Occurrence of flow parallel and flow transverse bedforms in Fehmarn Belt (SW Baltic Sea) related to the local palaeomorphology P. Feldens a, ⁎, M. Diesing b , K. Schwarzer a , C. Heinrich a , B. Schlenz c a Institute of Geosciences, Kiel University, Otto Hahn Platz 1, 24118 Kiel, Germany b Centre for Environment, Fisheries and Aquaculture Science (Cefas), Pakefield Road, Lowestoft, Suffolk NR33 0HT, United Kingdom c DHI-WASY GmbH, Office Syke, Max Planck-Straße 6, 28857 Syke, Germany abstract article info Article history: Received 21 March 2014 Received in revised form 28 November 2014 Accepted 30 November 2014 Available online xxxx Keywords: Sand ribbon Subaqueous dune Hydroacoustic data Seismic data Baltic Sea Sediment-starved shelf This study describes the spatial distribution of flow-parallel sand ribbons and flow-transverse large and very large subaqueous dunes in the south-western Baltic Sea offshore Fehmarn Island between 13 m and 20 m water depth, based on hydroacoustic and grain size data. The system of sand ribbons and dunes is intermittently active due to currents induced during major inflows of the North Sea water into the Baltic Sea. The sand ribbons are located on a lag deposit on top of glacial till, while the dunes rest on top of drowned Holocene nearshore deposits. The sand ribbons reach heights between 0.4 m and 0.6 m, with widths varying between 60 m and several hundreds of metres. The observed dunes have heights between 0.09 m and 2.35 m, while their wave- lengths range from 17 m to 120 m. Offshore Fehmarn Island, the transition from sand ribbons to dunes is most likely linked to a contrast in sediment supply, as reworked drowned nearshore deposits provide sediment available for transport in significantly larger amounts than glacial till. Similar to an earlier approach for river bed states, the dimensionless thickness of sediment available for transport is able to differentiate between the bed states. © 2014 Elsevier B.V. All rights reserved. 1. Introduction In subaqueous environments, the bed state depends on current and wave properties, flow depth, sediment grain size distribution, the amount of sediment available for transport as well as the abun- dance of benthic organisms (e.g., Allen, 1980; Rubin and McCulloch, 1980; Ashley, 1990; Southard, 1991; Mazumder, 2003; Best, 2005; Kleinhans, 2005). For conditions not limited by sediment supply, the developing bedforms include ripple, dune, antidune and plane bed states. Large subaqueous dune fields are generally formed in areas composed of sand subjected to sufficiently high current velocities. Apart from their occurrence in tidal seas (e.g. Stride, 1963; Bartholdy et al., 2002), examples of large subaqueous dune fields are known to occur in (epi)continental shelf seas, where they are shaped by unidi- rectional currents (Werner et al., 1974; Werner and Newton, 1975; Flemming, 1978; Kuijpers et al., 1993; Ikehara and Kinoshita, 1994; Kubo et al., 2004), or were preserved due to the rapid sea-level rise since the last glacial maximum (Flemming, 2013). Where not related to tidal currents or to continuous ocean currents, dune fields may only be intermittently active, as is the case in the Baltic Sea (Werner et al., 1974) or off Japan (Kubo et al., 2004). If sediment supply is insufficient for the formation of equilibrium dunes, the above-mentioned parameters will not be adequate to predict the stable bed phase. Large parts of continental shelf seas, including large areas of the south-western Baltic Sea, are sediment starved (Niedermeyer et al., 2011). A sediment-starved sea is defined here by the presence of an armour layer at the seafloor that the local water movements (with the exception of rare events) cannot entrain bed sed- iments. Bedforms that develop under such supply-limited conditions in unidirectional flows involve sand patches, flow transverse supply- limited dunes, including 2D dunes as well as barchanoid dunes, and flow parallel sand ribbons (Allen, 1968; Belderson et al., 1982; Kleinhans et al., 2002; Tuijnder et al., 2009). Emphasising the im- portance of sediment availability for sediment-starved conditions, Kleinhans et al. (2002) presented a conceptual model for the se- quence of sand ribbons (flow parallel) — supply-limited dunes (flow transverse) — dunes (flow transverse) in unidirectional flows, where the formation of the individual bedforms depends on the abundance of sediment available for transport and a shear stress-related mobility parameter. Conversely, in the classical series of bedforms in (tidal) shelf seas, the occurrence of sand ribbons has been related to current velocities (Belderson et al., 1982). In the example, sand ribbon formation was ascribed to current velocities between 75 cm/s and 150 cm/s. Higher velocities will be required for sand ribbon formation if the sand supply is abundant (Belderson et al., 1982; Kenyon, 1986; Johnson and Baldwin, 1996). With decreasing current velocity, bedforms change to Geomorphology xxx (2014) xxx–xxx ⁎ Corresponding author. Tel.: +49 4318805790; fax: +49 4318804432. E-mail address: pfeldens@geophysik.uni-kiel.de (P. Feldens). GEOMOR-04990; No of Pages 10 http://dx.doi.org/10.1016/j.geomorph.2014.11.021 0169-555X/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph Please cite this article as: Feldens, P., et al., Occurrence of flow parallel and flow transverse bedforms in Fehmarn Belt (SW Baltic Sea) related to the local palaeomorphology, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2014.11.021