Bart Schultz
Eastern Flevoland was the first to be reclaimed. The ring dike was built between 1950 and 1956. In terms of water management, this part of the polder consists of the High and Low Sections. The preferred water level in the High Section is 5.20 metres below NAP (Normal Amsterdam Ordnance Datum) and in the Low Section 6.20 metres below NAP. These levels were chosen to ensure that the polder could still be properly drained after the expected subsidence. However, there are areas with a higher water level, especially on the south side of the polder, which was achieved by means of weirs. The main canals and some submain canals that discharge into the main canals were dredged under water to ensure that the water to be pumped out could be properly conveyed to the pumping stations after the ring dike was closed.
There are three pumping stations in Eastern Flevoland: Wortman near Lelystad, Colijn on the eastern side, and Lovink near Harderwijk. Colijn and Lovink are electric pumping stations, while Wortman is diesel powered. Colijn can pump out water from both the High and Low Sections, Lovink can only pump out water from the High Section, and Wortman only from the Low Section. The discharge capacity of the pumping stations is based on the situation after the polder was drained and reclaimed. This meant that after the ring dike was closed and the pumping operations began, it took approximately nine months to pump out the water layer of about four metres.
Southern Flevoland was reclaimed later. Dike construction here took place from 1959 to 1967. The Knardijk, originally the western dike of Eastern Flevoland, was used as part of the ring dike for Southern Flevoland as well. In Southern Flevoland, the main canals and some of the submain canals were also dredged under water. Only the pumping station De Blocq van Kuffeler was added. This was originally a diesel powered pumping station, but is now also an electric pumping station that pumps out the excess water from the High and the Low Sections.
The total discharge capacity of the four pumping stations is 8,150 cubic metres per minute. This means that, converted over the entire area of Polder Flevoland, they can pump out a water layer of 12 millimetres per day. With the storage in the open watercourses and especially in the cracks in the top layers, this is such that there is virtually no need to worry about inundation due to rising water levels.
In Flevoland, the towns of Biddinghuizen, Dronten, Swifterbant, Lelystad, Almere and Zeewolde have been developed and are in the process of being further developed. In Lelystad and Almere in particular, most building sites have been raised with a metre of sand to allow building during summer and winter. While this has led to additional subsidence, the urban areas in question are still generally higher than the surrounding rural areas. In Lelystad, this has resulted in a water level of 5.40 metres below NAP being maintained primarily in the urban canals. Excess water is discharged via weirs into the Low Section.
In Almere, a similar situation exists. For example, Almere-Haven has predominantly a water level of 4.80 metres below NAP, and Almere-Stad has a water level 5.50 metres below NAP. This allows Almere-Haven to discharge freely into the High Section, and Almere-Stad into the Low Section, but there is also a pumping station to pump water to the High Section. When the development of Almere started, there were concerns that the accelerated discharge from the urban areas would overload the watercourses in the rural area. We then performed non steady calculations, which showed that under extreme conditions, the water surge from the urban areas discharged faster than the surge from the rural areas. Consequently, the Zuiderzee Project Authority, which was then responsible for managing the water in main system, approved the main principles of the proposed water management in Almere.
The Oostvaardersplassen and several other nature reserves are preferably kept marshy, allowing excess water from these areas to be discharged freely into adjacent open watercourses in the rural areas. Water in recreational areas and forests is generally in open connection with the watercourses in the rural areas.
This system, combined with the soil structure in Polder Flevoland, allows for adequate water management in both dry and wet periods. The Polder Flevoland is still in a developing stage. Time will tell which adjustments will be necessary in future.
