Chute cutoff as a morphological response to stream reconstruction: The possible role of backwater

Abstract

Stream restoration efforts often aim at creating new unconstrained meandering channels without weirs and bank revetments. In reconstructed streams, the initial morphological response of the new streams is often rapid, until a dynamic equilibrium is reached. Here we report on a chute cutoff that occurred within 3 months after realization of a stream restoration project, caused by a plug bar that formed in response to a backwater effect. The temporal evolution of the morphology of both the new and the old channels was monitored over a period of nearly 8 months, including precutoff conditions. The observations can be separated into three stages. Stage 1 is the initial period leading to cutoff vulnerability, stage 2 is the actual cutoff, and stage 3 is the morphological adjustment in response to the cutoff. In stage 1, a plug bar was deposited in one of the channel bends. Hydrodynamic model results show the location of the plug bar coincides with a region where bed shear stress decreased in downstream direction due to backwater. Longitudinal channel bed profiles show that the channel slope decreased soon after channel reconstruction. Hence, sediment from upstream was available to form the plug bar. After the plug bar was deposited, an embayment formed in the floodplain at a location where the former channel was located (stage 2). The former channel was filled with sediment prior to channel construction. It is likely that the sediment at this location was less consolidated, and therefore, prone to erosion. The chute channel continued to incise and widen into the floodplain and, after 6 months, acted as the main channel, conveying the discharge during the majority of time (stage 3). The cutoff channel gradually continued to fill with sediment, from the moment the plug bar formed until the chute channel incised into the floodplain. Sedimentary successions of the deposited material show upward fining, which is in agreement with observations of chute cutoffs in rivers. Although the artificial setting limits the degree in which the observed processes can be projected on natural rivers, the observations prompt to investigate the role of backwater effects in natural chute cutoff initiation.

Meander cutoffs nonlocally accelerate upstream and downstream migration and channel widening

“…Over shorter time scales, cutoffs act as "shot" perturbations [Camporeale et al, 2008] to river morphodynamics by increasing the bed slope and stream power both upstream and downstream [Biedenharn et al, 2000;Hooke, 2004;Jugaru Tiron et al, 2009], injecting downstream pulses of sediment excavated from the floodplain during chute channel formation [Fuller et al, 2003;Zinger et al, 2011], and substantially altering the local channel planform and hydrodynamics [Hooke, 2004;Zinger et al, 2013]. Considerable attention has been given to local cutoff-induced channel response immediately adjacent to and within cutoffs [Hooke, 1995;Fuller et al, 2003;Zinger et al, 2011Zinger et al, , 2013 as well as factors controlling their occurrence [Grenfell et al, 2014;Eekhout and Hoitink, 2015;Słowik, 2016]. However, the spatial and temporal extents to which cutoff perturbations induce nonlocal morphologic change remain unknown, largely due to difficulties of observing morphodynamics over sufficiently large spatial scales and high temporal frequencies to capture changes [Winkley, 1977;Hooke, 1995].…”

Monitoring of avulsion channel evolution and river morphology changes using UAV photogrammetry: Case study of the gravel bed Ondava River in Outer Western Carpathians

“…Other authors have reported significant effects on chute cutoff mechanisms. Constantine et al (2010) consider that flood events influence floodplain overflow and erosion and Eekhout and Hoitink (2015) indicate that in-channel changes affect hydraulic flow and channel sedimentation. Hooke (1995) describes cutoff adjustments in the English Dane and Bollin rivers with rapid sediment accumulation in the cutoff entrance, morphological adjustments of channel and vegetation succession.…”

“…Cutoffs are responsible for significant flow modification, changes in sinuosity and channel morphology and pattern. Eekhout and Hoitink (2015) report the lack of precise determination of chute cutoff mechanism despite extensive research in meandering river systems. Chute cutoff is defined as a new channel formation in the meander neck or inner part of the meander loop during flood events and it is a major planform control element in actively meandering rivers (Constantine et al, 2010).…”

“…Lewis and Lewin (1983) describe five types of cutoff and two commonly recognised main cutoff processes: neck and chute cutoff. Three main mechanisms of new chute cutoff channel creation and initiation are discussed and identified in nature (Constantine et al, 2010;Eekhout & Hoitink, 2015): (1) swale enlargement, (2) headcut extension, and (3) incision of an embayment. Eekhout and Hoitink (2015) report the lack of precise determination of chute cutoff mechanism despite extensive research in meandering river systems.…”

Flow thresholds for leaf retention in hydrodynamic wakes downstream of obstacles

“…Although single‐thread streams in lowland areas often appear to be highly sinuous, they remain virtually fixed in time. Active morphological processes such as the development of alternate bars and chute cut‐off may occur as a response to human measures (Eekhout & Hoitink, ; Eekhout, Hoitink, & Mosselman, ), but after an initial period of adjustment, the streams tend to maintain stable (Eekhout, Fraaije, & Hoitink, ). Eekhout, Hoitink, de Brouwer, and Verdonschot () showed that typical deteriorated lowland streams have cross‐sectional‐averaged flow velocities of 0.08–0.13 m/s and homogeneous bed substrate.…”