GRADED BEDDING AND CLIMBING RIPPLES

Provided by Gary Parker, St. Anthony Falls Hydraulic Laboratory, University of Minnesota.

Total Time= 1:19 (7.6 MB)

This shows a turbidity flow within a water-filled flume. The flow consists of a mix 10 cmture of fine sand, coal and water. The current is sustained but the coal is added intermittently to help clarify bedding.

The scale, visible along the bottom, is in meters. As the flow first enters the tank, from the right, rapid suspension settling leads to graded bedding. The coaly base of each bed is abrupt, coincident with the episodic introduction of coal. Over a short distance, 20 to 30 cm, the flow goes through a hydaulic jump and climbing ripples form.

0:03 &endash; 0:10 &endash; Two graded beds have been laid down and a third is just starting to settle (coal was just added to the mixture).

0:14 &endash; The hydraulic jump has migrated downflow over time. At the center of the screen in the lowest graded bed, you can see the rapid transition from horizontal bedding to low angle bedding. This boundary steps upsection and downflow over time and space.

0:22 &endash; Here the hydraulic jump is at the sediment/water interface.

 

0:29 &endash; First scour pit. The scour here is significant, cutting abruptly into the ripple immediately downflow. With imagination you can see counterflow by watching some of the black, coal, grains in transport.

0:31 &endash; 0:35 &endash; As the dune foreset first comes into view, note the avalanche of coal grains down the foreset. More avalanche motion takes place over the next few seconds.

0:44 &endash; 0:53 &endash; Abrupt close up of an avalanche on the dune foreset. Note that the motion seems to consist of a single grain thickness of coal grains.

0:55 &endash; 0:59 &endash; Note the toe of the previous avalanche is getting slightly swept back up the foreset by the counterflow in the adjacent scour pit.

1:00 &endash; 1:09 &endash; Another foreset avalanche.

1:10 &endash; 1:19 &endash; Pan back to show the train of climbing ripples

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