Abstract
AbstractCoastal 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.
Highlights
Wave conditions are projected to change for 50% of the global coastlines due to climate change (Morim et al 2019), which given the sensitivity of coastal ecosystems to hydrodynamic threshold exceedance could potentially lead to state shifts
To be able to quantify to which extent various coastal ecosystems can persist under potential future wave conditions, there is a need for an applicable method to quantify the hydrodynamic thresholds of ecosystem collapse
The TiDyWAVE is a small, portable field flume capable of inducing near-bed wave motion that was designed based on the proven principle of oscillatory flow channels
Summary
Only very few studies test ecosystem performance under hydrodynamic threshold conditions, often relying on laborious and potentially expensive ecosystems transplantation methods (see e.g., Rupprecht et al 2017; Marin-Diaz et al 2019). While laboratory flumes provide excellent control over the hydrodynamics, the studied ecosystem typically needs to be simplified in some aspects. Unless expensive and laborious large-scale ecosystem transplantation methods are used (Möller et al 2014; Rupprecht et al 2017), such ecosystem simplifications and alterations strongly restrict our ability to gain quantitative insight in the hydrodynamic thresholds of coastal ecosystems when using laboratory flumes. Field studies lack the hydrodynamic control that laboratory flumes provide. When the perfect storm is predicted to occur, it will be challenging to timely install all equipment robustly enough to conduct process measurements under these potentially hazardous conditions
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