Resilient coastal defences needed to battle climate change

5098223 - DHI~Not fit for purpose, polders in Bangladesh credit DHI (1)
Polders in Bangladesh. DHI

From simple hard engineering structures to complex ecological solutions that seek to work with nature, coastal defences have evolved to cope with natural erosion and climate change.

Consisting of nine low-lying islands, the remote Polynesian nation of Tuvalu is one of the countries most vulnerable to the effects of climate change.
Formerly known as the Ellice Islands and a British Commonwealth member, Tuvalu is the world’s fourth smallest nation with a population of around 11,000 and a total land area of 26 km2. It is located in the Pacific Ocean about halfway between Hawaii and Australia.

Tuvalu’s leaders have warned for decades that the islands could eventually disappear if nothing is done. Finally, in December 2018, the United Nations Development Programme awarded a contract to Netherlands-based global offshore and onshore geotechnical and survey services company Fugro to carry out surveys to improve the country’s coastal resilience.

Given the nine islands are scattered over 500,000 m2, Fugro will use airborne lidar bathymetry technology to provide accurate national coverage of shallow – 0–50 m depth – nearshore and lagoon bathymetry and island topography. The resultant data will primarily support the Tuvalu Coastal Adaptation Project’s island vulnerability assessment work that is vital in planning resilience responses. It will also feed into a very broad spectrum of infrastructure development, natural resource management, vulnerability adaptation, and environmental monitoring needs.

Speaking to DPC, Fugro’s business development and government relations director for Asia Pacific, Paul Seaton, noted, “The survey will be conducted during 2019 and have a duration of nine months. No surface survey vessels will be used, it’s solely an airborne survey.”

Though he couldn’t reveal the contract’s value, Seaton added, “Provision of this data is widely seen as an extraordinary step towards improved planning, understanding resources, and assessing national vulnerability. Important services, such as navigation in shallow waters, will also be greatly enhanced and the baseline data will be of key interest to development partners and local authorities.
“For the first time since rising sea levels have been a concern in Tuvalu, detailed and accurate information regarding the relationship between land height and sea level across all nine coral islands will be available.”

Beaches in Benin
Another area of concern, because of its geographical location, is the coastline of African country Benin, situated on the Atlantic Ocean. It is highly susceptible to erosion – further aggravated by the construction of paved dykes on land. Consequently, coastal tourism – seen as a boon to the economy of this very poor country – cannot develop.
A novel but sustainable solution was needed and, working with the Beninese government, Belgian dredging, and offshore giant Jan De Nul (JDN) has come up with just that – an offshore submerged dyke.

Filip Morobé, JDN’s Africa area director, told DPC, “Following a contract from the Beninese Ministry of Living Environment and Sustainable Development for our design of a submerged dyke, during the past year our engineers have carried out the necessary studies and demonstrated, based on model tests, that this concept is sustainable and that it works.”
About 150 m off the coast, JDN’s submerged dyke will have a wave-damping effect, Atlantic waves are broken before they reach the coast. A wave-free climate thus develops between the submerged dyke and the coast to significantly reduce the impact on beaches – sand will move less and erosion will decrease.

Initially, granite rock from national and international quarries was transported to the site near the coastal villages of Avlékété and Djégbadji. In December 2018, work began with the first installation executed by side stone installation vessel Pompei. Because the dyke is submerged, the view of the horizon will not be disturbed, which is very important in terms of government plans to fully develop coastal tourism.

“During the first phase, a 2-km dyke will be installed, followed by two more rock installation campaigns, each of 2 km,” Morobé concluded. “Trailing suction hopper dredgers will next dredge sand to restore the affected beaches, with work expected to conclude in 2021.”

Californian coastal lagoons

5098233 -M&N San Elijo project map credit Moffatt & Nichol (1)
San Elijo project map. Moffatt & Nichol

Moving to the US, the San Elijo Lagoon restoration project in Encinitas, California, is a model of innovative, sustainable techniques that have seen it win the 2018 Best Restored Beach award from the American Shore & Beach Preservation Association. One of the few remaining coastal wetlands in San Diego County, San Elijo lagoon provides vital habitats for resident and migratory wildlife, and sensitive, threatened, and endangered plants and animals. Urbanisation of the lagoon’s watershed demanded additional transportation infrastructure and sewage sludge disposal. Both negatively affected the lagoon, along with modification of the original full tidal system that with upstream urbanisation, both degraded habitats and impaired water quality.
Global infrastructure consultancy Moffatt & Nichol (M&N) took on the project to create a resilient and sustainable ecosystem by modifying both physical and biological conditions to improve the habitat function and value of the 388.5 ha lagoon complex.

M&N coastal scientist Chris Webb told DPC, “It consists of dredging and beneficial reuse of material to improve tidal hydraulics and hydrology, and to adjust elevations to become suitable for certain habitat types. Considerations of sea level rise and habitat migration are key factors for the long-term survivability of the lagoon ecosystem.

“About 344,000 m3 of beach-compatible sand were dredged by Oregon-based contractor Ross Island from within the lagoon and placed on two adjacent beaches. This created a large confined disposal pit in the lagoon, providing a placement location for dredged material unsuitable to be used elsewhere in the design – and a habitat opportunity for salt marsh creation.
“A majority of the lagoon channels are being dredged,” Webb continued, “and pumped back into the disposal pit. The intention is to expand the channel system and modify habitat elevations to expand habitat areas.

“The critical resiliency consideration of sea level rise is being accommodated: the lagoon habitat is designed to be sustainable through a future sea level rise of two feet [0.61 m] using elevation ranges and slopes to allow upward habitat transgression. The lagoon river valley extends upstream several miles and rises in elevation to provide a suitable ramp for wetland habitat transgression during any sea level rise greater than two feet.

“Additionally, two beaches were nourished as a proactive measure for sea level rise,” Webb concluded. “They protect a major highway in Encinitas and city storm water and lifeguarding infrastructure. Sand and cobble were also provided to the Cardiff Living Shoreline Project, which is a city of Encinitas project to improve resilience along Coast Highway 101. The highway has been damaged from flooding in the past and may experience more in the future during climate change and sea level rise.”

The long view

The Netherlands’ current flood protection programme also focusses on future-proofing, based on government requirements introduced in 2017, it aims to ensure the country’s primary flood defences can cope with the effects of sea level rise and climate change.

Drawn up by Rijkswaterstaat (Directorate General for Public Works and Water Management) and all the Dutch water authorities to protect the Netherlands from flooding until at least 2050, the programme will continue until 2028 and involves 300 projects on the coast, the major rivers and the largest lakes. More than 250 locks and pumping stations will be overhauled, together with more than 1,100 km of dykes. One of them is the north western Wadden Sea dyke between Eemshaven and Delfzijl, and Boskalis and its joint venture partner KWS Infra have the contract.
Boskalis Project Manager Gertjan Timmers explained to DPC that, “The project involves raising and widening the dyke over a distance of almost 12 km. It’s being executed for the Noorderzijlvest water authority and it is largely financed by the national Flood Protection Programme.

“It’s a priority because the region is affected by earthquakes and subsidence as a result of gas extraction. As a result, local people have gone through many dramatic events in recent years so it’s hardly surprising that they keep a critical eye on a project like this. Working with the water authority, we inform local people as much as possible about everything the work involves – through a website, a visitor centre, and information meetings at which the residents are given the opportunity to use virtual reality technology to experience the situation in the future.”

In the joint venture, Boskalis is responsible for around 70% of the work. “In simple terms, we strip the dyke, strengthen it with sand, and pack it in with clay again,” Timmers stated.
“Depending on the location, we will raise the dyke by between 50 cm and 2 m and widen it by 5 m to 25 m. That involves adding sand, clay, and soil to the landward side. On the outside, we are applying an innovative asphalt structure to some sections. In a few places, the existing revetment of Norwegian rock will be reinforced with concrete. That requires 1 million m3 of sand, 200,000 m3 of clay, 70,000 tonnes of asphalt, and additional earthmoving amounting to a volume of 450,000 m3.”

An international group of experts has looked at the measures needed to prevent dyke failures if there are severe earthquakes, Timmers noted, adding, “On the basis of that study, it was decided to install a number of depots holding a total of 30,000 m3 of clay behind the dyke that can be used for rapid repairs of any earthquake damage. The dike will also be fitted with an earthquake monitoring system.” Scheduled project completion date is 31 December 2019 – and as Timmers stressed, getting the logistics right is crucial.
“We need to get large quantities of raw materials into the right place in a very tight time window. The various itineraries and the operational phases must match, and we also have to make sure that construction traffic doesn’t need to go through the dyke-side villages.

“Most of the work consists of large-scale earthmoving and initially, we’ll concentrate on strengthening the landward side because we can’t strengthen the outer slope during the storm season,” Timmers concluded, noting that in a follow-up operation, Groningen and Delfzijl authorities will implement linked projects, “including the creation of nature areas, a cycle path on and alongside the dike, and a city beach near Delfzijl.”

5098226 - JDN Loading rock aboard Pompei credit Jan De Nul[1]
Loading rock aboard Pompei. Jan De Nul

Polders in Bangladesh
Think polders and you think of the Netherlands, but they are also important in Bangladesh. A new study commissioned by the Bangladesh Water Development Board, with funding from the World Bank, will focus on developing a sustainable solution for polders in the country. The 30-month project will be led by Danish water environment specialist DHI and Dutch research institute Deltares in association with the Bangladesh Institute of Water Modelling and the US universities of Colorado and Columbia.

DHI marine water vice president Henrik Kofoed-Hansen told DPC that polders were constructed in Bangladesh during the 1960s and 1970s to protect farmers from saline water intrusion and storm surges impacts – “Today, the polders are not fit for the future,” he explained.

“Two-thirds of Bangladesh’s coastal land has an elevation of less than 3 m, often leading to devastating flooding during cyclones in the Bay of Bengal. The combined effect of soil subsidence and sea level rise threaten the livelihood of more than 30 million people living in the country’s coastal area, and it is foreseen that flooding will occur more frequently and with bigger human, environmental, and economic impact.

“The aim of the project is to develop a clearer understanding of the relationships between the functioning of the polders and the ever-changing large-scale drivers, such as human impact on the environment across the entire river basin and changes in climate. State-of-the-art numerical models will be implemented at different scales, in combination with innovative monitoring techniques to quantify hydrodynamics, sediment load, and subsidence rates across the Bangladesh Delta. This will form the basis for the development of conceptual polder designs,” he concluded.