Lucien Masbernat

Floods effects on rivers morphological changes application to the Medjerda River in Tunisia

Abstract In Tunisia especially in the Medjerda watershed the recurring of floods becoming more remarkable. In order to limit this risk, several studies were performed to examine the Medjerda hydrodynamic. The analysis of results showed that the recurrences of floods at the Medjerda watershed is strongly related to the sediment transport phenomena. Initially, a one dimensional modelling was conducted in order to determine the sediment transport rate, and to visualize the river morphological changes during major floods. In continuity of this work, we will consider a two-dimensional model for predicting the amounts of materials transported by the Medjerda River. The goal is to visualize the Medjerda behaviour during extreme events and morphological changes occurred following the passage of the spectacular flood of January 2003. As a conclusion for this study, a comparative analysis was performed between 1D and 2D models results. The objective of these comparisons is to visualize the benefits and limitations of tested models. The analysis of the results demonstrate that 2D model is able to calculate the flow variation, sediment transport rates, and river morphological changes during extreme events for complicated natural domains with high accuracy comparing with 1D Model.

Langmuir circulations and enhanced turbulence beneath wind-waves

Abstract This paper presents a set of experiments which describes the mutual influences between wind-wave enhanced turbulence and secondary motions like Langmuir circulations (LCs). Different external driven forces were produced in laboratory wind-wave flumes, and mean and turbulent fields were measured for each experimental situation. A (k–e) model coupled to momentum equations are numerically integrated to investigate the influences of a diffusion–dissipation (Diff=e) equilibrium of turbulent kinetic energy (TKE) to represent surface boundary conditions under wind-waves. A new formulation for estimating surface energy is obtained from wave and wind stress parameters, expressed in non-dimensional grounds as k S =0.12(σa/u ∗ ) 2 +C μ −1/2 . The close fit between measurements and numerical results suggests that Craik–Leibovichs equations associated with (k–e) turbulence model provides a good description of flow characteristics. Model issues confirm that secondary motions have remarkable influences over the distribution of momentum and turbulence, indicating that wind-wavy boundary conditions resulting from a balance between diffusion and dissipation of TKE is capable of reproducing the realistic scales of phenomena.

On the influence of longitudinal mean flow over Langmuir circulations

The Craik-Leibovich equations including vortex forcing terms and a pressure gradient force in x-momentum are integrated numerically. Some typical environmental problems with strong longitudinal currents are analysed. Results are compared with Leibovich & Paoluccis stability analysis which allows to identify situations where Langmuir cells exist. An establishing characteristic time for the setting of cells has the same order of magnitude as in field observations. Its variation with Langmuir number (La) and nondimensional depth is pointed out. A scaling analysis of the momentum equations is made for vanishing La. Influence of the longitudinal current intensities over Langmuir circulations structure is showed and confirmed by numerical results : more energetic convective cells are verified for situations where stronger longitudinal velocities are present. Results indicate how the redistribution of the shear stress induced by secondary flows is important for mixing in the water column and for coastal sediment transport.