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BwN for coastal resilience. A pilot on rehabilitation of a mangrove-mud coast in Timbul Sloko (Java, Indonesia).

This pilot tests whether siltation in marshes can be triggered by building grids of permeable structures as a means of land reclamation and eventually, at higher elevations, mangrove growth. The ultimate aim is restoration of the natural mangrove – mud coastal system in Java. This approach is inspired by the Dutch salt marsh works. Fine sediments (mud) play a crucial role in the hypothesis tested in this pilot project. This application has strong links with:

The problem

For the past decade the people of Timbul Sloko Village (North Coast of central Java, Indonesia) have witnessed the devastating impact of converting mangroves into aquaculture ponds. The mangroves used to protect their village against the sea. The conversion of the mangroves into aquaculture ponds has disturbed the delicate sediment balance. The sediment input is blocked by the impermeable pond bunds and the sediment output is increased due to erosion by waves. As a result the coastline progressively recedes (see Figure 1). Housing, roads and valuable land is literally swept into the sea (see Figure 2). The erosion causes saltwater intrusion, affecting drinking water sources and agricultural production. Soil subsidence and projected climate change aggravated vulnerability to flooding during storm surge, high tides or periods of excessive rainfall.

Major investments have been made in traditional hard-structured infrastructural responses, like breakwaters, in an attempt to resolve the problem. However, such structures are expensive and often prove counterproductive for muddy coast. Moreover, they do not revive the mangrove values that were lost.

Mangrove belt establishment has been widely promoted as an alternative means to enhance coastal resilience. However, mangroves can only be successfully restored if the abiotic conditions are optimal (bathymetry, fresh water and sediment input). Most pilots do not reinstate these conditions and they fail to stabilize eroding coastlines. Long-term success rates of mangrove replantation are as low as 5-10%.

Proposed solution.

The proposed solution is to recreate a resilient mangrove buffer zone to protect from erosion. The first necessary step to stop the erosion process and to regain a stable mangrove coastline is to reverse the loss of sediment. The innovation lies in the approach. It is proposed to build grids from permeable structures made of local materials such as bamboo, twigs or other brushwood (see Figure 3).The permeable structures create low-energy conditions which favours siltation of fines. The structures are maintained until enough land is reclaimed for mangroves to naturally re-colonise the area.

By combining the permeable structures with engineering techniques, such as agitation dredging or mud nourishments, the process of mangrove belt re-establishment can be accelerated. 
This approach is named 'hybrid engineering'. Hybrid engineering combines engineering techniques with natural processes and resources, resulting in dynamic solutions that are able to adapt to changing circumstances. Hybrid engineering approaches can be applied in a variety of situations and ecosystems. Several examples exist - some are hundreds of years old and reflect local and/or traditional knowledge regarding ecosystem management. Since 2008, the Building with Nature program (De Vriend and van Koningsveld, 2012) has been implementing hybrid engineering approaches in different contexts.

Rationale.

Healthy mangrove mud coasts are in a dynamic equilibrium, with sediment naturally eroding due to wave action and accreting as a result of tidal action and river input. The mangroves’ root system helps to capture and stabilise the sediment. The tidal flat is convex-up, with a gentle slope and shallow water at the seaward edge of the mangrove forest. This is illustrated in Figure 4.

As a result of mass conversion of mangroves to aquaculture ponds, the delicate balance between erosion and sedimentation is disturbed. Erosion due to wave action is increased due to reflection of waves on the hard structures, and sediment import by the tide is blocked. As a result the coastline progressively recedes.

Hard structures, such as aquaculture pond bunds and breakwaters, exacerbate the problem in muddy coasts. They disturb the balance of incoming and outgoing sediment. Waves reflect on the structure, becoming bigger and taking even more sediment away. The tide cannot bring enough sediment in, as it is blocked by the hard structure. The tidal flat becomes concave-up, with steep slopes, and deep water at the seaward edge of the hard structure / mangrove forest. As a result, waves can penetrate further, enhancing their erosive forces. These processes are illustrated in Figure 5.

In order to stop erosion and regain a stable mangrove coastline, the first necessary step is to reverse the loss of sediment. The sediment input by the tide needs to be increased and sediment output by waves should be limited. 
To reduce sediment loss, permeable structures made of local materials such as bamboo, twigs or other brushwood are placed in front of the coastline. These structures let the sea and river water pass through, dissipating the waves rather than reflecting them. The permeable structures also let mud brought in by the tide pass through, while creating low-energy conditions which favour sedimentation. Sediment is trapped in the same way a natural mangrove system would. This is illustrated in Figure 6.

The permeable structures are applied in grids, to slowly but steadily reclaim land from the sea. Regular maintenance of the permeable structures is needed. New structures are placed at the seaward end once sufficient sediment has been trapped and the desired amount of land is reclaimed for mangroves to (naturally) re-colonise the area. The mangrove saplings are no longer washed away by the currents and a new mangrove belt can further break the waves and capture sediment in the long term. Mangrove management will maximize wave attenuation, for instance by opting for species with aerial roots and by aiming for a mix of trees of different age and size in order to continuously sustain and enhance coastal protection. This technique has been applied successfully in salt marshes in the Netherlands for centuries.

By combining the permeable structures with engineering techniques, such as agitation dredging, the amount of sediment suspended in the water can be increased and the process of land reclamation can be accelerated. Mud nourishments can be applied at locations where the tide has difficulties to bring fine sediments to the shore, or where too much sediment already has been washed away. It may also be feasible to sow mangrove seeds (propagules) and/or plant mangrove juveniles. This ‘recovery cycle’ is illustrated in Figure 7.

The approach described here is in general also applicable to many other tropical (former) mangrove areas suffering from erosion.

The pilot project

In this pilot permeable dams are used to re-establish the mangrove green belt.

The objectives of the pilot project are to:

  1. protect Timbul Sloko Village from further erosion,
  2. initiate mangrove restoration and 3) learn from this project.

This pilot is executed by a collaboration between Deltares with Wetlands International (a global NGO dedicated to the conservation and restoration of wetlands) and the Indonesian Ministry of Marine Affairs and Fisheries.
The first step was to design the permeable structures and to determine the location and layout. In close collaboration with local stakeholders a prototype design was designed for the permeable structures in Timbul Sloko, see Figure 8. The structure consists of wooden poles drilled into the muddy substrate and was then filled with brushwood. This design needs to be adapted to local conditions. Materials to be used for these permeable structures may be adapted to the local circumstances and availability. Materials should be sustainable from ecological, social and economic perspectives.

Three structures covering a total area of approximately 0,2 km2 were constructed at strategic locations in the village. In Figure 9 the layout of 2 of the structures is presented.

The first structures have been placed mid-September 2013 (see Figures 10 and 11) and the effect looks promising. It is clearly visible that the permeable structures weaken the force of the waves and at the same time let fine sediments pass through so that the land can build up again. After the first storm encouraging messages were received from the local Timbul Sloko community. Investigation are now being made into opportunities of upscaling the pilot for a larger part of the central Java coast.

Scale of the problem.

Large stretches of the sub-tropical and tropical coastlines of Asia, Africa, Oceania, the Americas and the Caribbean are fringed by mangroves. Rapid erosion due to massive conversion of mangrove forests in muddy coasts is a worldwide observed and recognised problem. The world has lost around 3.6 million hectares (ha) of mangroves since 1980, equivalent to an alarming 20 percent loss of total mangrove area. The total mangrove area has declined from 18.8 million ha in 1980 to 15.2 million ha in 2005. Much of this loss has been in Asia and North and Central America (FAO 2007). Since the 1980s no less than 1,300,000 ha of mangrove forest were converted in 2005, mostly in western Indonesia (FAO 2007). Coastlines continue to erode at alarming rates causing environmental and economic damage. The "alarming" loss of mangroves in many countries should be urgently addressed.

References

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