#average flow

The purpose of a hydrology study is to predict the variation in the flow during the year. Since the flow varies from day to day, a one-off measurement is of limited use. In absence of any hydrological analysis, a long-term measuring system may be set up. Such a system is often used to reinforce the hydrological approach and is also the most reliable way of determining actual flow at a site. One-off measurements are useful to give a spot check on hydrological predictions.

The flow measuring techniques described here are:

    * The weir method

  * Stage control method

  * The salt gulp method

  * The bucket method

  * The float method

  * Current meters

Measuring weirs

A flow measurement weir is a weir with a notch in it through which all the water in the stream flows. The flow rate can be determined from a single reading of the difference in height between the upstream water level and the bottom of the notch (see figure 40). For reliable results, the crest of the weir must be kept sharp and sediment must be prevented from accumulating behind the weir. Sharp and durable crests are normally formed from sheet metal, preferably brass or stainless steel, as these do not corrode.

Weirs can be timber, concrete or metal and must always be oriented at right angles to the stream flow. Siting of the weir should be at a point where the stream is straight and free from eddies. Upstream, the distance between the point of measurement and the crest of the weir should be at least twice the maximum head to be measured. There should be no obstructions to flow near the notch and the weir must be perfectly sealed against leakage.   

Temporary measuring weirs are used for short-term or dry-seasoned measurements and are usually constructed from wood and staked into the bank and stream bed. Sealing problems may be solved by attaching a large sheet of plastic and laying it upstream of the weir held down with gravel or rocks. It is necessary to estimate the range of flows to be measured before designed the weir, to ensure that the chosen size of notch will be correct. The use of permanent weirs may be a useful approach for small streams, but larger streams might better be measured by staging (explained below).    

Stage-discharge method

Once set up, this method provides an instant measurement of the flow at any time. It depends on a fixed relationship between the water level and the flow at a particular section of the stream. This section (the contour section) is calibrated by taking readings of water levels and flow (stage and discharge) for a few different waterlevels, covering the range of flows of interest, so as to build up a stage-discharge curve. During calibration the flow does not have to be measured at the contour section itself. Readings can be taken either upstream or downstream using, for instance, a temporary weir, as long as no water enters or leaves the stream in between. The stage-discharge curve should be updated each year. A calibrated staff is then fixed in the stream and the water level indicated corresponds to a river flow rate which can be read off the stage-discharge curve.     

'Salt gulp' method

The `salt gulp' method of flow measurement is adapted from dilution gauging methods with radioactive tracers used for rivers. It has proved easy to accomplish, reasonably accurate (error <7 %), and reliable in a wide range of stream types. It gives better results the more turbulent the stream. Using this approach, a spot check of stream flow can be taken in less than 10 minutes with very little equipment.

A bucket of heavily salted water is poured into the stream. The cloud of salty water in the stream starts to spread out while travelling downstream. At a certain point downstream it will have filled the width of the stream. The cloud will have a leading part which is weak in salt, a middle part which is strong in salt and a lagging part which is weak again. The saltiness (salinity) of the water can be measured with an electrical conductivity meter. If the stream is small, it will not dilute the salt very much, so the electrical conductivity of the cloud (which is greater the saltier the water) will be high. Therefore low flows are indicated by high conductivity and vice versa. The flow rate is therefore inversely proportional to the degree of conductivity of the cloud.

The above argument assumes that the cloud passes the probe in the same time in each case. But the slower the flow, the longer the cloud takes to pass the probe. Thus flow is also inversely proportional to the cloud-passing time. Detailed mathematics will not be covered here because the conductivity metre is usually supplied with detailed instructions.

The equipment needed for `salt gulp' flow measurement is:

a bucket,

pure table salt,

a thermometer (range 0 - 40° C),

a conductivity meter (range 0-1000 mS),

an electrical integrator (Optional).

Bucket method

The bucket method is a simple way of measuring flow in very small streams. The entire flow is diverted into a bucket or barrel and the time for the container to fill is recorded. The flow rate is obtained simply by dividing the volume of the container by the filling time. Flows of up to 20 l/s can be measured using a 200-litre oil barrel.    

Float method

The principle of all velocity-area methods is that flow Q equals the mean velocity Vmeans times cross-sectional A:

Q=A × Vmean (m3/s)

One way of using this principle is for the cross-sectional profile of a stream bed to be charted and an average cross-section established for a known length of stream. A series of floats, perhaps convenient pieces of wood, are then timed over a measured length of stream. Results are averaged and a flow velocity is obtained. This velocity must then be reduced by a correction factor which estimates the mean velocity as opposed to the surface velocity. By multiplying averaged and corrected flow velocity, the volume flow rate can be estimated.     

Current meters