INPUT REDUCTION FOR LONG-TERM MORPHODYNAMIC SIMULATIONS

Input reduction is imperative to long-term (> years) morphodynamic simulations to avoid excessive computation times. Here, we discuss the input-reduction framework for wave-dominated coastal settings introduced by Walstra et al. (2013). The framework comprised 4 steps, viz. (1) the selection of the duration of the original (full) time series of wave forcing, (2) the selection of the representative wave conditions, (3) the sequencing of these conditions, and (4) the time span after which the sequence is repeated. In step (2), the chronology of the original series is retained, while that is no longer the case in steps (3) and (4). The framework was applied to two different sites (Noordwijk, Netherlands and Hasaki, Japan) with multiple nearshore sandbars but contrasting long-term offshore-directed behavior: at Noordwijk the offshore migration is gradual and not coupled to individual storms, while at Hasaki the offshore migration is more episodic, and wave chronology appears to control long-term evolution. The performance of the model with reduced wave climates was referenced to a simulation with the actual (full) wave-forcing series. It was demonstrated that input reduction can dramatically affect long-term predictions, even to such an extent that the main characteristics of the offshore bar cycle are no longer reproduced. This was particularly the case at Hasaki, where all synthetic series that no longer capture the initial chronology (steps 3 and 4) lead to rather unrealistic long-term simulations. At Noordwijk, synthetic series can result in realistic behavior, provided that the time span after which the sequence is repeated is no