River bed evolution due to channel expansion: general behaviour and application to a case study (Kugart River, Kyrgyz Republic)

Abstract

We study the effect of spatial variations of river width on bed aggradation and degradation processes, making use of a one-dimensional numerical model of channel morphodynamics. We refer to a peculiar case, the downstream reach of the Kugart River (Kyrgyzstan). The river has been partly channelized in the recent past with the aim of reducing the flooding risk for the surrounding villages; the consequent reduction of channel width in some reaches was also expected to improve channel conveyance with respect to the high sediment load produced in the upper river basin. The resulting longitudinal sequence of relatively sharp channel expansions and contractions has, however, triggered rapid siltation rates, especially in the narrowest reaches. This motivated the application of a 1-D numerical model of river morphodynamics.Abrupt channel expansions are found to be the main driving forces for aggrading processes, which may extend for long distances from where they are generated. In order to obtain a thorough understanding of the morphodynamics of channel expansions, we first apply the model to simple test cases. This allows us to characterize the basic features of the problem and the dependence of bed evolution on the upstream Froude number Fr and on the expansion ratio r b , which are the most relevant controlling parameters. We invariably find that deposition occurs in expansion regions with bed aggradation observed both upstream and downstream. The deposition prism progressively increases its height and lengthens both in the upstream and downstream directions. The deposition process is particularly intense, in terms of deposition prism height, in super-critical conditions. Moreover, it is found that higher values of Fr strongly reduce the time scale of morphological processes and faster deposition rates are further facilitated by abrupt expansions.The present outcomes are relevant for assessing the expected altimetric response of river bed to the implementation of localized channelization works and to local river widening, a practise which is increasingly being employed within river restoration projects, with the aim of enhancing habitat diversity.

Channelization of a large Alpine river: what is left of its original morphodynamics?

“…The lower panel of (a) shows an actual example of such downstream morphological variability observed in the Adige River downstream of the confluence with the Isarco River (from Nowack historical map, lower panel). (c) Examples of channelized rivers showing different morphological responses to channelization: 1 Kugart River, near Jalal‐Abad, Kyrgyz Republic (Siviglia et al ., ); 2 Alpine Rhine close to Vaduz (Liechtenstein); 3 Adige near Auer/Ora in South Tyrol, Italy. Flow is from left to right.…”

“…Most common effects of channelization are channel narrowing, bed-level lowering, and simplification of the channel morphology (e.g. from multithread to single thread channel, Hohensinner et al, 2004;Zawiejska and Wyzga, 2010), but aggradation has been documented in some cases (Siviglia et al, 2008;Davies et al, 2013). Existing studies suggest that: (i) predicting morphological effects of channelization in gravel-bed rivers is not straightforward; and (ii) channel width is a key parameter controlling river morphodynamics (Garcia Lugo et al, 2015).…”

Biomorphodynamics of alternate bars in a channelized, regulated river: An integrated historical and modelling analysis

“…Petts, 1989) and is currently being widely implemented in many developing countries (e.g. Siviglia et al, 2008). The motivation for embankment construction varies from land reclamation to flood protection but it is almost invariably accompanied by a marked decrease in river channel width.…”

Eco‐hydraulic modelling of the interactions between hydropeaking and river morphology

“…The rationale behind these measures is that giving ‘more room to the river’ (Rohde et al , ) is normally expected to improve the health of river systems at least locally, because channel width represents a fundamental control on river morphodynamics (e.g. Siviglia et al , ; Crosato and Mosselman, ). Widening is then expected to promote self‐formed morphodynamics, leading to enhanced morphological diversity, and turn into more local hydraulic diversity (e.g.…”