Processes of neck cutoff and channel adjustment affected by seeding herbaceous vegetation and variable discharges

Although the inhibition effect of riparian vegetation on bank erosion and lateral migration of meandering rivers is well studied, its detailed role in the process of neck cutoff is largely unknown. We examined this role using a highly sinuous meandering channel created in a laboratory flume (25 m 6 m 0.4 m) that has a mobile bed and includes seven bends with the narrowest neck of 0.22 m, covered by dense herbaceous vegetation (i.e., Festuca elata) seeded 10 days before each of three experiments that had different vegetation density and discharge arrangements. We depicted the processes of neck reduction by examining temporal changes of the narrowest neck width and planform of the bend that had neck cutoff. We characterized development of the new cutoff channel by measuring temporal changes of the mean channel width and its final width/depth ratio (). We also investigated the upstream and downstream responses to neck cutoff by tracking the temporal changes of their mean slopes and . Our results revealed that (1) herbaceous vegetation can significantly extend the period of neck narrowing process such that neck cutoff may still take a long time even after neck width is about 0.4 of the mean channel width; (2) higher variable discharges only have limited impact on shortening this period; (3) neck cutoff is triggered by seepage flow that is incapable of generating sediment pulses and thus the morphological adjustment of the upstream and downstream reaches are mainly caused by changes of channel hydraulics rather than sediment deposition as in the case of chute cutoff. Our successful reproduction of neck cutoff shows a new way of replicating neck cutoff in flume experiments and our findings provide new insight into understanding processes of neck cutoff in natural highly sinuous meandering rivers.