Regime theories in gravel-bed rivers: Models, controlling variables, and applications in disturbed Italian rivers

<p>Downstream hydraulic geometry relationships describe the shape of alluvial channels in terms of bankfull width, flow depth,flow velocity, and channel slope. Recent investigations have stressed the difference in spatial scales associated with thesevariables and thus the time span required for their adjustment after a disturbance. The aim of this study is to explore theconsequences in regime models considering the hypothesis that while channel width and depth adjust quickly to changes in waterand sediment supply, reach slope requires a longer time span. Three theoretical models were applied. One model incorporates anextremal hypothesis (Millar RG. 2005. Theoretical regime equations for mobile gravel-bed rivers with stable banks.Geomorphology 64: 207–220), and the other two are fully physically based (Ikeda S, Parker G, Kimura Y. 1988. Stable widthand depth of straight gravel rivers with heterogeneous bed materials. Water Resources Research 24: 713–722; Parker G, WilcockPR, Paola C, Dietrich W, Pitlick J. 2007. Physical basis for quasi universal relations describing bankfull hydraulic geometry ofsingle-thread gravel-bed rivers. Journal of Geophysical Research 112, DOI: 10.1029/2006JF000549). In order to evaluate theperformance of models introducing the slope as an independent variable, we propose two modifications to previous models.The performance of regime models was tested against published data from 142 river reaches and new hydraulic geometry datafrom gravel-bed rivers in Patagonia (Argentina) and north-eastern Italy.Models that assume slope as a control (Ikeda et al., 1988; or Millar, 2005) predict channel depth and width reasonably well.Parker et al.’s (2007) model improved predictions because it filters the scatter in slope data with a relation slope–discharge. Theextremal hypothesis model of Millar (2005) predicts comparably to the other physically based models.Millar’s model was chosen to describe the recent changes in the Piave and Brenta rivers due to human intervention – mainlyin-channel gravel mining. The change in sediment supply and recovery was estimated for these rivers. This study supports theinterpretation that sediment supply is the key factor guiding morphological changes in these rivers.</p>