Sand Wave Morphology and Migration in the Main Channel of the Outer Tagus Estuary

Portela, L.I., 2024. Sand wave morphology and migration in the main channel of the outer Tagus estuary. 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. 387-391. Charlotte (North Carolina), ISSN 0749-0208. Sand waves are flow-transverse bedforms common in coastal environments, whose activity may affect navigation channels, offshore infrastructure and sediment management. Sand wave asymmetry is generally assumed to indicate the dominant direction of sediment transport, as well as the direction of bedform migration, but the latter assumption is not always confirmed by observations. The complex interaction of sand waves with larger-scale features (e.g. sandbanks, tidal inlet shoals) is also poorly understood. This paper examines the morphology and migration of sand waves in the seaward part of the main ebb channel of the Tagus estuary, based on two multibeam surveys conducted in 2019 and 2020. Sand waves are restricted to the channel side adjacent to the main ebb-tidal delta shoal. Geometric parameters (length, height, steepness and asymmetry) and migration rates were calculated over a 1400 m transect, at an average depth of 20 m, divided into two sections (landward, A, and seaward, B). The average wavelength and height were found to be 80 m and 2.1 m respectively (steepness of 0.027). The spacing versus height relationship is in good agreement with the global mean trend equation of Flemming (1988). The bedform asymmetry is always ebb-oriented (in 2019: 0.29 ± 0.14 in transect A; 0.39 ± 0.17 in transect B). The direction of sand wave migration obtained from the comparison between surveys is also ebb-oriented on average, but more variable (in 2019-2020: 5.3 ± 24.8 m year-1 in transect A; 9.9 ± 18.7 m year-1 in transect B). The observed evolution appears to be generally consistent with ebb dominance in the main channel. It is suggested that (a) sediment transported by waves and flood currents over the main shoal is partially mobilised by the ebb-dominant currents on its steep side, (b) the sand waves reflect a sediment transport pathway along the shoal edge towards the terminal lobe, and (c) sediment is eventually recirculated in a counterclockwise direction.