Morphodynamic adaptation timescales of the Guyana mangrove-mudflat system
Are coastlines shaped by migrating mudbanks more resilient against sea level rise?
U.S.N. Best (TU Delft - Coastal Engineering, IHE Delft Institute for Water Education)
Alexandre Legay (Université Grenoble Alpes)
Johan Reyns (TU Delft - Coastal Engineering, Deltares, IHE Delft Institute for Water Education)
Mick van der Wegen (TU Delft - Hydraulic Structures and Flood Risk, Deltares, IHE Delft Institute for Water Education)
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Abstract
In developing effective and resilient coastal management strategies, it is critical to consider the coastline's natural spatial and temporal variation. This is particularly relevant for Guyana's wave-exposed mud-dominated coastline, where longshore migrating subtidal mudbanks create a 30-year cycle of accretion and erosion of the coastline, driven by the presence or absence of a mudbank. These cyclic dynamics are integral to the broader coastal system, shaping the development of both the mudflat profile and the opportunistic mangrove vegetation. This study aims to enhance our understanding of the behaviour of mangrove-mudflat systems on the Guyana coast by examining: (1) intertidal sediment dynamics and vegetation dynamics on a mangrove/mudflat profile, (2) the morphodynamics associated with the cyclically varying forcing conditions and (3) the processes involved in the morphodynamic response under sea level rise. We utilized an open-source, 1D, cross-shore model (Mflat) that couples tidal flow, wave action, sediment transport and morphodynamic development to vegetation dynamics, including temporal and spatial growth of the tree population and bio-accumulation. The mudflat dynamics are highly determined by the cyclically varying sediment influx, wave height and period, whereas the mangrove vegetation tends to follow the evolution of suitable intertidal areas rather than impacting morphodynamic changes. An important finding is that the time scale of the cyclically varying boundary conditions (30 years) is much shorter than the system's characteristic morphodynamic adaptation timescale, so no formal equilibrium is reached for a given boundary condition. The cyclic mangrove-mudflat system shows considerable resilience to sea level rise due to the abundance of mud, though it gradually drowns under larger sea level rise scenarios. These insights on the survivability of these intertidal systems are relevant to many other mangrove-mudflats worldwide.
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