Bart Schultz
Regarding the canals, the requirements for the system in a new urban area were first of all important. Furthermore, the question of how the system should be integrated into the overall system of Polder Flevoland was a key consideration. Regarding the expected changes in the groundwater regime, in addition to the impact on the expected soil subsidence in the Almere area, the main concern was the drop in the groundwater level in the Gooi region.
In Polder Flevoland, water management consists of two sections: the High Section, with a preferred water level of 5.20 m-NAP (Normal Amsterdam Ordnance Datum), and the Low Section, with a preferred water level of 6.20 m-NAP. See my previous contribution to Flevolands Geheugen about the Pumped discharge of Polder Flevoland.
Regarding the canals, the main questions were which preferred water levels needed to be chosen in the various urban areas of Almere, what the percentage of open water would have to be, and how the canals in the urban centres would have to be connected to the main canals in the rural areas of the polder. The surface level in the urban areas and the expected amount of subsidence also played a role. It was important that, in the initial phase, the vast majority of the building sites were raised with a metre of sand, primarily to allow construction to continue throughout both summer and winter. The expected total subsidence in this situation at the time was over a metre.
Based on the available information, a preferred water level of 4.80 m-NAP was chosen for Almere-Haven, 5.50 m-NAP for Almere-Stad, and 5.80 m-NAP for Almere-Buiten. The preferred water levels for the two other urban areas could be determined later. A glance at the recent water level map of the Water Authority Zuiderzeeland shows that these levels are still maintained, more than forty years later. With these chosen levels, Almere-Haven and Almere-Buiten could discharge over weirs into the Hoge Vaart and Lage Vaart canals, respectively. Excess water from Almere-Stad had to be pumped out to the Hoge Vaart. For this purpose, the pumping station, aptly named Leeghwater, was built in 1981, after the famous 17th century hydraulic engineer Jan Adriaansz. Leeghwater. See my previous contribution to Flevolands Geheugen about Leeghwater's plans for draining the Haarlemmermeer.
For the separate sewer system that was to be implemented in Almere, excess rainwater would be discharged via underground sewers to the urban canals where a minimum of at least three percent open water was deemed necessary. The domestic and industrial wastewater would be discharged to a wastewater treatment plant, In Almere-Stad, a higher percentage was ultimately achieved by incorporating the Weerwater into the system.
Regarding the integration of Almere's water management system into the overall system of Polder Flevoland, there was concern that the accelerated discharge from the urban areas would create an additional burden on the canals in the rural areas. Using a specially developed non-steady state computer model, we calculated extreme situations in the canals in the High and Low Sections. This revealed that the discharge wave from the urban areas had already been partially discharged from the polder before the discharge from the rural areas started. Permission was subsequently granted to construct the systems in the urban areas on this basis.
Regarding groundwater, it was important to realise that Almere was built on a subsoil consisting primarily of a nearly five metres thick and virtually impermeable clay layer. Beneath this clay layer lies a more than one hundred metres thick, highly permeable, sand layer. The significant potential changes in the groundwater regime in the clay layer concerned the expected subsidence. Regarding the sand layer, the main question was what groundwater levels could be expected in Almere itself and in the adjacent Gooi region.
To gain insight into this, we used a specially developed steady-state computer model to simulate the situation before and after the completion of the development of Almere and its associated canal system, with the urban canals being a key factor. This revealed that the construction of the urban canals in Almere-Haven could lead to significant drops in the groundwater level in the sand layer and subsequent subsidence of the overlying clay layer. A certain drop in the groundwater level was also expected for the Gooi region. Consequently, the decision was made to provide the urban canals of Almere-Haven with a virtually impermeable base. Later calculations revealed that such an expensive construction would not be necessary for the canals in the other urban areas of Almere, as these areas are located further from the dikes.
Based on the principles described here, Almere's water management system was constructed and gradually expanded in conjunction with population growth. We are now close to the maximum population of 250,000 that was initially assumed. Further population growth and climate changes may necessitate further adjustments in the system in the near future.
