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Mangrove-mud coasts; a muddy story (4)

02-Jul-2013

By Han Winterwerp

In my previous blogs, I described the large losses of our mangrove heritage, in spite of the great value of these ecosystems. Today, I argue that these losses are caused by thoughtless land-use.

In my previous blogs, I described the large losses of our mangrove heritage, in spite of the great value of these ecosystems. Today, I argue that these losses are caused by thoughtless land-use.

Thoughtless, as fish/shrimp ponds are built too close to the waterline, infringing on the need for a sufficiently wide green belt. This green belt provides “room for the sea”. Such room is required for the deposition of sediments carried by the tide, and for reducing the energy of incoming waves.

Below you see a diagram depicting the changes in the entire coastline of Suriname (370 km long) over a period of about 50 years. The two rivers which border between Suriname and French Guiana, and between Suriname and Guyana are indicated. The capital Paramaribo is situated along the Suriname River.

Each colour represents the coastline within a particular period. The diagram demonstrates the following:

The coastline is very dynamic over time. Periods of accretion are followed by periods of retreat, and the other way around. This behaviour is very common, but exceptionally extreme along the coast of Suriname, because of the role of large mud-banks migrating along the coast.

In general, the coast accretes, except at two locations: Paramaribo and Coronie. Here, the forest is exploited too close to the waterline, whereas the rest of the coast is still almost pristine. Around 1840, Scottish settlers erected plantations at Coronie, and we have data demonstrating that the coast retreated by more than 7 km since the early 1900s.

Let us also further study the photograph in my first blog of a piece of coastline in Thailand (repeated here for convenience).

On the foreground, we see the Bay of Bangkok, and in the back, 500 m long and 100 m wide shrimp-ponds. Further, we see fringes of mangrove forest, and an isolated, detached breakwater parallel to the coast. In the middle, right part of the photo, we see the remnants of a hard coastal protection infrastructure. However, because of limited resolution this photo cannot reveal the bunds (small dams made of earth) erected around the ponds. These bunds are fragile, made from the locally available mud, and they may therefore breach easily due to wave impact. When that happens, the pond-owners erect a new bund a few tens of meters further inland. However, the bunds of adjacent ponds are then further exposed. When these fail, the farmers retreat further. In this way, the coastline slowly retreats, following the lay-out of breaching ponds. If you zoom in on Google Earth images of the north coast of Java, the coastline along the Gulf of Bangkok, and various coastlines in the Philippines, you can see similar patterns. Coastal erosion follows the breaching fish/shrimp ponds.
 
Finally, this photograph suggests that erosion was not stopped by the construction of the coast-parallel breakwater. In fact, we have data that the opposite was true, e.g. erosion increased. I will tell a bit more on this in my next blog.
 
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.

 

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