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To understand the influence of major water conservation projects on river channels and shorelines in the middle and lower reaches of the Yangtze River, current study comprehensively analyzed the historical underwater topography and water level data, field measurements of main channel, and laboratory testing. Some new progresses have been made. Firstly, a multi-mode sensor system is innovatively constructed to integrate the measurements of land and underwater hydrodynamic, sedimentary, and geomorphic characteristics and data acquisition. Secondly, it is found that the main channel of the Yangtze River experienced strong erosion, arc collapse, and strip collapse. Thirdly, the suspended sand and bed sand were coarsened. The resistance of the river bed declined, forming the erosional-type “chain bead”-shaped sand wave. The piers of the Yangtze River Bridges in Nanjing, Dashengguan, and Tongling were subjected to serious erosion. Finally, the tidal limit obviously moved up, and the geomorphology of the river changed significantly. From these findings, we put forward relevant suggestions and countermeasures. These outcomes are of great significance to the protection and restoration of the bank and beach of the Yangtze River, the waterway regulation, the coastal flood control, and the maintenance of the piers of the Yangtze River Bridge.

The tidal limit is the key interface indicating whether water levels will be affected by tidal waves, which is of great significance to navigation safety and regional flood control. Due to limitations in research methods, recent changes in the Yangtze River tidal limit, caused by sea level rise and large-scale engineering projects, urgently need to be studied. In this study, spectrum analysis was undertaken on measured water level data from downstream Yangtze River hydrological stations from 2007 to 2016. The bounds of the tidal limit were identified through comparisons between the spectra and red noise curves, and the fluctuation range and characteristics were summarized. The results showed that: (1) During the extremely dry period, when the flow rate at Jiujiang station was about 8440 m3 s−1 , the tidal limit was near Jiujiang; whereas during the flood season, when the flow rate at Jiujiang station was about 66700 m3 s−1, the tidal limit was between Zongyang Sluice and Chikou station. (2) From the upper to lower reach, the effect of the Jiujiang flow rate on the tidal limit weakens, while the effect of the Nanjing tidal range increases. The tidal limit fluctuates under similar flow rates and tidal ranges, and the fluctuation range increases with increasing flow rate and decreasing tidal range. (3) With the continued influence of rising sea levels and construction in river basin estuaries, the tidal limit may move further upstream.

Changes of subaqueous topography in shallow offshore water pose safety risks for embankments, navigation, and ports. This study conducted measurements of subaqueous topography between Datong and Xuliujing in the Yangtze River using a SeaBat 7125 multi-beam echo sounder, and the channel change from 1998 to 2013 was calculated using historical bathymetry data. The study revealed several important results: (1) the overall pattern of changes through the studied stretch of the river was erosion–deposition–erosion. Erosion with a volume 700× 106 m3 occurred in the upper reach, deposition of about 204× 106 m3 occurred in the middle reach, and erosion of about 602× 106 m3 occurred in the lower reach. (2) Dunes are the most common microtopographic feature, accounting for 64.3% of the Datong to Xuliujing reach, followed by erosional topography and flat river topography, accounting for 27.6% and 6.6%, respectively. (3) Human activities have a direct impact on the development of the microtopography. For instance, the mining of sand formed holes on the surface of dunes with lengths of 20–35 m and depths of 3–5 m. We concluded that the overall trend of erosion (net erosion volume of 468× 106 m3) occurred in the study area mainly because of the decreased sediment discharge following the closure of the Three Gorges Dam. However, other human activities were also impact factors of topographic change. Use of embankments and channel management reduced channel width, restricted river meandering, and exacerbated the erosion phenomenon.

Measurements of topography at the segment of bifurcation between the South Channel and North Channel in the Yangtze Estuary were conducted, and a new type of subaqueous dune was discovered. This structure, newly defined as a catenary-bead dune, consists of a catenary dune and its associated elliptical pit bedform. Based on this finding, the nomenclature of “morphology of dune associated with accompanying bedform” is first proposed. The measured data indicate a mean height and wavelength of 1.29 m and 31.89 m, respectively; wavelength/height ratio (L/H) of 14 to 56; and elliptical pits of mean and maximum depth 0.98 m and 1.98 m, respectively. Flow information was obtained using an Acoustic Doppler Current Profile (ADCP), and the bed material components were gathered with a bottom sampler. The results show mean flood and ebb velocities of 0.27 and 0.78 m s-1, respectively, with shorter duration of flood tide than ebb tide. The silt, very fine sand, and fine sand fractions were within the ranges 21.6–23.4%, 28.2–32.2%, and 39.7–41.6%, respectively, revealing complex bed material composition. Water depth at the study site varies from 13 to 17 m. This finding will enrich the study of dunes and provide important data for geomorphological research. Moreover, the results are significant for engineering applications to estuaries.

Recent bathymetric changes in the Changjiang Estuary under the influence of artificial regulation engineerings and basin reservoirs have been analyzed based on the maritime charts since 1997 and recent fieldworks. The results indicate a slight erosion of the channels in the upper and middle estuary, continuing deposition and seaward move of the mouth bar crest and intensifying erosion at the nearshore seabed. It is noteworthy that the morphological evolution caused by intensive human activities dominates over the changes from nature process. First, the riverbes are eroded overall in the South Branch (SB), the South Channel (SC) and the upper and middle reaches of the North Channel (NC). The nearshore seabed outside the river mouth is being eroded slightly, which is attributed to the declining sediment supply from the Changjiang Basin due to the construction of the Three Gorges Dam upstream. The sediment above the seabed is very active and coarsened, meanwhile, sand waves are becoming more distinct. Second, a deposition occurs in the North Brach (NB), the mouth of the NC, the mouth bars of the North Passage (NP) and the South Passage (SP) and especially the main channel of the NP, where it shows a massive siltation after the deep waterway project. The reasons for the recent changes are not only the dynamic structure in estuarine mouth bars, but also the supply of sediment resuspension in a local and offshore area. Meanwhile, the severe erosion and siltation in some reaches is related to the construction of estuarine engineerings. It is indicated that the Changjiang Estuary is gradually self-adjusted and adapting to the varying natural factors and intensive human activities. The study on the mechanism of self-regulation of the recent bathymetric changes in the Changjiang Estuary has important and practical significance.

Riverbed scour of bridge piers can cause rapid loss in foundation strength, leading to sudden bridge collapse. This study used multi-beam echo sounders (Seabat 7125) to map riverbed surrounding the foundations of four major bridges in the lower, middle, and upper reaches of the 700-km Yangtze River Estuary (YRE) during June 2015 and September 2016. The high-resolution data were utilized to analyze the morphology of the bridge scour and the deformation of the wide-area riverbed (i.e., 5-18 km long and 1.3-8.3 km wide). In addition, previous bathymetric measurements collected in 1998, 2009, and 2013 were used to determine riverbed erosion and deposition at the bridge reaches. Our study shows that the scour depth surrounding the bridge foundations progressed up to 4.4-19.0 m in the YRE. Over the past 5-15 years, the total channel erosion in some river reaches was up to 15-17 m, possessing a threat to the bridge safety in the YRE. Tide cycles seemed to have resulted in significant variation in the scour morphology in the lower and middle YRE. In the lower YRE, the riverbed morphology displayed one long erosional ditch on both sides of the bridge foundations and a long-strip siltation area distributed upstream and downstream of the bridge foundations; in the middle YRE, the riverbed morphology only showed erosional morphology surrounding the bridge foundations. Large dunes caused deep cuts and steeper contours in the bridge scour. Furthermore, this study demonstrates that the high-resolution grid model formed by point cloud data of multi-beam echo sounders can clearly display the morphology of the bridge scour in terms of wide areas and that the sonar technique is a very useful tool in the assessment of bridge scours.

Observation data of along-estuary and lateral current velocities over a transect located at the South Channel of the Yangtze estuary was obtained during a spring tide in August 2011. Harmonic analysis was done on the current velocities to get a mean component and a semi-diurnal component. Based on these two components, the driving mechanisms of mean lateral flow and M2 lateral tidal flow are shown and analyzed respectively. The dominant driving force of mean lateral flow is nonlinear advection and that of lateral M2 tidal flow is Coriolis force. The friction plays an important role near the bottom and surface for both lateral mean flow and M2 tidal flow.

The mouth bar in the Yangtze estuarine waterways has a significant influence on navigational transport within the estuary, flood discharge and construction of the Shanghai Port. In this paper the morphological evolution and mechanisms of mouth bar formation of the Yangtze estuarine waterways are studied by analyzing hundreds of years of historical data and the latest profile maps of some or the main mouth bar channels in the Yangtze Estuary. The results are shown as follows: The mouth bars in the North Branch have moved gradually from outside the mouth to the inside and formed a huge sand bar. In the North Channel, the head of the mouth bar has migrated about 30 kilometers downstream, and a channel bar has been developing since 2001. Two mouth bar tops, which always existed in the North Passage, disappeared in 2010. The head of the mouth bar in the South Passage has migrated downstream about 14 km and the number of tops increased at first but is reduced to only one now. According to the results, we can conclude that the evolution of the mouth bars differs depending on their location. In the North Branch it is directly related to large-scale reclamation in Chongming Island, but in the North Passage it has a close relationship with regulation of the Yangtze Estuary Deepwater Channel. However, the evolution of mouth bars in the North Channel and South Passage is not only connected with the Yangtze Estuary Deepwater Channel Regulation Project, but also with the reclamation in the East Hengsha Shoal and the closure of the Qingcaosha Reservoir.

Six bathymetric transect profiles were drawn from the nautical charts of 1942, 1962 and 1992 to show that the nearshore seabed remained unstable during the recent 50 years in the middle channel of the eastern entrance to the Qiongzhou Strait, South China Sea. Our results demonstrate that the multi-year averaged seabed aggradational rate was 25 cm/a and erosion rate was 12.5 cm/a. Lateral migration rate of the sea bedform identified from the historical contours was about 100 m/a in the SE direction. Bedform measurements were made using GPY Shallow Seismic Profiler in 1994 in the study area. The records revealed four types of distinctive bedforms that were composed of fine and medium sands. The average spacing of large and small-scale sand dunes is 416 m and 144 m and the average height remains 8.8 mand 4.9 m. The spatial and temporal equilibrium-range spectra of numerical bedform records were applied to estimate short term celerity of bedform movement. Results indicate that large and small dunes migrated at an average celerity of 0.02 cm/hr eastward and 0.09 cm/hr westward in the calm sea weather, while their celerity can reach 53 cm/hr eastward during typhoon season and is only 0.008 cm/hr westward when NNE winds prevail. The results also show that the larger the temporal and spatial scale is, the smaller the bedform movement celerity appears. On the other hand, the smaller-scale bedform celerity of the present study is much greater than that of flume, empirical and theoretical data, but close to the wind tunnel and field-measured data of similar grain size.