Bart Schultz
I have previously contributed a story to Flevolands Geheugen about seepage. This time, it is precipitation. Of particular importance here are the annual precipitation and the precipitation under extreme conditions. The annual precipitation and its fluctuations are especially important for determining how much water must be discharged annually through the discharge sluices or pumping stations. Additionally, the fluctuation over the course of a year is important for determining how well the desired water level in a polder can be maintained.
Precipitation under design conditions is particularly important for determining how much water needs to be stored in the polder's watercourses under such conditions, in combination with the amount of water to be discharged. In a number of polders with a clay topsoil, significant storage can also take place in the ground during design conditions. This is the case in Polder Flevoland, among other places, due to the clay soils containing many cracks.
The availability of long-term precipitation measurements is therefore of importance. This is generally the case in the Netherlands. For instance, a series is available from 1735 to the present, with measurements taken at Huize Zwanenburg in Halfweg from 1735 to 1866 and at Hoofddorp from 1867 to 2024. A problem here, however, was that a different rain gauge was used in the past than today, and that this affected the measured values. The older measurements have been corrected by A. Labrijn, a former employee of the Royal Meteorological Service of the Netherlands (KNMI). A gradual increase from an average of 680 to 890 millimetres per year can be seen in the values.
As stated, precipitation under extreme conditions is the determining factor for establishing the dimensions of the water management system. This was generally based on the highest daily precipitation in a year, on the basis of which the extreme nature of the precipitation was subsequently determined using a probability distribution.
The daily figures on which the aforementioned series is based are directly available only for Hoofddorp. Based on these figures, I have determined the series of the highest daily figures. These figures range from 11.7 mm/day (millimetres per day) in 2022 to 68.3 mm/day in 1972. A large margin is therefore apparent in the figures, as can be deduced from the distribution of these figures.
Across the Netherlands, the KNMI has an observation network consisting of 51 automatic weather stations, 37 of which are on land and 14 at sea, and 300 rain gauges that are read manually by volunteers every morning at 8 a.m. Since the beginning of 2025, the KNMI has started replacing the manual rain gauges.
The pumping capacity for the polders in the Netherlands varies considerably depending on the local situation, but averages around 15 mm/day counted over the area of the polder. When we compare this with the highest annual daily precipitation figures, as I have shown above for Hoofddorp, it appears that in about ninety percent of cases, the majority of precipitation per year amounts to less than 45 mm/day, which can then be pumped out over three days. Consequently, it may be necessary to temporarily store about 30 millimetres in the polder to be pumped out over the following two days.
However, two other factors play a role here. First, account must also be taken of the precipitation that occurs before and after the peak day; this can therefore increase the amount to be pumped out. However, it is also the case that extreme showers often occur locally and thus, especially in the larger polders, will not fall on the entire polder surface, which in turn can have a reducing effect.
As a resident of Lelystad since 1973 and interested in water management, I have visited relevant locations several times whenever a heavy shower occurred or had just fallen. Actually, I have only witnessed minor flooding once, and that was at the De Gordiaan shopping centre in 1998. At that time, there was still a sunken section of road there, which was later modified. On one occasion, there was an inundation for several hours, which was pumped away by the fire department.
I have also seen a few times that the street drain in front of my house, through which precipitation from the road is discharged into the rainwater sewer, could no longer cope, and that the water reached the edge of the sidewalk. That usually disappeared within an hour.
In this context, the weir in the urban canal on the north side of the Buitenplaats in Lelystad is also an interesting point. There you can see how high the water can stand after a heavy shower at a major discharge point of the urban canals, which for the most part have a level 80 centimetres above the polder water level in the watercourses in the surrounding rural area. Here, I have never seen an increase of more than about 20 centimetres. All in all, it therefore seems that Lelystad's water management system is well capable of handling extreme downpours.
