For finding the depth and discharge of river we stand/instrument on a stable point in river known as a reach of a section.It is the reach of a channel or a channel section where there is a unique relationship between stage and discharge.
TYPES OF CHANNEL CONTROL
Artificial Control Natural Control
It is an amended section or length of stream for measurement purposes. Examples are weir, flumes etc.River stage is a term used in stream gauging. It is the elevation of the water surface at a specified station above some arbitrary datum.The zero elevation is some times taken as mean sea level but more often it is set slightly below the point of zero in the stream. The river stage is denoted bt ‘The design of an artificial control should be based on the following considerations.The profile of crest should be such that a small change in discharge indicates appreciable change in stage (Maximum water level during some specified past time).The structure should not create undesirable disturbance in the channel above or below the control. (should be self cleaning and should not be subject to obstruction by debris and ice or to deposits of sand, gravel or silt either u/s or d/s)The control should have structural stability and should be permanentThe conditions regarding depth of water are depending upon the physical features of the stream on the downstream side. The profile of crest should be such that a small change in discharge indicates appreciable change in stage.The relation between stage and discharge is controlled by the physical features of the channel downstream from the gauge site. Essential requirements for stream gauging are:Permanence of controlOpportunity to install an artificial controlPossibility of no backwater effect from downstream tributaries or other sources.Availability of a good metering station where good discharge measurement can be made.Availability of a site for proper placement of stage gauge to record all fluctuations of water level and free from debris and floating matter with respect to the control and the metering section.Suitability of existing structures for use in making flood discharge measurements, or the availability of a site for the proper placement of a cable way for this purpose.here is no possibility of flow bye-passing the site in ground water or in flood channels.Availability of power line or telephone lines, where needed for special instrumentation.Accessibility of the site by roads, particularly during floods.Economic construction is possible.
The river stage is measured by two types of gauges
RECORDING GAUGES.Non-recording gaugesStaff gaugeFloat type gaugeElectric tape gaugeStaff GaugeVertical Staff Gauge Sectional Staff Gauge Inclined Staff Gauge
The simplest way to measure river stage is by means of staff gauge. A portion of the vertical staff gauge is immersed in the water at all times. The gauge may consist of a single vertical scale attached to a bridge pier, or other structure that extends into the low water channel of the stream. Figure 1 shows a typical vertical staff gauge.If no suitable structure exists in a location, which is accessible at all stages, a sectional staff gauge (as shown in figure 2) may be used.Short sections of staff are mounted on available structures or on specially constructed supports in such a way that one section is always accessible.Inclined staff gauge is placed on the slope of the stream bank and graduated so that the scale reads directly in the vertical depth.The inclined staff gauges are considered better than the sectional staff gauges Float Type GaugeThis gauge is generally used as an inside reference gauge and is installed in a stilling well to avoid wave effects.
It is built on side of a bank. A pipe connects well to river at its lowest level.The gauge consists of a float, graduated steel tape, counter weight and a pulley. The pulley is grooved to accommodate the tape and mounted on a stand. An arm extends from the stand to a point slightly beyond the tape to carry an adjustable index, which shows the gauge reading. The index is adjusted at the initial setting of the gauge at the site. If the adjustments to the tape readings are too large to be accommodated by further adjustments can be made with the help of a clamp connecting the tape to the float Electric Tape Gauge. One terminal of the battery is attached to ground connection and the other to one terminal of the voltmeter. The other terminal of the voltmeter is connected through the frame reel, and tape to the weight. The weight is lowered until it touches the water surfaceThis contact completes the electric circuit and produces a signal on the voltmeter. The tape reading is then taken at the index provided on the reel mounting. .
These are similar to the non-recording gauges but have some arrangement to give a continuous record. In recording gauges motion of a float for example, is recorded on a chart, and in a continuous recorder, the motion of the float moves a pen across a long strip chart. When the pen reaches the edge of the chart it reverses direction and records the next readings in the other directions across the chart. The chart roll contains enough paper to operate for a year at a scale of 6 cm/day.