flood future | Protect Southsea residents and businesses from the ocean

Southsea residents could find themselves in the sea instead of beside it without the £130m investment in coastal defenses currently underway.

JTwo piers, a castle, a museum, historic military defences, a pool complex and an aquarium, not to mention many miles of promenade, make the Southsea area of ​​Portsmouth a real draw for tourists.

Norway’s cranes, diggers, pile drivers and boats carrying thousands of tons of rock add to the region’s attractions now – and for the next seven years.

Protective infrastructure

This temporary addition is equipment used to provide new infrastructure that will protect existing attractions, as well as 10,000 homes and 704 businesses, from flood risk for the next 100 years.

The £130million Southsea Coastal Scheme will replace a wide variety of different sea defences, some dating back to the 1700s. It will protect the area from sea level rises which could rise by 600mm to 1 m over the next 100 years.

The need for 4.5km of new defenses is real – three major breaches of waterfront structures in the past seven years have given the project a sense of urgency.

“Emergency repairs create significant additional costs – repairs to the defenses of Southsea Castle in 2015 cost £660,000,” says Marc Bryan. He is Major Projects Manager at Coastal Partners, an alliance formed by five Hampshire local authorities to work on coastal issues along the county’s 100 miles of coastline.

The damage to Southsea Castle was just one of many failures and some structures have just five years of life remaining according to inspections carried out in 2017.

Analysis of existing sea defense structures also highlighted the risk that a storm surge combined with high tides and another major breach could bring 9m of flood water into some residential areas.

Despite the urgent need for investment, Portsmouth City Council’s Southsea Coastal Scheme project manager Guy Mason said it had taken 10 years to secure funding deals and permissions to start work.

The bulk of the funding for the work comes from the Department for Environment, Food and Rural Affairs with £100m awarded in 2020. Portsmouth City Council previously provided £6.5m to make advance the design of the work. The remaining funds were obtained from the Environment Agency, which authorized the start of work on the site in September 2020.

The £130m cost of the works is broken down into £7.3m for design, £97m for construction, £9.7m for delivery of the project and £16m sterling as emergency funding.

The works will protect Southsea from what is expected to be a flood every 200 years by the 2120s. The new structures also have a lifespan of 100 years.

Design and construction is undertaken by an integrated delivery team trained by Royal HaskoningDHV, supported by Atkins and LDA design. VSBW, a joint venture between VolkerStevin and Boskalis Westminster, is carrying out the construction work.

The project is divided into six sections – called facades – whose length is determined by the type of construction.

Facades are not built sequentially. Instead, the order is determined to minimize the impact on the public and to ensure that the most difficult sections are tackled first.

Eemergency repairs incur significant additional costs – repairs to Southsea Castle defenses in 2015 cost £660,000

Facade 1, which covers the 1.5 km section from Long Curtain Moat to Clarence Pier, was the first to disappear.

It lived up to expectations that it would be a tricky section with many heritage challenges and hitherto unexplored military defences, as well as known ones that need to be treated with sympathy. Work began in September 2020 and is expected to be completed in December.

The work on Facade 1 is divided into two parts – the western end where the heritage structures are located and the simpler eastern end which includes a single sheet pile wall, a capping beam and a raised parapet wall.

The sheet piles at facade 1 are almost complete

The new breakwater for the western part was built inside a double sheet pile wall and the front part will be cut at ground level once the works are completed. Permanent rockfill will create a protected area from the sea. The stone used for the new wall has been petrographically matched to the Purbeck quarry used to power the original structures.

While the historic military nature of the section was already known, other historic walls were discovered during the works. The project team had to liaise closely with Historic England for advice on how to conserve them.

Hydrodemolition techniques, now used on Facade 4, have proven useful in gently exposing the structures of Facade 1.

South Seas Castle Protection

Front 4 runs along the sea side of Southsea Castle and was the next to start in January this year. The project team expects this section to take two years. The works are currently being approached as a maritime operation with 140,000t of rock armor brought to the site to protect the castle while the existing massive concrete defenses are removed and replaced by a two-tier promenade with stepped revetments.

The team are currently using high-pressure hydro-demolition techniques to gently expose the structures surrounding the castle, as they anticipate more unexplored archeology there.

Archeology is not the only challenge for Façade 4 – the construction work and final design must take into account the resting and feeding sites of the Purple Sandpiper birds that winter near the castle from October to March.

The detailed design of the other four facades is in progress and should be completed by the end of this year.

To start the project, a planning permit was requested for the entire program according to the planned design and each façade. It is then resubmitted for consent as the detailed design is completed. According to Bryan, the integrated design team has helped improve the design since the original planning application was created.

The next façade to be delivered will be Facade 3 at the start of 2024, followed by Facades 5 then 6 with work ending in 2029 on Facade 2. Each has its challenges. Facade 3 contains a number of historic war memorials that will need to be moved and rebuilt or considered in the design. Facades 5 and 6 will involve beach nourishment and Facade 2, located primarily at the current project site, will create the only landward sea defense with earthen bunds, flood gates and road structures. raised.

Construction will take another seven years and VSBW Contracts Manager Jon Benton believes the working standard on Facade 1 will set the tone for the rest of the project. He expects the completion of work on this section later this year to give the public confidence in the purpose of the overall scheme, despite disruption to enjoyment of normal waterfront attractions.

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