VICTORIAN TECH RE-INVENTED TO RESTORE SALTMARSH
A drag box method dating back to the early 1900s has been re-invented by wet civils engineering firm Land & Water to restore saltmarsh in an English harbour.
Land & Water has carried out trials of the method with new sister company Earth Change under a seascape project to restore the saltmarsh in Chichester. More than 250 hectares of the saltmarsh have disappeared since 1945, the firm says.
Saltmarshes provide incredible environmental benefits including acting as a natural flood defence, an absorbent of harmful nitrates and phosphates and a haven for wildlife to increase biodiversity.
The method works with the excavator operator pushing the winch anchor blade into the ground to secure the winch then using it to pull the drag box backwards down the foreshore towards the deposited silt and pontoon.
When travelling in reverse, the drag box tailgate is pulled open and once it has travelled over the deposited silt, the excavator
operator reverses the winch to pull it back to shore, pulling the tail gate shut and loading the drag box as it is pulled through the silt.
A full cycle takes around two to three minutes. An amphibious excavator is on hand for general duties and to ensure the material is placed at the design height in the correct location utilising GPS positioning.
“The drag box methodology was used by Victorian engineers to dredge lakes, dragging a skid between two traction engines to recover silts and sediments ashore,” says Land & Water. “We borrowed modern hydraulic winching technologies from the Canadian Forestry Industry, coupled with a new/ enlarged drag box design to enable the efficient recovery of large quantities of sediment in very short time scales, and with negligible environmental impact.”
“Restoring saltmarshes within an intertidal habitat is not straightforward. The large barges and ships used in commercial dredging operations cannot directly access the shallow waters and mudflats where saltmarsh proliferates. The use of the drag box enables the larger vessels to deposit their muddy cargoes close to it at high tide for retrieval once the tide has ebbed away.”
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