Initial morphodynamic changes in the Voordelta in response to the Delta21 interventions

Abstract

In recent years, the awareness of climate change and sea level rise has grown. Especially in low lying countries like the Netherlands, with a large river delta with a high economical and natural value, extreme variations in water levels will have great impact. Former used solutions proofed to be expensive and in some cases disturbed the dynamics of nature. Consequently, the need of new solutions of flood protections grows in which the negative impact on nature environments should be limited. The Delta21 plan proposes a future-proof design for the Dutch Southwest delta, that aims to tackle the growing need for flood protection, while contributing to nature and storing energy all at once. However, the coastal structure, located in the Haringvliet mouth, is likely to produce large disturbances in the environment. Since the Haringvliet mouth is located in the 'Voordelta', a Natura2000 area, it is essential to study the initial response of the proposed interventions to the hydraulics and morphodynamics that shape the system. Hence, the objective of this research is to determine the initial impact of the Delta21 plan on the large scale multi-year average hydro- and morphodynamics of the Northern Voordelta. To this end, a literature review was performed to analyse the evolvement of the Northern Voordelta since the closure of the Grevelingen and Haringvliet estuaries by the Delta Works in 1970. This study showed that the closure caused the tidal prisms to reduce strongly resulting in the tidal propagation to change from cross­shore to longshore. The remaining ebb tidal deltas started to follow the characteristics of a short basin, in which water levels offshore and nearshore are in phase. Flow velocities reduced strongly and channels started to fill up, at the foreshore waves started to dominate the morphodynamics. Waves began to 'bulldoze' the former ebb-tidal deltas onshore, creating shore parallel sandbars like the Bollen van de Ooster and the Hinderplaat. Besides, the construction of the Maasvlakte 2 triggered further sedimentation of the Haringvliet mouth, landward of the Hinderplaat, by sheltering the area from NW waves. The initial response of the system to Delta21 has been studied using a short-term morphostatic 2DH computational model simulation. The complexity of the processes inside the study area caused the need of the process based model Delft3D, in which flow, waves and sediment transport computations can be carried out. To reduce computational times, the applied modelling setup made use of reduction techniques and schematisations of the hydraulic forcing, while maintaining the natural character of the system. The computed flow and sediment transport patterns were used as indicators, in order to predict and compare the feedback of the system of the various scenarios. Model results showed that the tidal propagation inside the Haringvliet estuary, which is opened in the Delta21 scenario, changes to cross-shore again. Similar to the situation prior to the closing, the area will start to follow the characteristics of a long-basin. This implies that water levels in the estuary are out of phase with the offshore water levels. A cross-shore tide dominated flow prevails in the area, while the Grevelingen delta remains unchanged in the Delta21 scenario. The cross-shore action influences the offshore wave- and setup-driven residual currents. In the same offshore area, the Delta21 pump discharge develops a similar cross-shore acting influence area. Model results further showed that the Delta21 geometry causes a 'tidal squeeze', similar to the present-day processes around Maasvlakte 2. Altogether, the large disturbances to the hydrodynamic forcing, triggers morphodynamic activity. Concluding from literature and the model results, this leads to the erosion and formation of ebb and flood channels in the Haringvliet mouth and the formation of an ebb- and flood-tidal delta around the inlet. Processes in and around the ebb-tidal delta adjust towards a balance between the wave and tidal action. These findings agree with the known processes of a mixed-energy tidal inlet system that is adjusting towards a dynamic equilibrium. Conceptualised results are shown in Figure 1. Concluding, the analysis on the initial response of the system, as well as the expected initial dynamic behaviour, provides useful insight and a good basis for future optimisation studies on the Delta21 design. This study reveals the disturbed areas caused by the presence of Delta2 l and relates it to the contribution of the various hydrodynamic processes. Furthermore, the development of this type of study can be significantly relevant to the evaluation of other preliminary studies on solutions to sea level rise.