Mangroves are an amazingly rich ecosystem, supporting biodiversity, providing natural resources and services – and thus supporting livelihoods – and storing carbon. They also offer a very effective defense against the effects of climate change on tropical coast, protecting deltas from shrinking and sinking. But deltaic and estuary mangroves forest have an Achille heal: they depend on a steady supply of sediment flows to strive and survive. Yet the delivery of sediment from most tropical rivers to their mouths has been drastically reduced over the past 3 decades. This rapid assessment tool has been designed to assist practitioners & decision makers in understanding and assessing the role of sediment flows while considering conservation, replantation or regeneration mangrove forest projects.
Section 1 of this report provides context on the role of mangroves, as a unique ecosystem, and their benefits to people. It also outlines why sediment supply is important for mangrove sustainability, the impacts of a diminishing sediment supply, and why one should consider sediment when undertaking mangrove conservation and restoration. Section 2 sets the objectives and frame of the rapid assessment tool. Section 3 presents method and guidance to understand and assess key processes, the impact of human interventions and how to measure this impact on sediment flows and deposition, while taking into account the state of relevant governance and policy frameworks at four spatial scales of analysis interlinked by sediment flow connectivity: (1) river basin scale, (2) delta scale, (3) coastal scale, and (4) site scale. Section 4 explains how to conduct a rapid assessment for non-experts when data is readily available – even if it is limited. Section 5 provides concluding remarks on the future of mangroves in the context of climate change and increasing anthropogenic pressures. Further guidance on how to use the tool is provided in the annex.
Mangroves are found at the junction of three biomes¹: terrestrial, marine and freshwater. They are found in the intertidal zone between high and low water on low-lying coasts, and they are one of the world’s most productive ecosystems for food, notably for rice and fish. Mangrove areas are therefore often densely populated by humans, which brings them under significant and ever-increasing pressures that have resulted in the conversion of large tracts of mangrove forests to coastal development, for example agriculture and aquaculture zones.
The exploitation of mangrove areas is however extremely problematic: these extraordinary trees that have the ability to thrive despite the presence of salt in water, play a unique role as biodiversity hotspots. They are also one of nature’s anti-tsunami and anti-cyclone defence systems. Cyclones are becoming more intense with climate change, making the pressures on mangroves a problem that needs to be addressed with extreme vigilance.
Pressures on mangroves are often direct. These include deforestation, either related to wood-cutting – for firewood and construction – or to the conversion of mangrove areas into farmlands, aquaculture farms, and grazing pastures.
Coastal erosion and increased subsidence in river deltas – which both account for more than 40% of the world’s mangrove areas – are also detrimental factors of mangrove degradation. Increased subsidence in river deltas is more difficult to ascertain, and is thus often overlooked. Coastal erosion and increased subsidence in river deltas are essentially due to insufficient river sediment supply, a phenomenon that can originate several thousand kilometres upstream in the river basins to which these deltas belong.
The net long-term sustainability of mangrove-bearing shorelines depends first and foremost on an adequate sediment supply. Mangroves – like all trees – rely on sediments, yet are at the same time, limited producers of sediments (meaning in situ organic production by mangrove roots, leaves and tree trunks.)
Mangrove populations are often found on coasts that are subjected to land subsidence and erosion processes that are in natural dynamic balance, meaning that natural forces continuously ensure that sediments are replenished. Without an adequate sediment supply, these processes become imbalanced, thus creating serious threats to the integrity of mangrove ecosystems.
MANGROVES AND SEDIMENTS
In considering the role of mangrove ecosystems, the free flow and availability of sediments constitute an overarching background factor without which (1) mangrove substrate elevations cannot be durably sustained, and (2) the commonly considered ‘land-building’ and ‘coastal protection’ roles of mangroves cannot be successful. The extent to which mangroves can be conserved, or the eventual outcome of mangrove rehabilitation or restoration projects, fundamentally depend on sediment supply. Many mangrove rehabilitation or restoration projects, often implemented at a considerable cost, unfortunately overlook this fundamental criterion, and therefore sometimes meet with failure.
Sediment replenishment to coasts by rivers is a natural process which is greatly hindered by the trapping effect of man-made structures, such as dam reservoirs, and by sand mining. This is further aggravated by groundwater extraction and by sea-level rise induced by climate change, both of which increase the need for more sediments to balance accelerating subsidence, and to fill the ‘accommodation space’ created by water-level increases. This is notably the case in delta areas where natural subsidence is common, and which can exacerbate sea-level rise. Further disruptions of sediment supply and dynamics are frequently caused by the presence of harbours, ports, and coastal protection structures such as groins and dykes.
THE RAPID ASSESSMENT GUIDE
The aim of this report is to provide an assessment guidance manual for the management of mangrove conservation and restoration. This manual takes into account sediment flows and sediment deposition as crucial factors in the success of mangrove projects. This will help local decision-makers in large tropical river deltas to better plan and manage mangrove conservation and restoration programmes. Derived from background literature, workshops in Tanzania, Vietnam and Myanmar, and questionnaires on policy and field practices in connection to mangroves in the Irrawaddy, Ganges-Brahmaputra, Indus, Mekong and Rufiji river deltas, the manual has two key purposes: one, to ensure mangrove practitioners, coastal zone managers and land-use planners in delta regions are aware of, and able to address sediment issues at the various spatial and temporal scales of analysis and planning – with special attention given to mangrove resilience to environmental stressors and perturbations – and to better understand the factors that lead to the success or the failure of mangrove replanting and rehabilitation projects. Secondly, this manual aims to provide guidance on best management practices, by integrating the sediment-supply dimension in mangrove conservation, rehabilitation or restoration.
The rapid evaluation guidance manual applies a multi-scale approach to the assessment of sediment supply and availability. It guides the collection of existing information on man-made major causes of loss of sediments, and the governance frameworks relative to their management. The four embedded scales are: the river basin scale, the delta scale, the coastal scale, and the local mangrove site scale.
For more information, contact Marc Goichot, WWF Greater Mekong Programme Office.
¹A biome is a community of plants and animals living together in a certain kind of climate. Scientists have classified regions of the world into different biomes. Source: NASA.