Abstract The southern coast of the Arabian Gulf is considered a good example of an arid carbonate depositional system. The objective of this field investigation is to better elucidate the dynamic controls on the character and pattern of the depositional belts in a carbonate-evaporite lagoonal setting. The interpretation of this lagoon is based on combined field examination and laboratory analyses of surface samples, and this has resulted in an updated depositional model for carbonate-evaporite lagoonal settings. This study highlights the effects of sediment transport by longshore currents, with the formation of sand spits that gradually closed the lagoon during their southwards migration. The associated narrow and elongated back-barrier settings consist of tidal-dominated muddy sediments affected by a network of tidal channels. This wave-dominated high-energy system has generated several carbonate barrier and back-barrier units that migrated southwards and seawards during the last few thousands of years. This updated depositional model significantly differs from shoreline-parallel facies tracts of the classical Trucial Coast model, where the dominant wind is orientated perpendicular to the coastline. Conversely to the sediment belts of the Trucial Coast, the Al Dakhirah lagoon displays a strongly asymmetrical pattern of the sediment belts, marked by a southwards and seawards migration.

Abstract Longshore sediment transport (LST) is an important component of many coastal systems. This common term is frequently expressed as an annual volume, either net or gross, passing a particular point. In spite of its importance as a factor in coastal morphodynamics it has received relatively little attention as a process. We review the mechanisms of longshore sediment transport and current approaches to its assessment using field measurement and geoindicators. We conclude that approaches to date have never succeeded in quantifying longshore sediment drift rates in an adequate way. This shortcoming is attributed to the spatial and temporal variability of contributing processes coupled with a technical inability to quantify simultaneously longshore sediment transport in all its modes. We also conclude that research involving the measurement of longshore transport can enhance our understanding of the processes of transport and rates of transport under certain conditions. However, the accurate quantification of drift rates as annual volumes is an unattainable goal. We propose that such quantifications be replaced by qualitative values (small, medium, large) or measures of the relative importance of longshore transport in effecting morphological change (e.g. drift-dominated, cross-shore dominated, mixed processes).

Abstract Currents associated with the nearshore cell circulation, including rip currents, redistribute beach sands into a variety of rhythmic topographies, but do not produce longshore sand transport continuously along the shoreline. Waves breaking at an angle to the beach generate longshore currents flowing parallel to the shoreline. These currents in turn interact with the wave surf to produce a longshore transport of sand. A simple equation has been found with which this longshore current velocity can be evaluated for the mid-surf position where data are available. Theoretical relationships have been formulated for the complete longshore current distribution across the beach width, but data are lacking. However, the distribution can be made to agree with the available data at mid-surf by the proper selection of the drag coefficient. Equations have also been obtained for the evaluation of the sand transport rate, caused either by waves breaking at an angle to the shoreline or by longshore currents generated in other ways. This gives the total sand transport rate. Theoretical relationships have been determined for the distribution of longshore sand transport across the beach width, but again data are lacking to test the equations. The distribution can be calibrated such that, when summed across the beach, it gives the correct total sand transport rate.