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Beach formation in sensitive areas

Presented during:

CEDA Dredging Days 2012 - Virtue, Venture and Vision in the Coastal Zone

Authors:

Coenen VJ and Timmer H - Witteveen+Bos Consulting Engineers, the Netherlands

Abstract: When it comes to dredging in a nature reserve, decision making is not only dominated by engineering, technical and financial concerns. It requires a profound understanding of environmental impacts and the aquatic and terrestrial effects of dredging and replenishment. An example from the Netherlands shows the type of information needed to allow a dredging and replenishment project in an intertidal nature reserve.

The Eastern Scheldt surge barrier was completed in 1987 to secure coastal safety. The tidal flats area behind it is an international nature conservation area and one of the most important shellfish areas in the Netherlands. The semi-open barrier however was found to have an adverse effect on the tidal system: sedimentation on the flats slowed down, but erosion did not change. An annual net loss of 1 to 1.5 million m3 sand and deterioration of the whole intertidal system makes it necessary to replenish sand periodically.

Fundamental issues surfaced: do you replenish on a large scale with long intervals between the replenishments? It is cost-effective, but covers large areas of tidal flats with significant amounts of barren sand. Or do you supply sand in small amounts with short intervals? This is not cost effective, but would have less impact on the ecology and fisheries.

A complex, multi-criteria cost-benefit analysis was made, based on detailed information on the environmental thresholds for various species and fisheries and detailed information on dredging and replenishment techniques. In two pilot projects on eco-friendly replenishment, calculations and assumptions were tested on validity.

We learned that detailed information on environmental impacts of dredging and replenishments is of vital importance for planning of works in sensitive areas. We discovered that this kind of information is not always available as the focus in dredging is on civil engineering aspects. More attention should be given to modelling and monitoring of environmental impacts.