Maintenance dredging in the Port of Rotterdam

Abstract

The Port of Rotterdam (PoR), located in the Rhine-Meuse estuary, is subject to sedimentation. In order to keep the port accessible for vessels with high drafts, maintenance dredging works are done. The maintenance dredging over the port area is executed by two parties, PoR itself regarding the harbour basins and Rijkswaterstaat (RWS) regarding the river and waterways.

Since 2013 a substantial increase of the yearly total maintenance dredging volume of the area under control of PoR is observed. The problem of this research is the increase in maintenance dredging volume, from an average of 5.2 mln cubic meters a year (over 2005-2012) to an average of 8.9 mln cubic meters a year (over 2013-2016). By analysis of the administrated maintenance dredging volumes database of PoR it is concluded that the problem is concentrated at Maasvlakte I. Including the maintenance dredging volumes data of RWS results in the conclusion that over the entire port area no occurrence of an increase in maintenance dredging volume is observed. A decrease administrated by RWS at the same period of time is concentrated at the area in front of Maasvlakte I, the harbour basin responsible for the increase in maintenance dredging volume of PoR. These findings lead to the conclusion that not an increase of sedimentation over the port area is responsible for the research problem, but a redistribution of the sedimentation rates from the area in front of Maasvlakte I to Maasvlakte I is.

An analysis of the events that are potentially of influence on the research problem is performed. Based on the correlation of time and potential impact on the hydrodynamics of the water system, the event 'Construction of Maasvlakte II' is selected for an assessment. Two simulations with an extensive hydrodynamic flow model managed by PoR are run. One simulation includes the layout of the Maasvlakte before the construction of Maasvlakte II, the other includes the layout of the Maasvlakte as it is today. Both simulation use exactly the same initial and boundary conditions. With use of the simulations, the impact on the hydrodynamic conditions within the area of interest is assessed. The results show a significant increase of the tidal filling volume of the Maasvlakte harbour basins with a factor of 1.4. This increase is associated with in particular a significant increase of the horizontal flow velocities, and strengthened by a higher horizontal density gradient as a result of higher mixing rates of fresh and saline water at the Maasvlakte. The increase of the horizontal flow velocity is in particular measured in front of Maasvlakte I and in the connection to Maasvlakte I itself. Within the Maasvlakte harbour basin, the velocities are quickly dampened by the large width of the basin.

The results of the assessment correspond accurately with the results of the data analysis. At the area subject to an increase of the horizontal flow velocity, a decrease of the maintenance dredging volume is observed. At the area where an increase in maintenance dredging volume is observed, no to slight changes of the flow velocity are measured. This is explained as follows. The increase of the tidal filling volume by the construction of Maasvlakte II, results in an increase of the horizontal velocities over the entire area connecting the North Sea to the Maasvlakte. Sediments that were able to settle within that connection before are now kept in suspension and transport to the Maasvlakte. The sediments kept in suspension reach the harbour basins where the horizontal flow velocities are quickly dampened by the large width of the basin, enabling the sediments to settle.

It is concluded that the dominant mechanism leading to the increase in maintenance dredging volumes at the Port of Rotterdam is a change in local hydrodynamics by the construction of Maasvlakte II, resulting in a redistribution of the sedimentation rates within CaBe-system. A potential reduction measure in the form of a sediment trap is recommended to improve the current situation, but is unable to bring the hydrodynamics within system back to the situation as before the construction. The research problem is one of the consequences of the construction of Maasvlakte II, and hence partly have to accepted as well. A detailed study to the design of the problem specific sediment trap is required. Other studies that are recommended to improve the understanding of the actual problem regard the used dredging strategy, the exact pattern of sedimentation and the development of the composition of the bed material in the area the problem is concentrated.