Updating of the Belgian Sigma plan on a risk-assessment basis

摘要

In Flanders (Belgium), the storm tide of 03-01-1976 inundated almost 900 houses.In TheNetherlands, where at that moment the Delta-plan was almost completely finished, no damages occurred.Thisdisaster gave rise to an accrued (but temporary) public awareness of inundation risk along the tidal reach of theScheldt (Sea Scheldt) in Flanders, and to the conception of a Flemish so-called Sigma plan for Sea Scheldt inthe beginning of the 1980's.This Sigma plan was composed of a tidal storm surge barrier downstream Antwerp,combined with a general heightening of the river-embankments and the construction of a number of controlledinundation areas.However, a socio-economic analysis performed in 1982, showed that a storm surge barriercould not be economically justified.Hence, the barrier was never constructed.From the other measures, about80% is in place nowadays (2004).In the meanwhile, it is generally accepted that the mean sea-level is rising: Belgian and Dutch authoritiesconsider a possible value of 60 cm rise within the next century.This fact will result in more frequent high waterlevels, and thus more inundation damages and a higher risk.Therefore the Flemish authorities decided to updatethe existing Sigma plan, taking into account the effects of the rising sea-level, and considering the principles ofrisk analysis combined with those of sustainable river management.The article describes the different phases of the study.An inventory of different possible building–stones forthe new Sigma plan is made and analyzed: storm surge barrier(s), heightening of embankments, areas prone tobe transformed into controlled inundation areas.Damages and risk are calculated through the use of a hydrodynamic model of the Western Scheldt and SeaScheldt, by simulating water levels, breaches, inundations and translate them into damages and finally into risk.Anoptimal solution is searched for, by combining the building-stones and analyzing the result through a social benefit-cost analysis.To determine the benefits, not only the avoided risks are considered, but also the secondary effectssuch as (in case of controlled inundation areas) effects upon local agriculture and benefits for nature development.The outcome of the study is that a storm surge barrier is not the best solution, although it has become aneconomically feasible solution because of the effects of sea-level rise.Better solutions can be found in appropriate combinations of controlled inundation areas with local heighteningof embankments.The study points out a best choice, and will be the basis of future modifications in theexisting spatial planning of the area.