Bedload transport in the Vistula River mouth derived from dune migration rates, southern Baltic Sea

Summary: In this paper, bedload sediment transport to the Baltic Sea in the main Vistula River mouth (Przekop Wisły) is estimated. For the first time in this area, investigations were performed based on the non-invasive measurement techniques with the using of hydroacoustic tools. Repeated bathymetric surveys were carried out using a multibeam echosounder for the period with hydrological conditions close to that of the long-term mean annual water discharge. Quantification of the bedload transport, as a main factor for the subaqueous Vistula delta development, involved applying the bedform tracking technique, and estimating the dune celerity by analysing the cross-correlation functions of bed elevation profiles (BEPs). The BEPs were obtained along two transects of 500 m in length situated in two different morphological parts of the river mouth – in the “shallow” and in the “deep” sites located upstream and downstream of the submerged sandbar, respectively. Contrarily to previous observations, the bedload transport was found to take place constantly. Moreover, a significant difference in a character of dune migration between the two sites of the investigated area was determined. The “shallow” dunes migrate 7 times faster (0.022 m/h) than the “deep” ones (0.003 m/h). Estimation of the daily bedload transport towards the Baltic Sea revealed values about 40.9 t/day and 8.4 t/day for “shallow” and “deep” sites, respectively. This result can probably indicate that a significant portion of sediments (ca. 80%) transported by the river during average hydrological conditions is deposited temporarily on the submerged sandbar, causing its growth. Keywords: Sand dune migration, Bedload transport, Vistula River mout