The diagram in my previous blog, depicting the evolution of the coastline of Suriname, shows that this coastline is very dynamic, accreting or retreating alternatively. Accretion occurs when sedimentation processes exceed erosion processes. Vice versa, retreating occurs when erosion processes dominate. In equilibrium, these two processes are equally significant, as sketched in this cartoon.
Cartoon illustrating the fine sediment balance of a healthy mangrove-mud coast.
The muddy foreshore of a mangrove-mud coast in equilibrium has a very gentle slope, somewhere between 1:1500 and 1:2000. Moreover, the seabed profile is a little curved, with the convex side up. Therefore, large parts of the mudflat and mangrove forest inundate during high water, while a similar area stays dry during low water.
Sedimentation is induced by the tide. With every rising tide, the sea carries fine sediments (mud) towards the shore, which are deposited on the mud flat and in between the roots of the mangrove forest, where flow velocities and wave action are small.
Erosion, on the other hand, is caused by waves. Waves stir up the sediments on the sea bed, which are then carried away by the flow. Such flow can be generated by the tide, or by wind. The waves also reduce sedimentation rates. These waves are somewhat damped on the very shallow foreshore, and further lose energy once they reach the mangrove forest. The amount of damping depends on the number of mangrove trees and the density of mangrove roots: the denser the forest, the more damping. This explains the favorable role of mangrove forests in coastal protection.
Photographs of the accreting (left) and retreating (right) of healthy mangrove-mud coasts along pristine parts of the Suriname coastline
The picture is a bit more complicated, though. Because, what is the source of the fine sediment carried by the tide into the mangrove forest? Well, this source is the muddy foreshore. The fine sediments on this muddy foreshore have been deposited there a long time ago, sometimes many hundreds to thousands of years ago. Waves not only erode the mangrove-mud coast, but also stir up the muddy foreshore. It is this sediment that is carried to the mangrove forest by the tide.
This is why one cannot protect an eroding mangrove-mud coast with a coast-parallel breakwater. Such a breakwater reduces the stirring up of sediments from the seabed, hence killing the source for sedimentation in the mangrove forest. Waves take and give! This is why erosion continues after the construction of the coast-parallel breakwater depicted in the photograph of Thailand coast in my previous blog. But I will tell you more about that in my next post!
Han Winterwerp works for the Dutch research institute Deltares and is a professor at the Delft University of Technology, where he teaches and develops instruments to measure and predict the behaviour and properties of mud. He specialises in the behaviour of mud in open waters.