Utrecht University: Migrating mud banks hinder coastal management

The Surinamese coast changes constantly. Mangrove tree roots retain sediment, causing the coast to rise along with sea levels, thus providing protection against flooding. But the supply of sediment is extremely variable. In phases of ten to fifteen years, mud banks erode and silt up again, raising or lowering the coast along with mangrove forests. These phases make it difficult to plan for long-term coastal management, especially when mangrove forests are cleared for agriculture: how can you ensure that the coast remains protected, even when these mud coasts erode and silt up as a result of natural processes?

Moving sediment
In order to prevent erosion, beaches in the Netherlands have to be regularly maintained by adding extra sand. This way the beaches are able to rise along with the sea level. In Suriname, this is not necessary: there is more than enough mud naturally present in the form of large mud banks that lie just under the coastal water surface.

The coast starts eroding almost as fast until the next mudbank arrives in ten years’ time.

Under the influence of tides and waves, these huge banks, some of which can be twenty to thirty kilometres long, migrate along the coast. These migrating banks protect the coast from incoming waves and provide new mud for land accretion; ideal for young mangrove seedlings to develop, or for mature trees to raise the land by retaining more sediment in their roots.

However, this becomes a problem when, after ten to fifteen years, a mud bank has passed the coast again, the natural flow of sediment ceases, and waves get a better grip on the new land, says Job de Vries, researcher at Utrecht University. The result is that the coast starts eroding almost as fast until the next mudbank arrives in ten years’ time.

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Mangroves as natural dikes
For coastal protection it is therefore important to know where the mud banks are and what effect migration has on a stretch of coast. During phases in which the coast expands, policy makers and coastal managers shouldn’t become dependent on land that will naturally erode again in ten years’ time. Because in addition to contributing to CO2 storage and biodiversity, mangrove forests also provide natural coastal protection.

There are already examples where too many mangroves have been removed in times of land accretion, which has resulted in insufficiently healthy mangroves remaining as a natural buffer during the subsequent erosion phase, illustrates De Vries. With the result that expensive dikes and intensive planting projects are ultimately required.

We can now say with greater accuracy and certainty where mud banks are located and can therefore expect changes along the coast in the near future.

Job de Vries
Researcher faculty of Geosciences
Natural cycles versus human action
Research by De Vries and his colleagues, who used time series of satellite observations since 1985, has made it possible to predict these phases in the shorter term. We can now say with greater accuracy and certainty where mud banks are located and can therefore expect changes along the coast in the near future. Even if these changes are more severe or different than we would expect, says De Vries.

Mangrove zaden op zand
Although much research has been done into these mud banks since the 1940s, until now it has been difficult to distinguish the natural effects of migrating mud banks from the effects of human intervention on a local scale. Satellite images of the entire coast can help. In order to predict how muddy mangrove coasts will adapt to changes in climate and human intervention, it is necessary to be able to make the distinction between natural effects and anthropogenic effects. The researchers state that modern methods allow them to compare the Surinamese coast with the coasts of neighbouring countries, French Guiana and Guyana, where human intervention differs again.