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Abstract: Submarine barchans in three new regions were described using new high-resolution multibeam bathymetric data off the Dongfang coast in Beibu Gulf. Currents, seismic profiles of horns, surface sediments and comparative surveys in 2007 and 2009 were obtained to study barchan mobility and sand transport in the barchan fields. The submarine barchans occur at depths of 15–25m, reaching up to 551.9m in horn length and 8.6m in height. Most barchans have asymmetrical horns and display wider plane shapes than subaerial barchans for the same length and height. The morphology of barchans exhibits regional variations that are mainly controlled by finite water depth, daily reverse currents, sand supply changes and interaction of dunes. Barchan asymmetries, internal bedding inclinations and grain size changes are in agreement with the dominant current directions. Results show that submarine barchans migrated 5–43m in two years (2007–2009) through comparison of bathymetric data. Because of the marginally cohesive fine sediment composition and long distance from contemporary sediment sources, it is believed that barchans in region C are not of modern origin. However, these bedforms can be also affected by high energy flow regimes. Collisions limit barchan widths and provide efficient transport paths between barchans on their horns. Large sand supply through the horns increases the barchan heights and also changes their shapes. Some small dunes are identified that are moving away from the barchans field in 2007 and have departed from the host barchans in 2009. These small barchans were probably caused by the separation of the faster-moving horn tails from the host barchans when extreme situations happen. The mechanism gives a new explanation for sand loss and size determination of submarine barchans. [Copyright &y& Elsevier]

Abstract: To improve navigability of the major access channel into Shanghai Harbor, a large-scale Deep Waterway Project was carried out in the North Passage (NP) of the Yangtze Estuary. In this paper, we investigate how the navigational works affected morphological changes of this channel, as well as those of the adjacent North Channel (NC), Hengsha East Shoal (HES), Jiuduansha Shoal (JDS), and the South Passage (SP). Morphological changes were assessed by analyzing digitized bathymetric data of this area prior to and after execution of the engineering works. The qualitative relations between these changes, along with the hydrodynamic changes as a result of the construction of engineering works, were subsequently investigated. The results reveal that the construction of training walls, groins and jetties resulted in decreased ebb transport in the upper reach of the NP and increased ebb transport in the upper reach of the SP. In turn, this led to intensive upstream erosion with large amounts of sediment transported seaward in the SP and associated sedimentation in the downstream area. At the same time, intense siltation occurred in the upper reach of the NP, while the main channel of the NP mainly experienced erosion caused by the construction of training walls and groins that concentrated ebb flow in the main channel. The waterway deepened significantly in previously shallow areas. Small tidal channels in the HES, which used to connect the NP and the NC, were cut off by the northern training wall. Consequently, residual flow in the middle reach of the NC flowed directly toward the northern bank of the HES, leading to local erosion. Flow obstruction by the southern training wall reduced the upstream propagation of the flood tide in the SP. As a result, a collection of small flood channels evolved under the flushing action of flood currents in the JDS, preventing the natural trend of horizontal extension of the JDS. However, the flow obstruction enhanced the vertical accretion of the JDS. [Copyright &y& Elsevier]