£1.9 million, five month project to create five new rock groynes between Branksome Chine and Branksome Dene Chine.
Lead Authority
Borough of Poole
Main Contractor
Budget
£1.9 million
Project start date
24 November 2008
Project completed
05 May 2009
Branksome beach 09.05.2009 - five new rock groynes of varying lengths
Poole’s beaches following the replenishment project 2005/06 were wide enough to function as a coast protection structure, and controls were required to ensure they remained that way for as long as possible.
In 2008 planning permission was granted to Poole Council’s Coastal & Drainage team for the construction of five new rock groynes with concrete walkways at Branksome Chine and Branksome Dene Chine, each measuring between 45m – 75m in length.
Some 20,000 tonnes of Portland limestone were delivered to the beach by low-loaders transporting up to 500 tonnes daily via the car park at Branksome Chine.
A bedding layer of stone prevents subsidence of the groyne structure
A bedding layer of stone to prevent subsidence of the groyne structure
Working shoreward walkway sections are put into position at the same time as rock is built up to the finished level
Working shoreward walkway sections are put into position at the same time as rock is built up to the finished level
Rock is stockpiled for work to commence on Groyne 4
Rock is stockpiled for work to commence on Groyne 4
Project context
The choice of rock groynes for this project followed more than three years of detailed investigations and analysis by HR Wallingford.
Engineers from Italy, Holland and New Zealand were involved in the first stage of the study which considered more than 20 options for beach control structures. A final report recommended four options for detailed consideration – three groyne options and one submerged reef option. An economic appraisal concluded that rock groynes were the preferred option.
This project at Poole is a move away from the traditional coastal protection scheme in that a post project appraisal and programme of beach monitoring is being funded by Defra as part of the overall scheme, and will compare outcomes with computer modelling by HR Wallingford. The results will inform Environment Agency research, and help the refinement of beach plan shape models in the future.
Beach access ramps
During this project large excavations were made at the back of the beach for construction of ramps. There foundations were taken to below the level of sand before beach replenishment (2006/07).
Ramps allow access across the beach for, for instance, the beach rake and tractor. They also provide access from the promenade to the beach and the groyne walkways, and allow easier access for the less able and those using mobility scooter or wheelchairs, buggies and prams. The size of the excavation is due to the working area required to provide the long 1:20 slope that complies with Disability Discrimination Act (DDA) regulations.
During construction sand from the excavation was placed seaward to give protection to the workforce and the working area. Once construction work was complete the sand was replaced in the excavation trench. Current beach levels mean that the majority of the ramps will be covered by sand but the extent of the ramps will ensure they can be utilised if / when beach levels drop.
Installing beach access ramps
Deep excavations at the back of the beach, Branksome Chine
Deep excavations at the back of the beach, Branksome Chine
An access ramp taking shape at Branksome Dene Chine
An access ramp taking shape at Branksome Dene Chine
Completed ramp
Completed ramp
Ramps and groyne complete
Ramps and groyne complete
By mid-June sand is beginning to cover the ramps & walkway
By mid-June sand is beginning to cover the ramps & walkway
The groyne plan above shows the position and relative size of each of the five groynes
Rock groyne construction
Portland limestone is normally used for rock groyne construction along Poole Bay, brought to site by road.
Each rock for the head section of groyne weighs between 3-6 tonnes, and for the section from the head back to the promenade 1-3 tonnes.
At the beach, sand is first excavated from the area where the groyne will be located. The area is then backfilled with a bedding layer of stone to prevent subsidence of the groyne structure. Using this method of construction the groyne is extended seawards with bedding material and rock, building up to a level that will permit plant to run on it.
As construction progresses to full groyne length manhole rings are used to create voids to take the piles for the walkway that allow public access to the groyne.
Walkways are independent structures consisting of concrete beams spanning piles driven along the length of the groyne. Walkway sections and pile heads are made at the beach from reinforced precast concrete and lifted into position.
Pile heads provide a stable base
Pile heads provide a stable base
Walkway sections are made on site and lifted into place
Walkway sections are made on site and lifted into place
Rock groyne, Sandbanks east (constructed 1995-96)
Rock groyne, Sandbanks east (constructed 1995-96)
Construction proceeds with the head of the groyne being built to its full height. Piles are then driven into position through the voids by a vibrating rig (and air hammer if required). Working shoreward pile heads and walkway sections are put into position at the same time as rock is built up to the finished level.
The whole operation is then able to retreat off the groyne leaving a near completed structure; just the in situ joints and infill between the walkways and pile heads are completed later.
Access ramps are situated between the back of the groynes and the promenade, made from concrete poured in-situ and tied back into the existing stepped sea wall. First the wall sections of the ramps are poured, then the ramp is filled with beach sand and the slab section is cast on top.
Groyne construction methods have changed little since groynes were first built at Sandbanks in 1996 and again in 2001 (photos on this page were taken during the 1996 project).