St Vincent – Black Point Beach Modelling

Abstract

Black Point on the East coast of St Vincent and Grenada was modelled using a storm impact model. A total of 5 different storm events were simulated, based on peak wave parameters taken directly from observations made in July-October 2018 and from numerical and parametric models of hurricane events. Impacts from these events were assessed by calculating wave run up and beach level erosion and deposition during the events. The results of the model simulations show that the 98th percentile (standard impact parameter R2%) of wave run up for the observed events were largely the same despite a noticeable difference in their boundary conditions. Therefore, for these events the impact on the coastline is largely the same. For tropical cyclone events, there is a variety in the impacts depending on the wind forcing used. ERA5 reanalysis winds are lower than those observed, but using Holland wind forcing, derived from the best track hurricane archive data for the same event (Hurricane Tomas in 2010) and results in much larger impacts, with wave runup over a metre higher than for the forcing taken from the observed events for the R2% assessment parameter. It has been noted that the parametric wind forcing, for a Category 4 hurricane in particular, results in waves that are outside the conditions that the model is known to be validated for and so should be considered suspect without further validation. Changes in beach levels during the simulations was also calculated, with the work focusing on the upper 250m of the beach. The beach profile at Black Point was reconstructed based on LiDAR bathymetry collected by the UKHO in 2016-2017. All events smooth out the beach profile, removing small bumps and berms. There are small changes during the observed events, with a small berm being removed at the top of the beach and a slight increase in the height of the beach itself. There are large differences for the tropical cyclone wind events, with ERA5 winds having a lower impact than the observed events, with just a smoothing of the beach. The higher wind forcing from Holland winds results in a greater change where from 150 m offshore there is erosion of the beach, leading to a steepening of the beach slope and a higher beach crest. This would reduce the impact of the extreme event itself. Finally, parametric waves for a Category 4 hurricane result in heavy erosion 250m offshore all the way to the end of the beach with the beach crest being totally removed. This would result in water over-washing the beach completely. However, as previously mentioned, these wave parameters are outside the model validation and the results for this simulation should be treated with caution. It does however indicate the potential severity of the impact of a Category 4 Hurricane.

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