Completed Projects

In Poole Bay, sandy beach material is currently our frontline defence. Our cyclic beach renourishments are a “soft” engineering approach which provides an economical, attractive and sustainable form of coast protection that works with natural processes.

Harder engineering approaches such as rock revetments or rock armour are the opposite. If used, this style of coastal defence would be placed at the back of the beaches making access to the sand and sea difficult. The levels of sand would continue to drop, and more rock would be needed to prevent the failure or collapse of the sea wall.

We continually analyse sea levels, wave parameters and storm data, as well as monitoring beach levels to ensure our coastline is maintained at the correct height and width. If beach levels were allowed to drop significantly, inshore wave energy would not be attenuated and sea water could run up and over the crest, flooding the areas behind the beach. Low beach levels would also cause the sea wall to be undermined, leading to sea wall failures or collapse and ultimately, the erosion of the cliffs would accelerate. Our modelling suggests this would be by up to 1m per year in some places.

It is now widely acknowledged that sea levels are rising because of climate change; +20cm since 1900. This may result in the need to raise the level of the promenade but for now, even small changes in sea levels require a higher and wider beach. This prevents overtopping and acts as a buffer to prevent erosion in-between renourishments. An additional benefit of a high and wide beach is the amenity value enjoyed by residents and visitors alike.

As sea levels continue to rise, it is anticipated there will be an increase in stormy weather too. In January 2021, Dr Matt Wadey, BCP Council’s Senior Coastal Scientist delivered the findings of a SCOPAC Storm Analysis Study* to the Royal Geographical Society. The study helps us to better understand how our region is affected by storms, how their frequency and intensity is changing, and their potential impact on beach loss and asset failures.

*The SCOPAC Storm Analysis Study technical report is available at southerncoastalgroup-scopac.org.uk

Until the 1970s, 10,000 years of erosion into our coastline’s soft cliffs deposited various sands, clays and gravels onto our native beaches. If you look at the exposed areas of the cliff today, you will see these differing materials capped by the river terrace gravels of the Stour.

To limit the process of the natural erosion of the cliffs and to prevent the coastline from rolling back, the sea wall was constructed. It was built in numerous stages; the Bournemouth, Boscombe and Southbourne sections date from the late 1800s, while other parts were built in the 1940s, during WWII. The beach levels started to fall because the material in Poole Bay is generally transported by wave action eastwards towards Hengistbury Head, before eventually being deposited offshore. This process is called longshore drift.

In the 1970s beach renourishments started to be used to sustain the beach levels and provide coast protection. This means that the BCP coastline beaches are now largely artificial, i.e. they are not a result of local cliff erosion, and with only a few per cent remaining of what was the original “native” material.

In Poole Bay, the strategically placed groynes help retain beach levels and slow down the process of longshore drift, but studies have shown that we continue to undergo net sand losses of approx. 70,000m³ annually. Cyclic beach renourishments are therefore required to ensure that we maintain a high and wide beach.

Most of the sand and beach material for the 2021 renourishments will be sourced from Area 501, a licenced dredge site on the UK’s east coast in the Thames Estuary region. During the renourishments, continual checks are made to ensure the imported materials closely match the existing (and previous) beaches whilst complying with specifications for coastal protection. All works are carried out in accordance with the Marine Management Organisation licence and planning consent.

Initially, the colour of the pumped sand and beach material may look slightly darker but after a short period will lighten-up as it dries out and is exposed to the atmosphere. Over the coming months the wave action will sort it into a more natural beach profile, and initially you may see the temporary formation of ‘cliffing’ along the water’s edge.

Currently, Area 501 in the Thames Estuary region is the closest site in the UK with the material grading we need to match the existing beach. As stocks become depleted and increase in price, we may have to travel further afield to Europe to source it but this would become uneconomical.

To plan for this eventuality, we are starting to investigate more sustainable ways of renourishing our beaches. One way is to recycle our own sandy beach material because the size and type matches our current beaches. In Poole, we hope to recycle material from the harbour’s main Swash Channel and during Spring 2021 we have a planned sand recycling scheme in Christchurch, from Mudeford Sandbank to Highcliffe 

The particle size of sand and beach material varies naturally across Poole Bay. Generally, at Sandbanks it is fine sand, grading to coarser and heavier material towards Hengistbury Head where the more aggressive wave climate removes the finer element. When imported beach material sorts itself into a natural profile, the finer materials remain on the surface while the heavier materials move downwards.

During storm events, the wind moves the finer particles to the back of the beach (and then onto the promenade) but in some areas, this natural process (along with dune regeneration work instigated in the mid-1990s*), has resulted in sand dune creation with self-seeded marram grasses holding them in place. Dunes provide an additional layer of tidal flood defence in front of the sea wall.

A high and wide beach is vital for coast protection purposes, fulfilling the prime objective of preventing reactivation of the cliff and loss of homes. Therefore, it is not an option to intentionally lower the beach levels to prevent sand from ‘creeping’ onto the promenade. Sand is cleared from the promenade, to maintain access for all beach users and emergency vehicles. In the future, especially with dwindling beach material supplies, we may need to consider slightly increasing the size of our sand and beach material. This would also be expected to help reduce the amount of sand blown onto the promenade.

* For more about the dune regeneration see Report-on-Sandbanks-Works-2003.pdf 

To carry out these coastal works, it is vital we understand the impacts on the marine environment and to use this information to inform what we do. For years we have also worked collaboratively with many stakeholders including researchers at the Universities of Southampton and Bournemouth, Fishery Liaison Officers, Environment Agency and Southern Inshore Fisheries & Conservation Authority to ensure any impact is kept to an absolute minimum.

Since 2015, we have carried out annual environmental assessments. Divers have undertaken surveys in numerous areas across the frontage including near Hengistbury Head to assess whether there has been an increase in fine sediment release. No significant increases in bare, un-colonised horizontal surfaces were noted between 2015 and 2018. This suggests that marine life (including lobsters and Sea Bream) is not being affected by beach renourishments or any other particulates within the water column. This has been evidenced in our marine licence application and as a result we received approval from the Marine Management Organisation to deliver Phases 2 and 3 of the Poole Bay Beach Management Scheme.