In the upcoming decades, the Netherlands is going to face challenges regarding the protection against flooding. The reason for this is the expected rapid change in climate around the world. As a result of the changing climate, the sea levels rise and the peak river discharges increase. These changes lead to higher water levels in the Dutch rivers. To deal with this increase, the current flood defence system has to be improved. A new method for Southwest Delta of the Netherlands is the Delta21 concept. 
The focus of the Delta21 concept is on the Southwest Delta, the region around Rotterdam and the Haringvliet. The idea is to create a lake, the Energy Storage Lake, next to Maasvlakte II in the North Sea. During high discharge conditions in the rivers of the Southwest Delta, the Rhine and the Meuse, the lake can be used to store excess river water. From the Energy Storage Lake, the water can be pumped into the sea. In combination with a closed storm surge barrier next to the Energy Storage Lake, the water level in the rivers upstream of the lake can be lowered. To achieve the lowering of the water levels in the upstream rivers, a spillway is required to let the water flow into the Energy Storage Lake. 
This thesis aims to come up with a conceptual design for the spillway discharging the water from the Tidal Lake towards the Energy Storage Lake. This design must fulfil the functional and structural requirements, must be affordable, constructible and must fit within the ideology of Delta21. To achieve this, the hydraulic engineering design method is used. The first step in this method is to analyse the problem, in which climate change plays a vital role. In the second step, the basis of design is elaborated, including the boundary conditions and the requirements. The main requirements for the spillway are the discharge capacity of 20 000 m3/s and that the water level near Dordrecht may not exceed NAP + 2.5 m.
In the third step, the location of the spillway is first decided upon. After this, four different concepts are generated that differ in, among other things, the type of flow and the way the structure is part of the surrounding flood defence structures. The different concepts are concept Siphon, concept Underflow, concept Caisson and concept Ogee.
The fourth step is the verification of the concepts. The concepts are verified on the functional requirements and the stability requirements. This results in the main dimensions of the concepts. After this step, the concept Ogee is rejected due to stability issues and a large flow velocity at the end of the spillway.
In the next step, the remaining three concepts are evaluated by a multi criteria-analysis and a cost estimation. This evaluation shows that the best concept is the concept Caisson. After the selection of the best concept, the strength verification is performed for this concept and the scour protection is designed.