In this case study we will investigate the formation of alternate bars with the Delft3D-FLOW module and compare the results with linear theory, as described in the ‘conceptual’ tab. The model domain is based on a laboratory flume, in which Zolezzi et al (2005) carried out some experiments to investigate the formation of alternate bars. The flume consisted of a 180° bend and two straight parts, 12 m upstream and 12 m downstream of the bend. The half-width-depth ratio, which is an important parameter for the behaviour of alternate bars is varied by varying the inflowing discharge.
Behaviour of small-amplitude bars
It was assumed by various authors that the behaviour of alternate bars is linear (not dependent on the bar height) if the amplitude of the bar is very small. To check this assumption, we have imposed alternate bars with small amplitude and various lengths. The migration and amplification rate of these bars was monitored by analysing the Delft3D output files with MATLAB.
Influence of the bend
Whether bars develop only downstream of the bend or both upstream and downstream, depends on the width-to-depth ratio (β) of the river. Laboratory experiments by Zolezzi et al. (2005) have shown that, only downstream of the bend non-migrating alternate bars occur under sub-resonant conditions (β < βres), but under super-resonant conditions (β > βres) the non-migrating alternate bars appear both upstream and downstream. To check this observation, we have done simulations with different discharge (thus different β).
ZOLEZZI, G., GUALA, M., TERMINI, D. & SEMINARA, G. 2005. Experimental observations of upstream overdeepening. Journal of Fluid Mechanics, 531, 191-219.