A crucial step in reclaiming coastal land is ensuring that sediment from
the Ganges-Brahmaputra rivers continues to flow freely and spread effectively
across the coastal area, as indicated by scientific research.
Nature is a remarkable healer, as evidenced by the resilience of river deltas. These ecosystems have a remarkable ability to resist rising sea levels by replenishing themselves through the continuous delivery and disposition of sediment. Likewise, the Ganges-Brahmaputra delta combats rising sea levels by perpetually transporting, dispersing, and depositing its substantial annual sediment load, totaling approximately 1 billion tons.
New hydrological models suggest that if the monsoon
rains get stronger in the 21st century, the amount of sediment
carried by the rivers could increase by 34-60%. However, other studies show
that if we go ahead with building dams and diverting rivers as planned, the sediment
could decrease by 15-80%.
In recent
research published in Nature Communications, researchers investigated the sediment
dynamics of the Ganges-Brahmaputra rivers. They developed a detailed, sediment
budget based on field data spanning different climate conditions throughout the
Holocene period. Their study corroborated earlier model predictions, suggesting
that increased sediment delivery could potentially mitigate the effects of
rising sea levels. This offers a glimmer of hope for the Ganges-Brahmaputra delta’s
future, but the implementation of proposed dams and diversions could hinder
these natural processes.
River deltas can be found all around the world,
occurring whenever a river flows into a body of water unable to completely
carry away its sediment load. Sediment plays a direct role in various aspects such
as reclaiming land, deciding where people live, and using vulnerable areas. A
crucial step in reclaiming coastal land is ensuring that sediment from the Ganges-Brahmaputra
rivers continues to flow freely and spread effectively across the coastal area.
Recent research
indicates a notable change in our understanding of how river deltas respond to
rising sea levels and decreased sediment flow. Past assessments frequently
predicted negative outcomes for deltas in the face of climate change and human
influence.
While
valid, these concerns tend to oversimplify the complex dynamics of deltaic
systems, potentially overlooking more positive outcomes.
One consequence of these negative and
simplistic assessments is the promotion of bleak narratives that advocate for
either constructing large engineering projects or abandoning vast coastal areas
as the primary responses to climate change. This approach can undermine local
and regional efforts aimed at maintaining sediment supply and preserving land in
these vulnerable coastal landscapes, explains the report.
A steady supply
of sediment from river is crucial for counteracting the rise in sea levels
in river deltas. However, we need better information and data on how sediment is
delivered and spread to ensure that we accurately understand how deltas will
respond in the future. This includes considering both the natural changes in delta
systems and the impacts of human development and land use. It is also becoming increasingly important to
use approaches that look at both human actions and natural processes together to
predict what will happen to river deltas. This is because, by the year 2021, as
many as 630 million people could be living in areas at risk of flooding or
becoming uninhabitable due to rising sea levels.
Recent studies
that have used detailed and up-to-date data suggest that more river deltas might
be gaining land than we previously thought. This means that overall, river deltas
might be more resilient than we believed.
The South Asian Monsoon affects the Ganges-Brahmaputra
System
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| Map of the Ganges and Brahmaputra catchments in South Asia. Image Source: Darby et al., CC BY SA 3.0 Deed |
The Ganges-Brahmaputra River systems’ water flow, weather, and sediment movement are mainly influenced by the South Asian monsoon, which changes a lot over long periods. Studies show that over thousands of years, the monsoon has had strong and weak periods, with more rain in the past compared to recent times. Climate reports suggest that the monsoon will become stronger and more variable in the future. Also, because the region is getting warmer, glaciers in the Himalayas might melt more, adding more water and sediment to the river for decades or even centuries to come.
Predicting the Ganges-Brahmaputra Delta’s future
The Bengal Delta, alternatively known as the Ganges-Brahmaputra Delta or Sunderbans Delta, stands as one of the planet's most expansive and productive river deltas. Situated in South Asia, it spans across regions of Bangladesh and India, where the Ganges and Brahmaputra rivers converge, emptying into the Bay of Bengal. Encompassing an area of about 105,000 square kilometers (40,000 square miles), this vast delta hosts a multitude of inhabitants.
Distinguished by its intricate network of watercourses including rivers, streams, and distributaries, the delta forms a complex maze amidst verdant mangrove forests, marshlands, and fertile plains. It represents a dynamic ecosystem shaped by ongoing processes of sedimentation, erosion, and tidal dynamics.
Observations suggest that the amount of
sediment reaching the Ganges-Brahmaputra delta varies widely, with estimates ranging
from less than 500 to over 1,100 million metric tons per year. This uncertainty
poses a significant challenge to understanding how the delta might respond to
rising sea levels.
In order to verify the accuracy of projected sediment delivery to the
Ganges-Brahmaputra delta, Raff and
colleagues undertook a thorough investigation. They gathered over 600
field-based measurements from 500 locations, providing crucial validation for
modeling studies and enhancing understanding of the river system's connectivity
from its source in the Himalayas to the Bengal deltaic margin. Through analysis
of previously unpublished borehole data and compilation of information from
various studies, they achieved unprecedented detail in their sediment budget,
covering volume, mass, grain size, and provenance. This new research represents a significant
advancement, offering insights into bedload contribution and sediment delivery
patterns, particularly in response to climate change.
To validate future sediment-delivery
projections, the researchers compared them with historical responses of
sediment load to climate change and monsoon strength during the Holocene period.
They carefully quantified the mass and grain-size distribution of sediment sequestered
in the delta and analyzed how it was distributed and stored during significant
climate and sea level changes. Additionally, they compared sediment reconstructions
with projected sea-level rise demands in the coming century.
What does the future hold?
The future of the Ganges-Brahmaputra delta is a complex interplay between natural processes and human intervention. On one hand, the resilience of river deltas, fueled by the continuous delivery of sediment, offers hope for mitigating the impacts of rising sea levels. Research by Raff and colleagues suggests that if the projected intensification of monsoon rains occurs, the sediment load carried by these rivers could increase significantly, potentially counteracting the effects of sea-level rise. However, this optimistic outlook is tempered by the threat posed by proposed dam construction and river diversions, which could disrupt the natural flow of sediment to the delta. The implementation of these projects risks undermining the delta's ability to replenish itself and adapt to changing environmental conditions.
Moreover, the uncertainty surrounding the amount of sediment reaching the delta complicates our understanding of its future trajectory. While studies indicate a wide range of estimates, ranging from less than 500 to over 1,100 million metric tons per year, the exact impact of this variability on the delta's resilience remains unclear. To address this uncertainty, recent research has provided crucial insights through comprehensive field-based measurements and sediment budget analyses.
Looking ahead, it is imperative to prioritize sustainable management of sediment resources and maintain the natural flow of sediment to the delta. This involves balancing the need for infrastructure development with the preservation of deltaic ecosystems and coastal landscapes. International cooperation and coordinated efforts among countries sharing the Ganges-Brahmaputra river systems are essential to ensure the long-term resilience and stability of this critical ecosystem. By embracing nature-based solutions and fostering a deeper understanding of deltaic dynamics, we can work towards a more optimistic future for the Ganges-Brahmaputra delta, where the natural healing powers of these remarkable ecosystems are allowed to flourish.
References:
Raff, J.L.,
Goodbred, S.L., Pickering, J.L., et al. (2023). Sediment delivery to sustain
the Ganges-Brahmaputra delta under climate change and anthropogenic impacts.
Nature Communications, 14, 2429. https://doi.org/10.1038/s41467-023-38057-9
Jarriel, T.,
Swartz, J., & Passalacqua, P. (2021). Global rates and patterns of channel
migration in river deltas. Proceedings of the National Academy of Sciences,
118(46), e2103178118. https://doi.org/10.1073/pnas.2103178118
Darby, S. E.,
Dunn, F. E., Nicholls, R. J., Rahman, M., & Riddy, L. (2015). A first look
at the influence of anthropogenic climate change on the future delivery of
fluvial sediment to the Ganges–Brahmaputra–Meghna delta. Environmental Science:
Processes & Impacts, 17(11), 1587-1600. https://doi.org/10.1039/C5EM00252D