Introduction

One hundred years ago, a long dike was constructed in the North of the Netherlands to protect large tracts of land from flooding.  This barrier dike transformed the Zuiderzee (South Sea) into a freshwater lake known as the Ijsselmeer (Ijssel Lake), and continues to separate the fresh Ijssel Lake from the salt water of the Wadden Sea. Sluices in the barrier dike only allow freshwater to drain into the salty sea water, but do not allow salt water to penetrate into the freshwater lake. This means that at present fish cannot migrate from the sea into the lake.

The Wadden Sea region is a highly appreciated nature area and various parties are concerned with the connection between the Ijssel Lake and the Wadden Sea. Different levels of government (municipality, province and national government) and commercial fishing companies are highly interested in investigating solutions that would restore the connection between the two water bodies while maintaining the safety standard of the barrier.

We are required to design a nature friendly solution for this problem. The solution must provide ecological opportunities for the Wadden Sea and Ijssel Lake while maintaining the flood safety standard and ensuring that the Ijsselmeer can still be used for freshwater supply.

Where is the study area located?

The study area is situated in the Northern part of the Netherlands, where the IJssel lake is separated from the Wadden Sea by a 30 km long barrier dam. Please note that the grey polygon in the map indicates the location of the existing lock and sluices at Kornwerderzand, and not necessarily the geographical boundaries of your study.

Additional information on the problem

• The Afsluitdijk barrier has two openings: the Stevin locks and sluices at Den Oever and the Lorentz locks and sluices at Kornwerderzand.
• The Afsluitdijk’s sluices discharge large amounts of fresh water from the IJssel Lake into the Wadden Sea, but do not permit saline water from the Wadden Sea into the Ijssel Lake.
• The barrier dam forms an obstacle to fish in migrating from the marine environment to freshwater to spawn and reproduce. Particularly herring, anchovies, bone smelt and salmon cannot reach their spawning grounds in the hinterland, and in the upper reaches of the Rhine.
• Fish stocks are also affected by the lack of a salinity gradient. Young fish and larvae also need a brackish water area of about 100 hectares to adapt physiologically from fresh to salt water. Fish-eating bird species are also affected by the declining fish stocks.
• The fresh water discharged from the Ijssel  Lake attracts migratory fish towards the barrier dam. Even if they could pass through the sluices, the current would be too strong for most fish species. They need places along a migratory route with current speeds lower than 0,5 m/s so that they can rest.
• Tidal exchange throughout the year (every day) would be most beneficial to fish.

Some hints

• The construction of the dam affected several species as well as commercial fishing.
• Re-opening the dam is not an option at this stage. The current dam has a safety level of 1/10,000 year and the protection against safety of large areas of the Netherlands would be affected negatively by its removal. It also provides a traffic connection between the provinces of Friesland and North Holland.
• Any connection between the Wadden Sea and the Ijssel Lake should be long enough to allow a gradual transition between salt and freshwater for fish, but the connection should also allow the Ijssel Lake to retain its freshness (not become brackish). Therefore, it is advisable that the sluices are not open (fully) during flood tide in the Wadden Sea.

Additional knowledge clips on river and coastal systems, and river and coastal interventions

Have a look at the videos below. These videos are part of the MOOC Water and Climate, also from Delft University of Technology. They will provide you with additional knowledge on river and coastal systems, and river and coastal interventions.