Human versus natural mud fluxes in the Scheldt estuary: are they significant and if so, how can they best be optimised?

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Thijs van Kessel, Gijsbert van Holland, Joris Vanlede

Monday 29 june 2015

14:05 - 14:17h at South America (level 0)

Themes: (T) Special session, (ST) Scheldt Estuary physics and integrated management

Parallel session: 2J. Special session: Scheldt Estuary physics and integrated management

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The mud dynamics of the Scheldt estuary is governed, like other estuaries, by the interplay between tidal flow, freshwater discharge, marine and fluvial mud supply and local sources and sinks, notably mud flats. Human impacts on the mud dynamics can be either direct or indirect. Direct impacts are caused by harbour and fairway maintenance dredging (i.e. dredging vessels transporting mud from A to B), indirect impacts are caused by human modifications of the estuary such as increasing channel depth by capital dredging or reducing the area of tidal flats by land reclamation, thus modifying the natural flux. Using a process-based mud transport model of the Scheldt estuary, these impacts have been quantified by evaluating different scenarios representative for the present bathymetry and maintenance dredging procedure or with a modified bathymetry or modified dredging procedure. The focus of this presentation is on the dredging procedure, as this can more easily be modified on the short term than estuarine bathymetry (both a reversion of land reclamation and channel depth would have large consequences for the economical function of the Scheldt estuary). The results show that although the ‘human’ fluxes caused by maintenance dredging are typically small compared to natural gross fluxes, they are very significant compared to natural residual fluxes, notably in the narrower section of the estuary near Antwerp. Here more than half of the available mud is ‘second-hand’, i.e. it has been dredged from and released back into the estuary at least once. This implies that an optimisation of the dredging and release cycles, including the smart selection of release locations, offers the perspective of smaller human impacts, possibly even at lower costs. A down-estuary shift of release locations would be favourable, whereas the present practice is that dredged mud is regularly released up-estuary of the harbour or access channel from which it has been dredged. Also, a shift to locations closer tidal flats may interrupt the vicious circle between mud disposal and dredging by enhancing the accretion rate of these flats. However, the surface area of these flats has to be substantial to provide more than just a short-term solution.