Numerical modelling of the evolution of a river reach with a complex morphology to help define future sustainable restoration decisions

Abstract

Abstract. The prediction of river morphology evolution is very complicated, especially in the case of mountain and Piedmont rivers with complex morphologies, steep slopes, and heterogeneous grain sizes. The Lac des Gaves (LDG) reach, located within the Gave de Pau River in the Hautes-Pyrénées department, France, has precisely the complex morphological characteristics mentioned above. This reach has gone through severe sediment extractions for over 50 years, leading to the construction of two weirs for riverbed stabilisation. Two large floods resulted in changes in the LDG's hydromorphological characteristics as it went from a single channel river section to a braided river reach. In this study, a 2D hydromorphological model is developed with the TELEMAC-MASCARET system to reproduce the evolution of the channel following a flood that occurred in 2018. The model's validity is assessed by comparing the simulated topographic evolution to the observed one. The results reveal the challenge to choose well-fitted sediment transport equations and friction laws that would make it possible to reproduce such complex morphology. Although the exact localisation of the multiple channels forming the braided nature of the LDG was challenging to reproduce, our model was able to provide reliable volumetric predictions as it reproduces the filling of the LDG correctly. The influence of the two weirs on the river's current and future morphology is also studied. The aim is to provide decision-makers with more reliable predictions to design suitable restoration measures for the LDG reach.