Long-Term (1986–2018) Evolution of Channel Bars in Response to Combined Effects of Cascade Reservoirs in the Middle Reaches of the Hanjiang River

Abstract

Channel bars are essential landforms and their evolution is crucial to aquatic and riparian biodiversity, river’s water-sediment process, and economic development. With the development of water conservation facilities and hydropower projects, numerous changes have been taken place in hydrological regimes and morphology. There have been many changes on channel bars in the middle reaches of Hanjiang River due to the combined effects of cascade reservoirs. However, little was known about such dynamics and their linkages to cascade dams across the entire downstream area. Using Landsat remote sensing images from 1986–2018 and the threshold binary Otsu extraction method, this study completed comprehensive monitoring of nine mid-channel bars (DX1–DX7, XZ1, and XZ2), and three shoal group (XZ3–XZ5) dynamics. Results showed that the mid-channel bars’ area in the reach from Danjiangkou to Xiangyang (DX) decreased over the past 33 years, with the exception of DX4, while the total area decreased by 23.19%, this channel bars’ area change was mainly influenced by backwater from the Cuijiaying Reservoir with high water level after 2010 (r = −0.93, p < 0.01). The total channel bar area from Xiangyang to Huangzhuang (XZ) decreased by 16.63% from 1986 to 2018. The total channel bar area in XZ had a strong negative correlation with runoff at Huangzhuang hydrologic station (r = −0.79, p < 0.05), which was partly attributed to upstream precipitation according to the high correlation between runoff and precipitation (R2 = 0.65). In general, the DX section was under equilibrium between scouring and deposition compared to downstream Xiangyang, the bars in DX section were mainly affected by water level, and bars in XZ section during 1986–2018 were complicated because it was upstream eroded and downstream deposited. In addition, vegetation cover, revetments, flood events, sand mining, land use, and over-exploitation may cause channel bar area dynamics. Hence, more continuous investigations are suggested to focus on effects of cascade reservoir operation on hydrological regime, as well as the changing morphology of channel bars in the middle reaches of the Hanjiang River.

Use of Sentinel-2 Images for the Detection of Sandbars Along the Lower Vistula

“…DOI: http://dx.doi.org/10.15576/ASP.FC/2020.19.2.23 30 www.acta.urk.edu.pl/pl 1989). Tracking the dynamics of changes is important due to the accumulation of data in order to assess the state of the river and take possible corrective or preventive actions (Zhang et al, 2020).…”

“…Researchers use traditional fi eld methods for this purpose, which, especially in high fl ow conditions, can be very diffi cult due to practical limitations (Nones et al, 2018). They also use historical maps (Scorpio et al, 2018), data obtained from unmanned aircraft (Langhammer et al, 2017), video camera image analysis (Nones et al, 2018), and images accessed remotely via satellites (Zhang et al, 2020;Kuenzer et al, 2019). All such studies need a clear distinction between the water and land surfaces, for which spectral water indices are used.…”

“…The data processing methodology was partly inspired by the work of Zhang et al, (2020). Several valuable comments were also drawn from the work of Robak et al, (2016), in which the water indices were tested to create a water mask in the shortest possible time for crisis management.…”

“…The lower Vistula riverbed is shaped by natural and anthropogenic factors (Łajczak et al, 2006;Kałmykow-Piwińska and Falkowski, 2012). The hydrological conditions expressed by the volume of flow and sediment transport of bed load have the greatest impact on the dynamics of the resulting channel forms (Babiński, 1992;Pieron et al, 2013;Bogucka-Szymalska, 2010;Nones et al, 2018;Zhang et al, 2020). In the case of the lower Vistula, its hydrotechnical regulation in the form of the river training structures is also important (Babiński, 1999).…”