Stream Transportation

So now that it is understood the ways diverse kinds of dredges work, we will now go into talking about how they suck up the gravel and sand from the stream bed. The reality is that they do not actually suck anything up; what actually occurs to it is that it is entrained just like the water close to the jet of the jet pump. What occurs is that the gravel becomes suspended, pulled into the suction nozzle stream and here is it carried away by the momentum of the water current.

People that know about stream transportation will know that when a current of water is flowing it starts to move the sands of the bed and that this first happens by saltation and then the velocity suspends the sand and carries it down the stream as if it were all floating. Well, the same event occurs close to the edges of the suction nozzle of the dredge. The pressures on the outside force the water to the nozzle with its lower pressure and therefore it creates a little current that looks a little bit like an eddy. This then causes the sand to suspend and it is then swept away in the water current of the suction tube.

When this event is taking place big stones can actually be mobilized. In most cases what occurs is that the sand around the stone is taken away and the pressure from the water current starts to focus its force on the stone. As soon as there is enough force underneath the stone it is lifted up and swept up by the tube. A dredge that is well made has the ability to lift up a stone that is two times more than its suction nozzle sometimes, however the results are not favorable in most cases. We will get into why later on.

So how is it that gravel is able to transport itself up the suction tube so easily? This is actually something easy to respond. If you are new at sluicing, keep in mind the weight of a stone when it is in the air. When a stone is under the water the weight of the stone is very reduced because of the principle of buoyancy. The fact that there is a lot less weight is a big advantage in getting the stone or gravel to the jet where it is then speeded up even more and very easily acquires enough momentum on its own to travel to the pressure box.

If less amount of weight were the only advantage the dredge obtained, it would not be quite as useful though. However keep in mind the law of the inclined plane. This actually does not lessen the weight of the stone but brings down the forces needed to keep it moving after the acceleration is the same as the velocity of the column of water that is moving.