Predicting erosion of vegetated dunes during hurricanes

Abstract

Coastal dunes serve as the primary defence mechanism against coastal storms for many coastal communities around the world. Vegetation plays a role in increasing dune resiliency as it enables dune growth, however not enough is known about its effects during storms. Research has shown that the current climate crisis will increase the intensity of coastal storms as the sea level rises and global temperatures increase. It is therefore of utmost importance to understand the effects of vegetation on coastal dunes. Particularly with current trends of nature-based solutions to plant vegetation in dune restoration projects.

In this master thesis, the impact of Hurricane Ian on two barrier islands is analyzed and modelled. The primary focus of the study was to investigate how dune vegetation influenced the erosional effects of the storm, with a particular emphasis on enhancing the existing methodologies for incorporating vegetation into morphological models such as XBeach. The research findings derived from the data analysis revealed that the most resilient dunes are high, broad and have a dense vegetation coverage. Moreover, the model outcomes highlight the substantial improvement in predictive accuracy achieved by integrating vegetation as a bed roughness coefficient within the XBeach model. Adding vegetation to the model directly influences current velocities, but does not affect water levels, wave heights, or infragravity waves. The primary influence of vegetation becomes pronounced when an island is inundated or breached, significantly reducing the currents and sediment transport caused by water level gradients. An additional effect of vegetation was observed in a comparative analysis between vegetated and unvegetated models. This analysis highlighted a delay in dune crest lowering when vegetation was present, showcasing the importance of vegetation in shaping the response of barrier islands during storm events.

Numerical modelling can help understand the complex processes that shape barrier islands during storms. This research emphasized the necessity of including vegetation in XBeach models to enhance their predictive capabilities. The best predictions occur when high-resolution bathymetric data is combined with land use land cover (LULC) data to include vegetation as a constant bed roughness parameter. Furthermore, reducing the land classes to four different ones based on dune vegetation zones improves the results of the model and facilitates the calibration of bed friction coefficients.

The most effective models applied in this study demonstrated impressive skill, ranging from good to excellent, accurately predicting breaches in the precise areas they occurred for both islands under investigation. This research contributes to the continued improvements of modelling with XBeach and provides a detailed method of analyzing the effects of dune vegetation on dune erosion to determine the impact of Hurricanes in coastal dunes.