TIDAL BORE IN THE NORTH BRANCH OF THE CHANGJIANG ESTUARY

摘要

The development, characteristics and causes of the tidal bore in the North Branch of the Changjiang Estuary are expounded, based on in-situ survey, measured and relative historical data. When the velocity of flood current exceeds the tidal wave celerity (the supercritial flow occurs), the water level abruptly jumps and a tidal bore is formed. The Froude number can be used to determine whether a tidal bore will form or not. A analysis indicates that narrowing channel increases flood current velocity and sloping riverbed slows down wave celerity. The North Branch channel has been narrowed and its bed has been silted because of natural processes and artificial reclamation in recent years, which are the main causes of frequent and strong bore occurrence. Furthermore the observed data in April 2001 when the tidal bore occurred at the Qinglong Harbor indicated that a tidal bore was produced when the Froude number F_r > 1. The converging funnel shape and submerged sand bars are necessary conditions for the formation of a bore. The Froude number begins greater than 1 at Lingdianxi (50 km upstream from the mouth) for either maximum or minimum bores, which indicates Lingdianxi is the initial site of North Branch bores.