Waterwatch Australia national technical manual

Module 4 - physical and chemical parameters
Waterwatch Australia Steering Committee
Environment Australia, July 2002
ISBN 0 6425 4856 0

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Methods

This section contains information about tests you can conduct; what they are for; and how to do them.

Some physical and chemical characteristics of water quality can be measured more simply and quickly than others.

There are three levels of difficulty — only the first two levels are described in this module.

The first (the 'very simple' level) includes flow, pH, salinity (total dissolved solids), turbidity (total suspended solids), temperature and dissolved oxygen. These can all be measured by anyone in the community including students at all levels of schooling.

The second — measuring concentrations of phosphate and nitrate - even though relatively easy to do, is suitable only for members of the community and for students in upper primary and secondary school.

The third — measuring levels of chlorophyll-a, faecal coliform bacteria (using filtration and culture equipment), pesticides and heavy metals - are not included in this module, but information about them will be available from your Waterwatch coordinator. It is recommended that only upper secondary school students (particularly those studying science) and trained members of the community be involved.

Flow

What is it and why does it matter?

Flow: the volume of fluid (in this case, water) that passes through a passage of any given section in a unit of time.

The water generally comes from surface run-off and from water that has passed through the soil and out into the waterway.

The amount of any particular substance carried in the water is known as the load. The faster and bigger the flow of the water, the stronger the current, and the heavier the load it can carry.

When there is little water in the waterway (low flow) most of the water entering the stream will be from underground seepage, and the flow rate is slow. Sediment settles quickly to the bottom, sections of the stream will become semi-stagnant resulting in low dissolved oxygen concentrations, algal growth will increase if there is adequate light, leading to algal blooms, and salinity and water temperature may increase to values that affect the biota in the waterway.

Moderate flows ensure good mixing of oxygen with water, and dilution and flushing of contaminants.

After heavy rainfall the water level rises or floods (high flow) because run-off rushes into the waterway increasing turbidity and the load of contaminants. During flooding, the concentrations of oxygen, turbidity, pH, salinity and nutrients can be expected to fluctuate.

For the purposes of measurement, flow is the velocity of water multiplied by the cross-sectional area of the stream. These two quantities must be measured as accurately as possible to avoid compounding errors when calculating flow.

What factors affect flow?

Flow is modified by conditions along and around the waterway, such as:

The size of a waterway and its flow rate affect its water quality. For example, discharges containing contaminants will have less effect on large swiftly flowing rivers than on small slow streams. This is one reason for measuring flow - to work out the load of contaminants and sediment the waterway is carrying.

Because velocity and flow have a significant effect on water quality, it is important that you record them at the time of sampling and, if possible, during the previous few days. It is particularly valuable to know if flows are at low, moderate or high level and if the level is rising or falling. This is because the concentrations of nutrients, turbidity and contaminants tend to be higher when the stream level is rising than when it is falling.

Suggested methods, equipment and reporting

There are three ways to measure water velocity. A simple method is to see how fast a floating object travels downstream over a chosen distance. This is called the float method. Secondly, flow data can be obtained from the local water authority, if your site is near a gauging station. The local water authority measures and keeps records of flow on a regular basis at gauging stations spaced out along all main waterways.

Thirdly, the head rod method can be used. Note, however, that head rods are limited to relatively shallow streams with medium velocities.

Float method for determining water velocity

The float method is easy to understand and something most of us have done as children. You simply float an object on the water and measure the time it takes to travel a set distance.

Equipment

The equipment you will need for this method includes:

Procedure

  1. Mark out a 10-metre length of the waterway, upstream of your sampling site. Choose a section of the waterway that is relatively straight and free of vegetation or obstacles. Avoid areas with a culvert or bridge because those structures will modify the true flow. If the flow is very slow, mark out a shorter distance.
  2. Position a person at each end of the 10-metre section.
  3. Place the ball on the surface near the middle of the waterway at least 2 metres upstream of the end of the tape so it has time to come up to water speed.
  4. When the ball is in line with the beginning of the tape, start the stopwatch.
  5. Stop the watch when the ball gets to the end of the 10-metre section.
  6. Repeat the procedure at least three times at this site and average the results.

To calculate the water velocity, divide the distance travelled in metres by the time taken in seconds. Then multiply by a correction factor of 0.9 to compensate for the variability in velocity with depth and across the channel, i.e. water will flow more slowly at the edges than in the middle, and more slowly near the bottom than near the surface. For example:

distance travelled = 10 metres
average time taken = 18 seconds
correction factor = 0.9
∴ stream velocity = (10 x 0.9) 18 = 0.5 metres per second

Head rod method for determining water velocity

The head rod method is quick and possibly more accurate than the float method for shallow streams.

Head rods are limited to use in relatively shallow streams. Also, they may be difficult to read for velocities of less than 0.5 metres per second and they are hard to handle in velocities greater than about 2.5 metres per second.

The method involves wading across the stream, and measuring the water depth at approximately equal intervals across the stream, including the deepest part. Five to ten measurements are recommended, and the more you take the more accurate your estimate will be.

Equipment

The equipment you will need for this method includes:

A head rod

A head rod is a 1 metre stainless steel ruler about 40 mm wide, or a wooden ruler with a bevelled edge. The bottom end should sit on a small flat disk to prevent it from sinking into the stream bed during use. The width can vary within the normal range of ruler widths.

Procedure

  1. Place the rod in the waterway, a small distance from the edge (up to 0.5 metre in larger streams) where there is measurable flow. Measure the depth of the stream in metres with the thin edge of the head rod facing into the flow. Record the height (m) of the water against the rod (depth 1).
  2. Rotate the head rod 90° so the flat side faces the flow, creating a standing wave or 'head' (see Figure 4.1). Record the new height of the water (in m) at the top of the standing wave (depth 2). The difference between depth 1 and depth 2 in metres is the head. Record this in your data sheets.
  3. Repeat these measurements at 5 to 10 different points across the stream.
  4. Calculate the average head (h) in metres from these measurements.
  5. Calculate the average velocity of the stream in metres per second (m/s) using the formula below.

Average velocity (m/s) = √ (2gh), where g is the gravitational constant of 9.81*

e.g. If the average head height is 0.5 m, the average velocity is √ (2 x 9.81 x 0.5) = 3.13 m/s.

If the average head height is 1 m, then the average velocity would be √ (2 x 9.81 x 1) = 4.42 m/s.

* The gravitational constant is not significantly affected by height above sea level

Figure 4.1: Using a head rod
Figure 4.1: Using a head rod
Method for determining stream cross-section

The cross section is determined by measuring the width and depth of the waterway, and multiplying these measurements together (see Figure 4.2). The depth will vary across the waterway and so the width and depth should be measured in small intervals and aggregated to determine the total area.

Use two posts or similar to stretch your measuring string (marked at 0.1 m intervals) across the waterway at the water surface.

Figure 4.2: Plotting the stream channel on graph paper.
Figure 4.2: Plotting the stream channel on graph paper.

Measure and record the depth of the water at each interval.

Multiply each depth reading by 0.1 m and add the results together to determine your cross-sectional area in square metres (m2).

At a culvert, the cross-section is usually either circular or rectangular. Simply measure the width or diameter of the pipe. On graph paper, make a scale drawing of the shape of the culvert and count the number of squares to find the cross-sectional area.

Equipment

The equipment you will need for this method includes:

Procedure

  1. Secure a marker peg or post, such as a star picket, in the stream bed. This will mark your measuring section and be your reference marker. Place it where it will be accessible when the stream is flowing but not so it obstructs flow. Set the reference marker up so it is above the deepest water height you expect to measure.
  2. Use star pickets (preferable) or tomato stakes or survey pegs or similar to hold your measuring tape across the stream adjacent to the reference marker and use a spirit level to ensure the tape is horizontal.
  3. Prepare a table with column headings for horizontal distance, depth and area (see Figure 4.2).
  4. Measure from the horizontal tape to the top of the reference post.
  5. Record this value.
  6. Take depth measurements from the horizontal tape to the bottom of the creek at 0.1 metre intervals, as described earlier.
  7. Add the value recorded in Step 5 to each depth.
  8. Enter this value in the depth column.
  9. Record the horizontal distance from your starting point and depth in this way as you work your way across the stream at each interval.
  10. From your recorded measurements draw the stream cross-section on graph paper and calculate the total area from the sum of the areas of the 0.1 metre intervals.

Reference markers

Establishing reference markers can make determining the cross-sectional area much simpler when you are sampling at sites on a regular basis. By setting up a reference marker in your waterway and developing a cross-sectional area graph you can simply read a depth level off the reference marker and determine cross-sectional area from the graph. After storm events or flooding, the waterway channel may change in shape and size; if so, you will need to recalibrate the cross-sectional area graph.

This use of reference markers is best for small streams, culverts, V-notch weirs, or sites located at bridges. You may be able to measure depth of slow-flowing rivers by carefully taking the measurements from a boat.

Data confidence

To make sure the flow data you collect are of good quality, repeat the velocity measurements (by float or head rod methods) 5-10 times each, and make 20 measurements of water depth. Once you have enough measurements over time, it is possible to make a flow rating curve (see Figure 4.3). Subsequent calculations of flow can be compared with that, to check they are within the normal range.

Figure 4.3: Flow rating curve for a stream monitoring site
Figure 4.3: Flow rating curve for a stream monitoring site

Interpreting your results

Velocity is relatively easy to work out and, by itself, provides a rough measure of the likely effect of contamination on water quality in the stream. By comparing the results of measurements at different times at your site you will be able to identify low, normal and high flows. You may find corresponding changes in other water quality results. For example, you may find that turbidity tends to increase as velocity increases.

By combining your flow measurements (velocity and cross-sectional area) with your corresponding measurements of, say, phosphorus concentrations, you can estimate the loss of phosphorus per hectare of catchment, along your waterway. Your results are only a guide to what is happening in the catchment at a particular time. The final result depends on the accuracy of both the water quality test and flow measurements, so you can expect errors in load estimates of up to 50%.

You can have more confidence in your data if they are for a range of flows. For example, if you sample only moderate flows, you are possibly under-estimating the loads lost from the catchment during the whole year because the highest loads are carried in floods. On the other hand, if you only sampled high flow conditions, the estimated loads are probably over-estimated.

Nevertheless, load estimates provide an interesting picture of what is happening in your catchment and allow you to identify some possible causes of problems, particularly when put together with other results and information.

How to calculate load estimates

You need to know the flow in your waterway to estimate how much of any particular substance is being washed downstream. For example, you may wish to know how much phosphorus (P) is moved by your waterway in an hour or a day. This sort of quantity is called the instantaneous load.

Instantaneous load of P = flow x phosphorus concentration

If your water monitoring gives a phosphorus reading of 3 mg/L and your flow is 37.5 L/s, then

instantaneous load of P = 37.5 L/s x 3 mg/L = 112.5 mg/s.

Multiply this by 3600/1000 to calculate grams of phosphorus per hour, and then by 24/1000 to calculate kilograms of phosphorus per day.

So, 112.5 mg/s x 3600 s/h = 405,000 mg/h = 405 g/h, and

405 g/h x 24 h/d = 9720 g/d = 9.72 kg/d - even though the initial P concentration seemed quite low!

Only small amounts of phosphorus naturally come from the soil. Most of the 10 kilograms of phosphorus calculated above would be the result of land use practices in the catchment. It could be due to fertilising in the area or direct dumping of wastes into the water.

If the phosphorus is leaching in from the soil we can calculate the loss rate of P, which is the rate of loss of phosphorus from the land into the water for a given unit of time. We usually think of loss rate in terms of hours.

Loss rate = instantaneous load (kg/h) ÷ catchment area (hectares)

So, for a 400 hectare (ha) catchment, loss rate = 405 g/h ÷ 400 ha = 1.0125 g/ha/h.

How to calculate the flow of water passing through your site

The flow (discharge rate) is the volume of water - in cubic metres (m3) or litres (L) - that flows past a specific site every second. We can express the flow in cubic metres per second (m3/s) or megalitres per day (ML/d).

Remember that 1 m3 of water = 1000 L.

To calculate the flow, multiply the cross-sectional area of the stream underwater at your site by the stream velocity.

Flow (m3/s) = cross-sectional area (m2) x velocity (m/s)

Example:

Cross-sectional area = 0.6 m2
Velocity = 0.3 m/s
Flow = 0.6 m2 x 0.3 m/s
  = 0.18 m3/s
  = 180 L/s

Record your results on a water quality results sheet.

Safety considerations when measuring flow

The flow rating (discharge) curve

If you have been calculating stream cross-sectional area and velocity at your own site, you can establish your own flow rating curve to make it easier to estimate future flows. The curve will also allow you to identify possible errors made in measuring velocity and cross-sectional area of your site.

When you have measured discharge for a range of water levels (e.g. Table 4.2), you can plot a graph which will allow you to estimate discharge by simply measuring down from your reference mark to the water surface.

To prepare a graph, as shown in Figure 4.3, plot your measured discharge values (flow) against the distance from the reference marker for each measurement.

Draw a smooth curve of best fit through the points.

As new measurements become available, add the values and adjust the curve as necessary.

Ideally, the points should fall on a smooth curve. However, this depends on the channel shape at your monitoring site. The accuracy of your flow estimates will be better at sites that are relatively regular in shape, such as an irrigation ditch, with straight smooth sides and a profile of the channel that follows the same contours both above and below the water.

 
Table 4.2: Example of discharge data
Date
and
time

Distance travelled by float
Time in seconds (s)
Velocity (m/s)
Corrected velocity (m/s)
Distance (m) of reference mark to water surface
Cross-sectional area (m2)
Discharge (L/s)
 
6
23
0.26
0.23
1.04
0.20
46.0
 
15
45
0.33
0.30
0.88
0.43
129.0
 
10
62
0.16
0.14
0.11
0.11
15.4
 
10
40
0.25
0.23
0.25
0.25
57.5
 
10
24
0.42
0.38
0.59
0.59
224.2

Your flow estimates for low to moderate flows should fall within 10% to 15% of a line forming a smooth curve.

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