A review of the application of the fetch effect to modelling sand supply to coastal foredunes

The fetch effect is an increase of the aeolian sediment transport rate with distance downwind over an erodible surface. The first observations of the fetch effect go back 70 years and the concept has been widely used in a variety of landscapes. This paper reviews the present state of knowledge of the fetch effect, with particular reference to its application in coastal areas, and compares findings from theoretical, wind tunnel, and fieldwork experiments. While wind tunnel experiments tend to show critical fetch distances of a few metres, studies in natural areas show that measured critical fetch distance can exceed one hundred metres. There is supporting evidence pointing to the role of soil clods/crusts and moisture content in increasing critical fetch distances in agricultural and coastal areas, respectively. In coastal areas tradeoffs imposed by the geometry of the beach over which the wind is blowing and wind direction determine the available fetch distance and thus the sediment transport rate downwind. A major challenge which needs to be addressed is the development of robust equations for predicting both the critical fetch length and the increase in the sediment transport rate with distance on beaches. There is also a need to obtain field data on the combined effect of moisture, angle of wind approach, beach width, and fetch length. Long-term monitoring using remote sensing techniques may provide valuable data to analyze the effect of fetch distances on the nature of transport events that deliver sediment from the beach to the foredune.