SRA Annual Report 2019-20: How water injection dredging works on the River Parrett
The photo below shows Van Oord’s water injection dredging vessel Borr in demonstration mode. The injection bar at the back, spraying water, is raised so that it can be seen.
When actually dredging the Parrett, the bar is lowered to a position just above the river bed and river water is pumped out through nozzles along the bar.
The bed of the Parrett is characterised by fine sediments including sands, silts and clays. The injection of water separates and mobilises these particles so they can be washed away in what Van Oord call a ‘density current’ in the lower third of the water column. The ‘density current’ travels downriver on an outgoing tide and disperses through natural processes, usually wave action. This disruptive force breaks the bonds in the ‘density current’ and releases sediments from suspension.
The process can shift 700m3 of silt an hour. There is roughly a 4-hour operating window for water injection dredging on each outgoing tide. A combination of outgoing tide and river flow is needed to make sure that sediments are carried away effectively. Sessions can be extended if the Parrett is in high flow. There are two outgoing tides a day and both are used, so a lot of work is done during winter’s long hours of darkness.
Hence the value of preparation and expertise. The Borr’s crew know where to aim jets of water because the results of regular silt monitoring are used by the Parrett IDB to create precise river profiles and to specify what changes they want dredging to make.
Design co-ordinates are loaded onto the Borr’s computers – then as dredging is done, they are updated in near real-time. As the vessel passes through a section of river, it re-measures it. On board, in the cabin, screens show the relative positions of riverbed and injection bar. This means it is possible to see what effect work is having, and to raise and lower the bar, and vary the pressure and volume of water, according to what is being achieved – and to what the SRA wants to achieve.
The goal is always to operate in the most effective ways possible in the time available. So, for example, if sediment half a metre deep needs to be removed from one section, the Borr’s crew will use their skill and judgement, and not just blast away, but take a surgical approach. Control and caution are essential so as not to compromise riverbanks’ function as flood defences in their own right.
Through such painstaking measures, water injection dredging is made successful.