Category: Flood Defence

Exploration of different methods of flood defence, their function and effectiveness and how the sea treats flood defences installed in an attempt to dissipate the energy of the waves

Havergate Island

Havergate Island is situated near the Suffolk village of Orford, at the meeting of the Ore and the Butley rivers.  The Ore divides in two around the two-mile long Havergate Island with flows splitting through what is known as the Gull on the north side of island and the Narrows on the south of island. Coriolis forces, roughly defined, state a force influenced by the earth’s rotation tends to deflect moving elements to the right in northern hemisphere and to the left in southern hemisphere. Such forces would suggest flood tidal flow would run through the Narrows and ebb flow through the Gull. Both flows are similar in width, but the Gull is much deeper.

A video, kindly provided by the RSPB taken by a warden at Havergate in 2019, shows a breach caused by a sea surge. The breach was at the narrowest part of the island, on the side facing the Gull, on Havergate Island.

A large sea surge in December 2013 breached the sea walls at Havergate. In order to limit damage from any future surges, the RSPB, introduced a scheme to allow certain areas (known as ‘sills) to overtop. The purpose is to allow the sills to flood in a relatively controlled manner to manage water inundation and enable areas recover relatively quickly.

This approach seeks to address the problem of storm surge sea water once it enters a reserve scouring (eroding) the insides of sea walls fundamentally weakening them. When this occurred during the December 2013 storm surge, the RSPB at Havergate built up damaged walls, but are aware the walls may never be as strong as they were.

Internal scouring of sea walls was one of the main causes of damage following the 2013 surge. As the reserve contains saline lagoons the flooding of salt water wasn’t as damaging as it was to the freshwater marshland at Hazelwood marshes. Wildlife has also recovered following the 2013 surge with the hare population recovering and uncommon yellow vetch growing again.

As the RSPB have lowered sections of the sea wall, they are able to utilise sluice gates to allow flood water to leave lagoons without surges causing lasting damage. The process for creating the lower sea walls in addition to reducing wall height involves laying wire netting on the lowered sea wall and sowing grass seed. Once the grass has grown, combined with the wall lowering and the net laying, it should enable defences to withstand several hours of overtopping. Such schemes are favoured as they enable constructive management of overtopping. Which often proves cheaper than dealing with breaches in sea walls which are seen as more damaging and costly to fix.

Regarding the future of Havergate Island, it has been predicted by site manager Aaron Howe that the reserve could have another 20-40 years before it succumbs to coastal erosion. Increasing storm surges and rising sea levels are developing alongside another process, known as Isostatic Rebalancing. This refers to the post-glacial rebound of northern Great Britain which is said to be happening at a rate of around 10cm per century. This is causing a corresponding downward movement of the southern half of Great Britain, which is developing at around 5cm per century. This could increase the risk of floods in southern England.

The sea is nibbling away at Havergate, and the RSPB believe tidal surges are increasing, but the wardens and site managers work hard to adapt and evolve. As do the many bird and wildlife species the island is famous for. In this way, Havergate, placed as it is, in the middle of a tidal estuary shaped by the North Sea, serves as a barometer of the evolution of tidal reserves and species, and the size and nature of sea surges. Like the boat October Storm that takes visitors, to and from the island, Havergate is pushing into its tidal fortunes, to innovate as currents and surges shape its environment.

Sudbourne Beach

Sudbourne Beach situated south of the Martello Tower near Aldeburgh on the Suffolk coast is a shingle spit that separates the open coast from the Alde/Ore estuary. Recently, the sea has eroded the shingle spit at a greater rate than has previously been experienced. 

Coastal Partnerships East and East Suffolk Council have a Shoreline Management Plan (SMP) which maintains a policy to Hold the Line up to 2025. This approach seeks to uphold or if necessary, add to and improve existing sea defences. These bodies also hold an interim position of No Active Intervention in the medium to long term, which would see no active investment in sea defences. However, concern about increased erosion has prompted these bodies to launch a public consultation to change the SMP position from No Active Intervention to Managed Realignment. A policy of Managed Realignment as defined by the Department for Environment, Food and Rural Affairs (DEFRA) means ‘allowing the shoreline to move backwards or forwards, with management to control or limit movement’. It is hoped this policy will enable maintenance of the ridge to prevent a permanent breach.

A Draft report, the SCF-130619-SMP7-Policy-Review-Slaughden undertaken by Jacobs Consultancy commissioned by East Suffolk council in partnership with a Client Steering Group, identifies the area of Sudbourne beach and Slaughden ridge as ORF15.1. This area begins at the termination of the concrete walls at the Martello Tower and extends southwards to Lantern Marshes. The open sea and the Alde/Ore Estuary exist as two distinct but linked elements, but the ORF15.1 shingle spit is very narrow, prompting fears of a breach at a future date.

As an indication of just how the state of ORF15.1 is constantly shifting, three photos show how quickly the sea can alter the beach and shingle bank. The first two photos are from the Draft SCF-130619-SMP7-Policy-Review-Slaughden. The first photo taken in July 2017 shows a substantial wide shingle mass on top of Slaughden Ridge.

The second photo taken in December 2018 shows a big reduction in the mass of shingle with a cliff formed of shingle on top of Slaughden ridge.

This later photo is similar to photos used again as a comparison by the Alde and Ore Association in their Spring 2019 newsletter. The first photo they use taken from above in December 2018 shows the steep shingle cliff with a wide section of shingle on the beach.

The second photo taken in January 2019, shows the sea has moved the shingle to the back of the beach raising it up to a much higher level. The shingle cliff looks much diminished from the December 2018 photo.

It is also worth considering from the point of view of wider consideration of coastal erosion in the Alde Ore Estuary that each feature involves distinct but connected alteration processes. Hazelwood marshes became an intertidal marshland following a storm surge that breached sea walls protecting the freshwater marsh.

Sudbourne beach and Slaughden ridge whilst processes are clearly not helped by storm surges, are subject to erosion caused by the regular attrition of changes in wave behaviour.

The next feature, Havergate Island could be said to be subject to both these processes, it has been struck by storm surges and regularly manages changing, escalating sea activity.