Assessing the Sources of Sediments and Mangrove Surface Elevation Changes in the Vanga Estuary, Kenya, Towards the Mitigation of Sea-Level Rise


Mangroves have tremendous social, ecological, economic and environmental values. They provide enormous goods and services for human communities (e.g., filter run-off, trap sediments and sequester carbon, shoreline protection) and other associated habitats (e.g., good nurseries for coastal fisheries). However, there has been a global decline in their coverage due to human-driven and natural causes. For example, between 1986 and 2016,>25 ha of Vanga mangroves were lost per year. Mangroves on low lying coastlines and deprived of sediments are vulnerable to sea-level rise (SLR), which has the potential to inundate, submerge and convert them into open seas. This would subsequently lead to the loss of the ecosystem’s services derived from mangroves. However, sedimentation has been heralded as one of how mangroves will be able to self-modulate and mitigate against SLR. Due to their extensive root structures, they can dampen tidal currents and necessitate sediment deposition and accretion. The accretion will allow mangroves’ surfaces to grow against the SLR, and if the rate of accretion exceeds the rate of SLR, then the mangroves will be able to compensate for the rising sea level. Therefore, it is vital to understand local sediment dynamics, including sediment supply, their sources and fate, surface elevation changes, and local rates of SLR. It allows for the evaluation of the possible response, adaptation and resilience of mangroves to current and predicted rates of SLR.

A combination of sediment provenance proxies and measurement of mangrove surface elevation changes provided insights into the sediment dynamics in the Vanga Estuary. This study showed that the mangroves of Vanga receive their sediments primarily from the Umba River, greatly influenced by its transboundary catchment. This is supported by the observed connectivity based on the similar geological source, mineralogy and provenance of Umba River sediments. They also receive sediments from the adjacent coastal waters through tidal fluxes. The current rates of sediment accretion (9 mm yr-1) in the Vanga Estuary exceed the local (3.8 mm yr-1) and global (3 mm yr-1) rates of SLR. This indicates the potential sustainability of the mangroves of Vanga in the face of SLR. However, human-driven exploitation and loss could hamper their long-term sustainability. Therefore, holistic management is recommended. This could be achieved through the proposed transboundary conservation area between Kenya and Tanzania by incorporating catchment-wide monitoring and joint-management to mitigate the adjacent hinterland dynamics, including land-use changes that would have a massive influence on sediment supply.