The Long-Term Response of Alternate Bars to the Hydrological Regime

Migrating bars are large-scale, alternate bedforms that often develop in channelized river reaches, as a consequence of an intrinsic instability of the erodible channel bed. Their behavior under steady flow conditions has been widely investigated by means of theoretical, experimental, and numerical models, which revealed that bar formation occurs when the width-to-depth ratio of the channel exceeds a critical threshold value. Conversely, no much information is available about the long-term, average characteristics of alternate bars in the case of a complex flow regime, which makes the width-to-depth ratio highly variable in time. Starting from the state-of-the-art theoretical models of bar dynamics, we propose a novel methodology to determine the long-term bar response to the hydrological river regime and the associated "bar-forming" discharge that, if applied steadily, would produce the same morphological response. We derive a generalized criterion to define whether bars are expected to form and to estimate the long-term bar topography, depending on flow probability density function and channel characteristics (width, slope and sediment size). Our procedure differs from the classical methods to define formative discharge, inasmuch as it accounts for the specific and reversible response of bar topography to the different flow stages that compose the hydrological regime. Application to four different gravel bed reaches in the Alpine region shows the capability of the procedure to interpret remarkably different riverbed morphologies and to provide a reasonable prediction of the observed bar height, thus suggesting its potential to analyze long-term morphological trajectories following hydrological alterations and river restoration projects.