The effects of the Delta21-project on the alongshore sediment transport and erosion

Abstract

The Delta21-project is a proposed alternative solution to the increasingly higher risk of flooding in the southwestern delta of the Netherlands. Located at the mouth of the Haringvliet, it has three main ambitions: flood protection, storage of renewable energy and the recovery and preservation of nature. Flood protection is achieved by the installation of high capacity pumps, that pump out water from the rivers towards the sea in the case of a high river level, in combination with a closed storm surge barrier during a severe storm at the North Sea. Storage of renewable energy is accomplished by an artificial lake in which turbines generate renewable energy and the aforementioned pump capacity can empty the water back out to sea. The project does however pose various challenges regarding the local ecology, morphology and coastal maintenance. In this thesis, the impact on the latter is studied. The objective is to study the consequences of this Delta21-project on the alongshore sediment transport and resulting erosion volumes. Using a sediment transport proxy, the offshore wave climate nearMaasvlakte 2 is reduced to 100 representative wave conditions. This reduced wave climate is then transformed to the nearshore by the wave-model SWAN, after which the coastline modelling software UNIBEST-CL+ is used to calculate the alongshore sediment transports and erosion for the coastal profile above NAP -8.0m. In Model 0, the models are firstly validated to check whether the models are an accurate depiction of the present conditions in the area of Maasvlakte 2. This validation is done by comparing the measured erosion and performed coastal nourishments for Maasvlakte 2 to that of the modelled erosion. This confirms the model’s validity and applicability for modelling the effect of the Delta21-project in Model 1. In addition, the implementation of tidal data from an external Delft3D-model is modelled. However, due to the inability of UNIBEST to model the tidal effect above NAP -8.0m accurately based on the available data and model, no conclusive remarks can be given on the effect of this. In Model 1, the alongshore transports and erosion due to implementation of the Delta21 project are then modelled. The extent and characteristics are summarised as being similar to those found at Maasvlakte 2. This study furthermore shows that the grain size can have a significant impact, with alongshore sediment transports and erosion a factor of 2.5 higher for a grain size of 160 mm instead of 370 mm. A large temporal and spatial uncertainty and variability in wave energy and consequent sediment transport and erosion for Delta21 can be expected, as was also observed for Maasvlakte 2. This leads to a total bandwidth of the estimated erosion for the current design of the Delta21 coastline in between 0.4 and 1.2 millionm3/year, for a grain size of 370 mm. This study shows that the current layout for the Delta21-project will behave similarly toMaasvlakte 2 in regards to alongshore sediment transport and erosion above NAP -8.0m.