Change in coastal sedimentation conditions due to positive shore displacement in Oeregrundsgrepen

This report is a part the SKB project 'SAFE' (Safety Assessment of the Final Repository of Radioactive Operational Waste). The aim of project SAFE is to update the previous safety analysis of SFR-1. SFR-1 is a low and intermediate level radioactive waste disposal facility, which is situated in bedrock beneath the Baltic Sea, 1 km off the coast near the Forsmark Nuclear Power Plant. At a possible discharge of radionuclides from SFR-1 today, ground water currents will presumably transport the radionuclides to the sea bottom. Many radionuclides are able to absorb on to fine particles, and it is likely that the radionuclides would adsorb on to particles in the sediment or particles suspended in the seawater. The adsorption is strong, and therefore the dynamics of the radionuclides are governed by the dynamics of the fine particles. A mathematical model has been developed to simulate the resuspension of fine particles caused by wave movement. The model simulates the wave-induced near bottom water dynamics based on meteorological data. First, the wave's characteristics in deep water are calculated. Then the gradual change of the wave's characteristics is calculated as it reaches shallower water. The maximum near-bottom orbital velocity is calculated for the entire fetch distance, and the wave's ability to resuspend fine-grained particles is determined using well-known semiempirical methods. A large variety of weather conditions are simulated and the results are shown as a map for two different bottom types: accumulated bottoms (continual accumulation of fine grained particles and any radionuclides present) and erosional/accumulation bottoms (periods with accumulation alternating with periods with erosion). The model has been calibrated for four areas close to SFR-1, which provides a range of different sedimentation conditions. Since shore displacement is positive, currently approximately 60 cm per century, sedimentation conditions have fluctuated and will continue to fluctuate over time. Areas with accumulation bottoms can switch to erosional bottoms and vice versa. For this reason, it has been necessary to simulate both previous and future conditions by adjusting the water depth conditions in the sea. The results of the simulations show that sedimentation conditions change quickly, but normally a clear pattern is evident. At the time the most recent glacial melted away from the region, the entire area was made up of accumulation bottoms. As the land rose and the area became more shallow, many of the bottoms became more erosional (at maximum approximately 14% 500 AD). Certain of these areas later went back to being accumulation bottoms as a result of the growing archipelago which provided protection from the waves of the open sea. Certain deep areas, such as the channel west of Graesoe have had accumulation conditions for the entire post glacial period. Large parts of the area close to SFR-1 have recently shifted from erosional bottoms to accumulation bottoms. This trend is continuing and will continue until the area becomes land (2,400-3,500 Ad). This means that should a release of radionuclides occur today, they could in part adsorb on bottom sediment near SFR-1, in part adsorb on bottom sediment in the Graesoe channel and spread to some extent into the deeper waters of the sea. Since the number of accumulation bottoms continually increases, the conditions improve for an every increasing amount of radionuclides adsorbing on bottom sediment close to SFR-1. As the area around SFR-1 rises to become land, a large number of small lakes will form and a large lake will form approximately 5,000 AD about 2 km north-east of SFR-1. After this point the conditions will change dramatically as the groundwater currents will flow into a drainage area on land or at the bottom of a lake. The ability of the radionuclides to spread will continue to decline since they will increasingly be able to adsorb to organic material in the drainage areas or on lake bottom sediment. It is clear that the greatest spread of radionuclides would occur if the release were to take place today. As time goes on, the ability of the radionuclides to spread would gradually decrease. After 5000 AD only very local spreading would occur