# Asian Megadeltas: Tackling Coastal Flooding Challenges

Melanie Becker, Katharina Seeger, Amelie Paszkowski

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Melanie Becker, Katharina Seeger, Amelie Paszkowski. Asian Megadeltas: Tackling Coastal Flooding Challenges. Eos, Transactions American Geophysical Union, 2025, 106, 10.1029/2025EO255008. hal-04988849

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Submitted on 13 Mar 2025

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# Asian Megadeltas: Tackling Coastal Flooding Challenges

*Integrating scientific insights into current actions is crucial for steering future research directions and underpinning informed management of coastal flooding in Asian deltas.*

M. Becker¹, K. Seeger²,³ and A. Paszkowski⁴

1. Littoral Environnement et Sociétés, UMR 7266 CNRS, La Rochelle Université, La Rochelle, France

2. Institute of Geography, University of Cologne, Cologne, Germany

3. Soil Geography and Landscape Group, Wageningen University & Research, Wageningen, The Netherlands

4. Environmental Change Institute, University of Oxford, Oxford, United Kingdom

<figure><img src="image_2.png" /></figure>

Drone view of Gabura Union, Ganges-Brahmaputra-Meghna Delta, October 2018. Devastated by Cyclones Amphan and Yass, this area faces ongoing erosion and rising salinity, threatening homes and livelihoods. Residents tirelessly repair the embankment, battling nature's incessant forces. Credit: Md Jamal Uddin Khan.

Asian megadeltas, including the Ganges-Brahmaputra-Meghna, Irrawaddy, Chao Phraya, Mekong, and Red River deltas, are home to millions of people and are vital for regional food security. However, these regions face escalating risks from sea-level rise, urbanization, socio-economic development, and climate change.
A *new article* in *Reviews of Geophysics*<fnref n="9" /> delves into the intricate dynamics of coastal flooding in Asia's largest river deltas, highlighting the interplay between local and global changes. Here, we asked the lead authors to give an overview of these Asian megadeltas, the recent scientific advances in understanding coastal flood risks, and the persistent challenges that remain.

## What is a megadelta?

A megadelta is a large delta system formed when one or more rivers flow into the sea. These deltas are characterized by extensive networks of channels, wetlands, and coastal features, and are formed by the deposition of sediments transported by rivers, which build up over time to create vast landscapes, often only just above sea level. Megadeltas are the largest and most populous deltas of the world. They are vital for agriculture, biodiversity, and have been the cradle of rich civilizations (e.g., in the Ganges-Brahmaputra-Meghna, Nile) that have learned to live with the consequences of natural hazards, such as coastal flooding.

## Which are the main megadeltas in South and Southeast Asia?

The Ganges-Brahmaputra-Meghna delta is the largest and most populous delta in the world, spanning 92,000 km² across India and Bangladesh, with 147 million inhabitants. The transboundary Mekong delta in Vietnam and Cambodia (50,000 km²), is home to 18 million people and is a critical "rice basket" for the region. The Irrawaddy delta in Myanmar is the third largest delta in the region (33,000 km²) with 13 million people, which accounts for 30% of the country's total population. Similar to the Mekong, it is highly agricultural. The highly urbanized Chao Phraya delta (24,000 km²) in Thailand provides a stark contrast, with most of the 26 million residents living in the megacity of Bangkok. Finally, with an area of 6,000 km² and 6 million residents, the Red River Delta in northern Vietnam is the smallest of the five Asian deltas but is crucial for the region's economic productivity.

## Why are they important but also challenging to study?

The Asian megadeltas are home to half of the world's deltaic population and play a critical role for food production and other economic activities in Asia. They are at high risk to climatic changes, with the vast majority of the world's coastal flood exposure observed in these systems. Rising sea levels and rapid socio-economic development worsen these vulnerabilities. Deltas are extremely hydrodynamic, with landscapes constantly shifting and changing. These complex natural processes (such as tropical monsoon, storm surges, and erosion) coincide with densely populated and highly dynamic human activities (like urbanization, land use changes, and resource extraction), creating a complicated web of interacting processes that makes delta environments particularly challenging to study. Likewise, data

<footnote n="9">⁹ Becker M, Seeger K., Paszkowski A., Marcos M., Papa F., Almar R., Bates P., France-Lanord C., Hossain Md S, Khan J., Karegar MA., Karpytchev M., Long N., Minderhoud P. S.J., Neal J., Nicholls R.J. and Syvitski J. (2024). Coastal flooding in Asian Megadeltas: recent advances, persistent challenges, and call for actions amidst local and global changes. Reviews of Geophysics, 62, e2024RG000846. https://doi.org/10.1029/2024RG000846.</footnote>
scarcity due to inaccessibility and/or the lack of measurements, as well as a lack of integrated approaches in assessing these systems, exacerbates the challenges of studying Asian megadeltas.

<figure><img src="image_3.png" /><figcaption>Figure: Location and setting of the five Asian deltas reviewed in this study. Credit: Becker et al. [2024], Figure 4(a,b)</figcaption></figure>

## What are the main geophysical drivers of coastal flooding in these Asian megadeltas?

The Asian megadeltas experience intense tropical monsoons with heavy rainfall, causing seasonal river discharge increases that can lead to fluvial flooding, overwhelm drainage systems, and cause waterlogging in low-lying areas. Tides are also a major driver, interacting with factors such as river discharge, topographic and bathymetric changes, and sea-level rise. These megadeltas face varying intensities of tropical cyclones: frequent, intense storms in the Red River and Ganges-Brahmaputra-Meghna deltas; fewer but stronger cyclones in the Irrawaddy delta; and tropical storms and depressions
in the Mekong and Chao Phraya deltas. These cyclones and storm surges can cause significant coastal flooding, especially when they coincide with high tides.

Land subsidence (i.e., the sinking of the land), caused by soil compaction, glacial isostatic adjustment, or tectonics, as well as anthropogenic drivers such as reduced sediment deposition across the delta and over extraction of groundwater and sand, also drives coastal flood exposure.

## How have human activities exacerbated coastal flood hazards in these regions?

Major cities in Asian megadeltas have tripled in population since 1950, and this rapid growth, alongside economic expansion, has heightened coastal flood risks due to urbanization, land use changes, groundwater extraction, infrastructure development, and sand mining. Since 1990, urban areas below 5 meters above sea level have expanded by over 70% in the Ganges-Brahmaputra-Meghna and Mekong deltas, and by at least 200% in the Chao Phraya delta. Excessive groundwater extraction has caused land sinking in all five megadeltas, with rates exceeding 1 cm per year, while high demand for construction has led to intensive river sand mining, resulting in erosion of riverbanks and shorelines.

Additionally, dams upstream regulate river flows and trap sediments, thereby altering downstream flow dynamics and reducing sediment delivery, which is critical for deltas to remain above sea level. Sediment levels in the Mekong and Red River deltas have dropped over 50%, raising concerns about the Ganges-Brahmaputra-Meghna and Irrawaddy deltas, where many dams are planned.

## How does coastal flooding reshape the landscapes, economies, and communities of these vulnerable regions?

Coastal flooding challenges agriculture, local economies, and livelihoods in these megadeltas, which are crucial for rice production and regional food security. The multifaceted driving forces behind coastal floods can generate profound and widespread impacts on deltaic environments and their inhabitants. For instance, the 2018 Son-Tinh storm devastated over 50% of rice fields in the Red River delta, while Cyclone Nargis in 2008 caused over US$4 billion in damages in Myanmar.

Beyond agriculture, coastal flooding disrupts critical infrastructure (e.g., transportation, electricity, and water supply), paralyzing communities and complicating recovery. The loss of homes and cultural sites leads to displacement, while the psychological impacts deepen poverty and inequality. Saline water intrusion further threatens health and food security by degrading soil and contaminating freshwater sources. Overall, flood impacts range from short to long-term consequences, and the threat of coastal flooding necessitates urgent strategies to mitigate risks and build resilience in these vulnerable regions.

## What methods are used to measure and map coastal flooding?

As flooded deltaic plains are often inaccessible during or immediately after coastal flood events, remote sensing technologies, such as optical sensors and radar imagery, offer a promising solution by providing large-scale, timely information on flood inundation. This data can enhance immediate relief
efforts and inform long-term flood management strategies. To understand human migration patterns in this context, researchers make, for instance, use of datasets from mobile phones and long-term nighttime satellite imagery. However, while these technologies excel in retrospective flood impact assessments, they fall short in predicting future flood behavior.

To anticipate future flooding, modeling approaches are necessary, ranging from simple bathtub models to complex hydrodynamic simulations that account for numerous interacting processes. The Mekong delta serves as an example where, for over 50 years of flood modeling, methods have evolved from the application of simple assumptions to complex hydrodynamic models, tailored to specific delta-related challenges, and the development of near real-time warning systems.

**Where is there plentiful data and where are there gaps in data and knowledge?**

For the Ganges-Brahmaputra-Meghna and Mekong deltas, abundant data from extensive research and monitoring efforts is readily available. However, significant data gaps persist in less-explored regions such as the Irrawaddy and Red River deltas, where essential baseline information on groundwater levels, land subsidence, and sediment transport is often missing or difficult to access. This data scarcity complicates accurate assessments and predictions of flood risks. Models can offer valuable insights, but they often rely on uncertain inputs, like bathymetry and terrain data. While water levels can be measured precisely, surface water flow and rainfall estimates have 10% to 20% accuracy. Modern elevation datasets map floodplains well, yet channel bathymetry often depends on assumptions. Future research should aim to improve data collection in these areas and enhance model accuracy.

**How can a systems perspective help develop more effective delta management strategies?**

A systems perspective highlights the interconnectivity of natural and human systems in delta regions, enabling the creation of comprehensive management strategies that tackle immediate coastal flood risks and promote long-term sustainability. By analyzing the complex interactions among different processes, decision-makers can target the root causes of flood risk rather than simply addressing symptoms. For example, the construction of upstream dams can reduce water and sediment flow to deltas, leading to altered sediment distribution and increased elevation loss due to subsidence. This process can exacerbate inland saline intrusion and necessitate greater groundwater pumping for freshwater, further intensifying localized subsidence. Such feedback loops can entrap communities in persistent cycles of risk. By addressing the root causes of flooding (such as groundwater extraction, sand mining, damming, land use changes, climate change, and sea-level rise) decision-makers can significantly enhance resilience to coastal flood hazards in the long term.

**What are some of the remaining questions where additional modeling, data, or research efforts are needed?**

The five Asian megadeltas still suffer from data scarcity. The generation of critical baseline data should be prioritized, especially in the least-studied Irrawaddy and Red River deltas. Substantial resources must be allocated for comprehensive data collection and monitoring, such as river and sediment
discharge, sea- and water levels, near-shore and channel bathymetric measurements, land subsidence, and elevation. Upcoming satellite missions offer unprecedented datasets on water and sediment dynamics, which together with ongoing field data collection are crucial to improve flood hazard assessments, modeling, and validation.

Aligning research with local delta management priorities can secure funding and resources, ensuring that scientific efforts are effectively co-produced with stakeholders. Integrated research across disciplines is crucial for informed decision-making, ensuring the health and resilience of these vital ecosystems.