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Sediment supply from rivers essential as nature-based solution for climate adaptation

  • Sediment supply from rivers essential as nature-based solution for climate adaptation

Free-flowing rivers carry a sediment load that is essential to maintain floodplains, coastal marshes and mangroves. An article in Forbes by Jeff Opperman, a freshwater scientist at WWF, explores the role of rivers as a specific type of nature-based solution (NbS), as part of a series that looks at NbS for climate adaptation in Africa. The series draws from a report titled Waterways to Resilience, by WWF, ABInBev and the African Development Bank.

A key function of many NbS is that they are able to turn sediment carried by rivers into land. An obvious example are deltas, formed when rivers deposit the sediment they carry as they arrive to the ocean. However, the sediment becomes eroded by the ocean over time and so it needs to be replenished by the river. When dams are built, the sediment becomes trapped in reservoirs: it is estimated that 25 percent of the sediment load carried by rivers each year is captured in reservoirs.

The reduction in sediment load can lead to the loss of land as the coastlines become more vulnerable to erosion from waves and storms, resulting in damage to infrastructure, increased flooding and saltwater intrusion. The ecology of deltas can also be affected, leading to declines in certain species and overall ecosystem changes.

Together with sea level rise, sediment capture is leading to the sinking and shrinking of deltas around the world. Given that deltas are home to 500 million people, and 4 per cent of the world’s food is produced on them, the implications are huge.

Managers try different interventions to try to protect deltas from land loss, using hard structures such as sea walls and soft structures such as sand dunes and vegetated buffer strips that dissipate the energy of waves and storms. They are also beginning to recognise the value of sediment carried by rivers as a NbS to protect deltas. Examples can be found in Louisiana, where opening gaps in levees is being planned to allow some of the river’s muddy water to flow out into marshes, and in the Netherlands, where the Rhine was reconnected to the sea in 2018 to restore the Rhine–Meuse delta.

Sediment deposition is also vital for healthy mangrove ecosystems. About 70 per cent of the world’s mangroves are in areas influenced by river sediment, where they perform NbS functions that include land building and coastal protection, thanks to the sediment supply from rivers.

The sediment load delivered by rivers is essential for coastal protection and the viability of NbS projects. Thus, existing and new projects to build dams or mine sediment along rivers should take into account the impact on the river’s sediment supply and the needs of downstream deltas, mangroves and marshes, and hence the people and economies that rely on them.

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