Deltas are necessarily transient: the supply of new river sediment compensates for the subsidence and helps to maintain the delta surface above sea level. But a new study has found that coastal deltas may lose more land than previously thought as rivers change course over time, informs Hakai Magazine.
Land in deltas is sinking as a result of sea-level rise, land subsidence, and reduced sediment supply. Groundwater extraction for agriculture and growing cities is a main driver of subsidence, through aquifer compaction. On the other hand, climate change is expected to lead to a small increase in sediment flows as increased rainfall washes more soil into rivers, but it will be more than offset by dams, which trap sediment.
River deltas throughout the world are home to millions of people, 339 million according to researchers from Indiana University. Many live in large cities, such as Kolkata in the Ganges delta, Bangkok in the Chao Phraya delta, or Shanghai, one of dozens of large cities in the Yangtze delta region. At the same time, their fertile soils make them hotspots for food production.
Previous models to forecast the evolution of deltas compared the pace of sea-level rise to sediment deposition, assuming sediment is spread evenly across the delta plain. Now a research team from the California Institute of Technology led by Austin Chadwick has analysed sediment deposition, taking into account that rivers periodically change course, redirecting sediment to different areas of the delta known as lobes. The researchers developed a model and tested it in a laboratory experiment, before applying it to river systems.
Their results indicate that, while new land is built on the active lobe, inactive lobes no longer receive any sediment and start to erode, gradually leading to delta land loss. “The previous work has been very much a best-case scenario,” says Chadwick, adding “a river actually can’t be everywhere at once, and it can’t fight back against sea level rise everywhere”.
Their projections are grim for some of the deltas they studied. The Mississippi River would need three times more sediment to maintain the current dry delta land. In the case of the Danube River delta, ten times more sediment would be needed to maintain the delta.
That is not the case with all the deltas, though, the Yellow River carries so much sediment that it forms a new lobe every ten years, and engineers have been able to divert the river to encourage sediment deposition in selected areas, maximising land building. Such diversion projects, although they have high engineering and environmental costs, ensure there is dry land in selected delta regions. These solutions, however, may not work in the future as the ocean continues to rise.
Sediment plays an important role in climate resilience for deltas across the world, particularly vulnerable to flooding and home to densely populated cities. Plans to build any new dams should assess the risks and consequences for downstream regions as a result of altered sediment transport, while international cooperation is crucial in deltas where sediment is delivered by rivers draining more than one country, such as the Ganges or the Mekong.