Aeolian transport on a beach

Abstract

Fire Island has been monitored since Hurricane Sandy (2012). During that period, Fire Island showed recovery (accretion on the island). Behaviour of the recovery is not yet well-known. Main processes during the recovery are also not known. Therefore, the emphasis of this thesis is on the understanding of the main processes of the recovery of barrier islands, with Fire Island, New York, United States as a case study. AeoLiS has been validated for the Sand Motor, Kijkduin in The Netherlands. From this study it is (1) not known whether AeoLiS would have the same predictive skill at other locations and (2) not clear what parameters at a new site would affect skill and model sensitivity the most. Fire Island differs from the Sand Motor as the beach width of Fire Island is significantly smaller. Therefore, the focus of this thesis is on the beach width and its effect on the aeolian sediment supply and transport. Testing the model at a new location will also indicate what additional data or processes are missing in the present AeoLiS model which are required to perform a hindcast. The main processes and behaviour in the recovery of Fire Island are analysed in a data analysis. The results of the analysis showed that, during the period 7 November 2012 until 26 September 2013, there was an accretion of the dunes. In this period the island was recovering from the impact of Hurricane Sandy. During this accretion period a welding bar was observed. This welding bar could be an important mechanism for the dune accretion growth as accretion was seen in the period after the welding of the bar. The reason for this accretion could be the increasing fetch and the addition of new sediment. Under idealized circumstances it can be assumed that the total flux settles in the dunes. The determination of the total transport under these circumstances using Bagnold’s formulation indicates a larger increase than is actually measured. This indicates that the actual accretion of the dunes is limited. The AeoLiS model was used to hindcast the recovery of Fire Island, during the period 7 November 2012 until 26 September 2013, by predicting the transport rate into the dunes. It is assumed that the total flux into the dunes will settle in the dunes. Comparing these results to the data analysis, showed that the hindcast was in the same range. The general validity of the results depends on several assumptions. The main assumption is; the model only included onshore transport, while the wind direction was variable and mainly offshore during the accretion period. The sensitivity analyses indicated that the model simulation of Fire Island is most sensitive for a variation in the beach width and the median grain size. Furthermore, it can be concluded that aeolian sediment transport on Fire Island is often transport limited (85%) but occasionally supply limited (15%). From this research it can be concluded that there are processes and data missing for a hindcast with the AeoLiS model. The improvements for the AeoLiS model can be summarized as improvements to the initial conditions, as follows: 1. The accretion of Fire Island in the data analysis is based on lidar data. The lidar data is compared to 1D transects. Those 1D transects are also imposed in the AeoLiS model. Using a 2DH bathymetry with both x-directions and y-directions could give more information about aeolian sediment transport. 2. The grain size distribution for Fire Island is not known in detail. The sensitivity analysis indicated that the effect of the uncertainty of the mean and standard deviation of the grain size distribution on the total transport is 13%.