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) t he selection of the duration of the original (full) time series of wave forcing, (2) the selection of the representative wa ve conditions, (3) the sequencing of these conditio ns, and (4) the time span after which the sequence is repeated. In step (2), the chronology of the original series is retai ned, while that is no longer the case in steps (3) and (4). The framework was applied to two different sites (Noordwijk, Net herlands and Hasaki, Japan) with multiple nearshore sandbars but contrasting long-term offshore-directed behavior: at Noordwijk the offshore migration is gradual and not coupled t o individual storms, while at Hasaki the offshore m igration is more episodic, and wave chronology appears to control lo ng-term evolution. The performance of the model with reduced wave climates was referenced to a simulation with t he actual (full) wave-forcing series. It was demons trated that input reduction can dramatically affect long-term predict ions, even to such an extent that the main characte ristics of the offshore bar cycle are no longer reproduced. This w as particularly the case at Hasaki, where all synth etic 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 sequenc e is repeated is not too large; the reduction of this time span has the same positive effect on the simulation as increasin g the number of selected conditions in step 2. It was further demon strated that, although storms result in the largest morphological change, conditions with low to intermediate wave en ergy must be retained to obtain realistic long-term sandbar behavior. The input-reduction framework must be applied in an iterative fashion as to obtain a reduced wave climate that simulates long-term sandbar sufficiently accur ately within an acceptable computation time. Given its potential huge impact on the actual simulation, we believe it is imperative to consider input reduction as an in trinsic part of model set-up, calibration and validation.