This morning I went to Torcross to review the recent works and hear about the long term plans to protect Slapton Line
I have received a copy of the Technical Briefing with regard to the Torcross Shingle Recycling and you can read this below:-
Why the need for Shingle Recycling?
The sea defence at Torcross is designed to withstand storm waves to protect homes, which it did successfully during the 1 in 60 year storms in early 2014. However, its design is based on waves breaking on to the revetment (the sloped retaining wall in the diagram) with the wave return wall acting as a barrier.
If the beach level drops significantly in front of the structure, the metal piling at the toe of the wall is forced to act as a wave return wall at low tide. Whilst this will not cause the structure to fail, we know that the wave action accelerates beach loss and the 'toe' piling suffers accelerated wear as a result of shingle "blasting". This has long term maintenance and life expectancy implications for the structure.
The objective of the shingle recycling1 programme, was to bring the beach level up so that the metal piling was covered and not exposed to wear from direct wave action.
It is not uncommon for the piling to become exposed at Torcross, as a result of storms changing the beach levels. Usually, the beach level will recover to some degree following storm events as is the way with this natural cycle.
After the storms in early 2014, the beach level was monitored regularly but after the summer period and heading into autumn, it was apparent that the beach levels had not recovered as much or as quickly as was hoped.
As a result of the continuing and prolonged low beach levels at Torcross and a surplus of beach material being available at the northern end of Slapton beach, a decision was made to replenish levels by transporting shingle. It was possible to fund this work from additional, one-off funding which central government made available to Risk Management Authorities for the repair of flood defences following the winter 2013/14 floods and storms. To ensure public money is spent in the best way, the outcome of the recycling project will be monitored but future beach recycling is dependent on funding being available if beach levels drop in the future
Why did the shingle move after it was placed?
As the beach is only accessible from one end it was decided that the best way to get the shingle down onto the beach was to run it along the top of the defences as high as possible on the sea defence to create a "haul road" (see picture 1) well above the tide.
Once that was complete and sufficient volume of material was placed along the length of the defence it needed to be spread to a more natural angle on the beach where it could fulfil its function of protecting the toe piling.
Rather than do this by machine, we simply let nature do it for us, and it did so very quickly! The result was a rise in beach levels of around 1.6m, completely protecting the toe piling (picture 2). Detailed topographical surveys at low tide show how the shingle has spread out beyond the low tide mark at the natural angle of repose of the beach. A volume check of this survey, revealed that after the shingle had spread, roughly the same volume that we imported was present on the beach.
Picture 1 – Haul road Picture 2 – Result of shingle recycling
We're now nearly ten years into the 30 years of further life predicted for the Slapton Line A379 road by the specialist engineering consultants' report (Scott Wilson) which recommended moving some sections of the road inland together with shingle movement in order to achieve this.
As recommended by the report, its recommendations are periodically revised to assess what has changed in the past ten years including beach recession rates, storm severity, as well as government policy and available funding. This review is planned to take place in early 2015.