The impact of the Rhine ROFI on the alongshore variability in cross-shore sediment transport of the Holland coast

Abstract

The Rhine ROFI ( region of freshwater influence) is a dynamic area where various processes and timescales come together. Research has shown that stratification caused by the freshwater outflow of the Rhine alters the tidal currents in front of the Dutch coast. In the case of the Netherlands the tide behaves like a kelvin wave, which causes rectilinear tidal flow. If the area is stratified this rectilinear flow becomes elliptic, the cross-shore component of the flow increases. As this happens the top and bottom layers of the flow become decoupled due to stratification. This results in two counter rotating ellipses. This behavior causes a cross-shore velocity shear in the water column. From high to low water the velocity shear in the bottom layer is offshore directed , from low to high water it is onshore directed. Research has shown that this cross-shore velocity shear causes extra sediment transport. This thesis describes the impact of the Rhine ROFI on the alongshore and cross-shore variability in annual cross-shore sediment transport in the depths from NAP-12 to NAP-20m between Hoek van Holland and Noordwijk. The stratification that drives the formation of tidal ellipses is dependent on the freshwater discharge from the Rhine, the spring neap cycle of the tide , the ebb-flood cycle and breakdown of stratification by wind and waves. To see the effect of stratification on the velocity field a D-Flow-FM model is used to simulate a two week period. This model includes the tide, wind and discharge. The necessary wave parameters to calculate the sediment transport are derived from a Delft3D-Wave model. This is an offline approach, so the impact of the waves on the stratification and flow is not taken into account. The flow field and waves are used to calculate the sediment transport with the total load Soulsby-Van Rijn formula. Five scenarios are run to assess the impact of different discharges. A reference scenario without the impact of stratification, three scenarios to simulate a year and an extreme scenario to see the impact of very high discharge. The effect of the wind is investigated by using a constant wind from the South-East and a no wind condition. From research it is clear that strong winds and high waves break down stratification. Due to the offline approach the effect of waves are not taken into account on the flow field. This has been taken into account in the sediment transport calculation by using flow results from the reference case that are without the effect of stratification for waves higher than 1.5m. The results show that there is an impact of the ROFI on the cross-shore sediment transport especially near the river mouth, this effect reduces in the direction of Noordwijk. The gradient in cross-shore sediment transport in the alongshore direction is negative, the transport reduces slowly in the direction of Noordwijk. The transport in cross-shore direction shows a high gradient near the river mouth at Hoek van Holland and a zero gradient further to the North. The results show that the effect of different discharges that represent the seasons is small. The overall impact of the ROFI is limited with its effect reducing in northward direction. The results demonstrate an onshore cross-shore sediment transport at the NAP-20m depth contour indicating that this area feeds the coast with sediment and it would have detrimental effects on the long term if this area was dredged. The overall results demonstrate onshore cross-shore sediment transport over a large part of the middle shoreface of the Holland coast.