Last update: 08 February 2008

Project questions or comments?  Please email David Harlow


 About the Project

 Project Diary

 Photo Galleries


 A4 Handout

 Fisheries News


About the Project

BIS4.1 Winter 2005/06

The Need for Beach Replenishment

The standard of flood and coast protection at Poole Bay depends mainly on the level and width of its beaches.

During the 20th Century the cliffs were protected from erosion by building concrete seawalls, promenades and groynes.  This allowed houses to be built on the cliff tops, but it also stopped the natural supply of sand and gravel to the shore.

Bournemouth beach has been replenished on four previous occasions in the last 36 years and is now effectively an artificial beach.  Those projects are called 'Beach Improvement Schemes' (BIS):



Quantity of sand (m3)
















*includes 15,000m3 stockpiled for the surf reef

BIS4.1 was part of last winter's project using material dredged from Poole Harbour.  The quantity of sand available was insufficient to replenish the entire beach so work continued during winter 2006/07 (BIS4.2).

The sandy material used for BIS4.2 was transported by the Oranje from a Licensed Dredging Area to the east of the Isle of Wight.  The dredger came in as close as possible to the beach but the waters of Poole Bay are shallow and the material was pumped ashore through a 1150m long pipe known as a 'sinkerline' which connected the dredger with the beach.

The sinkerline was constructed locally, at Branksome Chine on the Poole/Bournemouth boundary.

Bournemouth beach is interrupted by high level timber groynes, but the replenishment at Poole last winter created a wide, flat beach with the old groynes buried below sand.  The conditions at Poole were ideal for fabricating and welding the pipework.  Once completed, the sinkerline 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 offshore and towed to Bournemouth.

Sinkerline facts...

  • Each section of sinkerline is 18m long, 1.1m in diameter and weighs 13 tonnes

  • The steel that makes up the sections is 1" thick

  • The sections are so heavy that only two could be transported from the docks to the beach at one time

  • The sinkerline was last used on a replenishment project in Africa

How the sand gets to the beach

The sinkerline was towed to a position between Bournemouth and Boscombe piers.  With one end at the shoreline and the other out at sea, it was sunk to the seabed. 

At the seaward end the sinkerline is coupled to a flexible riser pipe section and flexible floating pipeline.  During operations the multicat (support vessel) Zwerver II  takes care of the floating section and connects it to the loaded dredger when she arrives at the Bournemouth coast 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 (approximately 12m long) can be coupled to discharge sand to the required locations.

Sand is mixed with a considerable amount of water in the dredger's hopper to assist it being pumped through the 1,150m sinkerline and along the onshore pipeline.

On the beach horseshoe-shaped sand bunds are formed; the sand/water mixture is pumped into these bunds and they act to retain the new sand on the beach avoiding losses to the foreshore.  Sand settles on the beach and water returns to the sea.  The settled sand is levelled by bulldozer and new sections are added to the onshore pipeline in preparation for the next load.  In this way replenishment work moves along 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 refer to as "pumped ashore" is recorded by the dredger and we have to allow for a 'bulking factor' of 1.2 due to the sand being mixed with water in the hopper.  For instance, if 10,000m3 is recorded has having been pumped from the hopper it would equate to about 8,333m3 material actually arriving at the beach.

We then allow for up to 15% of that 8,333m3 being lost to the foreshore during pumping & draining, so only 7,100m3 might be left on the beach.  Losses can depend on the weather; the stormier the conditions, the larger the losses are likely to be.

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; in that way we get an accurate picture of how much sand has actually been delivered.  Westminster Dredging were contracted to replenish the beaches by a minimum of 700,000m3 not simply to pump 700,000m3, so the amount recorded as pumped (e.g. the amounts mentioned in our project diary) actually totalled more than 1 million m3.

During operations

In the interests of health & safety it was necessary to close short lengths of beach while sand was being pumped ashore by the dredger and being moved by bulldozers.  The promenade remained open at all times.

Initially the new sand appeared darker than that already on the beach but soon lightened to a normal colour with exposure to oxygen and daylight. In the first few weeks small ledges appeared in the beach as wave action sorted the new sand into a natural beach profile.  The beach was regularly inspected for any unsuitable material.

About the Oranje

A trailing suction hopper dredger

To give her full title the Oranje is a trailing suction hopper dredger (or TSHD).  She trails a suction pipe on the seabed and loads sand into an open hopper.  When the hopper is full she raises the suction pipe and sails to the seaward end of the sinkerline at Bournemouth.  She is coupled with the floating section of the pipe and the sand is pumped from the hopper on to the beach.

Built in Holland in 2004 for Royal Boskalis Westminster, at 156m long Oranje is 60m longer than any of the dredgers that worked on the replenishment project last winter - and as long as the Barfleur.  Her hopper capacity is almost 16,000m3 - about 4x that of last year’s vessels and she’s quicker too, with a top speed of 16.2 knots.

The number of loads brought to the shore was much reduced on the previous phase due to the distance the Oranje travelled between dredging and pumping ashore.  Last winter we averaged 5 loads per day; for this project just one or two loads were delivered in a 24 hour period with an approximate ‘turnaround time’ of 10 hours.

Further Work

Bournemouth’s existing timber groynes will last another 8 years or so, after which consideration will be given as to what should replace them; perhaps rock groynes or offshore reefs.

In the past Bournemouth beach has been replenished at intervals of 15 years or so, by which time most of the sand has been lost.  For the next 3 years small scale top-ups will be used to keep the volume high.


• Home • About the Project • Project Diary • Photo Galleries • Map • A4 Handout • Fisheries News • Contact •

This product includes mapping data licences from Ordnance Survey © Crown Copyright and/or database right 2006 License Number 100024248

  © 2005-08 Borough of Poole Leisure Services & partners

Re web content or photos contact: Sarah Austin