Impact of the Beach Grain Size Variability on Observed and Calculated Aeolian Transport Rates

Lamy, A., Robin, N., Smyth, T.A.G., and Hesp, P., 2024. Impact of the beach grain size variability on observed and calculated aeolian transport rates. In: Phillips, M.R.; Al-Naemi, S., and Duarte, C.M. (eds.), Coastlines under Global Change: Proceedings from the International Coastal Symposium (ICS) 2024 (Doha, Qatar). Journal of Coastal Research, Special Issue No. 113, pp. 483-487. Charlotte (North Carolina), ISSN 0749-0208. Beaches can exhibit a wide range of grain sizes, from fine sand to boulders. Their temporal variability at a given location likely plays a role in aeolian transport rates, but it has not been thoroughly studied. Thus, grain size is commonly defined as a discrete and invariant value when calculating aeolian sediment transport using predictive formulae. This use of an unchanging grain size value likely introduces substantial errors to long-term calculations of sediment flux on dynamic beaches. In this study, a 16-month survey of environmental conditions (winds and waves) has been conducted on the wave-dominated microtidal Leucate beach-dune system in SE France, a beach influenced by offshore wind conditions. Five 1 to 3-day field campaigns were carried out to measure wind, sediment transport and beach grain size. This study shows that beach grain size can be highly variable over a one-year period, changing from medium sand to very coarse sand, with in a substantial impact on aeolian sediment transport flux. These grain size characteristics remained for days to months, depending on the wind and marine conditions. Under a mean wind speed of 10 to 14 m/s, the observed transport rates ranged from 21 to 154 kg/m/h for medium-sized grains to 0.4 to 50 kg/m/h for coarse-sized grains. Each observation was compared with potential sediment transport rates calculated from numerical formula and indicated an under or overestimation more than 10 times depending on the grain size used for this calculation. The potential total transport over the 16- months survey revealed up to two orders of difference depending on the grain size selected. This study highlights the caution that needs to be taken when estimating transport rates over long periods of time in an environment with significant temporal beach grain size variability.