 About
the Project
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):
|
Scheme |
Year |
Quantity of sand (m3) |
|
BIS1 |
1970 |
84,000 |
|
BIS2 |
1974-75 |
1,400,000 |
|
BIS3 |
1988-89 |
1,000,000 |
|
BIS4.1 |
2005-6 |
600,000 |
|
BIS4.2 |
2006-7 |
898,000* |
|
*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.
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.
|
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. |
|
