Hydrodynamic - dynamic hydro - moving water
Excavation - digging holes
Hydrodynamic excavation is a process where flowing water is used to dig holes. This is most often used subsea, where the availability of water is good.
A typical use of this method involves lowering a propeller down to just above the seabed (10-15 m above it). The propeller is weighed down with enough lead/steel to keep it in position and stable. The propeller is then started, and it builds up a column of water that punches through the seabed and lifts away the soil.
Because the process whirls up the seabed, visibility often goes down the drain - so observation of the dig using an ROV is no good.
Visibility down the column of water is very often good though, but as this is a co-axial picture it is hard to estimate the size of excavation. To help show the site the propeller often has mounted two (non-rotating) multibeam sonar bathymetry devices. When mounted at 90-degree angles, they will show the cross profile of the dig, and also the profile in the direction of movement. This gives good control of the excavation, as distances can be read on the sonar's monitor.