Developing a management plan for erosion of coastal defences in Sungai Nibung, West Kalimantan, Indonesia
In association with Planet Indonesia
Problem
Coastal erosion is a worldwide issue facing mangrove forests and demands immediate attention. Indonesia has one of the highest rates of sea level rise, estimated to be 7.5mm/yr, which will continue to increase. Mangroves are able to react to rising sea levels, unless the rate of increase exceeds approximately 10cm/100years. The forest line seaward of the village of Sungai Nibung is retreating, with noticeable dieback of mangrove occurring. This is likely due to rising sea level, as well as increased boat traffic and deforestation in the area. Furthermore, there are an increasing amount of aquaculture developments in the mangrove forests of Sungai Nibung and its surrounding villages.
Field observations
The village is located just north of a rapidly eroding coastline, made up of dense mangrove forest. The erosion can be seen from air and boat along a stretch of coastline approximately 5km in length. Since Oceanwise Australia’s first trip in October 2018, noticeable changes have been recorded. Terus River, that runs parallel to the coastline, has rapidly eroded to create a river mouth connecting it to the ocean (Figure 1). This river is comprised of mature Rhizphora species, not well-adapted to high wave energy zones (Figure 2). This erosion process will continue along the breach, accelerating the erosion of mangroves towards the village, due to the orientation of the river. Previously only smaller mud crab boats were accessing the river, which generate negligible wake and erosion. Now, due to the active establishment of a channel to the ocean, larger fishing boats are travelling through, creating larger wake and accelerating erosion along the river channel itself.
This presents a large issue because this 500m of forest between the coast and Sungai Nibung provides defence against extreme weather events. Without this barrier, the village will be exposed to major storms and extreme weather events. This will continue to erode the coastline and eventually the foundations of the buildings and homes of the village.
Figure 1. Aerial image of the Sungai Nibung community looking towards seaward mangrove boundary that is rapidly eroding. Terus River continues towards the top right of this image, eventually opening out into the ocean.
Figure 2. A view of the actively eroding southern coast of the Sungai Nibung land tenure. Such tall mature Rhizophora trees would normally be found within the interior of mangrove forests and not at the edge of a high energy coastline, indicating unprecedented erosion.
Satellite imagery
Analysis of satellite imagery since 1988 has revealed the coastline has retreated 174m at the newly formed Terus River channel (Figure 3). The rate of retreat has been fairly consistent throughout the past 30 years, with the exception of the last year. In the last ten months the tree line has retreated 16m, which is three times faster than the annual average. This follows strong onshore winds, large waves and increased boat traffic in December and January 2018/19. This increase is alarming given the proximity of Sungai Nibung to the seaward coastline (Figure 1).
Figure 3. The coastal retreat from 1988 to present, along with predicted coastal erosion if it continues at the rate experienced in the past 10 months. The base-map is from April in 2018, sourced from Google Earth.
If the rate of retreat of the past 10 months continues, it will take less than 30 years for the mangroves bordered by Sungai Nibung, Terus River and the newly formed channel to disappear (Table 1). The coastal erosion will be exacerbated from within the Terus River channel from the effects of increased tidal influence, wave action, vessel traffic and channel widening activities.
Table 1. Predictions for the amount of time before the coastal retreat reaches the village of Sungai Nibung.
Solution
Rehabilitating these areas is a difficult process that is far more complicated than just replanting trees or building a sea wall. Previous studies have discussed the most successful methods, of which there are few, and defines in detail the optimal methods, as well as highlighting the reasons why many rehabilitation efforts fail. Most successful cases require the restoration of shallow intertidal areas and previous hydrological conditions. A reduction in wave energy and the planting of native trees also helps to strengthen the resilience of the coastline. It is crucial that any structures put in place are not hard in nature, and do not change hydrology significantly.
The species that is planted is also pivotal. Currently the seaward coast consists primarily of Rhizophora species, which are highlighted as susceptible to coastal erosion. While the prop roots of Rhizophora are not suitable for high wave energy areas, other families of mangrove are better adapted to these conditions. Families that have pneumatophores and shallow roots are far more suitable to a high energy environment due to their ability to retain and trap sediment, and thus have greater net accretion than any other root type. This includes the genera Sonneratia and Avicennia, which we have observed in other nearby coastal areas. Sonneratia in particular is known to colonise areas and show resilience in areas of high erosuon. Due to the high density of their root systems sedimentation is increased, assisting to stabilise the soil. Furthermore, some species of this family are known to have pneumatophores up to 30cm high, allowing longer survival with a rising sea level.
Mitigation and management
The aim of the management and mitigation proposals are to:
Retain the integrity of pre-existing mangrove forest in the southern area of Sungai Nibung by reducing boat traffic and logging activities in the area
Repair some of the degraded areas by replanting in previously cleared and logged areas.
Re-establish coastal defences and combat the erosion of the seaward margins of mangrove forest by focusing on replanting historical losses.
This issue requires timely focused management and intervention. A number of community led initiatives should be implemented to:
Ban all forms of mangrove logging along Terus River.
Ban ring-barking of mangroves along Terus River.
Ban large fishing vessels using Terus River as a shortcut to the ocean.
Mangrove replanting under the current project should prioritise mitigating this issue in addition to other areas:
The logged areas along Terus River should be restored, particularly along the seaward side. This should consist of Rhizophora species.
Mangrove seedlings should be planted along the historical seaward margin. Most important species to plant in this area is those of the Sonneratia family, but Avicennia could also be used to prevent homogeneity.
All planting should be done at the beginning of the monsoon season to increase survival.