Surveys at Hengistbury Head Long Groyne
Following ground investigations last winter, we are continuing to carry out several surveys and investigations to help prepare for the upgrade and repair of the entire long groyne structure.
The Sandpiper jack-up barge
Activity around the long groyne includes underwater surveys to map out the depth and land beneath the surface while borehole investigations will show the composition of the ground below the structure.
Boreholes up to 20m in depth will be collected from both land and sea using a rotary drilling rig and a sea-based jack-up barge.
The jack-up barge is anticipated to arrive on site from early August 2022.
Working 24 hours a day, it will take boreholes from five seabed locations immediately beside the long groyne.
Councillor Mark Anderson, Portfolio Holder for Environment and Place said:
“The 83-year-old structure has been critical in stabilising and controlling coastal erosion in both Poole Bay and Christchurch Bay. Along with the beach and other shoreline structures, the long groyne helps protect local communities and infrastructure so it is vital we upgrade it so it can continue to perform effectively, particularly with the additional threat of sea level rise and an increasing number of storm events.
“Without the long groyne, beach material would rapidly wash away, and the cliffs could start to erode again. This would eventually lead to the sea breaching Christchurch Harbour at Double Dykes and Mudeford Sandbank and the impacts could extend to the Purbecks and New Forest.”
All data collected will help refine the most efficient and effective design for the new long groyne structure to ensure the continued stability of the coastline, while protecting it from sea level rise over the next 100 years.
The long groyne was closed to the public for safety reason following a detailed inspection of the structure in 2019 (using sonar and laser equipment) when it was revealed it to be in a very poor condition. Several voids were discovered in the foundations below the waterline, with an associated risk of localised collapse. The groyne is also regularly submerged in water.
The current national policy for most of this populated coastal frontage is ‘hold the line’. This means that existing coastal defences should be maintained, upgraded or replaced in their current position, where funding permits, to reduce the risk of coastal flooding and erosion.
A notice to Mariners was issued during w/c 20 June 2022.