Experimental study on the process of neck cutoff and channel adjustment in a highly sinuous meander under constant discharges

Neck cutoff is an essential process limiting long-term evolution of meandering rivers, particularly the highly sinuous ones. Yet, this short-term process is extremely difficult to be replicated in laboratory flumes or observed in nature. Here, we reproduced this process on the mobile bed (non-cohesive sand with a median size of 0.327 mm) in a laboratory flume (25 m long, 6 m wide, 0.4 m deep) by reducing at the 1/2500 scale the current planform of the Qigongling Bend (centerline length 13 km, channel width 1.2 km, and neck width 0.55 km) in the middle Yangtze River with geometric similarity. In five runs with different constant input discharges, hydraulic parameters (water depth, surface velocity, and slope), bank line change, and riverbed topography were measured by flow meter and point gauges; and bank line migration and neck cutoff process were captured by six overhead cameras mounted atop the flume. By analyzing the neck cutoff process, development of the cutoff channel, and adjustment of the old channel to cutoff, we found that (i) bank erosion around the upstream and downstream channel segments of the neck reduced its distance, subsequently increased downward water head gradient on both sides of the neck inducing the occurrence of neck cutoff in a short time period; (ii) the width of the new cutoff channel increased quickly after the occurrence of neck cutoff because the increased local slope generated a higher unit stream power in the cutoff channel; and (iii) neck cutoff significantly strengthened bank erosion and channel widening in both upstream and downstream channels as it is a gradual process compared with chute cutoff. These results indicated that bank revetment around the Qigongling Bend should be carried out to prevent the abrupt occurrence of neck cutoff in future.