Tack Dr. Hjulstom EarthCache
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This EarthCache is located on Minnesota State University Moorhead's outdoor Regional Science Center land. Park at the listed coordinates and follow the trails.
To claim a Find for this EarthCache e-mail me answers to the following questions;
1) How many seconds did it take your object to travel the 25m?
2) What was speed of the stream in cm/sec.?
3) At the speed you found; at what size will particles not be moved?
4) To the east of the main building by the parking area is a smaller building. What is written on buildings west wall?
It should not be a new idea for anyone that water in streams and rivers is often moving. Many of us have seen things being carried on the top of moving water, from sticks and toy boats in a creek or rill to houses in flooding rivers. But have you given much thought to the material that is carried or pushed by the water below the surface?
Some things are suspended in the water of streams, other material is pushed along the bed or bottom of the stream and some things are soluble, meaning they can dissolve in a stream and form a solution with the water. For this EarthCache we will be focusing on material that is suspended in or pushed by the current of the water.
The more force or energy the water has the bigger the material that can be moved. Material suspended in the stream will move with the current as long as it is lifted off the stream bed. Small particles, like fine clay particles, will stay suspended as long as the current is fast enough to overcome the downward pull of gravity. The faster a stream flows the higher the volume, or quantity, of water moving past a fixed point. As the volume of flow increases so does the stream's ability to carry, roll or slide material in the water.
As the water in a stream slows down the stream's ability to carry suspended particles goes down. Settling velocity is the speed at which suspended material settles, or falls, to the stream bed. A particles settling speed is dependent on it’s size. Swedish geographer Dr. Filip Hjulstom (1902-1982) studied soil erosion and transport. His studies lead to his developing the Hjulstom curve. The Hjulstom curve is used to find out what will happen to particles of various sizes in currents of different velocities.
Hjulstom Curve
The graph has three colors; green, yellow and red (I know, they are more like moss, tan and something that looks like the dusty rose of the late ‘80s, but let’s just go with green, yellow and red). On the left side of the graph you will find the speed of the rivers current in centimeters per second and at the bottom of the graph is where you will find the size of particles, given in millimeters.
If the stream is flowing at 1 centimeter per second (1 cm/sec) and you want to find out the current's effect on a piece of gravel that is 1 mm; simply find the point on the graph that is directly to the right of the 1 on the left side (speed) and directly above the 1.0 on the bottom. The place they meet is in the red area, indicating the force of the water will not move the piece of gravel. If the speed of the current was 100 cm/sec and the size of the gravel was 1.0 mm the point to the right of the speed and above the size would be in the green area. Indicating the current is strong enough to move the particle, so erosion would be taking place.
The posted coordinates will take you to a bridge crossing of the Buffalo River. Twenty-five meters west of the bridge, on the North bank is a steel rod with a pink flag and a yellow flag that has the number “1” on it. This is the part that will require two people; one person will stand on the bridge and the other will stand next to the steel rod. The person on the bridge will drop something in the water; the geologist I teach with has the students drop oranges, they are easy to see, biodegradable and by not using sticks she keeps scores and scores of middle students from braking branches off of the nearby trees. When the orange hits the water the cacher on the bridge calls out “Now” and the cacher on the stream bank starts their stop watch or makes note of the position of the second hand on their watch. The person the on the bank is in charge of measuring the number of seconds it takes the orange to travel the 80 feet from the bridge to a point in the river in line with the steel rod.
If an orange travels the 25 meters in 44 seconds we can find meters per second (m/sec) by dividing the distance, 25 meters, by the time it took the orange to travel the distance, 44 sec.; 25 meters ÷ 44 sec. = 0.568 m/sec.
To use the Hjulstrom chart above we will need to convert from m/sec to cm/sec. One meter is equal to 100 cm, so in the above we would multiply the 0.568 m/sec by 100 to find cm/sec; 0.568 m/sec * 100 = 56.8 cm/sec. With the stream current at 56.8 cm/sec we find 56.8 on the left side of the chart and look to the right to see how the current will affect different sized materials in the stream. Fine and medium sized sand of 0.02 mm size will be moved if present in the stream. Granules with a size of 7 mm or larger will not be moved by that current. Material that is in the Transition Zone may or may not be transported.
References
Press, Frank & Siever, Raymone (1993). Understanding Earth. New York: W. H. Freeman and Company. Farndon, John (1994). Dictionary of The Earth. New York: Dorling Kindersley Publishing, Inc.
Thank you to Tony Bormann and the Minnesota State University Regional Science Center for giving permission for the placement this EarthCache. The grounds and trails are free and open 6am to 11pm everyday. The Regional Science Center can be contacted at scicentr@mnstate.edu .
Brava to d.. for being the First to Log!
Additional Hints
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