Four ways to save deltas from sea level rise

River deltas are drowning in the face of rapidly rising sea-levels. To keep up with sea-level rise, delta land needs to be raised with sand and mud from rivers. But sand and mud supply from rivers is decreasing globally, and doesn’t make it to deltas anymore which causes flooding and land loss.

Utrecht University assembled a team of specialists from countries around the world to solve this problem. Deltares is part of this team.

The team investigated and reviewed existing and planned projects that are being explored globally to raise land, so-called ‘Sedimentation Enhancing Strategies’.

They found four ways to keep deltas dry:

• River diversions: diverting some water and sediment from the main channel of a river into adjacent floodplains,

• Tidal flooding: breaching existing dikes and allowing water and sediment to be delivered by the tides to adjacent floodplains,

• Sedimentation structures: placing structures (e.g. willow dams) in river channels to trap the existing sediment in the water,

• Vegetation planting: planting additional vegetation which traps sediment in its roots.

Jana Cox, river-delta researcher and lead author at Utrecht University, said: “All four of these strategies trap sand and mud onto delta lands and raise it above sea level.”

“They are effective and can offset even extreme sea-level rise conditions.

“Different projects are available at a variety of different scales for deltas of different sizes.

“Projects can last from ten to a hundred years, depending on what the desired outcome is.”

The review indicates that low-cost strategies only last 10-15 years and cover only small portions of the delta area.

Meanwhile, projects that last for 100+ years are very expensive (multi-million to billions of Euros) but can cover a large amount of the delta area.

The combination of several smaller scale projects will also be crucial in deltas where available land is low (e.g. Europe and SE Asia).