NED UNIVERSITY OF ENGINEERING AND TECHNOLOGY

STREAM FLOW MEASUREMENT TECHNIQUES

URBAN HYDROLOGY AND MUNICIPAL ENGINEERING

TABLE OF CONTENTS1. STREAM FLOW.................................................................................................................................................2

2. IMPORTANCE OF STREAM FLOW MEASUREMENT..........................................................................................2

3. STREAM FLOW MEASUREMENT METHODS.....................................................................................................2

4. VELOCITY-AREA METHOD (FLOAT-METHOD)..................................................................................................3

4.1 Equipment Needed....................................................................................................................................3

5. MEASURING DISCHARGE USING A WEIR..............................................................................................4

6. BUCKET METHOD............................................................................................................................................5

7. METERS.........................................................................................................................................................6

7.1 PYGMY METER...........................................................................................................................................6

7.2 Vortex meter.............................................................................................................................................6

7.3 Current meter............................................................................................................................................6

1. STREAM FLOWStream flow, or discharge, is the volume of water that moves over a designated point over a fixed period of time. It is often expressed as cubic feet per second (ft3/sec).

The flow of a stream is directly related to the amount of water moving off the watershed into the stream channel. It is affected by weather, increasing during rainstorms and decreasing during dry periods. It also changes during different seasons of the year, decreasing during the summer months when evaporation rates are high and shoreline vegetation is actively growing and removing water from the ground. 

2. IMPORTANCE OF STREAM FLOW MEASUREMENTStreamflow is of fundamental importance to virtually all environmental monitoring and affects almost all other environmental issues connected with water. The flow of rivers changes significantly in a very short amount of time due to plenty of factors. Snowmelt from mountainous regions has a major impact on rivers and can cause them to swell past their banks really quickly. Ice flows are also hazardous, especially when the temperature soars after having been below freezing for a long time. Thunderstorms also produce torrential downpours which results in flash flooding in many places. This is even more of a hazard because it can significantly increase the water levels in a small river or stream in a matter of minutes.

Streamflow measurements can yield information on changes in discharge that are valuable for predicting flooding, estimating long-term trends in water and sediment discharges, and for distinguishing possible long-term climate change.

3. STREAM FLOW MEASUREMENT METHODSIt is important to measure the stream flow as it assess the quantity of water that is available and also to check the quantity of water that is flowing through the treatment plants or system.

There are several techniques available for the measurement of flow of rivers and stream. Often all that is feasible to make an initiative assessment of flow as a part of the selection of water sources. This indicates that only a limited number of flow measurements can be taken. Now these limited numbers of measurements create a risk of not determining the appropriate measurement of the stream flow because there are variations in stream flow during different seasons. So due to this reason it is important to get information from the local community to get familiar with the high and low water flows and level that may be seen during the measurements.

The methods that are used widely for the measurement of flow on surface are as follows:

4. VELOCITY-AREA METHOD (FLOAT-METHOD)The velocity area method is a technique used to investigate the stage and discharge of a river. The discharge of a river is the volume of water which flows through a cross-section in a given time. It is measured in m3/s. Stream flow is affected by a number of factors including rainfall, temperature and snow melt. Measuring the discharge is important because first and foremost it is crucial in predicting floods. In addition the velocity and flow of a river affects food sources and the migration of fish amongst other wildlife.

4.1 Equipment Needed Tape Measure Waders Flow meter Pencil and notepad

In order to calculate flow of stream , it is important to first calculate the stream velocity using flow meter. It is also important to find out the cross section of the stream graphically. The simplest way to measure discharge is to divide the channel cross section into vertical rectangular subsections. Once the area (width X depth) of each of these subsections is established and multiplied by velocity to determine subsection discharge, the results can be added together to calculate total discharge.

Discharge=Velocity x Cross sectional area

Or

Discharge = (Water velocity) x ((Width) x (Depth))

Where

Discharge = Q ( m3

second¿∨( fte

second)

Velocity = Meter/second or feet/second

Cross-sectional Area= Cubic meter or cubic feet

5. MEASURING DISCHARGE USING A WEIRWeirs are overflow structures built across open channels to measure Suppressed rectangular weir the volumetric rate of water flow. Discharge in small streams can be conveniently measured using a weir. A weir is a small dam with a spillway, usually made of erosion-resistant material such as concrete, of a specific shape. Two common weir shapes are a 90° V-notch or a simple rectangular cutout. This method for measuring discharge involves creating a dam just downstream of the weir. This dam impounds in the weir, resulting in a more or less consistent stage height (e.g. a pool of more stagnant water without complications determining height due to waves or ripples). Using the height of water in the weir, one can determine discharge using one of the following empirically-derived equations :

For Rectangular weir,

Q=3.33 (L−0.2 H) H 3 /2

The above formula is used for measurements in feet.

Q=1.84 (L−0.2 H )H 2 /3

The above formula is used for measurements in meter.

For 90° V-notch weir,

Q=2.5 H 5 /2

For measurements in feet.

Q=1.379 H5 /2

For measurements in meter.

Rectangular Weir and V Notch weirs for measuring flow

6. BUCKET METHOD

The Bucket method is a simple way to measure the flow rate using household items. It requires a stopwatch, a large bucket, and preferably two to three people. To measure the flow rate using the bucket method:

Measure the volume of the bucket or container. Keep in mind that a typical 5 gallon bucket is often actually less than 5 gallons.

Find a location along the stream that has a waterfall. If none can be found, a waterfall can be constructed using a weir (see Figure Four).

With a stopwatch, time how long it takes the waterfall to fill the bucket with water. Start the stopwatch simultaneously with the start of the bucket being filled and stop the stopwatch when the bucket fills. The bucket should not be filled by holding it below the surface of the stream because it is not the true flow rate.

Record the time it takes to fill the bucket. Repeat steps two and three about six or seven

times and take the average. It is a good idea to do a few trial runs before recording any data so that one can get a feel for the timing and measurements required.

Only eliminate data if major problems arise such as debris from the stream interfering with the flow.

The flow rate is the volume of the bucket divided by the average time it took to fill the bucket.

In this method the Steam flow rate is determined by using the formula :

Q=VT

Where,

Q= Stream flow in cubic units per second

V= Volume filled in the bucket during the time

T= Time estimated for the bucket filling up to a certain level

7. METERSMeters are devices that measure the stream flow by directly measuring the current. There are many different types of meters by the most common is the Pygmy meter, the vortex meter, the flow probe, and the current meter: They are briefly described below.

7.1 PYGMY METERa wheel is rotated by water flow and the rate of the rotation signifies the water velocity. It is primarily used in measuring discharge.

PYGMY METER

7.2 Vortex meterVelocity is proportional to the downstream frequency of the vortex flow and is read on a digital readout. It is used for measuring flow in pipes.

VORTEX METER

7.3 Current meterElectronic pulses determine water velocity. Can be used in large bodies of water like oceans to measure the current.

CURRENT METER