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On the Long-Term Evolution of Alluvial Rivers

Author(s): Michael Nones, Arianna Varrani

Linked Author(s): Michael Nones, Arianna Varrani

Keywords: Alluvial rivers; Quasi-equilibrium conditions; Long-term modelling; Response time;

Abstract: Following appreciable variations of boundary conditions due to natural (e.g., climate change, extreme events, earthquakes) or anthropogenic (e.g., water extraction, damming, sediment mining) causes, alluvial rivers may experience long-term morphological changes, eventually affecting large portions of their watersheds.
Depending on the extent of the affected areas and the rapidity of the response, countermeasures are necessarily different, substantially falling into three categories. Relatively concentrated changes in time and space require a direct contrast of their causes at the local level. For more persistent and large variations, possible mitigation or compensation measures of negative effects may be considered. To contrast perturbations that can propagate over extremely long distances lasting for a very prolonged time, the only feasible strategy is preparing the system to progressively adapt to its final (quasi-equilibrium) conditions.
To evaluate this latter kind of evolution, a synthetic parameter of each river system can be introduced, aiming to characterise the basin beside the classical topographical, morphometric and hydrological parameters. In this respect, many researchers introduced the so-called “response time”, which could be defined as the time required by the watercourse to damp the perturbation applied at the upstream end of the river to a prescribed fraction.
As observed in the literature, for large rivers the response time spans between 103–105 years, with a large variability depending on the approach adopted and the hypotheses used to schematize the river basin. The present work compares the response times of fourteen large rivers worldwide, applying five different approaches to evaluate the long-term evolution of these watercourses. The first three approaches adopt diffusive models to describe the adaptation time scale, while the latter two use simplified analytical and numerical models (1D and 0D) to simulate the time required by a watercourse to adjust towards its quasi-equilibrium conditions, given an initial fixed perturbation.
Comparing the different approaches and the equations used to define the response time of large rivers, the study discusses the applicability field of each research, showing a large variability due to the different mechanisms accounted for. The analysis of the response time of alluvial watercourses could be of great importance for river engineers and geomorphologists to deal with the long-term evolution of watercourses, profile concavity and bottom fining, as well as to water managers in the energy sector for evaluating the prospected effects of planned impoundments or the opportunity of their removal.

DOI: https://doi.org/10.3850/38WC092019-0424

Year: 2019

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