Characteristics of high-resolution subaqueous micro-topography in the Jinshan Deep Trough and its implications for riverbed deformation, Hangzhou Bay, China

Subaqueous micro-topographic evolution and riverbed deformation have a direct impact on the safety of engineering projects in rivers, estuaries, and coastal environments. However, the relationship between micro-topographic evolution and riverbed deformation in strong tidal environments requires further study; therefore, this study collected high-resolution bathymetry data of the JinShan Deep Trough (JSDT), Hangzhou Bay, from 2013 to 2017 to interpret this relationship using a multi-beam echo sounder. Velocity data and sediment samples were used to simultaneously analyze the dynamic sedimentary environment. The results show that the strong tidal environment, with a maximum velocity of 1.89 m s−1 and a median riverbed sediment grain size of 9.89–25.21 μm, led to vigorous riverbed deformation in the JSDT. Riverbed deformation occurred via erosion in 2013–2015 (12.18 × 106 m3), deposition in 2015–2016 (2.28 × 106 m3), and erosion in 2016–2017 (4.17 × 106 m3) over an area of 21.64 km2. Overall, the JSDT underwent scouring from 2013 to 2017, corresponding to a total volume of 14.17 × 106 m3 or 1.97 million metric tons of sediment, assuming a bulk density of 1.4 t/m3 for riverbed material. Various types of micro-topography were observed in the JSDT, including scour troughs, collapse landforms, gullies, scarps, dunes, and pockmarks. According to correlation analysis of the development and evolution of micro-topographies and large-scale riverbed deformation, active micro-topographies (including dunes and pockmarks) indicated riverbed deposition, whereas stable micro-topographies (including scour troughs, gullies, scarps, and a smooth riverbed) indicated erosion. Deposition thickness was related to the scale and number of active micro-topographies. Furthermore, a reclamation project changed the flood current, which intensified erosion of the JSDT. Therefore, the development and evolution of micro-topographies can be used as indicators of riverbed deformation trends. These results are also applicable to other estuaries and coasts in similar strong tidal environments.