Clast Ploughing

School of Geography, University of Leeds


The applet below ploughs rocks (clasts) trapped in a glacier bed into a virtual slope of mud (till) until they stick in the slope (achieve lodgement). At the same time, it calculates the amount of ice that melts because of pressure on the rock, the force applied to the rock by the ice, and the length the clast travels.

This program is a development of that used in Evans (1998), please see chapter five of that work for a full explaination, though note the differences outlined below. For further information, please contact Andy Evans.

Launch applet in new window (may take a short while to appear)

The program runs in two modes. In the first, a single clast is ploughed into a slope using a given ice speed and till residual strength. In this mode the characteristics of the ploughing process are given for each day the ploughing occurs allowing a picture to be built up, for example, of the amount of sediment melting out of the ice at each point on the ploughing gouge.

In the second mode, a ploughing length is given, and various combinations of ice velocity and till residual strength are tried to see if they can produce the given ploughing length. If they can, a set of characteristics for the ploughing event are given, for example, the total amount of sediment melted out of the ice.

The applet can also be run as an application. If run as an application it allows you to specify an output file, which considerably speeds up runs.

If you run the program as an applet, you can still get results by using ctrl-x to copy the highlighted results from the applet.

To run the program as an application, download the file plough.jar and double-click on it in whatever file manager you use (e.g. Windows Explorer). If the program doesn't run, you'll need to install the JRE (this is installed on most machines already). You can find a copy at Sun.


General parameters

Inflow percentage
As the clast ploughs, it gouges through the till. This option allows you to set some percentage of the material gouged out of the way to flow back into the gouge.
Clast Width
This is the clast length in the direction of ploughing and also the length transfer to this.
Clast Height
The vertical length of the clast.
Slope the clast ploughes into
This is the slope angle in degrees. It could be used to indicate prow buildup, but the physics of this is less certain than that of ploughing into a slope.
Initial debris fraction
This is the amount, in kg, of sediment released from the ice by the melting of 1kg of water. Note that this is not quite the same thing as the ratio of sediment to water in the ice.
Till internal friction
The angle of internal friction of the till. 22 degrees is appropriate for a silt-clay mix with some sand.


Switch Modes
This button allows you to switch between the two modes described above.
Calculate for one speed/strength / Calculate for all speeds/strengths
Runs the program. As a minimum the program will tell you it's started and finished.
Clear screen
Clears the screen of results and messages.
Set output to file
This only appears if the program is run as an application. It allows you to divert results to a text file, rather than the screen. A few messages from the system are still shown on the screen.

Parameters for testing a single plough event

Till residual strength
The residual strength or residual supported stress of the till. This is the amount of stress the till can support after it has yielded. It is assumed in this model that the yield strength is zero.
Initial ice velocity
The ice velocity in metres per year.

Parameters for testing a multiple ice velocities / till strengths

Starting velocity / Ending velocity / Step between velocities
Allows you to set up the first and last velocities to try. The program will step between the two at a rate determined by the step value. Each velocity will be tried to see if it produces the requested ploughing length.
Starting till strength / Ending till strength / Step between strengths
Allows you to set up the first and last till strengths to try. The program will step between the two at a rate determined by the step value. Each till strength will be tried to see if it produces the requested ploughing length.
Minimum ploughed length / Maximum ploughed length
Allows you to set a length the clast can plough. The lengths between and including these two figures will be looked for as the program tries various ice velocities and till strengths. Any combinations that produce a length between these two will be saved.

Changes from Evans, 1998

For details of the changes, contact Andy Evans.

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