Abstract
An ecomorphodynamic model was developed to study how Avicennia marina mangroves influence channel network evolution in sandy tidal embayments. The model accounts for the effects of mangrove trees on tidal flow patterns and sediment dynamics. Mangrove growth is in turn controlled by hydrodynamic conditions. The presence of mangroves was found to enhance the initiation and branching of tidal channels, partly because the extra flow resistance in mangrove forests favours flow concentration, and thus sediment erosion in between vegetated areas. The enhanced branching of channels is also the result of a vegetation-induced increase in erosion threshold. On the other hand, this reduction in bed erodibility, together with the soil expansion driven by organic matter production, reduces the landward expansion of channels. The ongoing accretion in mangrove forests ultimately drives a reduction in tidal prism and an overall retreat of the channel network. During sea-level rise, mangroves can potentially enhance the ability of the soil surface to maintain an elevation within the upper portion of the intertidal zone, while hindering both the branching and headward erosion of the landward expanding channels. The modelling results presented here indicate the critical control exerted by ecogeomorphological interactions in driving landscape evolution.
Highlights
Mangroves are highly productive ecosystems that cover the intertidal area of many tropical and subtropical coastlines [1,2]
This drives a further concentration of the flow so that a positive feedback mechanism between erosion and channel formation is created [8,11,12]. In addition to this flow-sediment-topography feedback, mangroves are expected to interact with these physical processes and to change the dynamics related to the formation as well as the subsequent evolution of entire tidal channel networks
Numerical modelling studies of hydrodynamic processes have shown that mangrove trees have a significant impact on the flow structure in mangrove creeks by enhancing the tidal asymmetry [16,17]; this effect was attributed to the extra flow resistance in the mangrove forest
Summary
Mangroves are highly productive ecosystems that cover the intertidal area of many tropical and subtropical coastlines [1,2]. The question of how such biophysical interactions affect the morphological evolution of tidal landscapes over decades or even longer time scales has so far been largely unexplored This includes the influence of mangroves on the formation and evolution of tidal channel networks. The effects of mangroves on physical processes that have been included in the model are: (i) increasing flow resistance; (ii) increasing resistance of sediment to erosion by tidal flow; (iii) increasing resistance of sediment to slope-driven sediment transport; and (iv) soil expansion driven by the production of organic matter The latter process has received particular interest with respect to the ability of mangrove forests to adjust to sealevel rise [34]. We performed additional simulations to explore how the channel network might evolve under a rising sea level in both the presence and absence of mangrove vegetation
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