A Bypass Friendly Harbour

The objective of this study is to determine for which conditions the bypass capacity is optimized in combination with a limited influence on the navigational function of the harbour. The bypass capacity is defined as the capacity to bypass the sediment around the harbour under the presence of the defined driving forces (i.e. waves and/or tide). The conditions involve varying forcing, geometry of the breakwater, bathymetry, the presence of a dredged entrance channel and the grain size. The capacity for sediment bypassing depends amongst others on the sediment transport regime and the harbour geometry. For this research both phenomena have been categorized to make up a framework of transport regimes and types of harbours. This gives 6 archetypes with specific bypass characteristics. Only the most promising archetype has been studied more thoroughly. Therefore the hydrodynamic and morphodynamic processes of each archetype have been studied with the help of existing literature, reference situations and a sensitivity analysis of the relevant parameters. Based in this analysis it is concluded that a harbour which does not require a dredged channel and only services vessels with a small draft (approx. 2.5 m-3.5 m), along a coastline which experiences a large net transport is the most relevant and promising archetype to study. This is a relevant case because the downdrift coastline experiences erosion which opposes a problem for the coastal manager. The described case is a promising case because the small harbour blocks less sediment, there is no dredged channel and the contraction of the tidal current in combination with the wave-breaking-induced current is expected to give sufficient bypass capacity. The most promising archetype is studied more thoroughly with the numerical model Delft3D. From a sensitivity analysis and the literature study it was concluded that the parameters, as specified below, have significant influence on the bypass capacity. - The extension of the breakwater in the surf zone, related to the width of the surf zone; - The wave height; - The wave angle; - The shape of the breakwaters; - The current velocity; The influence of these parameters has therefore been assessed with Delft3D. Three criteria were of importance in assessing the results: - The amount of relative bypassing; - The depth in front of the harbour; - The depth in the harbour. The model calculations have provided output on the influence of each parameter on the bypass capacity and the serviceability of the harbour. The most promising situations are assessed on a longer time scale to see whether the promising situation holds. This gives the following conclusion (see next page): This research shows that a bypass friendly harbour is possible under certain circumstances. These circumstances are a harbour with streamlined breakwaters and an extension equal to the width of the surf zone at a coastline with significant wave and tidal influence. The streamlined geometry of the breakwaters leads to higher longshore flow velocities in front of the harbour, related to a harbour equal in size but with semi-streamlined breakwaters. The higher flow velocities enlarge the bypass capacity. The high flow velocities, in combination with the extent of the breakwaters also lead to a limited decrease of the depth in front of the harbour and very little sedimentation in the harbour. Both the limited decrease of the depth in front of the harbour and the little sedimentation in the harbour have a positive effect on the amount of bypassing. The bypass is also initiated quicker than that in case of semi-streamlined breakwaters because less sediment is required to make the new coastline orientation in case of the streamlined breakwaters.Coastal EngineeringHydraulic EngineeringCivil Engineering and Geoscience