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The effect of pontoons on flow fields in small harbours

Straatsma, Leonie (2013) The effect of pontoons on flow fields in small harbours.

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Abstract:Siltation is a problem in every harbour, suspended sediments come into the harbour with the flood tide or via flows from surrounding land. Because of the lower velocities in the sheltered basin the suspended sediments settle down. Many harbours at the northeast coast of Scotland are transformed from a fishing harbour into a recreational harbour which involves the placement of pontoons. In these new marinas an increase of siltation is detected which might be caused by the effect of the pontoons on the flow fields within the harbour. The aim of this research is therefore, to investigate these effects in small tidal harbours. A 3D hydro dynamical model for a schematic harbour was made in FLUENT. This Computational Fluid Dynamical software from ANSYS uses the Navier-Stokes equations to calculate the flow quantities, such as velocities and direction, inside the harbour domain. The schematic harbour domain was 120m long and 80m wide, with a 10 wide inlet located in the corner. A structured grid is used to divide this domain in multiple control volumes for which the equations can be solved. The tidal range used in this research resembles the spring tide in the northeast of Scotland and is 4.5m. The simulated period was 1 hour instead of the real 12hrs, which leads to higher velocities in the results. A velocity inlet was used to prescribe this time varying boundary conditions which are necessary for this tidal model. The Volume of Fluid method is used to solve the equations for two fluids (water and air) and to simulate the free surface. After the model was set up three different scenarios with pontoons were designed and simulated in the model. Each of the scenarios had different pontoon depths or number of pontoons but all simulations started with the same boundary conditions. Therefore the resulting differences in horizontal flow fields could be related to the differences in pontoon depth or the number of pontoons. The main conclusion of this research is that the placement of pontoons has an enormous effect on the flow velocities and circulation inside the harbour. The velocities in all scenarios with pontoons were more than twice as low as the scenario without pontoons. Furthermore in the scenario without pontoons one circulation cell was seen inside the harbour during the whole tidal cycle in contrast to the multiple circulation cells which occurred in the scenarios with pontoons during flood tide and the lack of circulation during ebb tide. The depth of the pontoons influences the circulations inside the harbour as well. In the case of deeper pontoons stronger circulations are formed between the pontoons. Secondly in the cases with the less deep pontoons the flow directions are more scattered and the flow underneath the pontoons is more similar to the scenario without pontoons. The number of pontoons changes the number of circulation cells in accordance to the number of pontoons and does not include other big changes in the flow fields. However the direction of these circulations can be different when the distance between the pontoons increases. To conclude, the pontoons have multiple effects on the horizontal flow patterns inside a harbour, depending on the depth and number of pontoons. The main effects are the decrease of velocities and an increase in the number of circulation cells inside the harbour. These effects may contribute to the sedimentation in the new marinas such as the Abroath harbour.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
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