Rehabilitation of mangrove coastal protection system using permeable structures (Objective 1)
Initiation for mangrove coast rehabilitation using permeable structures
Strongly eroded coastline in Northern Java
Communities in Northern Java are suffering from coastal erosion affecting hundreds of kilometres of coastline. In the district of Demak more than 3 kilometres of land including entire villages have already been swallowed by the sea.
It is expected that further sea level rise will by 2100 result in flooding 6 kilometres inland, affecting over 70,000 people and 6000 hectares of aquaculture ponds. In the long run 30 million people may be affected by coastal erosion.
Furthermore, subsidence of coastal land, for a good part caused by groundwater extraction, makes coastal erosion issues more severe and complicates the protective measures provided by mangrove restoration.
In the initiation phase a physical understanding of the coastal system has been obtained, with focus on the problem of coastal erosion and its root causes, with basic information on bathymetry, soil and oceanographic conditions.
To fight this coastal erosion in a sustainable manner, a ‘vision’ (or ‘dream’) has been developed.
The principal objective is to build a stable coastline with reduced erosion by stimulating the rehabilitation of mangroves of the most vulnerable parts of 20 kilometres of affected coastline in Demak District. The restored coastline should be able to adapt to sea level rise to some extent. A stable coastline will enable a revitalisation of the aquaculture in the communities in Demak, which will provide income for the inhabitants. Part of the income earned shall then be reinvested in maintaining and further strengthening of the mangrove coastline, thus obtaining a sustainable development cycle. Thereto this inspiring Building with Nature pilot is initiated, in which a paradigm shift towards water infrastructure solutions is triggered, aligning coastal protection with the interest of economic development and care for the environment. The alternative, Business as Usual scenario, shows a fully flooded area with evacuated villages and land loss due to land subsidence and erosion.
Planning and design for mangrove coast rehabilitation using permeable structures
In 2012 the Indonesian government asked for support to address the severe coastal erosion in Northern Java. Hard infrastructures like dams and sea walls had proven to be ineffective along the rural mud-coast, exacerbating erosion and being unstable and expensive. Furthermore, they failed to deliver the vital economic, environmental and social services that the mangrove belt, the original natural coastal protection, provided. A Building with Nature approach was proposed and jointly a small but inspirational pilot started in Timbul Sloko village to restore the mangrove coast, engaging communities, government agencies and knowledge institutes.
Planning and design studies for coastal rehabilitation focused on:
- Addressing the root causes of erosion
- The effect of mangrove belts on wave heights
- Mangrove rehabilitation using permeable structures
Implementation for mangrove coast rehabilitation using permeable structures
Mangrove rehabilitation using permeable structures
Permeable structures have been built from poles and brushwood to dampen the waves and capture sediment. Once the near shore bed level rises to around average sea level, mangroves will regenerate naturally, developing a natural defence that will protect the hinterland from further erosion.
New ponds, filled with sediment and mangroves,
starting to regenerate. (photo by Eko Budi Priyanto)
Natural mangrove restoration supports the development of mangrove forests with different species. This is because not all species can equally withstand the submerged conditions, wave exposure and salinity occurring at the seafront. Natural mangrove succession starts with pioneer species that facilitate colonization by many other species and results in a great variety in root types, tree sizes, foliage and fruits, fulfilling different functions and attracting diverse (fish) fauna. This in turn results in the provisioning of multiple goods (timber, fodder, honey, fruits, and fish) and services (enhanced coastal protection, carbon storage, water purification, fisheries enhancement). Ecologically restored forests are also likely to be more resilient to change because of this. For more information see the brochure: ‘Mangrove restoration: to plant or not to plant ’, which has been developed by the Building with Nature Indonesia consortium.
Where the coastline has not yet been eroded, the conversion of ponds into mangrove forests in close collaboration with local communities is encouraged.
The construction of a permeable structure is low-tech. It can be built by the local community with minimal tools or machines. At many locations where permeable structures should be constructed, it is difficult to bring mechanical equipment due to limited water depth and soft sea bed material. Machines can also more easily damage the material. In Demak one team of 6 to 8 workers can build 4m of permeable structure during a working day of 8 hours.
More information on construction of permeable structures can be found in Technical Guidelines #4: Building with nature to restore eroding tropical muddy coasts: permeable structures (in preparation).
Since 2015 the project has investigated several sediment nourishment options, notably offshore agitation dredging (to bring in additional sediment into the near-shore system) and infilling of (abandoned) ponds. However, the preliminary conclusion is that there is currently still sufficient sediment naturally available in the nearshore system to ensure natural infilling and (costly) mud nourishment is not required.
Operation and maintenance for mangrove coast rehabilitation using permeable structures
Monitoring and evaluation methods
Over the course of the project monitoring of the individual structures will be done to assess the durability of the permeable structures and implement gained information and further planning to increase the efficiency of the construction. Monitoring of the permeable structures is required at several stages and regarding different aspects.
BwN Indonesia is using a variety of methods to measure biophysical and socio-economic effects such as coastal risk reduction, land and water quality improvements, mangrove re-establishment and livelihood gains. This includes: physical field observations and measurements on variables such as sediment availability, salinity, water quality, mangrove extent and composition; satellite technologies to assess coastline change and erosion/deposition areas; drone images to assess on-the-ground mangrove recovery; and household surveys and community consultations to provide information about changes in local livelihood status and ecological condition.
Coastline development and performance of permeable structures
At the locations where permeable structures are built the coastal erosion was halted. In addition to that the first measurements in the first year showed 0.45 m of sedimentation. And mangroves settled once elevation was above mean sea level. This happened behind several structures within a year. However, in 2017 in many areas elevation lowered again and mangroves disappeared. In addition, monitoring poles and structures disappeared below sea level within a couple of years. Subsidence (having several causes, among which unsustainable groundwater extraction) seems a lot more severe in this rural area than initially thought.
The permeable structures function well when they are well maintained. The brushwood dampens the wave energy and sediment settles in the shelter of the structures. When the structures get damaged, by shipworm that eats wood and bamboo, compaction of the fill material or lack of maintenance the waves pass through the structures and erode the settled sediment. The design of the structures was updated, the vertical poles are changed to PVC filled with concrete and the bamboo is protected with wrapping. Also different types of materials are tested. Next to the update of the design the maintenance was intensified to keep sufficient fill material within the poles.
Maintenance of permeable structures
Permeable structures need continuous maintenance. Without maintenance the fill material lowers and decreases, and wave attenuation reduces, with the result that the structure can no longer perform. The material is also eaten by shipworm which reduces the lifetime of the structures. Tests are done with wrapping of bamboo and wood and the use of PVC poles to increase the lifetime. Also alternative fill materials are tested like the replacement of the brushwood by horizontal bamboo beams tied to the vertical poles.
Maintenance of the structures needs to follow the monitoring outcome and needs to be executed directly after monitoring shows it is required. If stormy conditions occur more often, the monitoring and maintenance frequency should be increased. This also implies that at the start of the project a life cycle cost analysis has to be made and budgets for the entire lifetime need to be allocated.
More information on monitoring and maintenance of permeable structures can be found in Technical Guidelines #4: Building with nature to restore eroding tropical muddy coasts: permeable structures (in preparation)