Construction of permeable dam structures as sediment traps and basis for mangrove rehabilitation
Technical measures to protect the coastline in Demak include restoration of the sediment balance using permeable dams and mud nourishments, alongside mangrove rehabilitation. – Grids of permeable dams are put in place to dampen erosive waves and to trap sediments, so that the disturbed soil profile is restored; – In this sheltered environment mangrove forests are rehabilitated. The mangroves stabilize sediment, further build up the soil and protect against salt water intrusion and flooding; This process is reinforced in the most severely degraded sites through sustainable sediment suppletions. Where needed agitation dredging and nourishment of sand banks may help shape the soil profile and shelter vulnerable parts of the coast.
Ownership and provisions for maintenance of the structures will be formalized during the project in co-management arrangements between communities and the local government. Communities will take full ownership over the structures ensuring their long-term maintenance. Maintenance costs will be covered via community-managed development funds.
The set-up was successful: we were able to trap up to 45 cm of mud and there are already Avicennia mangrove juveniles of 50 cm tall within 1.5 years. Some of the poles were damaged by shipworms and brushwood is disappearing – leading to the collapse of part of the dams during storm events. Some structures were damaged during the monsoon season. The pilot hence did its job: trapping sediment and the principle of the design is sound. We are now testing the best materials to use, for optimizing our design and providing lessons learned for scaling up. Other key lessons: place the poles deep enough, use sufficient fill material, add material frequently and keep the ropes really tight. We are developing guidelines on ‘permeable structures‘ to trap sediment for mangroves recovery as stakeholders want to replicate the approach Upcoming topics include system understanding, community planning, fish pond rehabilitation, mud nourishment and ecological mangrove rehabilitation (EMR).