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Annual aeolian sediment transport from the intertidal beach

Reim, Elisa (2013) Annual aeolian sediment transport from the intertidal beach.

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Abstract:This report presents a case study analysis of the quantification of sediment that is annually transported from the intertidal beach to the upper beach by wind at Egmond Beach in one particular year (2009). Special attention is paid to the relationship between the complex alongshore varying intertidal beach topography, which defines the fetch distance, and onshore directed aeolian sediment transport from the intertidal beach to the upper beach. The effect of a probable increased and varying moisture content was not yet accounted for. Semi-hourly collected ARGUS (digital monitoring system) video images from Egmond Beach, The Netherlands, of the year 2009 were used to identify the occurrence of aeolian sediment transport. Hourly averaged wind speed and wind direction data from IJmuiden and precipitation data from Wijk aan Zee were used to get insight into the effect of those on aeolian sediment transport occurrences. The sediment transport equation of Kawamura (1951) was used to calculate the amount of sediment that could theoretically be transported from the intertidal beach towards the upper beach and dunes during conditions in which actual aeolian sediment transport was observed. Moisture content was accounted for using the equation of Dong et al. (2002) and the effect of fetch limitation was accounted for by using the equation of Delgado-Fernandez (2010). It appeared that the hourly averaged wind speed data available from the long-term wind monitoring at IJmuiden was not an appropriate input value to calculate annual onshore aeolian sediment transport, as it lead to an underestimation. A new ‘representative wind speed’ was developed to account for gustiness of the wind throughout the hour. The translation to a representative wind speed value was developed based on an analysis of high resolution time series for wind speeds from wind stations near Salt Lake City, United States of America. The most aeolian sediment transport occurrences were observed while wind was blowing alongshore or nearly alongshore the beach, which in the case of Egmond Beach means wind directions from South-West. No single velocity threshold for aeolian sediment transport occurrences valid for all wind direction has been found. Nevertheless, below an hourly averaged wind speed of 6 ms-1 almost no aeolian sediment transport occurrences have been identified. The formula that is used in this study to calculate the annual aeolian sediment transport is most sensitive to changes in surface roughness length. Therefore, the surface roughness length is one of the key parameters of onshore annual aeolian sediment transport calculations. The dune volume change per year at Egmond Beach is measured to be 10 m3m-1y-1 (Arens, 2010). The total calculated onshore annual aeolian sediment transport in this study is 9.4 m3m-1y-1 which is almost the same amount as found by Arens (2010). However, only in 5% of the aeolian sediment transport occurrences, the actual fetch distance was smaller than the theoretical critical fetch. However, the effect of the alongshore varying fetch distances on annual onshore aeolian sediment transport has been found, as 0.88 m3m-1y-1. This is 8% of the total annual onshore aeolian sediment transport. It can be concluded that the small amount of cases, for which the actual fetch distance is smaller than the critical fetch distance, can result in a non-negligible difference in annual onshore aeolian sediment transport. During the hours that transport was identified in the ARGUS images, 75% of the calculated annual aeolian sediment transport took place. During the hours that no transport was identified in the ARGUS images, 25% of the annual aeolian sediment transport was calculated (due to transport enabling wind conditions). Therefore, the identification of sediment transport occurrences on ARGUS images is an important part of the annual aeolian sediment transport calculation method. Overall, in this study a method has been developed to calculate representative theoretical annual onshore aeolian sediment transport. This method consist of the analysis of ARGUS images, measurement of the fetch distances and development of a representative wind speed accounting for the gustiness of the wind.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
Link to this item:http://purl.utwente.nl/essays/64444
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