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Shingle recharge and timber groyne maintenance (Autumn 2023)

The cliffs in Christchurch Bay have been gradually eroded by the sea over the past few thousand years.

The eroded material is transported in a net eastward direction along the coast by the process of long-shore drift. The drift material forms beaches such as the one at Milford-on-Sea, which is comprised of sand and shingle.

The beach serves to protect the coastline by dissipating wave energy.

The importance of beach replenishment at Milford-on-Sea

At Milford-on-Sea, a series of coast protection works were constructed between 1936 and 1968, meaning that the cliff is better protected from erosion.

This also means that the local source of beach material is no longer available or significantly reduced. Additionally, the coastline at this location is essentially now 'fixed' and does not allow for natural changes to occur.

This has resulted in the need for beach control structures to reduce long-shore drift and thus retain material better and for longer, as well as the need to supplement beach levels with additional material (beach recharge). Without this intervention, it is likely that beach levels would reduce further making the seawalls much more vulnerable to future failure.  

Timing the works

As Coast Protection Authority, we can use permissive powers to undertake coastal works. However, it is important to recognise that the council does not have a statutory duty to undertake coast protection works. In this case, we are undertaking the work as a maintenance operation to protect the frontage, funding the works through the annual coastal maintenance budget.

The work is being carried out by local contractors, Earlcoate Construction & Plant Hire Ltd., who are experienced in undertaking coastal maintenance at this site and in the local area.

Due to its location and inherent risks involved, working on the coastline is very challenging. These works require both favourable weather and suitable tides.

Additionally, works are restricted during the year and cannot be undertaken during the summer due to possible impact on bathing water quality (under our Marine Licence Conditions) as well as the likely disturbance to beach users. It is also not advisable to work during the winter due to adverse weather and shorter length of days. This means that the available working window is quite limited and generally restricted to autumn and spring.

The contractor has indicated that they aim to complete in approximately 4-5 weeks (subject to suitable weather and tides).

Beach recharges

The beach is a dynamic first line of defence, offering protection from damaging waves by dissipating energy. This is especially helpful during heavy storms, so there is less impact on hard structures (such as the concrete seawall). Therefore, having a beach helps to protect the seafront at Milford-on-Sea. 

Once the material is placed, the effect of coastal processes (waves, currents and tides) will act to quickly move and sort the sand and shingle. This will mean the sediment will be naturally moved both up and down the beach (cross-shore) and along the coastline (along-shore).

This year (2023), approximately 4,500t of material will be placed on the beach, a similar amount to previous years. Since 2008, we have instigated a regime to undertake regular beach recharges to replenish the beaches at Milford-on-Sea.  During this period, well over 50,000t of material has been added to the frontage replacing the current volume of the beach at Milford-on-Sea at least 4-times. So, whilst beach levels have continued to decline, beach levels would have been more depleted had earlier recharges not been undertaken.

Whilst the amount the net amount of cliff erosion (further west) has reduced, which has led to a reduction the natural supply of material (and lower beach levels), it is possible that other factors may contribute to the cause of beach loss.

Recently, it has been seen that other beaches along the south coast are experiencing a decline in volume and there is emerging evidence to suggest that the frequency and intensity of storms could be increasing; it is possible that this may be as a result of climate change.

Source of beach material

This year (2023), the material is coming from New Milton Sand & Ballast and is the same mix and source as the 2022 recharge material. Due to its location, near to the recharge site, the material from the quarry is geologically very similar in composition to the naturally occurring cliff material in Christchurch Bay. Of particular importance are the Pleistocene plateau gravels, together with Eocene sands and clays (Barton Group, Headon Hill Formation, Solent Group) which comprise the cliffs in Christchurch Bay naturally erode to form the beaches.

The existing beach is analysed to determine the grading and size distribution. This then informs the specification for the recharge material. Being a processing facility, the quarry is, therefore, able to produce a recharge mix consisting of sand and gravel which closely matches the specification of the exiting beach.  

Protecting the future of the beach

Without the supply of additional material, beach levels will further deplete.  The level of the beach fronting the beach hut seawall at the eastern end of the frontage is currently very low. If beach levels drop too low, there is a potential that the seawall will become undermined, ultimately resulting in the failure and collapse occurring. It is likely that this would quickly lead the loss of the beach huts and the promenade, then other infrastructure further back. Property would also become increasingly at risk from erosion.  The beach recharge is intended to replace some of the recent losses and mitigate against possible failure of the seawall until additional protection works can be undertaken.

Groyne maintenance

The timber groynes along the Milford-on-Sea frontage require very regular maintenance. This is due to the location of the frontage and the composition of the beach. This time, in order to minimise disruption and to improve efficiency the beach recharge will be undertaken alongside repairs to the timber groynes.

Being located towards the more eastern end of Christchurch Bay, Milford-on-Sea is more exposed to high energy waves and damaging storms than further west in the Bay. Waves act on the beach by moving material up and down the beach as well as along the coastline.

Because material on the beach is mixed sand and gravel it is quite coarse and abrasive. When there are storms, the amount of abrasion is intensified. Any of the structures located in the beach (such as timber groynes or concrete seawalls) will be affected by this process and will wear away.

The timber groynes are made of Greenheart, which is a hard tropical wood, very resistant to the effect of seawater and to abrasion. Despite this, the timbers still get worn down and require replacing, typically every 3 to 5 years. Additionally, the stainless-steel fixings used to connect the components get worn down too and can fail after a few years, especially during storms when structures are under greater pressure.

In order to make these works as efficient as possible and reduce disturbance, timber groyne repairs are being undertaken at the same time as the beach recharge. Furthermore, having beach material available is necessary in order to access the beach across the groynes and to form protective bunds whilst excavating the beach during low water working periods.

The beach is dynamic, and the waves will act to move and reprofile the material, and this is how the beach naturally functions, and in doing so will dissipate the wave energy, thus protecting the seawall.  The photographs show the same area of beach before and after the works, with a marker indicating how the beach level has been raised against the seawall.  Also to note, the post works photograph is taken after Storms Agnes and Babet.

Comparison (PDF) [757KB]

 

Future works 

Beach levels at the eastern end of the beach hut seawall are currently extremely low, and it is becoming increasingly difficult to retain a beach. As a result, the seawall is becoming increasingly vulnerable to failure. Whilst beach recharges have sustained beach levels, in the long-term small scale beach recharges are not sustainable or efficient.

As such the council recognises, that further measures are now needed to protect the eastern end of the beach hut seawall, where the sheet piling is exposed. To prevent eventual failure, a proposal to install a rock revetment has been put forward and this has now received NFDC planning permission. The work also requires a Marine Licence from the Marine Management Organisation; the application for this is being undertaken simultaneously with the planning application. This is a long process and requires careful assessments in terms of the potential environmental impacts of the works. It is, however, hoped that a licence will be issued in time for the rock revetment to be constructed around March / April 2024.

For further information, please see the documents on our Planning website.

Moving forward, through the Christchurch Bay Strategy (see below) it has been identified that a scheme is now needed. However, this is likely to require significant investment and is likely to be a number of years until any scheme can be implemented. Until this time, the aim is to continue to undertake maintenance works in the short term, including further recharges, subject to funding.

Monitoring local beaches

Beaches in Christchurch Bay are regularly surveyed under the Regional Monitoring Programme by the team at the Channel Coastal Observatory (CCO). This is so that changes in beach levels and volumes can be checked over time.

Further details of the Channel Coastal Observatory, who are based at the National Oceanography Centre in Southampton can be found at:

Channel Coastal Observatory Welcome (coastalmonitoring.org)

Christchurch Bay

The beach at Milford-on-Sea is located at the eastern end of Christchurch Bay, which extends from Hurst Spit in the east to Hengistbury Head in the west.

Christchurch Bay comprises a 16km section of open coastline exposed to dominant waves from the south-west.  The beaches are generally comprised of finer beach material on the more sheltered western side of the Bay, with coarser beaches further to the east. 

A critical factor in the evolution of Christchurch Bay was the breaching of the Chalk ridge which previously extended between the Needles and Handfast Point, Purbeck.  It was not until the early to mid-Holocene period of sea-level transgression (12-5000 years ago) that the Chalk ridge was removed, and the barrier between Poole Bay and Christchurch Bay (now forming Christchurch Ledge) was cut back, and rapid erosion of Christchurch Bay proceeded.  Erosion of Christchurch Bay was also facilitated by its connection with the Western Solent, between 8,400 & 6,500 years ago, which created strong tidal currents causing scour in the eastern part of the Bay.

The dominant south-westerly wind and wave climate has created a littoral drift of shingle and sand along Christchurch Bay from the west to the east.

Further details of the littoral drift in Christchurch Bay can be found at the SCOPAC Sediment Transport Study 2012.

The coastline has been formed, and is characterised by, the cliff geology and the continual erosion.  There is a historic rate of erosion of the cliff line of approximately 1m per year.  Previously as the cliffs have eroded the supply of new beach material has been obtained naturally from the sand and gravels in these eroding cliffs.  However, with increased habitation along the coast over the years coast protection measures have been built along various lengths of the coastline.  The hard coast protection structures aim to retain the expanse of area above the cliff, often allowing homes and businesses to be built above and retaining amenity space.  This activity thus eliminating the supply of new beach material.  Beach material is the first line of defence for the coastline in that the material at the base of the cliff protects the cliff from erosion.

Shoreline Management Plan

A Shoreline Management plan is a strategy for managing flood and erosion risk, over short, medium and long-term periods of time. SMPs identify the best ways to manage coastal flood and erosion risk both to people and the developed, historic and natural environment.

The SMP divides the coastline into 'units' and each unit had a policy attributed to it. There are four main shoreline management policies:

  • Hold the Line: maintain and upgrade or replace coastal defences in their current position where funding permits.
  • Managed Realignment: manage coastal processes to realign the 'natural' coastline configuration, either seaward or landward of its present position.
  • No Active Intervention (do nothing): a decision not to invest in providing or maintaining defences or management of the coast.
  • Advance the Line: a decision to build new defences seaward of the existing defence line where significant land reclamation is considered.

The current policy (0-20 years) at Milford-on-Sea is Hold the Line, changing to Managed Realignment in the 2nd epoch. More information on Christchurch Bay and the Shoreline Management Plan can be found at:

Poole and Christchurch Bays Shoreline Management Plan (SMP2) 2010 (twobays.net)

A summary of each of the shoreline management policies for the local SMP Policy Units are indicated on the Policy Summary Map (twobays.net) .

The future of the Christchurch Bay Strategy

With increasing financial pressure and environmental considerations, the council aware that it needs to consider the long-term sustainability of this work. Looking further ahead (with climate change and sea level rise) the coastline will become under ever more pressure, so that the issues that are being faced today are likely to continue and become more acute.

Since 2021 NFDC has been working in partnership with neighbouring BCP Council on a Strategy to define the long term (100-yr) sustainable and integrated approach for implementing the SMP policy intent to manage coastal flood and erosion risks, whilst building resilience and adapting to future changes

The Strategy has identified where, when and broadly what type of works are needed to manage the risks of coastal flooding and erosion over the next century and roughly what they may cost. Through extensive engagement with stakeholders during 2022 & 2023, the Strategy has now developed Leading Options for the different frontage units around Christchurch Bay.

At Milford-on-Sea the leading option is to upgrade or refurbish the existing defences in the first 20-years, whilst undertaking much larger scale beach renourishment. The delivery of any such scheme will depend on the level of investment and funding available, which will dictate when and (ultimately if) a scheme can be implemented. Current assessments suggest that there may only be in the region 20% funding contribution available from Central Government, with the bulk of the investment having to be raised through local Partnership Funding.

The strategy will also consider methods of delivering better efficiencies in future working, such as joint working on beach management and considering alternative future coast protection options.  The result will be to produce a viable programme in the long-term to enable the undertaking of coast protection schemes.

Christchurch Bay and Harbour Flood and Coastal Erosion Risk Management (FCERM) Strategy | Have Your Say Bournemouth, Christchurch and Poole (bcpcouncil.gov.uk)

 

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