Rock arrives from Norway for Southsea flood defence

Thousands of tonnes of rock have begun to arrive on the beach at Southsea as part of major flood defence works that will stretch 4.5km along the south coast of England.

The £130 million (€156 million) work, funded by central government, is being done in a joint venture between Boskalis Westminster and engineering contractor Volkerstevin (VSBW).

A total of 140,000 tonnes of rock will be shipped to the coast from Norway over the two years that the work is expected to take.

“The rock used in the flood defence has to meet specifications ensuring it will last for at least 100 years; the granite we use has to be very dense and very durable, plus procuring it has to be economically viable,” said customer engagement officer Jeannette Moyse.

“The granite we are using from Norway best meets all our requirements, and although there may be granite rock closer it may not meet the specifications for performance that the flood defences need.”

The rock is delivered by coaster to the shore and stockpiled for access during the work, which is of course tidal dependent. A working platform is built out to sea, and long-arm excavators place geotextile material as a base for the rocks to be placed on to maintain stability.

Smaller rocks are then moved into position as a core, with larger rocks, each weighing about four tonnes, placed on top in a way that they interlock and cannot move.

The work is currently being carried out along a stretch of coast where in February 2014 a storm damaged sea walls. In 2016 there was significant flooding during the so-called ‘Storm Angus’. These areas will be protected by a 600m stretch of ‘rock armour’ that will go out to sea for about 35 metres, Moyse said.

“The impact of global warming and climate increases the flood risk at Portsmouth, which is low lying, and as a result, rising sea levels are a cause for concern,” said Moyse. “Also climate change does result in more frequent and stronger storm events, further increasing the chance of flood events.”

Boskalis has also been contracted to carry out protection and replenishment works along 40km of coast from Togo to Benin in West Africa.

Fifteen groynes will be installed and six existing ones refurbished, and 1 million m³ of sand will be deposited along the coastline.