Introducing the TiDyWAVE field flume: A method to quantify natural ecosystem resilience against future storm waves

Abstract

Coastal ecosystems are increasingly threatened by global change. Insight in their resilience against increased storminess is needed for their application in nature-based coastal defense schemes. This is often gained from flume experiments. Laboratory flumes provide excellent hydrodynamic control, but are restrictive in that it is extremely difficult to experiment on ecosystems with a naturally developed stability. Field flumes resolve the latter, but are limited to unidirectional currents. This study introduces an easily deployable field flume that mimics the near-bed water motion of waves: the Tidal Dynamics WAVE flume (the TiDyWAVE). The hydrodynamics of the TiDyWAVE are assessed and compared to natural waves. We also compare it with a more traditional unidirectional flow channel by measuring the erodibility (ucr) of (1) bare sediments of which ucr can be calculated and (2) a seagrass meadow. The TiDyWAVE can generate peak oscillatory currents up to 0.32 m s−1 with a maximum wave period of 3.5 s, corresponding to 0.42 m high waves for a water depth of 3 m. ucr measurements showed that bed shear stress in the TiDyWAVE mimics field waves well. In accordance with theory, the observed ucr on bare sediment is consistently lower for oscillatory flow compared to unidirectional currents. On Thalassia testudinum, ucr under unidirectional currents increases 3.5 times faster with increasing blade area than under oscillatory flow. The difference in hydrodynamic sheltering of the seabed by flexible vegetation under currents vs. waves emphasizes the need for imposing representative hydrodynamics to study hydrodynamic thresholds of coastal ecosystems.