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Hydraulic roughness is a fundamental parameter in the understanding and simulation of hydro- and sediment-dynamics over dunes in fluvial and coastal areas. Completely understanding the links between the turbulent flow field, sediment transport and morphodynamics is key to better quantifying the evolving roughness in time and space. This paper presents two surveys (one in the late flood season and the other in the late dry season), conducted at the tidal current limit of the Changjiang Estuary, analysing flow and bedform dynamics under different water/flow stages. Results show that when compound dunes generate in the late flood season, flow structure was affected by the bed morphology and the velocity profile was segmented into two parts. The lower segment is interpreted to reflect the friction induced by the superimposed small dunes, whereas the upper segment is likely attributed to the combined effect of primary dunes and superimposed small dunes. Moreover, as the lee-side angles of superimposed small dunes fluctuate around 10°, the increase of flow velocity could increase the possibility of generating Flow separation zone (FSZ), consequently resulting in a further effect on flow structure. The correlation between flow resistance and flow strength displays that both the shear stress and roughness length over relatively larger dunes in the deep water are more sensitive to the mean velocity. Low-angle dunes (LADs) with lee-side less than 3° have no effect on flow structure, while for LADs with lee-side angle between 3° and 5°, the roughness length related to bedform is positively correlated to flow velocity, when flow velocity is lower than the equilibrium flow velocity. This implies that the flow expansion over LADs is sensitive to both the lee face and flow strength, thereby affecting LADs' evolution. Furthermore, the bedload transport calculated from empirical equations shows a satisfactory agreement with that estimated from bedform evolution. This result implies that bedload transport induced by shear stress related to LADs’ form is little in this survey area. Besides, in the migrating progress of LADs, their deformation is innegligible and deformation-related bedload transport could occupy nearly 30% of total bedload transport. This finding is significant to studies on migration and deformation of low-angle dunes, and could support studies of numerical modelling, especially on geomorphic module.

This study analyzed bathymetric changes of the 77-km Yangtze River Estuary in China over the past ten years in order to understand the impacts of recent human activities on the estuary of a large alluvial river. Morphological changes were assessed by analyzing digitized bathymetric data of the estuarine channels from 2002 to 2013. Additionally, multi-beam bathymetric measurements made in 2012, 2014 and 2015 were utilized to investigate microtophographic bedforms of the lower reach of the estuary. Our results showed that the middle and upper reaches of the Yangtze River Estuary experienced substantial channel bed erosion in the past 10 years, and that the recent human activities have contributed to the change. These included the construction of a 70 km2 reservoir along the Yangtze River Estuary, the Qingcaosha Reservoir, for drinking water supply for the City of Shanghai, which has caused progressive bed erosion in the North Channel. The net volume of channel erosion in the Hengsha Passage from 2002 to 2013 was 0.86 × 108 m3. A large amount of the eroded sediment was trapped downstream, causing overall accretion in the upper reach of the North Passage. The middle and upper reaches of the South Passage also experienced intense erosion (0.45 × 108 m3) in the past ten years, while high accretion occurred in the lower reach because of the Deepening Waterway Project. The channel dredging left a large range of dredging marks and hollows in the North Passage. The increasing saltwater intrusion found in the Yangtze River Estuary may have been a consequence of either dredging or erosion, or both combined.

The response of subaqueous dunes to the variation of riverine sediment supply in an estuary is rarely reported. Five sets of field measurements of bed sediments and topography were made in the Changjiang estuary, China during dry and flood seasons for the past 9 years. Results show that dunes were mainly three-dimensional (3D) during flood seasons and two-dimensional (2D) dunes during dry seasons. Dunes were also much larger in flood seasons. Dunes mainly occurred within the main channels of the estuary although they extended 11.7 km seaward of their normal extent in the South Channel during dry seasons from 2002 to 2006. This extension of the dune field is likely due to a sharp decline of sediment discharge caused by large reservoirs such as Three Gorges.