'Winds of Opportunity': the Influence of Wind on Mudflats Accretion

The Windows of Opportunity (WoO) for salt-marsh development represent the low-disturbance periods needed for seedling germination and establishment . The WoO applies if one major condition is satisfied: the bed level has to be high enough (close to MHWL) and prone to limited dynamics. In the present study we investigate the conditions necessary for mudflat accretion, i.e. the conditions required for mud deposition and consolidation, with a focus on the upper mudflat zone.

A field campaign was set up on a mudflat in the Western Wadden Sea. Two transects with a reciprocal distance of ca 5 km were selected and two frames were deployed on each transect. Each frame had 1-2 ADVs, 1 ADCP, 2-3 OBSs and an a wave-logger. Measurements were carried out at high frequency (8Hz) during spring season and during winter season, covering diverse wind conditions.

Turbid fringes were identified: high concentrations occur during the first flooding stage and last ebbing stage were observed. The very shallow conditions provide even small waves the opportunity to stir up the sediment. Despite the limited depth, the water in the turbid fringe contains a considerable sediment mass and can therefore lead to substantial sedimentation.

Wind affects the tidal asymmetry in flow velocity, altering the transport capacity of the fringe. The Suspended Sediment Concentration (SSC) at the fringe also varies with wind speed and direction.

Onshore directed wind results in a water level set-up, leading to flooding of the upper flat and therefore creating the opportunity for mudflat accretion. The occurrence of deposition on the upper zone mainly depends on the duration of the inundation, on flow velocities and on wave conditions. Strong landward winds increase water depths, but generate also wavy conditions on the upper zone, reducing the opportunity for the sediment to settle on the bed.

Offshore directed winds result in a local water level set-down or even a set-down in the full estuary, leading to periods without inundation of the upper flat. These periods are favourable for consolidation of the mud, once deposition has occurred, and strengthening of the bed.

We therefore conclude that wind is a relevant factor for mud deposition on the upper zone of the mudflat. It provides crucial opportunity for mudflat accretion and therefore for the development of marsh vegetation.