Morphological characteristics and evolution processes of sharp bends in the Lower Yellow River

Abstract

Sharp bends in the Lower Yellow River (LYR) remain highly active under the intensive river regulation works, threatening the safety of levees and riparian villages. Based on remote sensing images and hydrological data, the evolution processes of sharp bends in the LYR were quantitatively investigated. A total of 16 sharp bends were identified during the period 1972–2019 and some of these bends were located in local highly active reaches. These sharp bends had highly sinuous planform morphology and deformed rapidly, with the curvature coefficients, amplitudes and migration rates ranging between 1.60–4.18, 3.3–6.7 km and 255–2633 m/a, respectively. A high occurrence frequency of bend cutoff was observed in the braided reach of the LYR. Cutoffs caused by ditch erosion and main flow scouring accounted for about 80% of the total cutoff events, implying the import roles played by the high floods in reshaping the channel planform in the study reach. A series of cutoffs occurred in a local reach in just a year, shortening the channel length by 76% and thus transforming the highly curved channel into a straight channel. The incompatibility between the regulation works and the changed flow and sediment conditions, has led to the inefficiency of these works in limiting the sharp bend evolution in local reaches, due to upstream damming and water and soil conservations. Some of these works even acted as stable controlling points and directly contributed to the development of distorted bends.