Gravity Current on a Slope


When two fluids of differing densities interact in such a way that a vertical interface exists between the fluids, the resulting motion consists of the heavier fluid flowing horizontally beneath the lighter fluid. Such a flow is said to form a gravity current.

Gravity currents are widespread in nature, and their common characteristics are observable in avalanches, heavy gas releases, turbidity currents, fresh and salt-water exchange, and sea breezes. An excellent review of the nature of gravity currents exists in the book by John Simpson, Gravity Currents in the Environment and Laboratory (Cambridge University Press, 1997.)

One of the important factors which contribute to the structure of a gravity current is the topography over which the current flows. Although a horizontal surface is the ideal case to study, such a situation is usually the exception rather than the rule in most physical applications. In a series of experiments in 1966, G. V. Middleton noted that in general, gravity currents flowing over small slopes (up to about 3.5 degrees) have similar properties to those flowing over horizontal surfaces. He also further quantified previously known results, stating that the front of the gravity current moves with a speed that is proportional to the height at the front of the gravity current, and a specific ratio of the density differences between the two fluids concerned.


This experiment is designed to study the influence of bottom topography on a propagating gravity current. It is limited to gravity currents created by instantaneous volume releases in a finite rectangular tank.

Experimental Set-up:

A large rectangular tank was used to created the gravity currents, with the following dimensions: 198 cm long, 17.5 cm wide, greater than 30 cm high. At one end of the tank, a plexiglass plate could be inserted vertically at diferrent locations, thus partitioning the tank into two sections. For the purposes of this experiment, only two positions were used: the left and right positions at 8.5 cm and 18.5 cm, respectively, from the end of the tank.

With the tank filled (up to a height of approximately 18 cm) with water and partitioned, salt was added to the smaller section at the left, giving that section a greater density than the tap water to the right. Lifting the partition thus created a gravity current propagating the the right along the bottom of the tank. To view this current, dye was injected into the salt (heavy) water. With a fluorescent light behind the tank, and a 5cm by 5cm grid drawn on the side, the entire flow was recorded on tape to be played back and analysed quantitatively later.

The problem of changing the slope of the bottom was overcome by placing foam plastic wedges of varying thickness underneath the tank, thus elevating one end while providing support at points along the tank's length. With the camera tilted appropriately at a similar angle to the tank, the analysis of the recording was simplified.

Experimental Procedure:

1. Insert lock and fill the tank with water.
2. Add salt and dye behind the insert, stir, and measure the density via optical density measurement.
3. Start the camera, and remove the lock.


Four experiments were conducted, from which captured images from the video recording are displayed below. Experiments 1 and 2 were initiated using similar densities from the left lock position, with the major difference that the tank was horizontal in Experiment 1 and inclined at 1.4 degrees in Experiment 2. Experiment 3 was conducted with the same slope and with a similar density difference as in Experiment 2, but with a larger volume gravity current created by using the right lock position.

The recording of the experiments allowed quantitative results to be obtained through use of the "digimage" program. Although these results are not displayed here, as they require some interpretation, quick calculations did produce results which were commensurate with Middleton's 1966 findings.

Several qualitative observations are listed below the gravity current pictures.
Experiment 1:
Experiment 2:
Experiment 3:
A movie of this experiment is available.
Some qualitative observations made during the experiments were:

1. The gravity current velocity increased with both volume of initial release and density difference between the fluids.
2. The experiments initiated at the left lock position (lower volume gravity currents) showed a slowing down of the gravity current in the later stages of flow. This was not observed in the large volume gravity current experiments, and is likely due to the increased importance of viscous effects as the gravity current thins.
3. Less diffusion at the interface was apparent in the experiments with a larger density difference.
4. The slope did not have a large effect on the form of the gravity current, although the speeds were typically higher in the experiments with nonzero slope.
5. The gravity current reflected at the wall, returning with a similar speed after reflection.
6. The front height was approximately twice the height of the following flow.
7. Interference by the bottom and side walls was slight unless the gravity current became thin (less than a couple of cm).
8. The gravity current was not steady - the following flow was seen to be moving faster than the front by observing small dirt particles in the fluid.

These experiments were performed and written up by
Patrick Montgomery

Last updated by:
Bruce R. Sutherland, Sept. 98.