FLOW DATA

Flow rate is the quantity of water available in stream or river and may vary widely over the course of a day, week, month and year. In order to adequately assess the minimum continuous power output to be expected from the micro-hydropower system, the minimum quantity of water available must be determined. The purpose of a hydrology study is to predict the variation in the flow during the year. It is important to know the mean stream flow and the extreme high- and low-flow rates. Whenever possible, stream flow data should be measured daily and recorded for at least one year; two to three years is ideal. If not, a few measurements should be made during the low flow season. If you are familiar with the stream, you might determine the low-flow season by keeping track of water levels and making several low-flow measurements for more than a week when the water level is at its lowest point during the year.

Information could be obtained from neighbors or other sources.

There are a variety of techniques for measuring stream flow rate; the most commonly used are;

For very small streams, a common method for measuring flow is the container method. This involves diverting the whole flow into a container such as a bucket or barrel by damming the stream and recording the time it takes for the container to fill.

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The rate that the container fills is the flow rate, which is calculated simply by dividing the volume of the container by the filling time. Flows of up to20 LPs can be measured

using a 200-litre container such as an oil drum.

Figure 4.3 Flow measuring container method

4.5.2 WEIR METHOD

A weir is a structure such as a low wall across a stream. A flow measurement weir has a notch through which all 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. For reliable results, the crest of the weir must be kept sharp, and sediment must be prevented from accumulating behind the weir.

Weirs can be timber, concrete or metal and must always be oriented at a right angle to the stream flow. The weir should be located at a point where the stream is straight and free from eddies. It is necessary to estimate the range of flows to be measured before designing the weir in order to ensure that the chosen size of notch will be adequate to pass the magnitude of the stream flow. Rectangular weirs are more suitable for large flows in the range of 1000 LPs, and triangular weirs are suitable for small flows that have wide variation. A combination triangular/rectangular compound weir may be incorporated into one weir to measure higher flows; at lower flows the water goes through the triangular notch.

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Figure 4.4 flow measuring weir method

4.5.3 FLOAT METHOD

Float method of finding flow rate required the following steps 1. Measure the speed of the water (in feet per second)

2. Determine the cross-sectional area of the water source (in square feet) by measuring and multiplying the average water depth (in feet) X the average water width (in feet)

3. Calculate the flow (in cubic feet per second) by multiplying the water speed X the cross-sectional area.

Water Speed

Determining the water speed is easy. Pick a representative segment of river or stream close to the expected water diversion point. Place two stakes 50 feet apart along the bank, marking the upper and lower limits of this segment. Drop a Ping-Pong ball (or other lightweight, floating object) into the current opposite the upper stake.

Time (a wrist watch with a second hand works great!) how long it takes for the Ping-Pong ball to travel the 50feet. Take this measurement several times and calculate the average time (add all times and divide by the number of trials). This is the speed of the water through the segment at the surface. Not all water moves as fast as the surface because there is friction at the bottom and along the banks. This velocity must

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then be reduced by correction factor, which estimates the mean velocity as opposed to the surface velocity. By multiplying averaged and corrected flow velocity, the volume flow rate is estimated. This method provides only an approximate estimate of the flow. Approximate correction factors to convert measured surface velocity to mean velocity are as follows:

Table 4.1 stream friction correction factor (n)

Stream Type Fraction Factor (n)

Concrete channel,rectangular,smooth 0.85

Large,slow,clear stream 0.75

small,slow,clear stream 0.65

Shallow(less than 0.5m/1.5ft.) turbulent stream

0.45

Very shallow rocky stream 0.25

Cross-Sectional Area

Now we can measure and calculate the cross-sectional area of a „slice‟ of the water. In the segment used above for determining water speed, select a spot that will provide a representative water depth and width for the 50 ft. segment. Measure and record the water depth at one foot increments along a cross section (water-edge to water-edge) of the river or stream at this spot. Laying a log or plank across the river or stream from which you can take these measurements is convenient. You can also wade (or boat) across but take care that you are measuring the actual water depth and not the depth of water affected by your presence in the water. Calculate the average depth of the water (as explained above during water speed).Measure and record the width of the river or stream (in feet and from water-edge to water edge).Multiply the average depth X the width. You now have the cross-sectional area (in square feet) of that „slice‟ of the river or stream.

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Figure 4.5 flow measuring Float method

Calculating Flow

The following equation is used to calculate Flow.

Water Speed (ft/sec) X Cross Sectional Area (sq ft) =Flow (cubic feet per second) X 450 = Flow (gallons per minute)

Calculate the flow in cubic feet/second first by multiplying the average speed (in feet per second) X the cross-sectional area (in square feet). Then convert the flow from cubic feet per second to gallons per minute (GPM) by multiplying the cubic feet per second X 450.