Modelling River Dunes in the Rhine Branches Under Extreme Floods

Satomi Yamaguchi, Sanjay Giri, Mohamed Nabi, Jonathan Nelson, Yasuyuki Shimizu

Tuesday 30 june 2015

13:00 - 13:03h at Oceania (level 0)

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

Parallel session: Poster pitches: 5B. Sediment - River

The Waal branch of Rhine system is considered to be an important fairway. The flood events and extensive navigation cause significant morphological changes, which create difficulties for safe and efficient navigation particularly during low water period. Therefore, it is important to predict the bed level changes, particularly evolution of bed forms during the flood event including both the high water and low water periods. Besides this, it is also important to understand the bed form evolution process and resulting flow resistance during extreme floods in order to quantify their effect on water levels. Many attempts have been made to improve both understanding and predictive capability of bed form evolution, transition and associated resistance under varying flow conditions. The interaction among the flow-field, bed forms and sediment transport is quite complex and difficult to capture in simple models. Giri and Shimizu (2006) and Giri et al. (2014) made some noticeable efforts to numerically replicate the dune formation and evolution processes. They proposed a morphodynamic model that successfully reproduces fluid and bed form dynamics in a coupled manner under arbitrary steady or unsteady flow condition. In this study, we have addressed the issue associated with bed form evolution process during floods including some test cases under extreme condition in the Rhine branches, particularly in the river Waal. At first, we analyzed the observed data during moderate floods in the Waal. The bed level data were measured during two different flood events in 1997 and 1998 respectively. A vertical two-dimensional morphodynamic model with a free-surface flow condition, which includes equilibrium, non-equilibrium bed load transport as well as suspended sediment transport modules, was used to replicate the bed form evolution processes under the condition of varying flows. The boundary condition for vertical two-dimensional model was extracted from calibrated hydrodynamic models (WAQUA and Delft3D). A preliminary result shows that bed forms are present even during extreme (design) condition. This study reveals the capability of a numerical model, which can be applied to predict real-world river dunes and drag evolution with hysteresis effects in physics based manner.