MORSPEED: a new concept for the speedup of morphological simulations

Numerical prediction of long term morphological evolution in rivers, estuaries and costal areas is computationally very costly. This problem is often tackled adopting morphodynamic upscaling techniques which allows to speedup the bed level evolution. The most popular approach implemented in numerical models is the morphological acceleration factor (MORFAC) (Roelvink, 2006). This is based on the assumption that the bed response to hydrodynamic changes is linear within a single time step and consists in multiplying the sediment fluxes in the Exner equation by a factor Mcs > 1. At present, the choice of the maximum speedup (Mcs) that can be used to accelerate the bed evolution is obtained empirically using mesh dependent criteria (e.g. Ranasinghe et al., 2011). This leads to accelerated bed level configurations in which a clear correspondence between the real and the accelerated time scale is not known. In this study a general criterion for the maximum speedup, which can be obtained under the assumption of linear bed response, is proposed. As a consequence a clear link between real and accelerated time scales can be established. Finally a new speedup concept (MORSPEED) which allows to reach larger values of speedup is also proposed, analyzed and tested.