Tide integrated hydrodynamic and sediment transport characteristics in tidal channels and the effect of deepening

Rinsema, J.G. (2016) Tide integrated hydrodynamic and sediment transport characteristics in tidal channels and the effect of deepening.

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Abstract:Estuaries are places where rivers meet the sea. Estuaries have different characteristics dependent on their location. An important factor is the shape of the estuary. Natural estuaries have a funnel or trumpet shape, which means the estuary, has a large width near the seaward boundary and is converging stream upward. Other estuaries are man-made and have a straight channel from the sea landward; these estuaries are called prismatic estuaries. The Rotterdam Waterway is an example of such a prismatic estuary. Estuaries are often used as access channel for harbors. Due to economic development and technology development ships size increases, which makes the harbors less accessible. To reduce this problem harbors are deepened to keep them accessible. The effects of the deepening on estuary processes are another key question which is unknown for prismatic estuaries. The sediment transport processes result in an Estuary Turbidity Maximum (ETM). The ETM is a suspended sediment front near the mixing of the saline and fresh water. The sediment is trapped at this location due to the estuarine circulation. The magnitude of the ETM is determined by several other processes determined by the boundary conditions of the study area. These processes include tidal asymmetry, internal asymmetry, tidal phase lag, turbulence damping and flocculation. The change of the ETM due to these processes in prismatic estuaries is relatively unknown. The Rotterdam Waterway is used as case study to evaluate the sediment transport characteristics of prismatic estuaries, using the process based numerical model Delft3D. A schematized study area is created which only consists of a straight channel including the fresh water boundary and a schematized sea of 40 kilometers along the coast and 20 kilometer perpendicular to the coast. The model is setup and validated based on available sources. A sensitivity analysis is done to evaluate the contribution of the different processes towards the hydrodynamics and the sediment transport characteristics. The wave conditions and discharge are changed in the sensitivity analysis. The fresh water discharge is changed towards the 5%, 25%, 75% and 95% discharge of the Rotterdam Waterway. The waves are changed towards the significant wave height during summer, during winter and during storm conditions at the North Sea. The change in discharge is an important driver for the salinity, hydrodynamics and the suspended sediment concentration. The internal asymmetry does not play a role with the changing fresh water discharge. The increase in tidal asymmetry with increasing discharge increases the available sediment in the water column. A combination of the increased estuarine circulation and the turbulence damping increases the sediment concentration in the lower layers of the water column. The sediment concentration in the top layer increases with decreasing discharge because the turbulence isn’t damped anymore. The waves have only small influence on the hydrodynamics and small influence on the suspended sediment concentration for the Rotterdam Waterway in the short term. If the waves occur for a longer period, the impact ETM increases resulting in the increase of suspended sediment concentration in the ETM. A scenario study is executed to evaluate the effect of the harbor basins and to determine the effect of deepening. The harbor basins are important for the suspended sediment concentration, in particular for the available sediment in the bottom layer. The sediment settles less in the Rotterdam Waterway, but it settles in the harbor basin instead where the velocities are lower and the turbulence is low. The effect of deepening is evaluated for two types of deepening. The first deepening is the deepening of the first step in the Rotterdam Waterway and the second deepening is near the location where the ETM moves to and fro in the estuary. The difference between the original depth and the deepened scenario is small for the deepening of the step. The salinity intrusion increased with 3 kilometers with increasing tidal prism. The step deepening results in decreased influence of the discharge on the location of the null point and leads towards a small increase in the suspended sediment concentration under average discharge and yearly average significant wave height due to increased ebb and flood velocities. The long term effect for the 5% discharge decreases, while the effect of the 95% discharge increases. The impact is small if the deepening is in the area where the ETM occurs. The salt intrusion length is increasing but for the same tidal prism. The salinity intrusion increased with 3 kilometers, and the near bed velocities increase in the first part of the estuary. The influence of the discharge on the salinity null point has also decreased for the ETM deepening, but the difference is smaller compared with the step deepening. The increasing velocities near the bed lead to increased suspended sediment concentration in the ETM. The long term change shows also decreasing effect for the 5% discharge and increasing effect for the 95% discharge.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
Link to this item:http://purl.utwente.nl/essays/71151
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