Permeable Dams Help Reclaim Demak’s coasts

Imagine watching powerlessly as neighbourhoods disappear under water

Practitioner

Kilometres of muddy coast in the province of Demark on the Indonesian island of Java have gradually been swallowed up by the sea over the past 35 years. For example, in the village of Bedono, two neighbourhoods - a total of 200 hectares - have disappeared. The cause: coastal erosion and rising sea levels.

Naturally occurring mangrove forests in the area have been increasingly chopped down to make way for fish and shrimp ponds as well as land clearance for urban development. Even though mangroves play an important role in coastal protection. Large quantities of groundwater are used for controlling the water quality of the ponds, causing sinking of the land. The increasing flooding of houses, agricultural land and roads means local fishermen and fish pond farmers have seen their incomes drop significantly, sometimes by as much as 80%.

There seemed to be no way to stop the erosion and floods. Major investments were made in sea walls and wave barriers, but all these efforts failed. In areas with soft muddy substrate like Demak hard structures only enhance erosion and often collapse. The floods kept creeping further into the villages and taking land by 100 meters every year. People became desperate and made plans to move away having witnessed neighbouring villages being swallowed up by the sea.

Created in partnership with: Deltares

Why not let nature do the hard work, with just a bit of help?

Researcher

But then a new solution was introduced, following the principle of ‘building with nature’. A group of international institutes and Indonesian ministries designed a plan to reclaim land by installing permeable dams. These dams break the waves and trap sediment. The dams are made of natural local materials such as bamboo, twigs or brushwood, and are cheap to construct and easy to maintain. They break the waves rather than reflecting them and let mud pass through. This approach requires enough availability of mud and mud transport toward the area.

The new permeable dams work effectively to provide suitable growing habitat for the mangroves. Once the erosion stops, mangrove seedlings regenerate naturally on the restored coastline. A new mangrove forest can further break the waves and capture sediment in the long term. For long term coastal safety, a combination of mangroves and permeable dams will always be necessary because of the rising sea level.

The regeneration of the mangrove forest has more advantages. It improves the eco system by providing spawning and nursery habitat which means bigger yields and higher quality fish and shellfish for harvesting. There are also more opportunities for harvesting wood, making prawn crackers, paint and syrup which will contribute to the socio-economic prosperity of the surrounding communities.

Created in partnership with: Witteveen en Bos

Commitment from the local population for a permanent solution

Researcher

Local villagers waited impatiently to see evidence that this new ‘permeable structure’ approach was successful. They had worked hard to contribute to the development and installation of the new dams. The concept of building with nature is innovative and its effectiveness is site specific. And it was being introduced in areas with limited understanding of currents, waves, sediment, and their dynamic changes. Therefore, a flexible, learning-by-doing strategy was used combining local expertise with international research. Everyone was delighted to see that their work was rewarded - the waves now are indeed much lower inside the grid of permeable dams than outside. In some cases, new mangrove trees are already testing the ground. In less than six months after the dams were installed!

A breakthrough came when the administrations of Bedono and its neighbours Surodadi and Timbulsloko signed a decree demarcating 100 hectares of the lost land as a protected area. This means that once the mangrove belt is restored, it will not suffer the same fate again. Once the construction was completed, the day-to-day management and maintenance of the permeable dams were taken over by the local governments. So far, several hectares of eroded coastal zone have been filled with sediment becoming suitable mangrove habitat. The expectation is that a significant mangrove forest will be in existence by 2030. Residents are now putting their heart and soul into the reconstruction of their own villages instead of looking for safer places to live.

Created in partnership with: Ecoshape