New Study Reveals Consequences of North Sea Sand & Silt Dredging

Through sand extraction and the disposal of dredged harbor silt, about 200 million tons of sediment are relocated every year in the coastal waters of the North Sea. The Wadden Sea is particularly strongly affected. This is the result of a new study by the Helmholtz Center Hereon, which for the first time evaluated comprehensive data on dredging activities along the North Sea coasts. The dredged material could potentially be used in the future to help raise the seabed in areas that do not naturally accumulate enough sediment to compensate for rising sea levels. The study was recently published in Nature Communications. Every year, specialized vessels extract large quantities of sand from the North Sea, which is used to build new port facilities or for coastal protection. Off the islands of Sylt and Wangerooge, for example, sand is regularly deposited because winter storms and wave action erode parts of the beach. Harbor basins and river shipping channels have to be dredged annually, otherwise they would silt up. This silt is "dumped," that is, redeposited at other locations off the coast. Researchers at the Hereon Institute of Coastal Systems—Analysis and Modeling have now calculated for the first time how much sand, gravel, and silt are transported back and forth in the Wadden Sea of the North Sea through dredging and dumping. The scale is enormous: a total of 200 million tons of material per year—about the same amount as is naturally transported by North Sea currents and all surrounding rivers combined. "For our study, we compiled and analyzed data on dredging activities from 30 years," says Dr. Lucas Porz, an expert in sediment transport at Hereon. Environmental burden or opportunity for coastal protection? Dumped sand and harbor silt do not remain where they are deposited but are gradually carried away by water currents. Often, the material accumulates again in ports or shipping channels, which then have to be dredged once more. The simulations conducted in the Hereon study make it clear that a large portion of the dumped material also settles in the Wadden Sea in the long term. In a previous study, a research group led by Porz's colleague and Hereon scientist Dr. Wenyan Zhang found that the Wadden Sea is not keeping pace with rising sea levels. The reason is that most tidal flats are no longer accumulating enough sediment naturally. Relocated sand and silt could counteract this. "If dredged material were strategically relocated, it could reach the affected areas and allow the tidal flats to grow again," says Porz. He is currently working with his colleagues to determine in detail which areas would be suitable for this. However, since harbor silt in particular can be chemically contaminated, the impacts on the marine environment would have to be carefully assessed in advance, and any targeted relocation of the material could only take place in harmony with nature conservation. Carbon is released However, dredging and extracting sand from the sea also has a significant drawback: It releases large amounts of carbon that have accumulated in the seabed along with the remains of algae and other marine organisms. In the current study, Porz therefore also determined the amount of carbon released each year by offshore dredging.