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Physical impacts of spoil dumping at sea in Hamina and Naantali

The impact of spoil dumping at sea on the water environment are generally clearly visible while the dumping is going on. Long-term impacts are more challenging to observe, and very little research is available.

Sito employee Tiina Vaittinen studied these long-term impacts in her master’s thesis. The purpose of the study was to generate data that will help justify and conduct impact assessments for colliery spoil dumping at sea and for determining the parameters for spoil dumping projects.

The study focused on two spoil dumping areas, in Naantali and Hamina. The Naantali study site is adjacent to the mainland and a busy harbour. The Hamina study site is in a spacious archipelago with considerably calmer flow conditions. At both sites, spoil dumping had been discontinued three years earlier. Sediment samples and sonar provided depth data that indicated how stationary the dumped spoil had remained.

The literature review in the study focused on the environmental impacts of spoil dumping projects observed in monitoring reports on spoil dumping at sea and earlier research, and on factors influencing them.

The findings show that the Hamina site is an excellent dumping site, the spoil having settled onto the sea bed. The sediment thus formed is also of a physically markedly higher quality than the original anoxic top sediment layer. The spoil dumping had the effect of isolating the original decaying sediment with a high oxygen demand from the bottom water.

At the Naantali site too, the dumped spoil seems relatively well stabilised, but not as clearly as at the Hamina site. This is largely due to the constant strong propeller flows in the area. Also, in Naantali it was quite hard to distinguish the dumped spoil, because it is very similar to the original sediment in terms of its physical properties.

Sediment top layer sampling and simple physical characterisation proved to be a good method for surveying the settling of the dumped spoil. Sediment samples are sometimes taken in the statutory monitoring of spoil dumping projects, but their analysis generally focuses on levels of harmful substances. Analysing the properties of sedimentation at the same time would yield valuable data on the stability of the dumped spoil. Depth data are also suitable for the monitoring of spoil dumping sites, although it is difficult to assess the reasons behind topographical changes on the basis of depth data alone.

The project was a public R&D project of the Finnish Transport Agency.

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