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Deltares | R&D Highlights 2015 Ecosystems and Environmental Quality
Lessons from full scale
flow slides
A careful balance has to be maintained between flood risk management, navigation demands and ecological boundary conditions in the Western Scheldt estuary. This task has been delegated to the Flemish-Dutch Scheldt Commission (VNSC). Deltares has been responsible since 2008 for Dutch knowledge input in this commission.
Research in 2015 focused on how intertidal areas respond physically and ecologically, in the context of the long-term drivers, to sudden changes in morphology. The ultimate aim of this research is to optimise the current sediment dredging and disposal management strategy (‘Flexibel Storten’) in order to combine channel-depth maintenance and ecosystem protection on the tidal flats and shoals by means of sediment nourishment. There was a large flow slide on the southern edge of the Walsoorden tidal flat on 22 July 2014. Flow slides are slope failures in channel or river banks, beaches or shoal margins that can have a major impact given the volumes of sediment involved. They can damage dikes and foreshores and have been reported along the Eastern and Western Scheldt for centuries. Before the Delta Works were completed in 1987 and hard bank protection was introduced on many vulnerable foreshores, dike collapse due to flow slides represented a major flood risk.
The 2014 flow slide opened up a large gap and 850,000 m3 of sediment was deposited in the adjacent navigation channel in a short time. The navigational depth was reduced by over 7 m and additional dredging was needed. In the context of the ‘Flexibel
Storten’ strategy, monthly measurements were being performed at several disposal locations and these measurements were extended to the location of the slide. Meanwhile, the Walsoorden flat was also selected for a validation test in the IJkdijk / Flood Protection research programme. This programme included the initiation of several small flow slides by dredging, with morphological changes being continuously surveyed with three vessels. Vibrocore boreholes and bottom samples were analysed. Several steep, six-metre-high breaches developed and slowly retrogressed into the shoreline but eventually stopped. The volumes involved remained small by comparison with the flow slide on 22 July 2014.
The detailed and frequent bathymetric surveys in combination with the soil investigation have produced a unique database of the geomorphodynamic recovery of the system after a large sudden change in the channel-shoal interface. Measurements show that the deposited sand moves along the channel bed in the form of migrating dunes and that the original channel bed was restored after about one year. This unique dataset allowed us to develop and validate a Delft3D morphodynamic model that can subsequently be used to support sediment management. A detailed analysis of the sand transport patterns provided unexpected new insights. The sand transport patterns associated with migrating dunes in the channel and sand transport along the shoal margin are in opposite directions. Further analysis and modelling will help us to understand these patterns in particular and the Western Scheldt sediment dynamics in general.
Further reading:
Van Schaick (2015). Morphological development after the July 2014 flow slide on the tidal flat of Walsoorden, MSc thesis Delft UT (via repository.tudelft.nl)
marcel.taal@deltares.nl T +31(0)6 2241 6276 dick.mastbergen@deltares.nl T +31(0)6 1517 9162
Bathymetry measurement Bocht van Walsoorden, Westerschelde (21 August 2014), the 22 July 2014 flow slide deposited 7 m of sand from the shoal margin (red) into the channel (blue) (Van Schaick, 2015)
< Tidal flat of Walsoorden, Wester-schelde with gap of the 22 July 2014 flow slide (Photo E. Paree, aerial view September 2014)
Walsoorden tidal flat view from the gap made by the 22 July 2014 flow slide to the south (Photo Deltares/ D. Mastbergen, October 2014)
