Over the last decades, mangrove forests have suffered immense and rapid losses worldwide. In recognition of their important socio-economic and environmental functions, many attempts have been made to both protect the remaining mangrove coastlines and restore eroding sites. Unfortunately, many rehabilitation attempts have failed, lacking a thorough system understanding of mangrove-mud coasts. Some mangrove-mud coasts are protected on their seaward side by sandy ridges (called `cheniers'). They protect against wave attack and can help to protect vulnerable mangrove-mud coastlines. In order to sustainably restore mangrove coasts, chenier dynamics need to be understood at the temporal and spatial scales relevant for mangrove establishment (daily to yearly variability driven by waves and tides). This dissertation aims to advance our understanding of chenier dynamics within the context of an eroding mangrove-mud coast. The severely eroded coastline of Demak, Indonesia, is used as a case study. We started with a field campaign in Demak, observing the cross-shore dynamics of a single chenier. The observations revealed that cheniers can be very dynamic in relatively calm conditions. Using velocity moments as a proxy for the sediment transport, we have explored the role of tides and waves in the observed chenier dynamics. Tides drive the chenier landward, especially when the water depth over the chenier crest is low (high crest level relative to mean sea level). Waves only generate substantial sediment transport when the chenier is submerged. Overall, the cross-shore chenier dynamics are very sensitive to the timing of tides and waves: most transport takes place when high water levels coincide with (relatively) high waves. While our observations showed the chenier to be highly dynamic in the short term, satellite images reveal that over longer timescales the position of the chenier remains more or less stable within the intertidal zone. This is in contrast to cheniers described in literature, which only migrate landward until they reach a stable position above tidal influences. We have developed an idealised chenier model to explore this dynamically stable position. The model simulates cross-shore chenier dynamics under daily wave and tidal influences and is able to predict both onshore and offshore migration. Onshore migration is mainly driven by wave action, while offshore migration is induced by a tidal phase lag or storms. This phase lag is caused by drowning of the coastal plain due to subsidence. For certain combinations of waves and tides, the model predicts a dynamically stable chenier. In the absence of a phase lag and storm season effect, the model yields a `classic' stable chenier that welds onto the shoreline by onshore migration. We used Delft3D to explore the formation of cheniers through wave winnowing (the sorting of sand and mud by waves). We have identified three phases of chenier development: (1) a winnowing phase, during which mud is washed out of the seabed initially consisting of a mixture of sand and mud, (2) a sand transport phase, when the sand in the upper layer is transported onshore, and (3) a crest formation phase, during which a chenier crest rapidly develops at the landward limit of onshore sediment transport. The main mechanism driving onshore sand transport is wave asymmetry. During calm conditions, sand transport takes place within a narrow band limiting the volume of sand delivered nearshore, and therefore no chenier develops. In contrast, average storm conditions mobilise sufficient sand for a crest to develop. Our results thus reveal that chenier formation through wave winnowing does not require extreme storm conditions. Our study also shows that chenier formation through wave winnowing is a relatively slow process, with the largest time scales associated with the the first two phases of chenier development: winnowing and sand transport. Overall, this dissertation contributes to our understanding of cross-shore chenier dynamics. While very dynamic in the short term, cheniers can maintain a stable position in the intertidal zone for certain combinations of waves and tides. As such, they can contribute to mangrove rehabilitation by creating windows of opportunity for mangrove establishment. Due to its rapid subsidence rates, the coast of Demak provides an analogue for a global drowning of coastlines under anticipated accelerated sea level rise. In fact, cheniers may form a natural defense mechanism of drowning coastal plains. As a result, small changes to the coastal plain (e.g. constructing a dike) could have a significant impact, disturbing the chenier dynamics and interrupting their negative feedback on coastal erosion. This work has illustrated the complexity and interconnectedness of coastal systems, a crucial notion in designing successful protection strategies for mangrove-mud coasts.@en

A chenier is a beach ridge, consisting of sand and/or shells, overlying a muddy substrate. In this paper, we explore the cross-shore dynamics of cheniers in their ‘active’ phase, i.e. the phase between their formation and their landing on the shore and can no longer be reached by daily wave and tidal influences. While cheniers described in literature are known to only migrate onshore until they reach a stable position with their crest level above tidal influences, observations in Demak suggest the existence of an alternative stable state, highly dynamic on the short term, but stable on the longer term. To explore this alternative stable state, we developed an idealised chenier model to investigate cross-shore chenier dynamics under daily wave and tidal influences. The model is able to predict both onshore and offshore migration; onshore migration is mainly driven by wave action, while offshore migration is induced by a tidal phase lag, or the effect of the storm season. For certain combinations of waves, tide (incl. phase lag) and a storm season effect, the model predicts a dynamically stable chenier. In absence of a phase lag and storm season effect, the model yields a ‘classic’ stable chenier that welds onto the shoreline by onshore migration.

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Cheniers are ridges consisting of coarse-grained sediments, resting on top of the fine sediment that forms the otherwise muddy coast. In this paper, we use Delft3D to explore how cheniers are formed through wave winnowing. We identify three phases of chenier development: (a) a winnowing phase, during which mud is washed out of the seabed initially consisting of a mixture of sand and mud, (b) a sand transport phase, when the sand in the upper layer is transported onshore, and (c) a crest formation phase, during which a chenier crest rapidly develops at the landward limit of onshore sediment transport. The main mechanism driving onshore sand transport is wave asymmetry. During calm conditions, sand transport takes place within a narrow band limiting the volume of sand delivered nearshore, and therefore no chenier develops. In contrast, average storm conditions mobilize sufficient sand for a crest to develop. Our results thus reveal that chenier formation through wave winnowing does not require extreme storm conditions. Furthermore, our study showed that chenier formation through wave winnowing is a relatively slow process, with the largest time scales associated with the winnowing and sand transport. Once sufficient sand is available in the intertidal zone, the crest develops rapidly.

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Mangrove-mud coasts across the world erode because of uninformed management, conversion of mangrove forests into aquaculture ponds, development of infrastructure and urbanization, and/or extraction of groundwater inducing land subsidence. The accompanied loss of ecosystem values, amongst which safety against flooding, has far reaching consequences for coastal communities, exacerbated by sea-level rise. To halt erosion various nature-based solutions have been implemented as an alternative to hard infrastructure sea defenses, including mangrove planting and erection of low-tech structures such as bamboo fences, permeable brushwood dams, etc. These structures have been designed on the basis of best-engineering practice, lacking sufficient scientific background. This paper investigates the use and success of permeable dams over a period of about 15 years, describing their application in Guyana, Indonesia, Suriname, Thailand and Vietnam, summarizing the lessons-learned, and analyzing their functioning in relation to the physical-biological coastal system. Also an overview of relevant costs is given. The basic philosophy behind the construction of permeable dams is the rehabilitation of mangrove habitat through re-establishment of the (fine) sediment dynamics – we refer to Building with Nature as the overarching principle of this approach. Our main conclusions are that a successful functioning of permeable dams requires (1) a thorough understanding of the physical-biological system and analysis of the relevant processes, (2) patience and persistence, including maintenance, as the natural time scales to rehabilitate mangrove green belts take years to decades, and (3) intensive stakeholder involvement. We give a list of conditions under which permeable dams may be successful, but in qualitative terms, as local site conditions largely govern their success or failure.@en

Cheniers are important for stabilising mud-dominated coastlines. A chenier is a body of wave-reworked, coarse-grained sediment consisting of sand and shells overlying a muddy substrate. In this paper we present and analyse a week of field observations of the dynamics of a single chenier along the coast of Demak, Indonesia. Despite relatively calm hydrodynamics during the one-week observational period, the chenier migrated surprisingly fast in the landward direction. The role of the tide and waves on the cross-shore chenier dynamics is explored using velocity moments as a proxy for the sediment transport. This approach shows that both tide and waves are capable of transporting the sediment of the chenier system. During calm conditions (representative for the south-east monsoon season), the tides generate a landward-directed sediment transport when the chenier crest is high relative to mean sea level. Waves only generate substantial sediment transport (direct, via skewness, and indirect, via stirring) when the chenier is submerged during periods with higher waves. The cross-shore chenier dynamics are very sensitive to the timing of tide and waves: most transport takes place when high water levels coincide with (relatively) high waves.

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During recent decades, mangrove forests have experienced severe degradation due to unsustainable land use. Restoration of mangrove ecosystems requires the recovery of their habitat, considering ecology, hydrology, hydrodynamics, and sediment transport. In a first pilot in 2013, brushwood dams were built on the eroding coast of Demak, Indonesia, in order to emulate the function of mangrove roots and provide the physical conditions for natural colonization. However, at present there is little research on how soft structures affect the local hydrodynamics. The present study aims to improve the understanding of wave attenuation by permeable brushwood dams in Demak, combining field observations and hydrodynamic modelling using Delft3D. The findings of the study will be used to develop a landscape bio-morphodynamic model, which will be applied for planning future mangrove restoration efforts.@en

In this paper the typical hydrodynamics on mangrove-mud coasts are studied. Worldwide, these coasts experience serious erosion problems, and while the importance of mangrove ecosystems is becoming widely recognised, mangrove restoration projects frequently fail due to poor understanding of the system, especially the hydrodynamics. Therefore, a landscape model of the eroding coastline of the Demak district in Indonesia is developed to analyse the typical hydrodynamics associated to mangrove-mud coasts. Owing to the fine sediment, these coastlines are characterised by gentle slopes, in the order of 1:1000 or less. Both the theoretical and numerical wave transformation have to be re-evaluated on such slopes, which is done by combining models with field measurements. Also the current patterns and density effects are studied in detail to generate a full understanding of the hydrodynamics on mangrove-mud coasts.@en