Coastal defence by sandscaping: don't just deposit sand, but think about the geochemical processes too
The Sand Engine might not achieve all its goals
Environmental scientist Iris Pit from Utrecht University investigated the environmental effects of the Sand Engine, an large artificial hook-shaped sand spit off the Dutch coast that inspired a coastal protection scheme in Norfolk (UK). Besides protecting the Dutch coast, the Sand Engine also provides space for recreation and nature development. However, Pit found three risk factors that influence life in the sand. As a result of this, the Sand Engine might not achieve all of its goals. ‘Environmental geochemical processes must, therefore, be included in the development of more sustainable coastal defence technologies.’
The Sand Engine has a volume of 21 million cubic metres of sand, which is supplied from elsewhere in the North Sea. Pit investigated this suppletion sand for possible contaminants. Firstly, she concluded that due to changes in the conditions of the sand from anaerobic (no oxygen present) to aerobic (oxygen present), heavy metals dissolved in the water between the sand grains. ‘The concentrations found are so low that these pose no risk for beach users and swimmers, but for soil-dwelling organisms, these heavy metals do constitute an environmental hazard’, explains Iris Pit. Secondly, the sand contains many bog iron ore fragments which contain a high concentration of arsenic. In the coming years, a larger quantity of bog iron fragments will probably become visible on the surface of the Sand Engine. ‘With such large quantities of bog iron ore, weathering can ensure that small particles of bog iron ore can easily be transported by the wind to places with anaerobic conditions where they are broken down, for example in the lagoon of the Sand Engine.’ The arsenic present can then mobilize there. This could lead to locally high concentrations of arsenic with possibly harmful consequences for soil organisms. Finally, she found that these organisms are mainly exposed to arsenic at dynamic locations where fine sediment is deposited. ‘That is because arsenic binds better to such particles than the larger sand grains.’
Sediment composition
These environmental risks obviously have an impact on the Sand Engine's nature development objectives. ‘Environmental geochemical processes must therefore be considered when improving the sustainability of coastal defence technologies. The composition of the sediment in the sand extraction area should be carefully examined and the possible chemical processes should be included in the guidelines for the environmental impact report before a following beach suppletion is designed.’
Calcium and pyrite levels
In particular, she argues that the sand suppletion material should always first be investigated for the presence of lime and pyrite. ‘If the lime content is too low, then there is a higher chance of acidification as a result of which heavy metals will dissolve more easily. A low pyrite content, however, is favourable because that often naturally contains a lot of arsenic and heavy metals.’ If the quantities of both minerals are known, then the chemical processes that will occur during the beach suppletion can be estimated. ‘With this knowledge you can then adjust the design for the sand suppletion.’
Norfolk
The world's largest cities are found in coastal areas. Due to rising sea levels, the coast is under pressure as a result of which many beaches are being eroded. That also applies to vulnerable beaches along the Dutch, British, French and other coasts. Since 1990, large-scale measures have been taken to protect the Dutch hinterland against floods. Sand is deposited on and along the coast to protect the Dutch coastline. The frequency and volume of these so-called sand suppletions are expected to increase. The Sand Engine was constructed in 2011 as a pilot for a more sustainable form of coastal management. The sand from this artificial sand spit must spread along the coast under the influence of the natural elements and therefore contribute to the coastal defence of the province of South Holland. Meanwhile, similar protection measures have been taken along the English coast near the villages of Bacton and Walcott in Norfolk. The Dutch Sand Engine has a life expectancy of 20 to 30 years.
Publication
On September 17, Iris Pit obtained her PhD degree following the succesful defence of her PhD thesis The shifting sands of the Sand Engine : Using environmental geochemistry to evaluate the Sand Engine’s ecological and toxicological risks.