After years of repairs to its sea wall, Tywyn in North Wales is getting a complete set of new sea defences. Martina Booth reports.
The small Welsh coastal town of Tywyn is susceptible to tumultuous weather, and its coastal structures have repeatedly suffered from storm damage for decades. Consequently, sections of the town’s sea wall have been beefed up over the years by adding sheet pile and concrete revetment protection at the toe. But with 78 homes at risk of flooding, and its beach receding annually, Tywyn has been in need of a longer term rescue plan.
Since the mid-1990s the local authority, Cyngor Gwynedd, has explored various schemes to defend Tywyn’s 1.8km long coastal frontage, including providing rock armour along the sea wall. But this proposal was scrapped, as it was judged to be detrimental to the beach’s and the Victorian promenade’s character.
In 2009 a £6.4M Atkins-designed scheme to shore up the Tywyn sea defences was approved - backed by £3.8M of Welsh Assembly Government money and £2.6M from the European Regional Development Fund. The scheme involves constructing a new large rock breakwater above low tide level, which is designed to break waves and mitigate overtopping at the sea wall. It has been designed to be built using granite, with individual rocks weighing between 10t and 12t on the crest and 6t to 10t beneath.
In addition, rock groynes and a rock revetment will be constructed, the beach’s deteriorating timber groynes (which currently extend up to 120m out to sea) will be replaced with 27 new timber structures, a new concrete stepped revetment to the base of the Victorian promenade will be built, and the sea wall will be repaired.
Atkins principal engineer Rob Morgan says: “The rock breakwater and extra beach nourishment - along with rock groynes, revetment and the repair of existing defences - have been designed to protect the town from the sea without causing any secondary impacts further down the coastline.
The materials used have been selected to be in keeping with the surroundings, and the local community has been incredibly supportive of this scheme, which will mitigate against future flood events.”
Around 60,000t of rock has been used in the scheme, with 40,000t sourced from nearby Penmaenmawr and the remainder coming from France and Norway. Two-thirds of the rock will be delivered by sea.
In addition, 3,000m3 of concrete has been used for the step revetments and accesses, and 5,300m3 of shingle/sand used for beach nourishment. In all 350 timber piles - equivalent to 1,500m2 of planking - have replaced the old timber groynes.
“We have been restricted to tidal working windows and have had to have rock delivered by sea overnight”
John Dielhof Jones Bros
In January Jones Bros won the contract to construct the scheme, and has had up to 30 workers on site in order to complete it by August - one month early.
One of the key challenges Jones Bros has faced is ensuring that the ships bring the rock into shore safely - a procedure that has to be timed and planned around tide times and weather.
“The breakwater is offshore, so we have been restricted to tidal working windows and have had to have rock delivered by sea overnight - retrieving the rocks at low tide, then building the rock structure offshore. It is a 24-hour operation,” says Jones Bros contracts director John Dielhof.
Ahead of schedule
“However, we’ve been very lucky with good weather. In terms of quietness of wind we had one of the best winters on record, which enabled us to complete the rock work almost two months ahead of schedule.”
Once the main breakwater had been built the main task was to remove all the old timber groynes, which ranged from 60m to 120m long, and replace them with fewer, shorter groynes of between 35m and 45m. Again, the work has had to be done at low tide.
Jones Bros had initially planned to use new 305mm by 305mm square section greenheart timber piles for the replacement groynes, but these would have taken up to five months to arrive on site, which could possibly have delayed the start of the piling and adversely influenced the project’s overall programme.
Instead the contractor found some second hand greenheart piles that had only been used once on a temporary scheme in Scotland, and were available at short notice.
As these were in very good condition, Atkins and Cyngor Gwynedd agreed they could be recycled and reused with pyramid-shaped steel pointed shoes and steel reinforcing top rings fitted.
Aarsleff Piling, which had previously worked with Jones Bros at the Tullo wind farm in Scotland, won the contract for the groyne piling. The specialist piling contractor had also installed all the timber groyne piles on the smaller, but similar, Atkins-designed scheme at nearby Aberaeron.
Piles and groynes
Using a Banut 700 self-erecting piling rigs, and working between the tides, Aarsleff installed the 5m long piles up to 3.5m into the beach sand. The new groynes are spaced 50m apart along the beach, and each row of groyne piles was driven in a straight line at 2.5m centres down the beach to a ±50mm position tolerance. The top levels of the piles gradually reduce towards the sea, with those furthest from the shore standing about 1.5m out of the sand.
The Banut’s hydraulically operated 5t free fall drop weight can be adjusted for drop height and blow rate to increase or decrease the impact energy transferred to a pile, which helps to overcome pile resistance in varying ground conditions.
This feature was especially useful at Tywyn when initially tapping in the piles before steadily increasing the blow height and rate to compensate for the increased resistance as penetration progressed deeper into the beach sand.
The piling contract involved installing almost 400 greenheart piles and, again, this element of the project was completed quicker than anticipated.
“The job went better than expected and we finished the groyne piling about one and a half weeks ahead of our programme,” says Aarsleff project engineer Dan Broadley.
After the piles were installed, Jones Bros completed the construction of each groyne by bolting on new greenheart waling bars and planks to the faces of the timber piles.
The entire project was programmed so that all the work on the south section of the beach was completed first, giving locals and holidaymakers the chance to enjoy it over the summer holidays.
Piling rig: Banut 700
Maximum pitched pile length: 16m
Minimum working height: 13.7m
Hammer: 5t hydraulic free
Client: Gwynedd Council
Project manager: Gwynedd Consultancy
Contractor: Jones Bros
Design and site supervisor: Atkins
Piling contractor: Aarsleff
Construction period: Jan-Aug 10