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

The goal of this study is to quantify morphodynamic roles of riparian vegetation and variable discharges in the process of neck cutoff, which is difficult to determine in natural meandering rivers due to the prolonged process and unpredictable occurrence of neck cutoffs. We achieved the goal in a highly sinuous flume channel (25 m × 6 m × 0.4 m) that has a mobile bed and includes seven bends with the narrowest neck of 0.22 m. Its banks and floodplain were 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. They are the first set of experiments of achieving neck cutoffs in a laboratory flume channel with vegetation. We examined temporal changes of the narrowest neck width and planform of the bend that had neck cutoff, measured temporal changes of the mean channel width and its final width/depth ratio, and tracked the temporal changes of their mean slopes and width/depth ratio. 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 experiments show 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.