Modelling fine sediment transport over an immobile gravel bed

Fine-grained sediment represents a significant component of the total transport load in most fluvial systems around the world that are not limited to the alluvial rivers with sandy beds. A variety of natural or human actions, such as fire, logging, flow diversion, road construction, and urban or agricultural development can increase the supply of sand to a gravel- and cobble-bedded rivers. From hydrodynamics point of view, if the coarsest part of the sediments mixture composing the bed grain size composition cannot be transported, a coarser immobile layer can develop through vertical sorting of grain size fraction. This layer has influence on the grain size transport rate describes qualitatively as a competition between absolute and relative grain size effects. The absolute size effect causes the inherent mobility of sediment grains to decrease with increasing grain size. The relative size effect tends to increase the transport rate of larger grains and decrease the transport rate of smaller grains, characterizing the supply-limitation conditions for the fine fraction of the bed composition. Two-fraction approximation (i.e. sand and gravel/cobble fraction) of widely sorted sediment might capture mixed-size transport dynamics of practical significance is suggested by a number of observations, because the fines content of the bed fs may often be more variable than that of the coarse fraction (i.e. partial mobility condition), and whose passage, intrusion, or removal may be a specific environmental or engineering objective. The problem of sand transport over gravel bed has been addressed in several recent field and laboratory studies. Bed load transport rate and suspension entrainment rate are related, among other, to the sand elevation hs in the gravel framework, because the parameter hs not only controls the amount of volume of sand available to be transported by the flow, but also affects the relative sand coverage with respect to the gravel rough elements. To this regard, experimental studies showed that the function hs controls the geometrical transition from gravel framework (i.e. fs 0.3 - 0.4), depending on the diameter ratio of sand and gravel fraction. Moreover, sand elevation hs is also a measure of the sediment supply-limitation that, in its turns, controls the sand bedform development. A limited volume of bedload sediment leads either to smaller dimensions, the sediment starved bedform or fewer isolated bedforms. Bedform types that are typically associated with partial mobility condition are: sand ribbons, barchanoid dunes, isolated dunes and sediment starved dunes, bedload sheets and low-relif bedforms. The state of knowledge suggests that there has been relatively little attention paid to understand physically which are the hydrodynamics mechanisms that control the sand transport in a gravel bed. The relevance of the present work is mainly in offering a mechanistic tool that can be used to better understand which physical phenomena control the development of sand bedforms when sand is transported over an immobile gravel bed, specifically the present research aims to: - understand physically how the local sand surface elevation hs affects the characteristic spatial scales of the bed with sand level-dependent roughness; - understand physically how the local sand surface elevation hs influences the transport phenomenon of the sand fraction when the gavel framework is at rest; - integrate the analysis of already published work often with different specific goal compared to that stated above; - determine the hydraulics conditions that controls the sand dunes formation when the sand bed elevation hs varies in the gravel framework. On the basis of the above main objectives, the present research propose a conceptual morphodynamic model accounting for the key processes of sand transport over a gravel matrix, taking into account near-bed conditions locally adapted to the evolving sand surface patterns relative to the turbulent near-bed characteristics and to the transport characteristics of fines