The environmental impact of flooding of the dutch ‘delta-metropole’

Abstract

Model studies into the consequences of flooding events usually focus on damage to buildings, infrastructure, economic losses and casualties yet ignore the risk of environmental damage. In this project, a model study was made to assess the environmental consequences of the release of pollutants during the flooding of a polder district in the Netherlands following a river dike breach. A conceptual framework was established for the sequence of events or ‘chain reaction’ during a flood. A 250 m wide dike breach is formed, the scour of the flood waters and/or high water levels may damage or destroy objects like homes, industrial complexes and farms; damaged objects release hazardous substances such as suspended matter and chemicals that will be dispersed in the flood waters and will affect people and ecosystems in the inundated area. The analyses were made in the 50,000 ha case study area ‘Krimpen’, located in the western Netherlands near the cities of Rotterdam, Delft and The Hague. The simulated period of flooding was ten days. The failure of objects where hazardous chemicals are stocked was linked to water height and -velocity. The release and migration of pollutants like volatile aromatics, germs, sum PAH, PCB, LNAPL, DNAPL, heavy metals, nutrients and pesticides was simulated with an innovative, integrated modelling tool. The novelty of the method required the collection of large numbers of data that had not been compiled earlier. More than once data were obtained through expert opinions and best estimates. The assessed impact of the pollutants on the environment was found to be substantial. Small yet numerous sources like cars may release substantial amounts of toxic chemicals to the flood waters and suspended sediments during and after the flood, while large installations like chemical plants only give problems near the dike breach where high flow velocities prevail. The simulated concentrations of toxic substances frequently exceeded legal threshold levels.