Beach replenishment – how it was done
Work was carried out by the Fareham-based Westminster Dredging Company with the Dutch dredger “Oranje”.
The dredger came in as close as possible to the beach but the waters of Poole Bay are shallow and the new beach material was pumped from the dredger through a 1,150m long pipe, submerged to the seabed and known as a ‘sinkerline’.
The sinkerline was constructed locally, at Branksome Chine on the Poole/Bournemouth boundary. Bournemouth beach is interrupted by high level timber groynes, but the wide, flat beach created by the replenishment project at Poole last winter had buried old groynes there below the sand – ideal conditions for fabricating and welding pipework for the sinklerine. Once completed, it stretched from Branksome Chine to Flaghead Chine; small sand ‘bridges’ were created during fabrication to allow public access to the shoreline until it was moved off the beach and towed to Bournemouth on 9th December.
Sand was collected from a Licensed Dredging Area off the Isle of Wight and transported to the Bournemouth coastline; with each trip Oranje delivered between 11,500 and 13,000 m³ of sand to the beach.
A trailing suction hopper dredger (TSHD) is so called to reflect the fact that this type of vessel dredges while slowly sailing, trailing one or two suction arms off the side of the hull. At the entrance of each suction arm is a draghead; the draghead collects sediment (sand, silt and gravel) from the seabed in a similar way that a vacuum cleaner works. Dredged material is pumped into the hopper, where it is stored for transport. When the hopper is full, the dredger sets sail for the discharge location.
The welded steel sinkerline was floated offshore at Poole, towed to a position between Boscombe and Bournemouth Piers and sunk to the seabed where it remained during replenishment work. Use of a sinkerline reduces interference with other marine activities in the area.
At the seaward end of the sinkerline is a 36m flexible riser pipe section, followed by a 100m flexible floating pipeline which connects to the steel section which lies on the seabed. On arrival at the discharge point, with assistance from a multicat (small workboat), the dredger is manually coupled to the flexible riser via a bespoke coupling ball-joint system.
At the landward end the sinkerline is fitted with a steel flange from which sections of onshore pipeline (each section approximately 12m long) can be coupled to discharge sand to the required locations. Y-pieces with valves can be introduced to the pipeline to direct the loads to different areas of the beach.
On the beach
Sand bunds are created at the end of the shoreline pipe; sand is pushed both toward the seawall (for protection) and towards the shoreline to retain the new sand on the beach and avoid losses to the foreshore. A sand/water mixture is pumped from the dredger through the sinkerline; it flows from the shoreline pipeline into the bunds where it settles on the beach, with the water returning to sea. The sand that remains behind is then moved around and levelled by bulldozers. Additional sections are added to the pipeline and the process is repeated further down the beach.
Measuring quantities pumped ashore
There are two factors to take into account when estimating the quantity of sand reaching the beach. The amount we claim as pumped ashore is recorded by the dredger and we have to allow for a “bulking factor” of 1.2 due to the material being mixed with water in the hopper. For instance, if we’re told that 3,000 m³ has been pumped from the hopper it would equate to 2,500 m³ actual material dredged.
We then allow for an estimated 20% of that 2,500 m³ being lost to the foreshore during pumping, so only 2,000 m³ might be left on the beach.
These figures provide guidelines. Beaches are independently surveyed on a daily basis, comparing levels and widths to a baseline survey carried out before work commenced, and in that way we get a far more accurate idea of how much sand has actually been delivered.