Stage

The water level at a gauging station, the most important measurement in river hydrom-etry, is generally known as the stage. It is measured with respect to a datum, either a local bench mark or the crest level of the control, which in turn should be levelled into the geodetic survey datum of the country (Ordnance Survey datum in the UK). All continuous estimates of the discharge derived from a continuous stage record depend on the accuracy of the stage values. The instruments and installations range from the most primitive to the highly sophisticated, but can be grouped into a few important categories.

7.3.1 The staff gauge

This is a permanent graduated staff generally fixed vertically to the river bank at a stable point in the river unaffected by turbulence or wave action. It could be conveniently attached to the upstream side of a bridge buttress but is more likely to be fixed firmly to piles set in concrete at a point upstream of the river flow control. The metre graduations, resembling a survey staff, are shown in Fig. 7.5 and they should extend from the datum or lowest stage to the highest stage expected. The stage is read to an accuracy of ±3 mm. Where there is a large range in the stage with a shelving river bank, a series of vertical staff gauges can be stepped up the bank side with appropriate overlaps to give continuity. For regular river banks or smooth man-made channel sides, specially made staff gauges can be attached to the bank slope with their graduations, extended according to the angle of slope, to conform to the vertical scale of heights. All staff gauges should be made of durable material insensitive to temperature changes and they should be kept clean especially in the range of average water levels.

Depending on the regime of the river and the availability of reliable observers, single readings of the stage at fixed times of the day could provide a useful regular record. Such measurements may be adequate on large mature rivers, but for flashy streams and rivers in times of flood critical peak levels may be missed. Additionally, in these days of increasing modification of river flow by man, the sudden surges due to releases from reservoirs or to effluent discharges could cause unexpected irregular discharges at a gauging station downstream, which could be misleading if coincident with a fixed-time staff reading. To monitor irregular flows, either natural storm flows or man-made interferences, continuous level recording is essential.

7.3.2 Float-operated recorders

A reliable means of recording water level is provided by a float-operated recorder. To ensure accurate sensing of small changes in water level, the float must be installed

Enlargement

Enlargement

A 1 m length

Fig. 7.5 A staff gauge. Stage reading at X = 0.585 m. (Adapted from BS 3680-7: 1971, British Standards Institution.)

A 1 m length

Detachable plate showing metre numeral in red

Fig. 7.5 A staff gauge. Stage reading at X = 0.585 m. (Adapted from BS 3680-7: 1971, British Standards Institution.)

in a stilling well to exclude waves and turbulence from the main river flow. Two different mechanisms are used to record the travel of the float:

7.3.2.1 Chart recorder

The float with its geared pulley and counterweight turns the charted drum set horizontally and the pen arm is moved across the chart by clockwork or an electrical mechanism (Fig. 7.6). The timescale of the chart is usually designed to serve a week, but the trace continues round the drum until the chart is changed or the clock stops. With this instrument all levels are recorded, but the timescale is limited. On visiting a gauging station, a hydrologist can see at once whether or not a current storm event has peaked. However, chart records require careful analysis and time must be spent in abstracting data on a digitiser tablet.

7.3.2.2 Shaft encoder

The float with its geared pulley and counterweight turns a shaft encoder (also called rotary encoder). The encoder comprises of a disc with concentric, metal rings with breaks, plus a series of fixed contact wipers. As the disc rotates on the shaft, those

Flo;

Flo;

Pulley

Beaded float line

End hook

Fig. 7.6 A float-operated chart recorder (Type F system). (Reproduced with permission of Stevens Water Monitoring Systems Inc.)

Beaded float line

End hook

Pulley

Fig. 7.6 A float-operated chart recorder (Type F system). (Reproduced with permission of Stevens Water Monitoring Systems Inc.)

wipers that are in electrical contact with the metal ring sections create a unique binary code, which can be recorded on an eight-bit data logger. Some chart recorders (e.g. Stevens units) can be retro fitted with shaft recorders. An example of a shaft encoder is shown in Fig. 7.7, and similar devices are used within the Environment Agency's network of river gauges in England and Wales.

7.3.3 Electronic pressure sensor

The measurement of stage by pressure sensors, an indirect method converting the hydrostatic pressure at a submerged datum to the water level above, are widely used for gauging small rivers and streams. Those pressure sensors used within these applications typically use piezo-resistive, silicon strain gauges, and are called pressure transducers (Fig. 7.8). Versions of these sensors with on-board signal amplification and current output are called pressure transmitters. Differential pressure transducers (or transmitters), where the differential pressure between water-level pressure and atmospheric pressure (observed by means of an air pipe running inside the cable connecting the sensor to data logger) are normally used. The calibration of the pressure sensor may change over time and this is normally checked annually.

7.3.4 Gas purge (bubbler) gauge

With gas purge devices (Fig. 7.9), nitrogen from a cylinder or air compressed from a pump, is allowed to bubble slowly out of the end of a tube located close to the river bed.

Fig. 7.7 Shaft encoder-based float-operated recorder. (Reproduced with permission of OTT Hydrometry Ltd.)
Fig. 7.8 A submersible pressure transducer used for river-level monitoring in the UK. (Reproduced with permission of Campbell Scientific Ltd.)
Fig. 7.9 A gas purge gauge. (Reproduced with permission from Herschy, R. W. (2009) Streamflow Measurement, 3rd edn. Taylor & Francis, Abingdon.)

When the rate of bubble production is sufficiently small, the pressure in the line is static so that the pressure at the orifice is the same as the pressure at the other end of the tube in the instrument itself. This allows the pressure to be measured in the instrument rather than in the river, and is usually measured with an electromechanical balance (using bellows or mercury-float device) or pressure transducer. The bellows-based electromechanical balance comprises an arm connected to pressure-activated bellows and a counterweight, and a servo-mechanism. The servo-mechanism is used to balance an arm, and its movement is transferred via gears to a pen on a chart. With the mercury-float device, the servo is used to balance a float and counter-weight system where the float is contained within a mercury reservoir. The principal advantage of the gas purge gauge is that no sensors need to be installed within or near the river, only a plastic pipe; the sensors can be housed within a building at some distance from m*

Fig. 7.10 A radar-based river level sensor. (Reproduced with permission from Vega UK.)

the river. This means that the expensive sensors and recording devices can be more easily protected from damage during flood flows. Gas purge gauges using a cylinder gas supply are widely used on large rivers within the tropics, while those using a pump (e.g. Seba PS-Light-2) are increasingly used on European rivers.

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