Wind waves on a mudflat: The influence of fetch and depth on bed shear stresses

Wind waves were measured in the Willapa Bay mudflats, Washington State, USA, for two months. Wave height, period, and bed shear stresses were modulated by water depth (0–3.5 m), wind speed (0–20 m/s), and fetch (1–5 km). Good agreement was found between the measured waves and predictions of the wave spectral model SWAN using either simplified 1D flat bottom or 2D geometries. The relationship between bed shear stress and water depth shows a dependence on fetch: the decay of bed shear stress with increasing water depth is gradual for long fetch and rapid for short fetch. This difference is explained by the coupled effects of water depth, wave height and wave period. Due to the fetch-dependent bed shear stress, different morphological consequences for tidal flats of different size are predicted. In small (∼2 km) and sheltered tidal flats, waves cause the largest sediment resuspension when water levels are near mean sea level. In extensive tidal flats (∼20 km) or in flats exposed to waves propagating from deep water, waves also are effective in causing substrate erosion during high tides or large storm surges.

► Wind waves measured on a meso-tidal mudflat.
► Wave reproduced using SWAN model 2D and 1D geometry.
► Short fetch: decreasing bed shear stress with depth.
► Long fetch: increasing bed shear stress with depth.
► Different morphological consequences for tidal flats of different size.