**Maurizio Tavelli \*,†, Sebastiano Piccolroaz †, Giulia Stradiotti †, Giuseppe Roberto Pisaturo and Maurizio Righetti**

Faculty of Science and Technology, Free University of Bozen-Bolzano, Universitätsplatz - Piazza Università 5, 39100 Bozen-Bolzano, Italy; spiccolroaz@unibz.it (S.P.); gstradiotti@unibz.it (G.S.); gpisaturo@unibz.it (G.R.P.); mrighetti@unibz.it (M.R.)


Received: 14 December 2019; Accepted: 18 February 2020; Published: 3 March 2020

**Abstract:** The selective trapping and erosion of fine particles that occur in a gravel bed river have important consequences for its stream ecology, water quality, and overall sediment budgeting. This is particularly relevant in water bodies that experience periodic alternation between sediment supply-limited conditions and high sediment loads, such as downstream from a dam. While experimental efforts have been spent to investigate fine sediment erosion and transport in gravel bed rivers, a comprehensive overview of the leading processes is hampered by the difficulties in performing flow field measurements below the gravel crest level. In this work, a new two-dimensional, semi-implicit numerical scheme for the solution of the Navier-Stokes equations in the presence of deposited and erodible sediment is presented, and tested against analytical solutions and performing numerical tests. The scheme is mass-conservative, computationally efficient, and allows for a fine discretization of the computational domain. Overall, this makes the model suitable to appreciate small-scales phenomena such as inter-grain circulation cells, thus offering a valid alternative to evaluate the shear stress distribution, on which erosion and transport processes depend, compared to traditional experimental approaches. In this work, we present proof-of-concept of the proposed model, while future research will focus on its extension to a three-dimensional and parallelized version, and on its application to real case studies.

**Keywords:** sediment transport; sediment entrainment; clogging; colmation; numerical modeling
