Natural adaptation of atoll islands to sea-level rise offering opportunities for ongoing human occupation (ARISE)

Increasing our understanding of overwash processes to develop practical management tools
Atoll Island

Project context

Originally submitted as an ERC Advanced Grant, but now funded by UKRI, the £2.8M ARISE project, led by Professor Gerd Masselink, is a follow-up from the GCRF funded project ‘Physical impacts of climate change on coral reef islands.
Due to their low-lying nature, coral atoll islands are widely acknowledged to be amongst the most vulnerable environments to climate change. Most of them are predicted to be uninhabitable by the mid-21st century because of sea-level rise.
However, these forecasts are based on relatively simple hydrodynamic models that consider the islands immobile, whereas, when overwashed during storms, the islands can vertically accrete due to sediment deposition. Repeated overwash can enable atoll islands to keep up with rising sea level.
This potentially provides opportunities for island communities to prolong habitability through innovative adaptation strategies, instead of having to construct expensive coastal defences or traumatically relocate to regions with no flood risk.
Atoll Island
Atoll Island
Atoll Island

Aim

It is generally accepted that overwash is key to atoll island survival, but further research is required to increase our quantitative understanding of overwash processes and transform the enhanced insights into practice by developing management tools.
The overarching aim of this project is therefore to ‘revolutionise our capability to model the physical impacts of sea-level rise on atoll islands to aid in the formulation, development and implementation of transformative climate-change adaptation strategies for atoll island communities’.

Atoll islands

Atoll islands are wave-built accumulations of gravel or sand that sit on top of coral reef platforms. The island sediments are derived from the breakdown of calcium-carbonate secreting organisms, such as corals, bivalves, gastropods and foraminifera, that dwell on the adjacent reef systems.
The existence of these islands is intrinsically linked to the reef ecology, as they rely on the reef’s production of sediments; however, the formation, maintenance and dynamics of atoll islands are primarily governed by physical processes involving water level, waves and currents.
Key hydrodynamic processes include:
Atoll Island
1. Breaking of ocean waves at the seaward edge of the platform.
2. Loss in wave energy as waves continue to break in the shallow water across the reef platform.
3. Generation of low long-period waves and wave set-up across the reef platform.
4. The combined wave motion being released at the island beach as wave runup.
5. If the waves are very energetic and the water level is high, wave runup may extend to the top of the island and flood the island by overwash.
Sediment produced by the reef ecosystem is transported by these complex hydrodynamic processes from the reef edge, to the reef platform and the beach, and onto the island in case of overwash, ultimately leading to island change.

Approach

This ARISE project comprises three main types of activities – data collection and analysis, numerical model development, model application for management – which are arranged through eight interlinking Work Packages. New knowledge and understanding of atoll island response to SLR will be obtained using unprecedented laboratory experiments (in the Delta Flume, Netherlands) and field measurements (in the Maldives and Pacific).
The unique data sets acquired will be used to develop, calibrate and validate hydro- and morphodynamic numerical models (using the XBeach suite of models). These models will then be deployed in an innovative modelling framework to evaluate the role of the various processes involved in the island response. Finally, the modelling tools will be deployed to enable atoll island communities to implement adaptation strategies that maximise opportunities for continued habitation.

Data Collection
Current Meter
Survey
Pressure sensor
 

Follow the project's progress

 

Maldives field campaign – January 2025

A subset of instruments remained deployed after the large 2024 field campaign to build a long-term dataset. These sensors were retrieved and redeployed for another year. UAV flights and multibeam surveys were carried out to complete the full digital elevation model. Additionally, ADCP surveys were conducted in the main channel between the islands of Fioyaree and Dighelaabadhoo to get a handle on the tidal currents.
Marine science students from the University of Plymouth were part of the January field trip as part of their degree programme. Thus, research was combined with educational activities to enhance their understanding of coastal processes on atoll islands. Before returning to the UK, Gerd Masselink presented a talk, reporting on the 1 July 2022 island flooding event, to the Maldives National University.
Teaching in the Maldives
Drone flight in the Maldives
Wave mapping in the Maldives
Aerial view of the Maldives
 
 
 
 
 
 
 

Maldives field campaign – August 2024

During this campaign, all the instruments deployed over the spring 2024 field trip were recovered. These data are being utilised for various analyses, including tidal fluctuations, sediment transport, groundwater hydrodynamics, wave dynamics and current patterns, and will be fundamental for numerical model development, calibration and validation.
The results from this campaign play a critical component of WP4 stage and serve as baseline datasets for the island. Preliminary outcomes include the development of a detailed digital elevation model and an extensive archived hydrodynamic data set. Results will be presented in April 2025 at the Coastal Dynamics Conference in Aveiro, Portugal. The data will form the basis for much further analysis over 2025 and 2026.
Preparing equipment in the Maldives
Diver off the Maldives
Diving equipment in the Maldives
Data collection device used under water in the Maldives
 
 
 
 
 
 
 

Delta Flume experiment – summer 2024

From July to September 2024, extensive physical modelling was conducted in the Delta Flume in the Netherlands, representing WP1 of the ARISE project. A 1:3 scale model of an atoll island with reef platform was constructed and this system was subjected to a range of wave and water level conditions, specifically to investigate how atoll islands respond to sea-level rise.
The tests show that to some degree islands can keep up with rising sea level by increasing their elevation due to overwash-induced sedimentation. As part of the test programme, and in collaboration with Dr Marion Tissier (Technical University Delft) and a Dutch start-up (CoastStruction), the efficiency of artificial reef structures placed on the reef platform, in reducing wave energy at the island shore was also investigated.
Researchers working at the Delta Flume wave installation in the Netherlands
Delta Flume
Delta Flume
Delta Flume
 
 

Maldives field campaign – spring 2024

A large field campaign was held over spring 2024 in the Maldives, representing WP3 of the ARISE project.
Working with colleagues from the universities of Bath and St Andrews, and international researchers from the USA, Australia, Singapore and the Netherlands, more than 80 individual instruments were deployed on the island of Dighelaabadhoo.
The measurements generated by the instrumentation will constitute the largest field campaign ever to be staged on an atoll island, and the instruments were deployed from March to August.
Researchers deploying instruments on Dighelaabadhoo island during the ARISE project
LiDAR tower in the Maldives as part of the ARISE project
Concrete blocks on a boat in the Maldives as part of the ARISE project
Pressure sensor submerged in shallow sea water in the Maldives as part of the ARISE project
 
 
 

Academic and technical staff

Project partners and collaborators

Research students

Publications

CMAR, 2024. Sea-level rise and coastal changes in the Maldives: Review for the World Bank . Report 2318, University of Plymouth Enterprise Limited, 72 pp.
Gea Neuhaus, A., Scott, T., Masselink, G., Lindhart, M., Roelvink, F., Vila-Concejo, A. and Kench, P., 2025. Coral rubble mobility on an intertidal reef flat Huvadhoo atoll, Maldives . Coastal Dynamics, ASCE, Alveiro, Portugal.
Holsclaw, A., Lindhart, M., Masselink, G. and Davidson, M., 2025. Observations of wave setup on a coral atoll . Coastal Dynamics, ASCE, Alveiro, Portugal.
Lindhart, M., Masselink, G., Holsclaw, A., Ganderton, P., Poate, T. and Robinson, E., 2025. Shallow reef-lagoon exchange flow on a coral atoll . Coastal Dynamics, ASCE, Alveiro, Portugal.
Masselink, G., Lindhart, M., Kench, P., Roelvink, F., Waheed, A.A., 2024. Report prepared by CMAR for the World Bank to support background analytics related to Sea-level rise (SLR) and coastal changes for the Country Climate and Development Report (CCDR) for the Maldives . Coastal Marine Applied Research (CMAR), Coastal Processes Research Group, University of Plymouth, 73 pp.
Masselink, G., Roelvink, F., Rose, S.T., Tissier, M., Zwanenburg, S. and Doeleman, M., 2025. Large-scale modelling of hydro- and morphodynamics associated with reef platform and island systems . Coastal Dynamics, ASCE, Alveiro, Portugal.


Roelvink, F., Masselink, G., Stokes, C. and McCall, R., in prep. Climate adaptation for natural atoll islands: the long-term morphological response of coral islands to sea level rise and hazard mitigation strategies. Earth’s Future.
Rose, S.T., Blenkinsopp, C.E., Masselink, G., Turner, I.L. and Storlazzi, C. D., 2025. Measurements of wave runup on an atoll island using LiDAR . Coastal Dynamics, ASCE, Alveiro, Portugal.
Scott, T., Gea Neuhaus, A., Masselink, G., Lindhart, M. and Saunders, J., 2025. Morphodynamics and mobility of coral rubble tracts: Huvadhu Atoll, Maldives . Coastal Dynamics, ASCE, Alveiro, Portugal.
Waheed, A.A., Masselink, G., Poate, T., Biermann, L. and Mohamed, A., 2025. Maldivian island typologies for coastal adaptation to future sea level rise . Coastal Dynamics, ASCE, Alveiro, Portugal.
Winter, G., Storlazzi, C., Vitousek, S., van Dongeren, A., McCall, R., Hoeke, R., Skirving, W., Marra, J., Reyns, J., Aucan, J., Widlansky, M., Becker, J., Perry, C., Masselink, G., Lowe, R., Ford, M., Pomeroy, A., Mendez, F., Rueda, A. and Wandres, M., 2020. Steps to Develop Early Warning systems and future scenarios of storm wave-driven flooding along coral reef-lined coasts. Frontiers in Marine Science, 31.
Atoll Island
GPS Survey
Atoll Island