Computation of bed shear stresses in natural torrential channels using smooth particle hydrodynamics

Elvira Džebo, Dušan Žagar, Simon Rusjan, Matjaž Mikoš, Matjaž Četina

Thursday 2 july 2015

17:30 - 17:33h at Amazon (level 1)

Themes: (T) Water engineering, (ST) River and coastal engineering, Poster pitches

Parallel session: Poster pitches: 13B. River Engineering

Flow in natural mountain torrents is very complex and typically characterised by non-linearity due to high roughness, steepness, wide range in torrent sediment sizes, presence of large woody debris, etc. Such flow is highly turbulent and difficult to describe with Eulerian grid-based methods. Smoothed Particle Hydrodynamics (SPH) method is a suitable Lagrangian method for modelling flow in torrential streams. Although developed primarily for simulation of astrophysicalopen boundary problems, the use of method is growing rapidly in other fields of research, too. Bed shear stress is the key factor in studies of torrential streams. Estimation of shear stresses is necessary to determine bed resistance which highly affects the accuracy of the results and stability of the computational procedure and for evaluation of sediment transport rates. Although numerous methods are being used for estimating bed shear stresses, only a few are applicable with natural torrents. In our study, we propose a new approach for calculating bed shear stresses in steep channels, natural streams and mountain torrents by using SPH method. We expected that our approach will be more useful for computing bed shear stresses in natural torrential channels than conventional methods. In the proposed equation, bed shear stress depends on (water) particle size, wall turbulent viscosity, water density, velocity of particles and the smoothing kernel. Bed shear stress can be computed at any chosen point. Particles within twice the distance of smoothing kernel from the chosen point influence on the value of bed shear stress. A series of steady flow simulations in 10 m long channel have been carried out and bed shear stresses with new equation were computed and verified using data of other method for estimating bed shear stresses. The channel slope was 2 %, values of particle size were between 0.025 - 0.1 m and channels width was between 0.2 – 0.6 m. Finally, the proposed approach was used on a real case study (the Kuzlovec torrent). The Kuzlovec torrent has their source in mountain forest. Bed shear stresses in the Kuzlovec torrent were estimated using a new technique and computed shear stresses are satisfactory.