Methods of stream gauge reading

The stage (gauge height) of a river is the water-surface elevation above a datum. It is observed in the following ways.

1. Non-recording (manual) gauges

a) Staff gauge: a graduated vertical enamelled plate (in cm), fixed on a pier, bridge abutment or wall. It is read by the observer 2 or 3 times a day. For wide stage ranges, sectional staff gauges are set in steps up the bank, or an inclined gauge is laid on the slope of the bank.

   |  |  <- 3.0 m
   |  |
   |  |~~~~~ water level
   |  |  <- 2.0 m
   |__|

b) Wire (chain) gauge: a weighted chain or wire lowered from a fixed point on a bridge until it touches the water. The length of wire let out gives the stage below a known datum.

c) Float and tape: a float connected by a tape to a scale with counterweight.

d) Crest-stage gauge: a pipe containing cork powder or a similar substance that marks the maximum flood stage.

2. Recording (automatic) gauges

a) Float-type recorder: a float in a stilling well (connected to the river by intake pipes) is attached to a counterweighted wire that turns a pulley and drives a pen on a clock-driven drum. It gives a continuous record (stage hydrograph).

   recorder drum
      O--pen
      |  wire & counterweight
  ----|------- bridge/shelter
   |  |  |
   | [F]  |  <- float in stilling well
   |  ~~~~|~~ intake pipe

b) Bubble (pressure) gauge: gas is bubbled through a tube at the bed; the pressure needed is proportional to the water depth above the orifice.

c) Pressure transducer / data logger: a submerged sensor records pressure.

d) Ultrasonic or radar gauge: non-contact sensors measure the distance to the water surface.

Data may be sent by telemetry. Automatic gauges are used for floods and flood forecasting; staff gauges serve as the reference check.