Spin-up phenomenon in morphodynamic modelling

Despite the substantial progress in last decades in modelling short-term hydrodynamic and sediment transport processes as well as long-term (centuries) morphological development, the ability of morphodynamic models to simulate medium-term (decades) development of estuaries is still very limited. One of the causes for the limitation is the problem of spin up. Many application of morphodynamic models show that at the beginning of the model simulation model results are unrealistic, and only after a certain period the model gives meaningful results. A extreme example is presented by Dam et al. (2016), who simulated the morphological developments of the Western Scheldt estuary (The Netherlands) for decades to more than a century. They concluded that the performance of the model, expressed in the Brier Skill Score (BSS), first decreases and then increases. The model results are "good" only after many decades. In this study, we aim to obtain more insight into the problem. We try to answer the following questions: x What are the causes of the spin-up problem? x What is the time scale of the spin-up? x What can we do to avoid or minimise the problem?