Design of the navigation and discharge channel in the tidal lake of the Delta21 project

Abstract

The awareness of climate change has grown worldwide in the past years. Possible consequences of climate change are sea level rise, heavier storms and high river discharges, but also prolonged drought. For a country such as the Netherlands, these water level variations, high discharges and droughts could have a great impact. Therefore, solutions of flood protection and water management are required to cope with these possible consequences.

The Delta21 project proposes a solution for flood protection of the Southwest delta in the Netherlands, while contributing to energy storage and nature preservation. The Delta21 project is a dam structure located in the Haringvliet mouth and, when constructed, results in a large change of the environment in the Haringvliet mouth. The remaining area of the Haringvliet mouth becomes the tidal lake. The important functions of the tidal lake are to be navigable and to have the capacity for extreme discharges to flow through the lake, without posing threats to safety. The functions are fulfilled by a channel through the tidal lake. The current channel (Slijkgat) requires highly frequent maintenance to allow for navigability and contains bottlenecks to meet the discharge capacity function. Hence, the objective of this research is to determine a suitable channel configuration in the tidal lake of Delta21, considering navigability and discharge capacity.

A suitable channel configuration is determined by designing a channel, accompanied by a morphological modelling study. The functional requirements and criteria for the design are defined based on the stakeholders interests. The requirements and the expected hydrodynamic and morphodynamic conditions in the tidal lake have resulted in three channel concepts that are considered for the design process. One of the concepts is the current bathymetry of the Slijkgat. The other concepts are Channel Concept 1 and Channel Concept 2, which are the same in cross-sectional area but different in orientation and location. The orientation of Channel Concept 1 is a small adjustment from the current Slijkgat, where the bend is slightly straightened. Channel Concept 2 is an almost straight channel and has the shortest length through the tidal lake.
The numerical modelling study is applied on the design in order to determine the morphological response of the system to the concepts and indicate the influence on the navigability, discharge capacity and maintainability. A Delft3D-FLOW model is applied for two simulation periods; normal conditions of one year of morphological modelling and extreme conditions for one week. The normative hydrodynamic drivers for normal conditions are the tide and the river discharge and for the extreme conditions only (extreme) river discharge.

The hydrodynamic and morphological results from the modelling study are processed in order to evaluate the three concepts in a qualitative and quantitative manner. The evaluation criteria are based on navigability, maintenance and nature preservation. Based on the results, Channel Concept 2 is most suitable in terms of navigability and maintenance with respect to the other concepts. Therefore, evaluation has led to the selection of Channel Concept 2 as the most suitable channel configuration. Additionally, an optimisation is proposed for the selected channel concept in order to improve the fulfilment of the maintainability criteria. A dam around the north western end of the channel is proposed in order to decrease the sedimentation volume in the channel. Modelling computations including the dam resulted in a significant decrease of sedimentation in the channel. Hence, the dam is an improvement to the selected channel concept.

Concluding, Channel Concept 2 is considered the most suitable channel configuration in the tidal lake with respect to the other concepts. Implementing the dam in the tidal lake further improves the channel concept in terms of maintainability and navigability.