Alluviation of a side-channel by bed material load. Field measurements and modelling

Abstract

In this research, we explore the alluviation process of an artificial side-channel built in August 2015 and designed to alleviate bank erosion risks on the main river. The study area is located on the lower Gállego River, a meandering-wandering gravel-bed river draining the Southern Pyrenees. This river reach is still dynamic (i.e. chutes, meander migration and other riverine processes are often observed during floods) despite the presence of three dams in the upper catchment, encroachments and extensive mining during the 1960s–80s that led to severe incision. In this research, we focus on the effects of bankfull-like flood events (Qpeak = 354 m3/s) between February 2016 and February 2017, which alluviated 62% of the side-channel. This research aims at finding the physical mechanisms that explain the alluviation process and at putting them in the general context of the current geomorphic trend of the Gállego River in response to past and ongoing impacts.The causes of the alluviation are studied through collecting and analysing field data, which in turn, are used as input in a novel modelling strategy that includes one-dimensional hydraulic modelling, semi-empirical expressions to obtain the sediment supply entering the side-channel system and a morphodynamic model able to deal with granular sediment mixtures in compound channels. The analyses of the numerical results along with the recent history of impacts and general channel pattern adjustments help us understand about the morphodynamics mechanisms responsible for the alluviation of the side-channel.