed material should not be so finely grained as to unduly impede
the passage of river water to the well point and thereby cause
lag in the recorded stage. To prevent clogging ofthe well-point screen,
the screen should be made of material that will inhibit chemical
reaction with substances in the water and (or) in the bed material. A
stainless-steel screen set 1 to 3 ft (0.3 to LO m) below the streambed is
recommended.
The bubble gage is used primarily at sites where it would be expensive
to install a stilling welL It is also used on sand-channel streams
because the gas tends to keep the orifice from being covered with sand
and the tube may be easily extended to follow a stream channel that
shifts its location. However, the float stilling-well installation is
cheaper to install at many sites, and its performance is usually more
reliable than that of the bubble gage. The two systems have about the
same accuracy-±O.Ol ft (0.003 mi. The choice of systems thus depends
on the characteristics of the gage site.
WATER-STAGE RECORDERS
A water-stage recorder is an instrument for producing a graphic or
punched-tape record of the rise and fall of a water surface with respect
to time. It consists ofa time element and a gage-height element
which, when operating together, produce on a chart or on a tape a
record of the fluctuations of the water surface. The time element is
controlled by a clock that is driven by a spring, by a weight, or by
electricity. The gage-height element is actuated by a float or a bubble
gage.
Ifa float sensor is used, the float pulley is attached to the recorder.
The float and counterweight are suspended on a perforated steel tape
or on a plain or beaded cable. Cone-shaped protrusions on the circumference
of the float.tape pulley match performations in the tape_ As
the float rises or falls the float pulley rotates in proportion to the
change in stage; the rotation of the pulley is transmitted to the recorder
and the appropriate gage height is thereby recorded. A copper
float 10 in (0.25 m) in diameter is normally used, but other sizes are
also used depending on the type of recorder, gage-height scale, and
accuracy requirements.