Page 277 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

256 Maritime Archaeology: A Technical Handbook, Second Edition
Figure 9.12 Airlift in operation on a shallow water site with the pipe running on the surface discharging spoil off the site. (Courtesy of Patrick Baker, Department of Maritime Archaeol- ogy, Western Australian Maritime Museum.)
site. This enables the spoil to be deposited outside the site (Figure 9.12). In deeper water, where the airlift pipe can be made quite long, the orientation of the airlift and the location of the discharge is important. No one wants a situation where the discharge spoil simply rains back down over the site, redistributing it uniformly over the site to be subsequently re-excavated. The solution is to ensure that the discharge is far enough off the site that it is clear of archaeological parts. If there is a current, this will have to be taken into account, as the current should carry the finer sediments away from the site. There will generally be a collection of the heavier material directly below the discharge and then finer and finer material off down current. In some cases the current can change direction, due to tides or other influences, and the excavator needs to be aware that the location and direction of discharge may need to be changed when the current changes.
C. WATER DREDGE
The water dredge consists of a long tube with a bend at one end forming an obtuse angle. High-pressure water (usually from a fire pump) is injected into the tube at the bend and directed so that the flow is axial with the long pipe. The flow of water along this pipe causes an induced suction at the working end (Figure 9.13). It is possible to attach a flexible tube to the