Effects Of Suspended Kelp Laminaria Japonica Cultivation On Hydrodynamic Conditions And Sedimentary Environments

Based on various measurements and observations, including hydrology, suspended sediments, surface sediments, shallow sediment cores and water depth at the typical suspended kelp Laminaria japonica cultivation area of Heini Bay, this thesis calculates the boundary layer parameters, the fluxes of erosion/deposition, as well as the rate of sediment transport in different seasons within and around the cultivation area. In addition, numerical simulation, grain size trend analysis and topography comparison are applied to distinguish the feature of flow structure, sediment distribution and transportation in and around the cultivation area among different seasons. Ultimately, this thesis discusses the influence of suspended kelp cultivation on local hydrodynamic conditions and sedimentary environments.The results revealed that due to the widespread distribution of suspended culture community in the study area, the current velocity in culture area has been reduced and the vertical distributions of flow velocity have been changed. Three current velocity profiles were identified within and to the landward side of the culture area. Each profile can transform into another as the flow velocity and direction changes.The emergence of suspended kelp may have attenuated the shear stress of the bottom boundary layer, and thereby can restrain the resuspension of sediments. The concentration of the suspended sediments within the cultivation area is much lower than that within the natural waters. However, due to weakened shear stress of flow velocity and turbulence caused by interaction between kelp beam and current, the vertical mixing process of water in the cultivation area is strengthened.Under normal condition, the study area is generally in a state of being eroded. The eroded area mostly appears in the regions occupied by the bedrock headland, where the primary sediments are comprised of coarse particle matters such as sand and gravel. Meanwhile, in nearshore area with shallow water of the Heini Bay, depositing process takes place more often. After kelp culture activities arose, the sedimentary patterns in the study area are changed; the cultivation area is subject to dramatic deposition environment, while depositing appearing in the shallow waters is impaired along with the coarsening of the sediments. Erosion around the bedrock headland is attenuated as well. At the west side of the culture area, deposition area with a range of fine particle emerges.In addition, due to the seasonal change of kelp growth, the hydrodynamic conditions over the culture area change in a cycle manner, and hence the steady sedimentary environments are uneasy to take shape. Eventually, erosion and deposition process take place alternately in the culture area, which may lead the dramatic changes of sedimentary patterns in the study area.