Geomorphology and biotic colonization of a recently restored tidal wetland in the Schelde estuary (Belgium).

With the updated Sigmaplan the Flemish government aims to harmonise flood control and ecological rehabilitation in the Schelde estuary. Tidal wetland restoration and creation are essential to the success of the plan. In managed realignment projects crucial design issues include creek precursors, breech dimensions and whether or not to remove dikes. In 2012 a sand stock on the historic polder of Lillo, was lowered back to a level between 0.5 and 1m below mean high water level (MHW). The polder is situated in the brackish part of the estuary where the mean tidal range is 5m. Two managed realignment methods were applied, separated by a peninsula. In the northern part (5.2 ha) the dike along the river was removed over a distance of 150 m to the level of the adjacent tidal mudflat. In the southern part (3 ha) the dike was lowered to 0.25m above mean high water spring level (MHHW). Both compartments were breached with a creek precursor of 10 - 15 m wide and with a bottom level at 1,5 – 2m below MHW. To enhance creek erosion in the adjacent mudflat a deeper dug out area (> 2 m below MHW) was created behind the dike prior to breaching. The first months after managed realignment very high sedimentation rates occurred and high rates persisted during more than two years. This is most probably due to the proximity of a turbidity maximum in the estuary. Removal or preservation of a (breached) river dike did not lead to differences in sedimentation rate, plant or benthos colonisation in the realigned sites after 2.5 years. Both methods led to the development of a dendritical creek system in the deposited sediment. In the restored site typical benthos species reached after two years comparable densities as in e nearby reference site. Differences in sedimentation patterns on the mudflat in front of the realigned area indicate more dynamic conditions after complete dike removal.