Dam construction and other human activities are putting the world’s deltas at risk, as they reduce the delivery of sediment to delta areas. River sediment is crucial to counter the rapid sinking of deltas due to sea level rise and subsurface mining among other pressures. Urgent action is needed to minimise the disruption of sediment delivery and improve the resilience of deltas. These are the conclusions of a new study lead by Dr Frances Dunn in Environmental Research Letters.
Publication in Environmental Research Letters
Human intervention increases risk of delta drowning
The world’s deltas are home to over 350 million people. They are fertile environments that support intensive agriculture, important expanding cities, and unique ecosystems, such that their social and economic importance extends well beyond their immediate locales.
Creation of deltas
Deltas are created when rivers enter a body of water, such as the ocean, and deposit the sediment they carry. This newly created land tends to sink as the freshly deposited sediment settles and compacts, which means that more sediment must be deposited for deltas to remain above sea level.
Influence of human activities
Currently many deltas are sinking rapidly relative to sea level. This is due to climate change induced sea level rise and subsurface mining of water and fossil fuels which lead to subsidence. There is concern that the current combination of pressures is presenting the world’s deltas with a sustainability crisis.
Importance of sediment supply
Delta 'drowning' can be offset by a sustained supply of sediment. It is therefore crucial to know how sediment delivery will change in the future. If enough sediment is not retained on deltas then, as they sink, salt intrusion, erosion, flooding, and eventually permanent inundation can threaten the lives and livelihoods of delta inhabitants.
Modelling 47 deltas
Frances Dunn with colleagues at the University of Southampton and elsewhere explored this question using computer modelling to project changes in sediment delivery. They modelled changes in 47 deltas over the current century based on future climate and dam construction data as well as population and wealth indicators.
The researchers found that river sediment delivery to many deltas is expected to decrease over this century under most of the investigated scenarios. Dam construction in particular is likely to have a substantial influence on sediment delivery to deltas, as the effect of existing dams is compounded by further construction of planned dams. Other anthropogenic activities such as land cover change, erosion control measures, and river channel engineering are likely to also noticeably reduce sediment delivery to affected deltas. Climate change alone is projected to cause a small increase in sediment delivery to most of the deltas. Nevertheless, the relative influences of these different environmental changes vary between the deltas.
Urgent action needed
The consequences of reductions in fluvial sediment delivery to deltas provide a clear rationale for reducing these impacts. The research shows that human activities occurring within river basins are the main drivers of change in future global sediment delivery. The projections for individual deltas indicate where the main threats to future sediment delivery in this century are likely to come from. This way they can be used to better manage sediment connectivity within river basins. International cooperation on improved river management and engineering for sediment transport is urgently needed to minimise the disruption of fluvial sediment delivery and improve the resilience and sustainability of deltas.
Water, Climate & Future Deltas
Frances Dunn is a researcher in the hub Water, Climate & Future Deltas of Utrecht University’s strategic theme Pathways to Sustainability. In the hub, researchers from varying disciplines at Utrecht University cooperate with external partners to design and evaluate pathways to sustainable delta development.
Frances Dunn*, Stephen Darby, Robert Nicholls, Sagy Cohen, Christiane Zarfl, Balázs Fekete
Environmental Research Letters 14, 2019
* from Utrecht University