Nonlinear modeling of river dunes: Insights in long-term evolution of dune dimensions and form roughness

River dunes are large-scale (primary) bed patterns commonly occurring worldwide. Capturing these features in process-based morphodynamic models is challenging, partly because of steep gradients at the lee side of the bedform. Here, we present a new morphodynamic model with a hydrodynamic module (solved within OpenFOAM) that is capable of capturing lee side effects such as flow separation, and with a sediment transport formulation that suppresses steep lee slopes. Model results suggest that river dunes develop as free instabilities of the flat bed, characterized by initial exponential growth. After the initial phase, dunes reach a quasi-equilibrium, with the wavenumber of the dominant topographic mode decreasing over time; this holds for a range of parameter settings. Furthermore, we show that the spatially averaged water depth – a proxy for roughness – increases with about 3–5 %; the effective roughness length increases with about 50–100 %. • Our new nonlinear morphodynamic model of river dunes includes non-hydrostatic effects • We model the formation of an irregular and migrating dune pattern • Dune wavelengths decrease for smaller water depths • Modeled form roughness is of the same order of magnitude as the grain roughness