Bedform characteristics during falling flood stage and morphodynamic interpretation of the middle-lower Changjiang (Yangtze) River channel, China

A survey was conducted to examine river bedforms using side-scan sonar, sub-bottom profiler and an Acoustic Doppler Profiler in the middle-lower (mid-lower) Changjiang (Yangtze) River (downstream of Wuhan), China, during August 14-20, 2003 when the discharge was 27,000-36,000m ³s ^-1. The results show bedforms of a range of sizes occur in the channel and their heights and lengths follow an exponential correlation below Flemming's maximum line. Not all bedforms were formed under the flow conditions experienced during the survey, as shown by comparison to three existing models in the literature. Small-scale bedforms with a length <60m, defined as flat bed, mega-ripple and small dune in the present study, are mostly in equilibrium and are probably formed under the existing flow conditions. The large dunes with a length of >60m are not in equilibrium and are believed to be the relicts of previous large discharges now undergoing adjustment. Different bedform types occur in the five river reaches in this section of the Changjiang (Wuhan-Tianjiazhen, Tianjiazhen-Anqing, Anqing-Maanshan, Maanshan-Jiangyin and Jiangyin-estuary downstream). In the narrow and single channel reaches above Anqing, large/small dunes are limited to the channel with higher water surface slope where stream power is strong enough for riverbed sediments to be moved frequently. In the downstream reaches between Anqing and Jiangyin, characterized by anabranching channels, small-scale bedforms superimposed on large dunes occur widely due to intensive riverbed sediment transport from frequent diverging/converging flow during both floods and low-flow periods. Mega-ripples and flat beds prevail in the lowermost channel below Jiangyin where wider cross-sections result in dispersed stream power.