Application of the morphological acceleration factor in fluvial hydraulics.

Benjamin Dewals, Sebastian Erpicum, Pierre Archambeau, Michel Pirotton

Tuesday 30 june 2015

14:50 - 15:05h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) River morphodynamics

Parallel session: 6B. Sediment - River

The morphological acceleration factor (MORFAC) has been used for about two decades to speed-up morphodynamic simulations. It was originally introduced in the context of coastal applications, with the purpose of efficiently grasping the overall morphological effect of a high number of successive tides. The value of this morphological factor has been mainly set by trial and error and little theoretical background is available to identify an optimal value of MORFAC for a given application and a target level of accuracy. Only Li (2010) and Ranasinghe et al. (2011) provide an insight into the influence of the morphological factor on the mathematical and numerical properties of the system of governing equations. They focus however on coastal applications, restricting thus the analysis to relatively low Froude numbers (Fr < 0.6), roughness heights and sediment transport rates (in relative terms). In the present research, we generalize the previous theoretical analyses regarding two main aspects. First, the range of the considered parameters (Froude number, relative roughness, transport rate …) has been considerably extended to cover typical values characterizing fluvial applications such as reservoir sedimentation. Second, we tested the influence of MORFAC for different mathematical formulations of the governing equations in the hydrodynamic model. Indeed, depending on the context of application, models described in existing literature use either the water depth or the free surface elevation as main unknown in the mass conservation for the flow, and either the unit discharge or the depth-averaged velocity as main unknown in the momentum balance. We show here that the optimal value of MORFAC depends greatly on the specific mathematical formulation used and on the flow regime. The theoretical results are tested against numerical experiments.