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Poohsticks EarthCache

Hidden : 03/28/2021
4.5 out of 5
4 out of 5

Size: Size:   other (other)

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Geocache Description:

Any survey of a river or stream generally includes measurement of several cross-section profiles and the water flow through that point. A channel cross-section is essentially a "slice" through the channel, made at right angles to the flow. Over time, such cross sections can be used to examine changes that occur in the profile as a result of natural or human induced events.

You are going to measure a cross-section and the river flow of the upper reaches of the North Pine River.  At this location the river is generally shallow, though can be deep in places.  You will need to wade across the river taking depth measurements.

It is important that you consider your safety at all times when around water.  Keep an close eye on any children. This area can flood after heavy rain. Do not attempt this cache if conditions are not safe.

The GZ is usually a suitable location for taking measurements. However, if current conditions make it too difficult to perform the tasks at the listed location, then do them as close as possible to the listed location.  If not measuring at the GZ, you need to provide coordinates of the location measured and provide a photo of the location.

To make your cross-section, firstly identify a point on each side of the bank forming a line at right angles to the flow of the river and crossing the posted coordinates.  The points on the side of the river form an imaginary line where you will take your measurements.  If you have a long piece of rope, you may like to attach it across the river to make keeping track of your distances easier.

You will need to take at least 5 depth measurements across the river.  At each location, measure the distance to the bank (or the distance from your last measurement), and the depth of the water (in centimetres).  There are multiple ways to measure the water depth - you may use a wooden pole with distances marked on it, or a weighted piece of string.  Measurement points do not have to be regularly spaced, but where possible, should represent changes in the river bed profile.

Once you have the measurements, draw a scale map of the river cross-section profile simlar to the example below:

You can calculate an approximate area of each part of the cross-section by treating the segments as trapeziums.   Then the area is calculated as 1/2 * (h1 + h2) * w, where h1 and h2 are the heights of the two sides of the segment and w is the width.  Sum all the segments together to obtain an estimate of the cross-sectional area of the water. 

Now we will measure the velocity of the water in each section. To do this, we will take a simple approach.  Place a Poohstick (or a leaf, or anything floating), a short distance back from where you measured the cross section.  Wait for it to cross your measurement line (as close to the middle of the segment as possible) and measure how far in cm the stick moves in around 30 seconds.  Then calculate the average distance moved per second (if you waited 30 seconds, divide your meaure by 30). Finally, calculate the volume of water moving through each part of the cross-section by multiplying the average distance per second by the cross-section area.  Do this for all sections and add the amounts together. (The proper way to measure velocity is much more complicated as velocity changes with depth and is affected by what materials are on the base of the river.)

Finally, convert your flow measurement to L/s.  Assuming all your measurements were taken in centimetres, you can divide your total by 1000 to get L/s.


To log this Earth cache, you are required to send the following to the cache owner:

0) If you were unable to perform the measurements at the listed location, provide the coords and a photo of where you measured the river.

1) A picture of your cross-section, including measurements
2) Provide your estimate of the area of the cross-section of the water
3) Your estimates of the velocity of the water in the middle of each section of the cross-section
4) Your estimate of the river flow in L/s.

Optional: Post a photo of yourself measuring the river.


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