Modeling Multi‐Fraction Coastal Aeolian Sediment Transport With Horizontal and Vertical Grain‐Size Variability.

Grain size affects the rates of aeolian sediment transport on beaches. Sediment in coastal environments typically consists of multiple grain‐size fractions and exhibits spatiotemporal variations. Still, conceptual and numerical aeolian transport models are simplified and often only include a single fraction that is constant over the model domain. It is unclear to what extent this simplification is valid and if the inclusion of multi‐fraction transport and spatial grain‐size variations affects aeolian sediment transport simulations and predictions of coastal dune development. This study applies the numerical aeolian sediment transport model AeoLiS to compare single‐fraction to multi‐fraction approaches for a range of grain‐size distributions and spatial grain‐size scenarios. The results show that on timescales of days to years, single‐fraction simulations with the median grain size, D50, often give similar results to multi‐fraction simulations, provided the wind is able to mobilize all fractions within that time frame. On these timescales, vertical variability in grain size has a limited effect on total transport rates, but it does influence the simulation results on minute timescales. Horizontal grain‐size variability influences both the total transport rates and the downwind bed grain‐size composition. The results provide new insights into the influence of beach sediment composition and spatial variability on total transport rates toward the dunes. The findings of this study can guide the implementation of grain‐size variability in numerical aeolian sediment transport models. Plain Language Summary: The growth of coastal dunes is caused by the wind, which moves sand from the beach to the dunes. The sand grains on the beach have different sizes. For instance, the size of the sand grains can vary from the waterline to the start of the dunes. Small sand grains are more easily picked up by the wind than larger, heavier grains. Thus, the size of sand grains and how they are spread over the beach can impact how much sand is moved by the wind. We use computer simulations to calculate how much sand is moved by the wind under different conditions. We investigate how different assumptions about the grain size on the beach influence these calculations. The results show that the calculations with one single sand size, in most cases, give comparable results to simulations with more complicated sand size variations. The simplified approach is beneficial because it reduces the need for detailed field data of grain sizes for future calculations. Key Points: The effects of multi‐fraction transport and spatial grain‐size variations were examined with the numerical aeolian transport model AeoLiSThe D50 can be used as a representative grain size in aeolian sediment transport modeling on a time scale of days to yearsThe bed surface grain size in the upwind source area might be the most relevant to include in aeolian sediment transport models [ABSTRACT FROM AUTHOR]