3D Numerical Model of Non-uniform Sediment Transport using Lagrangian Method and Bed Load Layer Concept

This study presents a novel and robust 3D numerical model for non-uniform sediment transport. The model consists of (1) the practical Lagrangian method tracking a parcel of bed load particles and (2) bed load layer concept to consider particle size distribution of bed load and bed-material.

Since the present model adopts the “bed load layer concept”, the present model is more robust than the conventional model using the “mixing layer concept”. The conventional “mixing layer concept” defines the sediment mixture at the mixing layer just below the bed surface. The inherent problem of the “mixing layer concept” is that the thickness of the mixing layer strongly influences the numerical results. On the other hands, the “bed load layer concept” treats sediment mixture at the bed load layer which includes sediment exchange among stationary bed, bed load, and suspended load. In the present model, the thickness of the bed load layer is given by the result of the Lagrangian computation. Thus, the present model needs no empirical parameter of mixing later thickness, leading to more robust simulation of non-uniform sediment.

For simulation of water flow, the VOF method is used to capture water surface, and k-omega SST model is used to simulate turbulent flow. The open-sourced CFD software OpenFOAM was utilized to implement the sediment transport model. Note that the model was extended from author’s previous model for uniform sediment transport (Ota et al., 2016).

The present model was validated in a laboratory experiment of sediment transport in a meandering channel. The model simulated very well the bed shape and particle size distributions at the meandering channel. Specifically, the model well reproduced deposited region with finer sediment at inner bank and scoured region with coarsen sediment at outer bank. Besides, the present model allows for tracking the non-uniform sediment in a Lagrangian manner, which becomes very beneficial in the practical view for river engineering (e.g., the effect of sediment replenishment).

Reference:

Ota, K., Sato, T., Nakagawa, H., & Kawaike, K. (2016). Three-dimensional simulation of local scour around a weir-type structure: hybrid euler-lagrange model for bed-material load. Journal of Hydraulic Engineering, 143(4), 04016096.