GM6.9

EDI
Sinking, shrinking and saltier river deltas: processes, interactions and responses

Worldwide over 500 million people live in low-lying coastal deltaic areas, existential to global food security, economic activities and biodiversity. Despite climate change severity at global scale, in many densely populated deltas its effect is currently evidently dwarfed by anthropogenic pressures in the river basin such as river flow modifications, damming and the overexploitation of the natural resources groundwater or sand, as well as profound land use changes and process such as urbanisation. As a result, many major deltas rapidly sink and shrink because of accelerated land subsidence and erosion rates. This increases relative sea-level rise and vulnerability to floods and storms, increases salinization of surface and groundwater and reduces freshwater availability, leading to significant losses in biodiversity, habitat degradation, reduced agricultural and economic productivity. A fundamental change in management approach is required to address these trends and challenges to sustain deltas environments, economies and populations through the 21st Century.

The processes resulting in sinking, shrinking and saltier deltas are interconnected and developing sustainable and inclusive management requires a multidisciplinary system approach. For this, we need to understand the full range of interrelated disciplines, including, amongst others, geology, river and estuarine dynamics, sediment dynamics, hydrology, hydrogeology, geomechanics, bio-morphodynamics as well as the human dimension of delta demography, economy and land use. This session aims to bring together contributions from the full range of scientific disciplines involved in understanding and managing the combined integrated environmental threats that our world’s deltas face. These includes recent advancements in measuring, modeling and projecting environmental dynamics, especially focused on distinguishing (quantifying) anthropogenic and climate change impacts on observed natural dynamics. In particular, inter- and multidisciplinary contributions on the interactions between different environmental processes and efforts towards developing integrated management and development strategies for our sinking, shrinking and saltier deltas are warmly welcomed.

Public information:
Worldwide over 500 million people live in low-lying coastal deltaic areas, existential to global food security, economic activities and biodiversity. Despite climate change severity at global scale, in many densely populated deltas its effect is currently evidently dwarfed by anthropogenic pressures in the river basin such as river flow modifications, damming and the overexploitation of the natural resources groundwater or sand, as well as profound land use changes and process such as urbanisation. As a result, many major deltas rapidly sink and shrink because of accelerated land subsidence and erosion rates. This increases relative sea-level rise and vulnerability to floods and storms, increases salinization of surface and groundwater and reduces freshwater availability, leading to significant losses in biodiversity, habitat degradation, reduced agricultural and economic productivity. A fundamental change in management approach is required to address these trends and challenges to sustain deltas environments, economies and populations through the 21st Century.

The processes resulting in sinking, shrinking and saltier deltas are interconnected and developing sustainable and inclusive management requires a multidisciplinary system approach. For this, we need to understand the full range of interrelated disciplines, including, amongst others, geology, river and estuarine dynamics, sediment dynamics, hydrology, hydrogeology, geomechanics, bio-morphodynamics as well as the human dimension of delta demography, economy and land use. This session brings together contributions from the full range of scientific disciplines involved in understanding and managing the combined integrated environmental threats that our world’s deltas face.
Co-organized by HS13/NH1/NP8
Convener: Philip S.J. MinderhoudECSECS | Co-conveners: Charlotte Marcinko, Robert Nicholls, Gualbert Oude EssinkECSECS, Pietro Teatini
vPICO presentations
| Wed, 28 Apr, 15:30–17:00 (CEST)
Public information:
Worldwide over 500 million people live in low-lying coastal deltaic areas, existential to global food security, economic activities and biodiversity. Despite climate change severity at global scale, in many densely populated deltas its effect is currently evidently dwarfed by anthropogenic pressures in the river basin such as river flow modifications, damming and the overexploitation of the natural resources groundwater or sand, as well as profound land use changes and process such as urbanisation. As a result, many major deltas rapidly sink and shrink because of accelerated land subsidence and erosion rates. This increases relative sea-level rise and vulnerability to floods and storms, increases salinization of surface and groundwater and reduces freshwater availability, leading to significant losses in biodiversity, habitat degradation, reduced agricultural and economic productivity. A fundamental change in management approach is required to address these trends and challenges to sustain deltas environments, economies and populations through the 21st Century.

The processes resulting in sinking, shrinking and saltier deltas are interconnected and developing sustainable and inclusive management requires a multidisciplinary system approach. For this, we need to understand the full range of interrelated disciplines, including, amongst others, geology, river and estuarine dynamics, sediment dynamics, hydrology, hydrogeology, geomechanics, bio-morphodynamics as well as the human dimension of delta demography, economy and land use. This session brings together contributions from the full range of scientific disciplines involved in understanding and managing the combined integrated environmental threats that our world’s deltas face.

vPICO presentations: Wed, 28 Apr

Chairperson: Philip S.J. Minderhoud
15:30–15:35
Keynote
15:35–15:45
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EGU21-283
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solicited
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Highlight
Jaia Syvitski

Many studies rank deltas as a continuum, as conditioned by hinterland drainage area, river discharge, sediment load, ocean energy, or even human interaction.  This scaling has helped advance our understanding on how these important coastal deposits develop. From such studies, equilibrium states have been identified, such as the balance between sediment supply and the subsequent dispersal of incoming sediment.  Conceptual equations are used to track the influence of changing boundary conditions such as sea level rise (or fall) that can then expose the role of anthropogenic influences such as groundwater mining.  However, scaling may not reveal important differences between small-scale deltas, that globally number in the thousands, and large regional deltas that number in the dozens.  For example backwater effects appear important in large delta systems but can often be ignored in many smaller delta systems with steeper fluvial gradients. Large deltas are home to large human populations and their infrastructure, but does this influence scale with delta size? This overview presentation explores the use of conceptual equations to determine if there is a limit to scaling.

How to cite: Syvitski, J.: Large deltas versus small deltas: Two different beasts, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-283, https://doi.org/10.5194/egusphere-egu21-283, 2020.

PICO Two-minute presentations
15:45–15:47
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EGU21-15474
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ECS
Florin Iulian Zăinescu and Edward Anthony

There is increasing recourse to Big Data in the geosciences as in all other spheres of research. This is an important development in the pursuit of global statistics or unifying rules on environmental change. However, the finality can only be justified if such data are rigorous and checks equally rigorous, because the objective is to derive and eventually propose reliable quantified trends or functional laws.

River deltas, a hot topic because of their exposure to hazards, increasing vulnerability and assumed loss of resilience caused by climate change and human intervention, are witnessing an upsurge of analysis based on available satellite and model data. A recent database (Nienhuis et al., 2020) comprises ~11000 identified ‘deltas‘ along with surface area changes for each delta based on Aqua Monitor (Aqua) and Global Surface Water Explorer (GSW) datasets derived from Landsat images, alongside with fluvial, wave and tidal sediment fluxes from global models and estimations. The authors claim that deltas globally have gained an area of 54 ±11.8 km2/yr over the last 30 years due partially to human interventions in drainage basins, and they attributed land loss in about 1000 deltas to recent reductions in sediment supply. However, these findings are, unfortunately, beset with flaws.

Prompted by the inventory of numerous ‘river deltas‘ in regions such as the British Isles and Britanny, France, where a Web-of-Science bibliographic check yielded no modern river deltas, we randomly selected 108 deltas from the dataset (n = 1%), checking for delta presence and obtaining change rates from manually-drawn buffers. We obtained no agreement with the original data of Nienhuis et al. (2020), and found the same disagreement when we tested the data against an already published dataset. We consider that the database of Nienhuis et al. (2020) is replete with errors that render the derived delta area changes unreliable. We raise fundamental concerns about their methodology and the criteria they use to define river mouths as deltas.

Our caveat here is that while Big Data certainly provide a way forward for the global analysis of river deltas (and other landforms), there is a need for awareness of current pitfalls in datasets and their handling. Nienhuis et al. (2020) proposed their definition of river deltas. There is indeed a need for community consensus on delta definition, but this could be a hard task. Considering just deltaic coastal change, some guidelines for rigorous analysis of data are: (i) better and more robust buffers that delineate only such change. Is this even achievable by automatic means?; and (ii) better filtering of anthropogenic modifications which, in many deltas, are dominating area change, so that new coastal reclamation projects can be more robustly detected in improved landcover databases. There is also a need for accurate datasets on water surface change. Do these exist? A comparison, for instance, of Aqua and GSW datasets on deltaic coastal change shows significant discrepancies between the two.

How to cite: Zăinescu, F. I. and Anthony, E.: Big Data-driven geomorphic analysis of the world’s river deltas: a need for caution and rigour, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15474, https://doi.org/10.5194/egusphere-egu21-15474, 2021.

15:47–15:49
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EGU21-12309
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ECS
Anita Rigoni, Philip S. J. Minderhoud, Claudia Zoccarato, and Pietro Teatini

Most of the world major deltas are threatened by relative sea level rise, i.e. land subsidence and sea level rise, caused by a combination of anthropogenic pressures and natural processes. This study focuses on the natural components of land subsidence directly and indirectly related to the Holocene delta stratigraphy. Firstly, subsidence is caused by natural compaction of the Holocene sediments following deposition over time under their own weight. Secondly, subsidence is caused by the visco-elastic deformation of the Earth crust driven by cumulative load of the Holocene delta (so-called Sediment Isostatic Adjustment). These two processes are obviously connected and call for a proper evaluation of the weight of (the Holocene portion of) a delta. This requires a proper quantification of specific weight and degree of compaction of Holocene deposits with depth to arrive at a first-order assessment of Holocene delta weight.

 

This study proposes an innovative methodology to address the following two questions: 1) What is the proper weight of a (Holocene) delta? 2) How much have deposits been compacted since their deposition during Holocene delta formation? Our approach integrates knowledge and data on deltaic depositional environments, stratigraphic information, geomechanical properties and other characteristics of the Holocene sequence. 

 

The developed approach is applied to eight major deltas worldwide selected from a larger database according to the availability of lithostratigraphic and geomechanical information. The analysis is conducted at the scale of an entire delta, thus required the upscaling and interpolation of datasets generally available from a few wellbores only. Lithostratigraphic data  is combined with a backwards modelling procedure to decompact the Holocene delta sequence to their decompacted thickness to provide a proper  estimation of their weight, which takes into account the (computed) in-situ compaction degree. The results show a large variability in compaction and specific weight distribution for the different deltas which underscores the substantial role of natural compaction on delta evolution. 

How to cite: Rigoni, A., Minderhoud, P. S. J., Zoccarato, C., and Teatini, P.: Decompacting Holocene deltas to quantify their (proper) weight, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12309, https://doi.org/10.5194/egusphere-egu21-12309, 2021.

15:49–15:51
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EGU21-7931
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ECS
Li yueting, Pietro Teatini, Shujun Ye, Andrea Franceschini, Matteo Frigo, and Claudia Zoccarato

Aseismic earth fissures due to the excessive groundwater exploitation have caused seriously damage in many subsiding sedimentary basins worldwide. Generally, multiple fissures almost parallel to each other with equal distances are prone to develop where a compacting aquifer system overlies impermeable and/or incompressible ridges. Here, an advanced finite-element interface-elements modelling approach is employed to understand this process within unfaulted sedimentary sequences. A simplified geological setting is initially used to investigate the effect of the ridge slope on ruptures behaviors. Then, we reproduce the case of Guangming village, China. In both the proposed scenarios, the model simulates the occurrence of multi-fissures that initiate at land surface and propagate downward, as observed in the sites. The earth fissures are formed as a result of the combination of tensile stress (bending condition) and shear stress (shearing conditions) accumulation around and above the tip and the slopes of the ridge, respectively. The numerical outcomes indicate that the steeper ridge results in higher magnitude stress accumulation above the ridge tip which favors the formation of fissures with significant opening and small or null offset, but at expense of the reduction in stress accumulation area and fissure distribution. In Guangming case, the outcomes show that two ruptures started sliding and only one year later a central fissure opened and propagated down to 15-30 m depth. The simulated maximum opening and sliding of the central and side fissures, respectively, approximate 30 cm, which are almost in agreement with the observations. The numerical results prove that the proposed modeling approach is an effective way to predict and analyze multi-fissure onset and development in subsiding basins.

How to cite: yueting, L., Teatini, P., Ye, S., Franceschini, A., Frigo, M., and Zoccarato, C.: Modelling multi-fissure zones above buried rock ridges in subsiding basins, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7931, https://doi.org/10.5194/egusphere-egu21-7931, 2021.

15:51–15:53
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EGU21-13231
Yujuan Sun, Lucy Bricheno, and Kevin Horsburgh

The annual mean combined river discharge from the Ganges-Brahmaputra-Meghna (GBM) riverine system is 100,000 – 140,000 m3/s (EGIS 2000), draining to Bay of Bengal, covering 83% of total area of Bangladesh, and making Bangladesh delta more vulnerable to both the freshwater and the mixing with sea water. This estuarine environment varies spatially and temporally, over all multiple time scales, due to its funnel-shaped vast river networks, strong tides, and saltwater intrusion. Recent studies reported a drastic salinity increasing at the end of the dry season in the past 20 years (Murshed et al., 2019). Significant salinity intrusion appears from the Sundarbans (over 20ppt in 2015), and then extends inland, which makes salinity a key factor for changing land use and demographic migration.

We examine volume and salt flux transports at multi-river channels where the GBM drains to the Bay of Bengal, using our unstructured-grid Bangladesh-FVCOM model (Bricheno et al., 2016). This realistic simulation of the whole delta has been shown to reproduce the present-day river flow circulation, tidal dynamics, and salinity stratification.

We then summarise results from the detailed hydrodynamic numerical model into a simplified flow budget, to summarise the climate impacts on salt-intrusion in the delta. In this way, we can investigate the mechanism of salt flux transports in Bangladesh delta, and improve our understanding of the controlling processes driving salinity intrusion in this region.

How to cite: Sun, Y., Bricheno, L., and Horsburgh, K.: Mechanism of salt flux transport in a tidal dynamic delta, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13231, https://doi.org/10.5194/egusphere-egu21-13231, 2021.

15:53–15:55
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EGU21-2326
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ECS
Constantinos Matsoukis, Laurent O. Amoudry, Lucy Bricheno, and Nicoletta Leonardi

High salinity values in deltaic regions can be detrimental for agriculture, aquaculture and human consumption. Salinity levels in river deltas could significantly increase due to sea level rise and infrastructure works such as river diversions or dam constructions. River flow and tides have a large influence on salinity concentrations and it is thus important to understand their combined role. In this paper, a 3D model is built for an idealized delta. A series of simulations is carried out to investigate salinity fields developed under the combined action of tidal amplitude and fresh water flow. Simulations are classified based on the ratio between fresh water and tidal range. Both tide influenced and river dominated cases were considered. Results emphasize the importance of tidally driven mixing which can establish fresher conditions in the delta for certain amplitudes. Tidal amplitude increase enhances the flow in the delta and enlarges the fresh water layer thickness and length. On the other hand, the maximum tidal ranges can limit significantly the fresh water volume. The spatiotemporal salinity distribution is described in terms of delta topology and network geometry. Salinity and river discharge were found to be negatively and exponentially correlated by an equation that resembles solutions of the 1D advection-diffusion equation. Large bathymetric differences between delta areas were identified to play a key role on the salinity patterns as they can modify the nature of the extracted relationships and correlations.

How to cite: Matsoukis, C., O. Amoudry, L., Bricheno, L., and Leonardi, N.: Modelling the salinity response to fresh water flow and variable tidal amplitude in an idealized river delta , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2326, https://doi.org/10.5194/egusphere-egu21-2326, 2021.

Mekong delta system
15:55–15:57
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EGU21-2069
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ECS
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Highlight
Sepehr Eslami, Maarten van der Vegt, Philip Minderhoud, Nam Nguyen Trung, Jannis Hoch, Edwin Sutanudjaja, Dung Do Doc, Tho Tran Quang, Hal Voepel, and Marie-Noëlle Woillez

In the context of global rising temperatures, rapid urbanization and excessive demand for natural resources (e.g., freshwater and sand) stress the livelihood of the world deltas. Sea Level Rise, land subsidence, discharge anomalies, floods, drought, and salt intrusion are common challenges facing these ecologically essential and economically crucial coastal landscapes. Climate change projections in deltas regularly isolate climate-driven stressors and disregard anthropogenic environmental drivers. This often leads to insufficient socio-political drive at times when the short window of opportunity to save the world’s largest deltas is closing. Here, by integrating both climatic and anthropogenic drivers of exposure and vulnerability, we project salt intrusion within the Mekong mega-Delta for the next three decades. Leveraging modern numerical codes and computation capacity, by applying a high-resolution 3D model we capture the 3D dynamics of saline water intrusion, and by covering the entire delta (from 400 km upstream to 70 km offshore) we eliminate/minimize the boundary effects at the areas of interest. We differentiate the relative effects of various drivers and demonstrate that while sea level rise can increase areas affected by salinity by 5-19%, anthropogenic drivers such as extraction-induced subsidence and riverbed level incisions due to sediment starvation can further amplify that by additional 10-27%. The results are crucial input for climate adaptation policy development in the Mekong Delta and provides a blueprint for systemic assessment of environmental changes and developing environmental pathways at scale of a delta.

How to cite: Eslami, S., van der Vegt, M., Minderhoud, P., Nguyen Trung, N., Hoch, J., Sutanudjaja, E., Do Doc, D., Tran Quang, T., Voepel, H., and Woillez, M.-N.: The past and future dynamics of salt intrusion in the Mekong Delta, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2069, https://doi.org/10.5194/egusphere-egu21-2069, 2021.

15:57–15:59
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EGU21-16509
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Highlight
Rafael Schmitt, Matteo Giuliani, Simone Bizzi, Mathias Kondolf, Gretchen Daily, and Andrea Castelletti

Rising sea levels, accelerated land subsidence, and changes in water and sediment supply from upstream basins put the major livelihoods and agriculture in global river deltas at risk. Identifying effective and robust strategies to make deltas more resilient will require to systematically address uncertainty while consider the coupling between global, basin and delta scale processes.

Here, we demonstrate a bottom-up exploratory approach to forecast land loss in the Mekong Delta by 2100 and to identify most effective management levers to fight that land loss through management on different scales. To our knowledge, this is the first time that such a robust approach is applied to study coupled delta and basin systems, thus considering the full range of drivers behind land loss and delta degradation.

For this analysis, we couple a network-scale river sediment model and a conceptual model of delta morpho-dynamics. Our land loss estimates cover a large range (20 – 90 %), driven mostly by uncertainty about accelerated subsidence from groundwater pumping. However, sediment supply from the basin plays an important role to maintain delta land, especially for low and moderate scenarios of accelerated subsidence. However, sediment supply from the basin is a function of counteracting and uncertain processes. Population growth and agriculture expansion are expected to increase erosion and sediment yield from the basin, but most of this increased sediment load will be trapped in existing and planned hydropower dams, ultimately reducing sediment delivery to the delta as a function of dam siting and design.

Using more than 2 million Monte Carlo runs of a river sediment model, we find that placement of hydropower dams is the dominant control on sediment supply, far outweighing increases in sediment yield due to land conversion or reduced sediment trapping in dams because of better sediment management. Thus, the future of the Mekong delta will be determined by renewable energy policies in the basin that strategically avoid excessive sediment trapping in dams as well as by effective water management in the delta.

Our results demonstrate (1) the need for connecting delta and basin scales for managing river deltas world-wide, (2) the contribution of basin-scale sediment management to maximize the resilience of delta land, and (3) the crucial control that dams and reservoirs exert on sediment continuity between rivers and deltas.  

How to cite: Schmitt, R., Giuliani, M., Bizzi, S., Kondolf, M., Daily, G., and Castelletti, A.: Robust multi-scale strategies for increasing the resilience of the Mekong Delta , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16509, https://doi.org/10.5194/egusphere-egu21-16509, 2021.

Ganges-Brahmaputra-Meghna delta system
15:59–16:01
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EGU21-6562
Michael S. Steckler, Bar Oryan, Md. Hasnat Jaman, Dhiman R. Mondal, Céline Grall, Carol A. Wilson, S. Humayun Akhter, Scott DeWolf, and Steven L. Goodbred

Deltas, the low-lying land at rivers mouths, are sensitive to the delicate balance between sea level rise, land subsidence and sedimentation. Bangladesh and the Ganges-Brahmaputra Delta (GBD) have been highlighted as a region at risk from sea level rise, but reliable estimates of land subsidence have been limited. While early studies in the GBD suggested high rates of relative sea level rise, recent papers estimate more modest rates. Our objective is to better quantify the magnitude, spatial variability, and depth variation of compaction and subsidence in the GBD in order to better evaluate the processes controlling it and the pattern of relative sea level rise in this vulnerable region.

With support from the Bangladesh Water Development Board, we have rehabilitated previously installed GNSS and installed new GNSS co-located with Rod Surface Elevation Tables (RSET) to better understand the balance of subsidence and sedimentation in the coastal zone in SW Bangladesh, which is less affected by the active tectonic boundaries to the north and the east. The continuous GNSSs installed in 2003 and 2012 were mounted on reinforced concrete building roofs. GPS stations in the area yield subsidence rate estimates of 3-7 mm/y.  To densify the subsidence data, in early 2020 we resurveyed 48 concrete Survey of Bangladesh geodetic monuments in SW Bangladesh that were installed in 2002. Although only measured at the start and end of the period, the time span between the two measurements is ~18 years enabling us to estimate subsidence over this timespan.

Preliminary results show that about ½ the sites yielded very high subsidence rates; repeat measurements confirm the suspicion that the monuments at these sites are unstable and have undergone localized subsidence from settling or anthropogenic activity. The remaining sites show an increase in subsidence from the NW to the SE, consistent with estimates of average Holocene subsidence (Grall et al., 2018). However, rates from the campaign stations are much higher than those from continuous GNSS sites, but only slightly higher than an RSET site. We interpret that the continuous building GNSS omit very shallow compaction-related subsidence, while RSETs neglect deep subsidence. This is further reinforced by results from a compaction meter consisting of 6 wells from 20 to 300 m depth with vertical optical fiber strainmeters in each well. They show a decrease in compaction with depth. While initial results require further investigation, we highlight the importance of multiple methodologies for interpreting subsidence rates--deep, shallow, natural, anthropogenic--in vulnerable delta regions.

How to cite: Steckler, M. S., Oryan, B., Jaman, Md. H., Mondal, D. R., Grall, C., Wilson, C. A., Akhter, S. H., DeWolf, S., and Goodbred, S. L.: Recent measurements of subsidence in the Ganges-Brahmaputra Delta, Bangladesh, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6562, https://doi.org/10.5194/egusphere-egu21-6562, 2021.

16:01–16:03
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EGU21-13928
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ECS
Carol Wilson, Sharmin Akter, Masud Rana, Michael Steckler, and Bar Oryan

In the Ganges-Brahmaputra Delta (GBD) and other tide-dominated low-lying regions, periodic tidal and cyclonic storm surge flooding of the land surface promotes sediment accretion and surface elevation gain which offsets elevation losses from eustatic sea level rise and subsidence. However, over the past several decades, anthropogenic modification of the GBD tidal deltaplain through embankment construction has precluded sediment delivery to densely populated embanked islands, locally-termed polders, resulting in landscapes 1-1.5 m lower than adjacent natural mangrove platforms. Recent discussion on GBD sustainability includes whether land surfaces (natural or anthropogenic) are keeping pace with local sea-level rise rates, and the quantification of continued elevation change, vertical accretion, and land subsidence. To provide local-scale, longitudinal trends of landscape dynamics, an array of Rod Surface Elevation Tables (RSETs) and sediment marker horizons was deployed in natural and embanked settings near Polder #32 and monitored seasonally over the past 6 years (expanded throughout the SW delta in 2019). These data are compared to existing and new co-located continuous GPS measurements (also expanded 2019). Near Polder #32, elevation gain is taking place in both natural and embanked regions (1-3 cm/yr), though it appears to be slightly greater (30%) within the poldered areas. This may be due to increased accommodation space and/or embankment sloughing. There also is a distinct seasonal pattern in both regions, with greater elevation change documented after the wet monsoon season (May-Sept), and either less elevation gain, or even elevation loss after the winter dry season (Jan-May). Elevation gain is a direct result of exceptionally large sediment vertical accretion (2-3 cm/yr), as measured from marker horizons and sediment tiles, and rates appear to be keeping pace with local effective sea-level rise documented by Pethick and Orford (2013). Seasonal shallow subsidence (0.8-1.1 cm/yr) is also observed, exacerbated in poldered regions during the dry season. These measurements of shallow subsidence are 30-50% greater than deeper subsidence measured with GPS (0.3-0.7 cm/yr) but consistent with resurveys of geodetic monuments (see Steckler et al. abstract). Preliminary results delta-wide show shallow subsidence can be as much as 3 cm over the course of one year. These data provide critical information to local stakeholders about the natural versus human-altered delta dynamics, and have cross-disciplinary implications for ecological productivity, social well-being, and flood risk mitigation.

How to cite: Wilson, C., Akter, S., Rana, M., Steckler, M., and Oryan, B.: Impacts of poldering: elevation change, sediment dynamics, and subsidence in the natural and human-altered Ganges Brahmaputra tidal deltaplain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13928, https://doi.org/10.5194/egusphere-egu21-13928, 2021.

16:03–16:05
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EGU21-475
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Highlight
Stephen Darby, Md. Munsur Rahman, Anisul Haque, Robert Nicholls, and Frances Dunn

The Ganges-Brahmaputra-Meghna (GBM) delta is one of the world’s largest deltas, and consists of large areas of low flat lands formed by the deposition of sediment from the GBM rivers. However, recent estimates have projected between 200~1000 mm of climate-driven sea-level rise by the end of the 21st century, at an average rate of ~6 mm/yr. Eustatic sea-level rise is further compounded by  subsidence of the delta, which in the coastal fringes varies from 0.2 to 7.5 mm/yr, at an average value of ~2.0 mm/yr. Therefore, the combined effect of sea-level rise and subsidence (termed relative sea-level rise, RSLR) is around 8.0 mm/yr. Such high values of RSLR raise the question of whether sediment deposition on the surface of the delta is sufficient to maintain the delta surface above sea level. Moreover, as the total fluvial sediment influx to the GBM delta system is known to be decreasing, the retained portion of fluvial sediment on the delta surface is also likely decreasing, reducing the potential to offset RSLR. Within this context, the potential of various interventions geared at promoting greater retention of sediment on the delta surface is explored using numerical experiments under different flow-sediment regime and anthropogenic interventions.  We find that for the existing, highly managed, conditions, the retained portion of fluvial sediment on the delta surface varies between 22% and 50% during average (when about 20% of the total floodplain in the country is inundated) and extreme (> 60% of the total floodplain in the country is inundated) flood years, respectively. However, the degree to which sediment has the potential to be deposited on the delta surface increases by up to 10% when existing anthropogenic interventions such as polders that act as barriers to delta-plain sedimentation are removed. While dismantling existing interventions is not a politically realistic proposition, more quasi-natural conditions can be reestablished through local- sediment management using tidal river management, cross dams, dredging, bandal-like structures and/or combinations of the above measures.

How to cite: Darby, S., Rahman, Md. M., Haque, A., Nicholls, R., and Dunn, F.: The physical sustainability of the coastal zone of the Ganges-Brahmaputra-Meghna delta under climatic and anthropogenic stresses, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-475, https://doi.org/10.5194/egusphere-egu21-475, 2021.

16:05–16:07
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EGU21-1404
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Highlight
Attila N. Lazar, Robert J. Nicholls, Craig W. Hutton, Andres Payo, Helen Adams, Anisul Haque, Derek Clarke, Mashfiqus Salehin, Alistair Hunt, Andrew Allan, W. Neil Adger, and M. Munsur Rahman

Deltas occupy only 1% of global land surface area, but contain 7% of the global human population (ca. 500 million). The influence of changing and interacting climates, demography, economy, land use and coastal/catchment management on deltaic social-ecological systems is complex and little understood. We apply a new and innovative integrated assessment model: The Delta Dynamic Integrated Emulator Model (ΔDIEM) to coastal Bangladesh to explore a range of plausible future scenarios and quantify the sensitivities of selected environmental and socio-economic outcomes to key external and internal drivers. ΔDIEM is a tightly coupled integrated assessment platform considering climate and environmental change, demographic changes, economic changes, household decision making and governance, and designed to support the delta planning in Bangladesh. ΔDIEM allows the testing of a large number of water-based structural and policy interventions within a robust scenario framework, as well as quantify different development trajectories and their trade-offs. In this sensitivity analysis, we quantified the impact of (i) climate (precipitation, temperature and runoff), (ii) relative sea-level rise, (iii) cyclone frequency, (iv) embankment maintenance, (v) population size, (vi) economic changes at household level such as selling price of crops, cost of food, etc., (vii) land cover, and (viii) farming practices on trajectories of inundated area, soil salinity, rice productivity, poverty, income inequality and GDP/capita, assuming two contrasting scenarios in a more Positive and a more Negative World. Trajectories of these plausible futures showed a clear separation and the long-term trends are greatly influenced by the combinations of scenario assumptions. Our systemic results indicate a diverse potential set of futures for coastal Bangladesh, where good governance and adaptation could effectively mitigate the threat of sea-level rise-induced catastrophic inundation and other adverse impacts of the changing climate. However, societal inequality requires special attention otherwise climate-sensitive population groups may be left behind.

How to cite: Lazar, A. N., Nicholls, R. J., Hutton, C. W., Payo, A., Adams, H., Haque, A., Clarke, D., Salehin, M., Hunt, A., Allan, A., Adger, W. N., and Rahman, M. M.: Potential social-ecological development of coastal Bangladesh through the 21st century, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1404, https://doi.org/10.5194/egusphere-egu21-1404, 2021.

Sunderbans Biosphere Reserve
16:07–16:09
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EGU21-11090
Sugata Hazra, Sourav Samanta, Ananya Halder, Robert Nicholls, and Jon French

The Sundarbans Biosphere Reserve is situated near Kolkata in the western part of the Ganges-Brahmaputra Delta. The Sundarbans mangroves together with the areas in Bangladesh are the world’s largest mangrove forest and home to the iconic Royal  Bengal Tiger. It is a Ramsar and World Heritage site. Over the last 20 years the mangroves have retreated from 10 to 50 m/yr along the open coast with the loss of 145 km2 area of the biosphere reserve , 40% of which constitute the  mangrove forest. This erosion reflects a response to waves in the Bay of Bengaland relative sea-level rise of about  5 mm/yr since 1948 which increased further during the last decade. In percentage terms this observed forest land loss is manageable. However, it will continue and almost certainly accelerate with sea-level rise. As well as open coast erosion, inundation will also occur within the mangroves. Hence over many decades,Sundarbans mangroves will be progressively degraded endangering  their iconic species. We are using these observed data and the Sea Level Affecting Marshes Model (SLAMM)to explore possible trajectories of the Sundarbans evolution under different sea-level rise scenarios and management interventions. The areas to the north are densely populated and increasingly influenced by the expansion of Kolkata. Discussions with stakeholders suggest a managed retreat does not seem feasible or practical due to the large displaced populations.The paper will discuss theinter linkages of the slow onset hazard in a sinking and shrinking delta to explore pathways to achieve sustainable outcomes in south Asian deltas.  

How to cite: Hazra, S., Samanta, S., Halder, A., Nicholls, R., and French, J.: The future of the Sundarbans mangroves in India, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11090, https://doi.org/10.5194/egusphere-egu21-11090, 2021.

16:09–16:11
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EGU21-14104
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ECS
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Highlight
Oindrila Basu, Isha Das, Sudipa Pal, Tim Daw, and Sugata Hazra

A range of ecosystem services provide critical direct benefits to poor households living in the Sundarban Biosphere Reserve in India. These include artisanal fishing in creeks and rivers, crab collection, prawn seed collection, brackish and fresh-water aquaculture, fuel, fodder and honey collection from forests, and marine fishing in mechanized and non mechanized boats. The roles of these ecosystem services are largely invisible to official data. Triangulating between available statistics, key informant interviews and a new household survey, we estimate that nearly 30% of the 4.6 million population, mostly poor people rely on these ecosystem services. Ecosystem services supplement traditional rainfed agriculture, providing over 30% of household livelihood requirements. The availability of these ecosystem services is declining in per-capita terms due to the rapidly rising population in addition to ecosystem degradation. The area and health of mangrove is affected by sea level rise, differential subsidence, reduction of sediment and freshwater supply due to human obstruction and abstraction, increased salinity, high intensity cyclones, monsoon instability and temperature rise. Under a business as usual scenario, sharp decline of provisioning and regulating ecosystem services available per capita by 2030 is envisaged resulting in the threatening to increase poverty in the Biosphere Reserve. We review policy options to protect and enhance these critical ecosystem services for poor households including restoration of the estuarine mangrove habitat through river reconnection and rejuvenation and  fresh water provisioning and desalination, scientific plantation and shore protection using building with nature concept, regulating marine fishery and aquaculture practices , land use planning and population realignment.

How to cite: Basu, O., Das, I., Pal, S., Daw, T., and Hazra, S.: Shrinking Ecosystem Services in a Sinking Delta – Maintaining livelihoods in the Sundarbans Biosphere Reserve, India, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14104, https://doi.org/10.5194/egusphere-egu21-14104, 2021.

16:11–16:13
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EGU21-16485
Tim M. Daw, Sandip Giri, Partho Protim Mondal, Sourav Samanta, Sugata Hazra, Craig Hutton, Duncan D. Hornby, Andrew Harfoot, Kasturi Mukherjee, and Radhika Gupta

Land in the Indian Sundarbans Biosphere Reserve (SBR) has been extensively (and illegally) converted from agriculture to aquaculture over the last two decades, with implications for Sustainable Development Goals addressing food, poverty, employment, terrestrial and marine ecosystems and inequality. The economic returns from aquaculture are higher than agriculture, but more unequally shared, demand for labour is lower (and often fullfilled by non-local workers) and the expansion of brackish water aquaculture, in particular can contribute to the salinization of land through seepage from ponds, and intentional water management to bring saline water to farms. While remote sensing can demonstrate the conversion, the drivers behind are less clear. Much literature, along with commonly articulated stakeholder perspectives strongly suggest that sea-level rise and cyclone impacts lead to salinization, resulting in reduced agricultural productivity, leading farmers to convert to saline aquaculture as an adaptation. However, this is unclear in the Indian Sundarbans where the highest rates of conversion are not in areas which have suffered saline inundation. SBR-wide factors that affect rates of conversion include international demand for prawns, technology development and transfer, availability of seed, legal frameworks and land tenure. At a more local level, connectivity (for inputs and for marketing product), proximity to water sources, levels of cyclone inundation, salinity and agricultural productivity, existing aquaculture areas, extension services and local government (dis)incentives may explain spatial patterns of differing conversion rates. In this paper we use a two-decade long timeseries of remotely sensed data on land cover and agricultural productivity along with spatially explicit data on connectivity to evaluate which factors were most associated with conversion from agriculture to aquaculture in the past two decades. We then project future possible conversion based on scenarios of how these drivers may change over the the next decade and discuss their implications for Sustainable Development Goals.

How to cite: Daw, T. M., Giri, S., Mondal, P. P., Samanta, S., Hazra, S., Hutton, C., Hornby, D. D., Harfoot, A., Mukherjee, K., and Gupta, R.: What is driving conversion of land to aquaculture in the Indian Sundarbans?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16485, https://doi.org/10.5194/egusphere-egu21-16485, 2021.

16:13–16:15
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EGU21-8764
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ECS
Charlotte Marcinko, Robert Nicholls, Tim Daw, Sugata Hazra, Craig Hutton, Chris Hill, Derek Clarke, Andy Harfoot, Oindrila Basu, Isha Das, Sandip Giri, Sudipa Pal, and Partho Mondal

The United Nations Sustainable Development Goals (SDGs) and their corresponding targets are significantly interconnected, with many interactions, synergies and trade-offs between individual goals across multiple temporal and spatial scales.  We propose a framework for the Integrated Assessment Modelling (IAM) of a complex deltaic socio-ecological system in order to analyse such SDG interactions. We focus on the Sundarbans Biosphere Reserve (SBR), India within the Ganges-Brahmaputra-Meghna Delta. It is densely populated with 4.4 million people (2011), high levels of poverty and a strong dependence on rural livelihoods. It is only 50 km from the growing megacity of Kolkata (about 15 million people in 2020). The area also includes the Indian portion of the world’s largest mangrove forest – the Sundarbans – hosting the iconic Bengal Tiger. Like all deltaic systems, this area is subject to multiple drivers of environmental change operating across different scales. The IAM framework is designed to investigate current and future trends in socio-environmental change and explore associated policy impacts, considering a broad range of sub-thematic SDG indicators. Integration is achieved through the soft coupling of multiple sub-models, knowledge and data of relevant environmental and socio-economic processes.  The following elements are explicitly considered: (1) agriculture; (2) aquaculture; (3) mangroves; (4) fisheries; and (5) multidimensional poverty. Key questions that can be addressed include the implications of changing monsoon patterns, trade-offs between agriculture and aquaculture, or the future of the Sundarbans mangroves under sea-level rise and different management strategies, including trade-offs with land use to the north.  The novel high-resolution analysis of SDG interactions allowed by the IAM will provide stakeholders and policy makers the opportunity to prioritize and explore the SDG targets that are most relevant to the SBR and provide a foundation for further integrated analysis.

How to cite: Marcinko, C., Nicholls, R., Daw, T., Hazra, S., Hutton, C., Hill, C., Clarke, D., Harfoot, A., Basu, O., Das, I., Giri, S., Pal, S., and Mondal, P.: A framework for the Integrated Assessment of SDG trade-offs in the Sundarbans Biosphere Reserve., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8764, https://doi.org/10.5194/egusphere-egu21-8764, 2021.

Endnote
16:15–16:17
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EGU21-9305
Robert Nicholls, Neil Adger, Craig Hutton, Susan Hanson, Attila Lázár, Katharine Vincent, Andrew Allan, Emma Tompkins, Iñaki Arto, Munsur Rahman, Sugata Hazra, Sam Codjoe, and Stephen Darby

Populous deltas exemplify many of the diverse social and environmental changes and challenges that are emerging across the planet during the Anthropocene. Loss of relative elevation due to relative sea-level rise (combining climate and subsidence effects) is one major threat, but there are others such as catchment changes (e.g., water extraction and dam construction) which reduce water and sediment inputs. Rapid socio-economic changes within a delta (e.g., migration, urbanisation, economic transition and land use change) are also widespread and frequently add further pressures on the environmental resources  contained within delta systems. There are long histories of evolving adaptation practice at household to community level. In the long-term (i.e. 2100 and beyond), given relative sea-level rise, there are three distinct (but not necessarily mutually exclusive) policy choices for deltas: (i) retreat and progressive abandonment of the coastal zone; (ii) protection with ever-higher defences, growing pumping needs, and residual risk issues; or (iii) raise land elevation by controlled sedimentation. Building elevation is an attractive option if sufficient sediment is available, and there are now a few innovative examples that show it can be delivered to the delta surface— for example, through strategic raising of agricultural and natural areas with controlled sedimentation. One challenge is to accomplish this in a way which does not disrupt the livelihoods of the delta residents. Further is sufficient sediment available now or in the future, and what about growing urban areas where flood defence is likely to remain the norm? This raises the question about the trade-off between elevation and wealth. Many deltas cope with ‘lost elevation’ via defences: the Netherlands is most advanced in this approach, but such defences are expensive, require access to technology, and require sophisticated governance arrangements to deliver. A range of potential adaptation options at different scales and with different levels of cost will be required to sustain delta futures. This presentation examines potential adaptation options and trade-offs and delta trajectories in a range of examples including the Volta delta, Ghana, the Mahanadi delta, India and the Ganges-Brahmaputra-Meghna delta, India and Bangladesh.

How to cite: Nicholls, R., Adger, N., Hutton, C., Hanson, S., Lázár, A., Vincent, K., Allan, A., Tompkins, E., Arto, I., Rahman, M., Hazra, S., Codjoe, S., and Darby, S.: Sustainable Deltas in the Anthropocene, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9305, https://doi.org/10.5194/egusphere-egu21-9305, 2021.

Central discussion
16:17–17:00