The stage of a river is defined as its water-surface clevation measured above a datum. This datum can be the Mean-Sea Level (MSL) or any arbitrary datum connected independently to the MSL.

Stage can be measured by :

1) Manual Gauges 

2) Automatic Stage Recorders

1) Manual Gauges 

Manual Gauges are further classified as :

i) Staff Gauge :

Staff Gauge can be used in natural channels such as streams and creeks.

It can be easily integrated into common primary flow management devices such as weirs and flumes.

It can be installed with minimal effort, meaning you can start taking readings after just a few minutes.

The simplest of stage measurements are made by noting the elevation of the water surface in contact with a fixed graduated staff. 

The staff is made of a durable material with a low coefficient of expansion with respect to both temperature and moisture.

It is fixed rigidly to a structure, such as an abutment, pier, wall, etc. 

The staff may be vertical or inclined. 

Sometimes, it may not be possible to read the entire range of water-surface elevations of a stream by a single gauge and in such cases the gauge is built in sections at different locations. 

Such gauges are called sectional gauges. When installing sectional gauges, care must be taken to provide an overlap between various gauges and to refer all the sections to the same common datum.

ii) Wire Gauge 

It is a gauge used to measure the water-surface elevation from above the surface such as from a bridge or similar structure. 

In this, a weight is lowered by a reel to touch the water surface. 

A mechanical counter measures the rotation of the wheel which is proportional to the length of the wire paid out. 

The operating range of this kind of gauge is about 25 m.

2) Automatic Stage Recorders 

Automatic Stage Recorders are further classified as:

i) Float – Gauge Recorder

The float-operated stage recorder is the most common type of automatic stage recorder in use. 

In this, a float operating in a stilling well is balanced by means of a counterweight over the pulley of a recorder. 

Displacement of the float due to the rising or lowering of the water-surface elevation causes an angular displacement of the pulley and hence of the input shaft of the recorder. 

Mechanical linkages convert this angular displacement to the linear displacement of a pen to record over a drum driven by clockwork. 

The pen traverse is continuous with automatic reversing when it reaches the full width of the chart. 

A clockwork mechanism runs the recorder for a day, week or fortnight and provides a continuous plot of stage vs time. 

A good instrument will have a large-size float and least friction. 

Improvements over this basic analogue model consists of models that give digital signals recorded on a storage device or transmit directly onto a central data-processing centre.

To protect the float from debris and to reduce the water surface wave effects on the recording, stilling wells are provided in all float-type stage recorder installations. 

 The water-stage recorder has to be located above the highest water level expected in the stream to prevent it from getting inundated during floods. 

Further, the instrument must be properly housed in a suitable enclosure to protect it from weather elements.

On account of these, the water stage-recorder installations prove to be costly in most instances.

ii) Bubble Gauge 

In this gauge, compressed air or gas is made to bleed out at a very small rate through an outlet placed at the bottom of the river.

A pressure Gauge measures the gas pressure which in turn is equal to the water column above the outlet.

A small change in the water-surface elevation is felt as a change in pressure from the present value at the pressure gauge and this in turn is adjusted by a servo-mechanism to bring the gas to bleed at the original rate under the new head. The pressure gauge reads the new water depth which is transmitted to a recorder.

The bubble gauge has certain specific advantages over a float-operated water stage recorder and these can be listed as under:

i). There is no need for costly stilling wells;

ii). A large change in the stage, as much as  30 m, can be measured; 

iii). The recorder assembly can be quite far away from the sensing point; and 

iv). Due to constant bleeding action there is  less likelihood of the inlet getting blocked or choked.

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