River, Coastal and Estuarine Morphodynamics: RCEM 2009 – Vionnet et al. (eds) © 2010Taylor & Francis Group, London, ISBN 978-0-415-55426-8 Tide-controlled variations of primary- and secondary-bedform height: Innenjade tidal channel (Jade Bay, German Bight) V.B. Ernstsen, C. Winter & M. Becker MARUM—Center for Marine Environmental Sciences, University of Bremen, Germany J. Bartholdy Department of Geography and Geology, University of Copenhagen, Denmark ABSTRACT: Primary- and secondary-bedform height dynamics were investigated during two tidal cycles in the Innenjade tidal channel in the Jade Bay, German Bight. Repetitive, simultaneous measurements of high- resolution swath bathymetry and flow velocity were combined with detailed spatial mapping of bed material characteristics. The bedform tracking tool (BTT) developed by van der Mark and Blom (2007) was further improved for an objective and discrete analysis of primary- and secondary-bedforms. Primary-bedform heights generally decreased during ebb tide and increased during flood tide. This was due to erosion and deposition of the crests, as the troughs remained practically constant. Crest erosion occurred at high energy stages during ebb tide, and overall crest deposition during flood tide (build-up of flood cap). We assign the low erosion in the troughs to a combination of low flow velocity and armoring through shell lag-deposits. Secondary-bedform height generally increased with increasing mean flow velocity and decreased with decreasing mean flow velocity. We hypothesize that this was due to an excess build-up of the crests primarily through internal sediment turnover, i.e. merely the shape of the secondary-bedforms changes. At maximum flow velocity, the secondary-bedform crests and troughs eroded with the erosion of the crests exceeding the erosion of the troughs, leading to a decrease in secondary-bedform height at maximum and initially falling flow velocities. We suggest that the relatively dynamic secondary-bedform troughs were due to the absence of a shell lag deposit in the secondary-bedform troughs. 1 INTRODUCTION Subaqueous bedform fields in nature are often com- plex systems with larger primary-bedforms superim- posed by smaller secondary-bedforms (e.g. Bartholdy et al., 2002). Field observations of primary-bedform height dynamics in unsteady flow at seasonal scales have been reported since the late 1960s: e.g. Rio Paraná River, Argentina (Stückrath, 1969), Elbe and Weser River, Germany (Nasner, 1974 and 1978), and Fraser River, Canada (Kostaschuk et al., 1989; Kostaschuk and Illersich, 1995). Also at shorter time scales, during flood hydrographs, primary dune height dynamics in response to an unsteady flow regime has been reported: e.g. River Rhine (Julien et al., 2002; Kleinhans, 2002; Wilbers and ten Brinke, 2003) and Fraser River, Canada (Villard and Church, 2003). More recently bedform evolution models capable of predicting dynamic primary dune height in response to flood hydrographs have been suggested (Giri et al., 2007; Paarlberg, 2008). The variability of primary dune height during neap-spring tidal cycles has also been investigated: e.g. Lifeboat Station Bank, Wells- next-the-Sea, UK (Allen, 1976), Ossenisse intertidal shoal, Westerschelde Estuary, The Netherlands (Terwindt and Brouwer, 1986), and Spiekeroog back- barrier channel, The Wadden Sea, Germany (Flemming and Davis, 1992). More recently primary- bedform height variability was also investigated during time scales as short as single tidal cycles: Fraser Estuary (Kostaschuk and Best, 2005) and Grådyb tidal inlet, The Wadden Sea, Denmark (Ernstsen et al., 2006b). Hence, there is a considerable amount of detailed field investigations on dynamic primary-bedform height at various temporal scales. However, Julien et al. (2002) stated that a composite analysis of primary- and secondary-bedforms is recommended for future studies on resistance to flow; and this is still lacking. In this paper, we report on variations of primary as well as secondary-bedform height in the Innen- jade tidal channel located in the Jade Bay, German Bight, within two tidal cycles at falling and rising tide based on repetitive high-resolution swath bathymet- ric measurements. We provide process-based expla- nations of the bedform height dynamics based on simultaneous high-resolution measurements of flow 779