Impact of embankments for reversing neck cutoff on flow structure in a Zoige meandering river

Cutoff occurrence is a pivotal process of the forward long-term evolution of meandering river. Here a neck cutoff occurred unexpectedly in a highly sinuous bend of a meandering river in the Zoige basin on the Qinghai-Tibet Plateau in July 2018. Nonetheless, for protecting the grassland inside this bend, subsequently two artificial embankments reversed this neck cutoff (backward evolution) in 2018-2019 and strongly affected three-dimensional flow structure according to field surveys using an unmanned aerial vehicle and acoustic Doppler Current Profiler from 2018 to 2021. This rare case for inhibiting natural cutoff remains an unknown geomorphic process, and furthermore the inverse impact of human intervention on an occurred cutoff is still unclear. The artificial earth embankment was breached in the 2019 flood season and left the broken debris at both ends. Soon afterwards the second rockfill embankment was built in the late 2019 to force the flow back to the original bend so far. Some main results are summarized: (i) Flow structure in the new cutoff channel was intensely adjusted in combination with locally increased channel slope by the cutoff and the first earth embankment built in 2019. Conversely, flow velocity and circulation in the upstream straight reach was less affected by neck cutoff and artificial embankments, while the flow velocity in the bend section was obviously adjusted after neck cutoff and two embankments built. The lateral distribution of the maximum velocity and circulation intensity at the apex of the bend are altered. (ii) After the cutoff occurred, the separate zone shifted to the erosion side of the new cutoff channel in 0.3 times channel width in 2019. At the cross-section of the apex, the clockwise circulation was generated with the maximum streamwise velocity close to the outer bank. The maximum streamwise velocity moved to 0.2 times channel width. (iii) The artificial embankment is a driving factor to generate the strong alteration on the bend completeness and hydrodynamic adjustment along the course in this unique case. It is of great importance on understanding the inverse process for implementing engineering measures to restore the original sinuous flow path and sustain an intact meander landscape after a cutoff occurred. Given that the intervention of reversing neck cutoff is a mandatory task required by local people, it is a better choice to wait 2-3 years until the cutoff channel reaching the quasi-equilibrium state.