Riverbed Micromorphology of the Yangtze River Estuary, China

Abstract

Dunes are present in nearly all fluvial channels and are vital in understanding sediment transport, deposition, and flow conditions during floods of rivers and estuaries. This information is pertinent for helping developing management practices to reduce risks in river transportation and engineering. Although a few recent studies have investigated the micromorphology of a portion of the Yangtze River estuary in China, our understanding of dune development in this large estuary is incomplete. It is also poorly understood how the development and characteristics of these dunes have been associated with human activities in the upper reach of the Yangtze River and two large-scale engineering projects in the estuarine zone. This study analyzed the feature in micromorphology of the entire Yangtze River estuary bed over the past three years and assessed the morphological response of the dunes to recent human activities. In 2012, 2014, and 2015, multi-beam bathymetric measurements were conducted on the channel surface of the Yangtze River estuary. The images were analyzed to characterize the subaqueous dunes and detect their changes over time. Bottom sediment samples were collected for grain size analysis to assess the physical properties of the dunes. We found that dunes in the Yangtze River estuary can be classified in four major classes: very large dunes, large dunes, medium dunes, and small dunes. Large dunes were predominant, amounting to 51.5%. There was a large area of dunes developed in the middle and upper reaches of the Yangtze River estuary and in the Hengsha Passage. A small area of dunes was observed for the first time in the turbidity maximum zone of the Yangtze River estuary. These dunes varied from 0.12 to 3.12 m in height with a wide range of wavelength from 2.83 to 127.89 m, yielding a range in height to wavelength of 0.003-0.136. Sharp leeside slope angles suggest that the steep slopes of asymmetrical dunes in the middle and upper reaches, and the turbidity maximum zone of the Yangtze River estuary face predominantly towards tides because of the ebb-dominated currents. Sharp windward slope angles in the lower reach of the North Passage show the influence of flood-dominated currents on dunes. It is likely that the scale of dunes will increase in the future in the South Channel because of a sharp decline of sediment discharge caused by recent human activities.

Study on the Spillover of Sediment during Typical Tidal Processes in the Yangtze Estuary Using a High-Resolution Numerical Model

“…Overall, for the North Branch, the deposition at the entrance reach will facilitate the shrinkage of its upper reach and further prevent upstream inflows from entering during the ebbtide duration. This conclusion is consistent with field observations and former studies [5,30,[35][36][37][38].…”

“…With three-level bifurcations and tens of islands or shoals, the Yangtze Estuary has complex river regimes [30]. The common resolution of bathymetry graphs, for an accurate description of the local river regimes of the Yangtze Estuary, is listed in Table 1.…”

The morphology of fluvial‐tidal dunes: Lower Columbia River, Oregon/Washington, USA

“…Three modern environments have received the most attention regarding dune morphology: (i) relatively deep (H ≥ 2.5 m; Bradley & Venditti, 2017) rivers (e.g., Bradley & Venditti, 2017;Cisneros et al, 2020;de Almeida et al, 2016;Galeazzi et al, 2018;Harbor, 1998;Knox & Latrubesse, 2016;Kostaschuk & Venditti, 2019;Nittrouer et al, 2008Nittrouer et al, , 2011Sambrook Smith et al, 2013), (ii) shallow marine combined flow wave-tidal settings (Barnard et al, 2011;Carle & Hill, 2009;Ernstsen et al, 2010;Fraccascia et al, 2016;Lefebvre et al, 2011;Wu et al, 2016), and (iii) meso-to macro-tidal estuaries or deltas (e.g., Aliotta & Perillo, 1987;Dalrymple, 1984;Dalrymple et al, 1978;Dalrymple & Rhodes, 1995;Davis & Flemming, 1991;Elliot & Gardiner, 1981;G omez et al, 2010;Harris, 1988;Hendershot et al, 2016;Langhorne, 1973;Sherwood & Creager, 1990;Wright et al, 1973).…”

A Case Study: Response Mechanics of Irregular Rotational Tidal Flows to Outlet Regulation in Yangtze Estuary

“…With three-level bifurcations and tens of islands or shoals, the Yangtze Estuary has complex river regimes [22]. The common resolution of bathymetry graphs, for an accurate description of the local river regimes of the Yangtze Estuary, is listed in Table 1.…”