Katwijk Coastal Defence

The North Sea coast is the epitome of Dutch expertise in the field of coastal defence. Most of the Dutch coast is protected from the sea by dunes, supplemented by the well-known delta works consisting of movable dams and sluices.  

In recent decades, the Dutch Ministry of Infrastructure and Water Management has reinforced several weak links along the Dutch coast. In many places, dunes and dikes have been heightened or widened to guarantee the low-lying hinterland's safety for the next 50 years. This means that the water defences must be able to withstand normative water levels with a probability of occurrence of 1/10,000 per year. 

The coast at Katwijk was one of the weak links; the village was situated mainly outside the dikes. The need to strengthen the coast was seized upon to develop the village into a tourist attraction. 

Relation between village and beach 

For the coast to be safe, it was necessary to widen and raise the dunes. In the first proposals, engineers suggested a dike. A dike would offer safety, but the distance between the village and the sea would increase, and in many places the view of the sea would be lost. 

OKRA's design for the coastal reinforcement of Katwijk was the result of an extensive interactive process with the residents and entrepreneurs of Katwijk. In several meetings, the wishes and ideas of the population were inventoried. One of the most important pillars of the plan was distilled from these, namely, the connection between the village and the beach. Also, a large number of parked cars on the boulevard was mentioned as a problem. 

OKRA has focused on creating the best possible relationship between the coastal village and the sea, which was made possible by a dike-in-dune coastal defence. As a result, a low row of dunes is sufficient. The wide landscape dune offers the same safety as a higher narrow dike and fits the landscape much better with the seaside resort. Katwijk is a friendly seaside resort that is more suited to a dune than a rigid coastal defence construction. 

A second important cornerstone of the plan is the integrated approach to the area. In our integral design, we have chosen to combine the problems of parking in the seaside resort, the tourist-use of the beach and sea, and the ecological value of the coast. The dyke-in-dune coastal defence made it possible to build an underground car park behind the dyke and to integrate it into a natural-looking dune landscape. An intensive network of footpaths and cycle paths has been constructed to connect the village and the beach and provide a view of the sea.  

The heart of the coast 

The dune crossings that form the entrance to the beach are designed to be accessible to everyone and to provide a direct route to the beach. A long path of shells has been laid in the new dune landscape. The path follows the relief of the dunes and offers alternating views of the sea and the village. Near the centre of Katwijk, this path widens and becomes a wooden boardwalk with viewing points and seats. The dune path, the boardwalk and the dune crossings form a functional and recreational network of paths, which can be seen as an extension of the street pattern of Katwijk. 

The close relationship between the seaside resort and the sea is obvious at the centrally located dune crossing, which provides a comfortable passage from the village's main street to the beach. The wide dune crossing is the highlight of the design. Seating objects make it a place to stay. At the seaside, there is a beach square, where events are organised. This area forms the lively heart of the coastal town. 

Natural dune landscape 

The coastal reinforcement itself consists of a dike-in-dune construction. A stone-clad dike with a 1:3 gradient is covered and reinforced by a seaward row of dunes. The new dune landscape is approximately 120 metres wide and is laid out in a seaward direction from the boulevard. The dike's height is NAP +7.5 metres, and the dune that lies over the dike varies from NAP +8 metres at the height of the centre to NAP +11 metres locally NAP +12 metres at the height of the northern and southern sections of the boulevard. Small sloping dune tops alternate with light dune pans. The average height of the dune meets the normative calculations. Adjacent to the project area, the new dune landscape connects to the natural dunes.  

A special aspect of the project is the construction of a parking garage landside of the dike. This garage (designed by RHDHV) offers space for more than 550 parking places. Because OKRA has positioned it in the dune and has pulled the dune over the parking garage, it is incorporated into the dune landscape and hidden from view. 

The sand package of the new dune on the sea-side of the dike is dimensioned in such a way that during a storm, the sand package can break away to the dike construction. When the new dune landscape was created, the beach was widened by 80 to 100 metres seaward. The original width of the beach has thus been restored, as has the beach bank. On this 30-metre-wide zone, at NAP +4.5 m, the beach pavilions were returned to their designated spots.  

The intention is for the dune to develop naturally and for succession in the vegetation to occur. The top layer of the existing dune was excavated to accelerate the development of natural dune vegetation. The top layer was restored after the sand suppletion and profiling of the new dunes. The dunes have been encrusted with marram grass (Amonphila arenaria) and enriched by the top layer's seeds to form varied dune vegetation. Sea buckthorn (Hippophae rhamnoides), dog rose (Rosa canina) and eglantine (Rosa rubiginosa) will soon establish themselves.  

Unique collaboration 

The people of Katwijk have significantly contributed to the realisation of this project. Through good cooperation and considering the coastal defence of Katwijk as a joint project, the commissioners were able to achieve more than with other coastal defence projects in the Netherlands. Intensive cooperation between authorities, experts and other stakeholders has led to a unique project in which the opportunities that existed have been seized.