Chenier Dynamics

Abstract

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.