Last update: April 3, 2012 09:26:32 AM E-mail Print



F S Greyvenstein & C P M de Villiers


What is a Jack?

It is a structure, formed by three poles, mutually perpendicular to one another and tied together as shown in the sketch.


Are all the poles of equal length?



What is a practical length for such poles?

Three metres.


How are the wires kept in position?

By drilling a hole 150 mm from the end of each pole through which, say an 8 mm steel rod fits tightly with the ends protruding about 30 mm. The rods will prevent the wires sliding down against the pole. Another method is to make a notch at the end of each pole in which the wire is held in place. This is not as effective and also weakens the pole as it decreases its effective thickness.


What is the purpose of such Jacks?

The purpose is to form an obstruction at localities where they are placed in the watercourse, causing the velocity of the water to be retarded and sediment to be deposited. Such localities are selected where stabilization and reclamation is essential.


Do jacks form an efficient obstruction?

Yes, definitely. It is remarkable to see how much debris is caught by the wires and the poles of such a jack and to what extent sediment is deposited in their immediate vicinity.


Where are jacks most often used?

Especially on bends of water courses but also at other places where erosion occurs and where it is necessary to gradually force the flowing water further away from eroding banks.


How are jacks placed?

As shown in the sketch, stones are tied to the jacks and serve as anchors. Each jack in the row is tied to a cable the ends of which are anchored to a log or a stone. The latter is buried in a hole dug in the ground.

There is no definite rule about the spacing of jacks. This depends on circumstances at each site, ego stream velocity. It is obvious that the closer the jacks are spaced, the more obstruction is caused and the greater the extent to which stream velocity will be reduced, in other words, the goal will thus be achieved quicker and better. As a practical guide the spacing of five metres should generally not be far off the mark.


Have Jacks already been erected in South Africa for reclamation purposes?

In the neighbourhood of Aliwal North jacks have been placed at various localities.


Were results satisfactory?

It actually surpassed all expectations. The extent of the reclamation that occurred within a relatively short time, was really surprising.

The accompanying sketch clearly indicates the construction of a jack.



The illustration also shows the method of placing the respective jacks so that maximum obstruction can be achieved, and to ensure the highest degree of protection.



Karoo Regional Newsletter June 1977