Morphodynamic processes controlling neck cutoff Link
A salient feature of neck cutoff is that it occurs after the meander bend experiences a long-term evolution. As such, neck cutoff is very hard to observe in reality. Perhaps the closest direct observation of neck cutoff was in River Bollin and River Dane, northwest England when two neck cutoffs occurred in November, 1980 and March, 1987, respectively (Hooke, 1995, Geomorphology). Since then, the studies on neck cutoff are primarily based on identification of neck cutoff from remotely sensed images and field measurements years before or after their occurrence. A big challenge is to capture physical processes around the time when neck cutoff occurs for better understanding mechanisms of its occurrence and impact on river migration.
Our research on this topic has been focusing on various tributaries of the Upper Yellow River, the Black River, located in the Zoige basin. By artificially creating a neck cutoff in a bend upstream of the Black River, we measured water discharges and channel cross section profiles over multiple years. Our analyses indicated that channel adjustment immediately after neck cutoff in this meandering river is similar to that of meanders in many other regions (see details in Li et al, 2019).
By performing flume experiments in a highly sinuous meandering channel constructed in a laboratory, we reproduced the physical processes during the last stage of neck cutoff and quantified these processes (see details in Li et al, 2019). Later, we examined these processes under the influence of vegetation on channel banks and proposed empirical equations that can predict the time periods needed for the occurrence of neck cutoff under different hydrological and vegetation conditions (see details in Li et al, 2021). Here is an example of neck cutoff process.
These results were from laboratory experiments, which are not exactly the same as what happened in natural meandering rivers. We then explored the evolution process of neck cutoff in a highly sinuous bend based on our multi-year field observation and measurements and image analysis. By extending our analysis to one other bend with the occurrence of neck cutoff and twelve other bends close to neck cutoff, we developed a general empirical equation that characterize the time period needed for neck cutoff and discovered that it takes about 50 to 110 years for a bend from when its channel width is close to the neck width to the occurrence of neck cutoff, which is much longer than generally thought. We will continue this investigation by performing more intensive field measurements, and photogrammetric and modeling analysis.
