The impact of extreme events on mudflats - The Guyana case

Abstract

Mudflats are coastal features present in numerous locations around the planet. Depending on the latitude and local conditions, mudflats can be vegetated by salt marshes or by mangroves and play an important role in coastal evolution. Vegetated mudflats can create a remarkable sea defense and their high ecosystem value has been proven by various studies worldwide. In this study, the response of a vegetated mudflat under extreme hydrodynamic forcing is analysed using, as case study, a specific part of the Guyana coast at South America. The sediment dynamics of the Guyana coast are dominated by Amazon River plume that shifts northwards and travels along the coast of South America till the Orinoco River delta in Venezuela. Mudflats are quite wide reaching tens of kilometres in width because of the high supply of sediment by Amazon and the formation of mudbanks along the coast travelling northwards and then westwards. The hydrodynamic environment is relatively mild as Guyana is located far from the track of tropical hurricanes; however swell events can occur caused by the North Atlantic cyclones. Despite of its mild hydrodynamic environment, there are often cases of overtopping of the seawall along Guyana coast every year with a severe and characteristic one been held on October 2005. This was the motive for the mangrove restoration project of the study area in 2011 and for the initiation of this study. The purpose of this study is to analyze the response of a vegetated mudflat under extreme swell and storm events and investigate the impact of vegetation on this response. Furthermore, the impact of the extreme hydrodynamic event on probable damage of vegetation is tested presenting the complete interaction between hydrodynamics and vegetation. The study area is the area of Chateau Margot, southern of Georgetown, where a mangrove restoration project took place in 2011 by planting Avicennia Germinans mangroves and field measurements were applied in the end of 2019 by the researcher Üwe Best. Using the 1-D process based model Mflat, based on an open source Matlab code, a 3.5km portion of the mudflat extending offshore is modelled. Vegetation dynamics and its influence on the flow is part of the model, so a full interaction between hydrodynamics, sediment dynamics, vegetation and morphological evolution can be simulated, establishing a complete bio-geomorphological model. Using field measurements and literature data, the model is calibrated for hydrodynamic conditions and for the vegetation parameters giving a profile in equilibrium. A sensitivity analysis is applied to understand the behaviour of the model and the interaction between various hydrodynamic and sediment parameters and morphodynamics. After calibration, two extreme cases are tested in the model: an extreme storm that could be expected in Guyana coast and an extreme swell event based on the real case occured in October 2005. The Mflat model, after calibration, is able to reproduce a stable profile with mangrove vegetation similar to the observations. Vegetation appears to be quite effective in reducing the wave energy and thereby protecting the coast behind it during the extreme events, while in long term results in accretion of the landward part of the mudflat. The impact of the extreme hydrodynamic conditions on vegetation is difficult to be quantified, but for the extreme cases corresponding to the hydrodynamics of Guyana coast, vegetation is expected to be resilient enough having minor damages that could be restored in a short time period. The very mild slope of the coast is a factor that prevents the bed level of being highly dynamic contributing to the stability of the profile.