I. Colosimo1, D.S. van Maren1,2, P.L.M. de Vet1,2, M. van Regteren3, B.C. van Prooijen1
The Windows of Opportunity for salt-marshes to develop on a bare mudflat depend on the emergence inundation regime and hydrodynamic conditions. We argue that the mudflat can only accrete to a adequately high level if four conditions are met: (1) sufficient sediment available; (2) a long enough mudflat inundation time; (3) sufficient sediment deposition and (4) consolidation of the bed. We analyse the effect of different hydrodynamic forcing on the four conditions, and we compare the effects of deterministic (tide) vs stochastic (wind) input.
Three field campaigns were set up on a muddy intertidal flat, in the Wadden Sea, close to Harlingen. Two transects with a reciprocal distance of 5 km were selected and two frames were deployed on each transect. Each frame had 1-2 ADVs, 1 ADCP, 2-3 OBSs and 1 wave logger. Continuous measurements were carried out over at least 30 days at a high frequency (8 Hz), during 2 spring seasons and 1 winter season. This provided a rich data set covering diverse meteorological conditions.
Based on the extensive dataset, we identified the four different conditions necessary for the bed-level accretion. The data suggest that the wind has an eminent role in all four conditions. For example, Figure 1 shows the high correlation between sediment concentration and wind speed (condition 1). We observe that wind has a strong influence on the high water (HW) and low water (LW) levels, determining the conditions of submergence or emergence of the upper flat zone (condition 2 and 4). Eventually, wind-waves reduce the opportunity for deposition (condition 3), with maximum wave heights occurring during HW, when the slack water should give opportunity for sediment settling.
The stochastic component of the hydrodynamic forcing is crucial for the bed level to accrete to a sufficient level to achieve salt marsh development. Furthermore, the temporal succession in which the four conditions are fulfilled is essential for a successful accretion. We conclude, therefore, that the Windows of Opportunity for bed level increase are strongly influenced, in some conditions entirely controlled, by wind.
Figure 1 Maximum Wind Speed and log(max(SSC)) per tidal phase (flood/ebb).
Location: F1, the frame installed at MLW (i.e. -1m MSL).