THE WATER DREDGE

There are two common techniques for the removal of overburden and excavation of wreck sites;  the airlift and the water dredge.  The principle of the airlift is probably the best understood and an article detailing the construction and operation of one particular design was the subject of an article in Diver April 1982, Vol. 24, No.4.  This section will hopefully give divers a better understanding of the principles, construction and use of the dredge, and attempts to highlight any advantages or disadvantages when it is compared to the airlift.

PRINCIPLES & CONSTRUCTION

A dredge is shown schematically in Figure 1 above and can be constructed very simply using plastic drainpipe and fittings (100-150 mm diameter are the most practical).  Design features like the  50mm swivel coupling, which avoids problems with the hose kinking on the seabed, and the inspection hatch on the side of the 'Y' section drainpipe are not essential, but make the basic design easier to use and maintain underwater. 

Unlike the airlift which uses air (and Boyles law) as its power source, the dredge uses a combination of water pressure and water flow from a powerful water pump to create the necessary suction at the nozzle.  Many people consider that the dredge is only a shallow water device, this is not the case, they can been used very effectively at 30 metres.  There is no need to worry about the pressure to be overcome at the working depth as is the case with the air supply to the airlift.  The water pressure at any depth is proportional to the weight of an imaginary column of water above, plus atmospheric pressure.  Since the hose merely contains this ‘column’ the pump does not have to do any extra work in pumping on the surface or underwater to 30 metres.  Flow losses do occur due to resistance as the hose length is increased; these losses are generally acceptable however.

The water enters the dredge via the supply hose and control valve (optional) to a jet fixed in a blanking plate mounted in the blind arm of the ‘Y' pipe coupling.  The jet, made from suitable copper pipe and fittings, must have a smaller cross-section than the supply hose so that pressure builds up behind it, directing pressurised water down the centreline of the discharge tube.  The velocity and flow of water at the jet creates a suction at the nozzle, commonly known as the ‘Venturi’ effect.  An example of dredge head construction is shown in Figures 3 to 6.

Providing the pump is powerful enough and power losses are kept to a minimum, a well designed dredge will suck up almost similar quantities of material to a comparable diameter airlift.  The only thing it will not do is bring the material to the boat unless the water is very shallow.

ANCILLARY EQUIPMENT

The major items are the pump and the hose.  Correct choice of water pump is essential if the optimum performance is to be obtained from the dredge.  Flow rate is major consideration but equally critical is the head pressure which is maintained behind the jet at the designed flow rate.  Figure 7 shows the performance of the 50mm ALCON Pump which is one of the more use to power our 150 mm diameter dredge.  It can be seen that the petrol engine option provides a significant increase in pump performance when compared with the diesel version.  Figure 7 also shows the effect of suction height (or suction head) and the need to keep the pump as near to the waterline as possible.  It will not matter if the intake hose has to rise over the gunwale or inflatable tube and then back down to the pump inlet since the rise and fall cancel out.  It is the pump which must be as low as possible.  75mm pumps having similar or higher flow rates to the 50mm versions often develop lower head pressures and are not always as effective for dredge use.  For short term projects, pump hire is probably the most cost effective;  however these pumps are relatively cheap at £700 to £900 and for longer term use it may be worth buying one.  If purchase is considered it is worthwhile stripping the pump and applying extra paint  and/or corrosion inhibitor while new to give added protection.  The pump must be cleaned thoroughly after each use, especially when used in salt water.  Salt water resistant pumps are available but these tend to be more expensive and still require regular cleaning because the engine which drives the pump is unlikely to be protected.

The ideal hose is the ‘lay flat’ or ‘fire’ type since this will roll up neatly and take up little space.  15 metres of hose plus couplings will roll up into a coil about half a metre diameter.  You will need a length at least twice the working depth and fittings to suit the chosen pump.

PERFORMANCE

The performance of any dredge will depend on many factors, the principal ones are as follows:-

All these factors are interactive and it is therefore very difficult to give precise performance data for any particular size of dredge.  It is generally a matter of 'suck it and see (sorry about that), however the table below gives a guide to the requirements and dimensions with an 'estimate' of performance.

OPERATION

The dredge is normally used in a horizontal attitude i.e., laying across the seabed, unlike the airlift which must be nearer to the vertical.  It can be operated at a small angle and, in fact, dredges are used to bring material to the surface from shallow lakes and rivers.  Too steep an angle or too long a discharge pipe however, causes loss of power and consequent blockage.  The major advantages of the dredge technique are, in my opinion, the small size of the power source, i.e. the water pump, and the fact that the discharge tube need only be 2-3 metres in length.  All the equipment necessary to operate a 150 mm unit can be comfortably carried in a 5 metre inflatable.  It is worth remembering that a similar diameter airlift requires a small road compressor weighing about 700 Kg., whereas the ALCON 800 litre/minute pump weighs a mere 60 Kg., and measures 50 cm x 50 cm x 40 cm.

Handling the dredge is in many ways similar to the airlift.  The lay flat hose however needs careful handling to avoid any kinks; ideally keep it neatly coiled on a drum and once the dredge is attached, pay it out carefully from the boat as the diver descends to the site.

Anchoring the dredge needs careful thought so as to allow adjustment for manoeuvrability and also allow collection of the spoil in a small area for subsequent investigation of ‘losses’.  (Remember that the exit of the discharge tube must be positioned carefully to throw the spoil clear of the site and, of course, down tide.) Some excavators fit sieving systems to the discharge tube on the seabed and bring the contents to the surface for inspection.  Operation in a strong tide is difficult and will be impossible without rigid anchoring points, (found from bitter experience) since the force of water acting on the cross section of several metres of delivery hose can be considerable.  To overcome the problem, the delivery hose should be clipped at intervals to a strong mooring line and a long section of hose allowed to lay on the seabed (hopefully out of the tide) to allow manoeuvrability of the dredge.  Try and complete the preparation at slack water.

During dredging, nozzle suction can be reduced by means of a valve (see Figure 1) but it is preferable to move the nozzle away from the work area since flow reduction in the discharge tube can cause blockages.  Unless it is a matter of clearing overburden, manoeuvre the nozzle to allow objects to be found before they are sucked up.  

In conclusion then, the choice of techniques, airlift versus dredge, will ultimately depend on the location of the site, longevity of the project and probably funds, since airlifting generally requires a much larger boat, and compressors are expensive.  This article I hope, will enable divers contemplating underwater excavation, to consider the pros and cons of both these techniques and in the case of the dredge, build effective and robust equipment.

Due to popular demand I have updated this article to the version in the Abergavenny and Dorset Trilogy CD's.  Original article; Published in Diver/Underwater World - August 1987, p. 24.

E. M. Cumming - April 2008