Tidal currents,bed sediments,and bedforms at the South Branch and the South Channel of the Changjiang (Yangtze) estuary,China: Implications for the ripple-dune transition

Measurements were made of tidal currents, bed sediment particle sizes, and bedform dimensions at the South Branch and the South Channel of the Changjiang estuary, China, during the dry season in 1997 and the flood season in 1998. The near bottom current speed and direction were measured by a mechanical current meter for 10 h in 1997. The near surface current speed and direction were measured by a Current Meter of Endeco/YSI Inc. 174 SSM for 14 h in 1997 and 1998. Nine bed sediment samples were taken and their particle sizes were analyzed with sieves and siphons. The bedforms were nautically detected by an echo sounder and a side scan sonar. Results show that the ebb tides had larger near-bottom and near-surface current speeds and longer durations than the flood tides, in which the former occurred during the flood season in 1998 and not in the dry season in 1997. The bed sediments were composed of coarse silts and very fine sands during the dry season but of fine sands during the flood season. Bedforms were dominated by ebb tidal currents, the height∶length ratios of dunes and lee face angles were low, and heights and lengths were larger during the flood season in 1998. The ebb and flood tidal currents, bed sediment sizes, and dune morphology were largely controlled by the seasonal runoff variations. A new tentative boundary might be proposed for natural dunes in very fine sand with the availability of additional field data in the future.