Hydrodynamics over low-angle dunes at the tidal current limit of the Changjiang Estuary

Abstract

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.