Bart Schultz
In addition, water can be admitted during dry periods, water can enter through shiplocks, and leakage can occur through hydraulic structures in the dike. Of these components, besides rainfall, dike seepage and deep seepage are generally the most important for deeper polders. This is especially the case in the drained lakes – reclaimed areas that were previously permanently under water – because these are the deepest polders.
To prevent problems with dike seepage, a seepage ditch is normally located on the polder side of the dike. This ditch ensures that the groundwater flow through the dike between the outer and inner sides is such that the dike's stability is not compromised. In the IJsselmeerpolders, where the difference between the outer and inner water levels can be more than six metres, a seepage ditch alone was insufficient. Subsurface drainage has been installed between the seepage ditch and the inner side of the dike to keep the groundwater level on the inner side of the dike sufficiently low.
As far as I know, Jan Adriaansz. Leeghwater was the first to recognize the phenomenon of seepage. In the 4th edition of his Haarlemmermeer book from 1641, he wrote:
"I am well aware that there are some lakes that are not successfully drained; but there is a reason for this: either because the clay is too deep, or because these lakes are situated on mountainous terrain or coarse sand, which does not retain water, as I have seen and found; or because the ground is raised and covered with shrubs or tree leaves, and therefore remains leaky and spongy. It also appears that some of these lakes do not freeze solid, no matter how hard the frost is; which is a sign that the ground is open, spongy, and leaky."
Various methods have been used to determine the total seepage rate in a polder. For example, based on data from existing reclaimed areas during the winter months, when evaporation is low, the seepage rate was determined based on precipitation and the amount of water pumped out. Dike seepage can be easily determined by measuring the discharge of the seepage ditch. Deep seepage is more difficult to determine. Various formulas and methods have been developed for this purpose over the years.
As far as can be determined, the first study of deep seepage was conducted around 1901 by Reintjes in the Groot-Mijdrecht Polder. By damming ditches, he temporarily divided the polder into sections. Seepage in the central section proved to be considerably smaller than near the dike, but still amounted to approximately 5 millimetres per day. Later, various measurements of seepage in polders were conducted in a similar manner. Of particular note are the ice surveys conducted at various locations in the Noordoostpolder and the Flevopolder. These surveys assigned a value to the amount of seepage based on the quality of the ice in the ditches.
The first study on expected deep seepage was conducted prior to the draining of the Wieringermeer. This study concluded that deep seepage would have a limited impact on the required pumping capacity, but a significant impact on the total annual water to be pumped out. Seepage is a more or less continuous process, while precipitation has more extreme periods that determine the required capacity of the pumping stations.
The first formulas for determining deep seepage also originated from the aforementioned study for the Wieringermeer. In the studies preceding the reclamation of Oostelijk Flevoland, extensive attention was paid to deep seepage. This was not so much due to its impact on the water management system of the reclaimed land, but rather to the potential drop in groundwater levels in adjacent areas. The reason for this was that the reclamation of the Noordoostpolder had caused desiccation damage in the adjacent areas. To prevent such damage, it was decided to construct peripheral lakes - Veluwemeer, Wolderwijd, and Gooimeer - between the polder and the so-called old land. Similar studies were also conducted to investigate the potential impact of the construction of Almere on the groundwater in the Gooi region, as well as effects of a potential reclamation of the Markerwaard on the groundwater in the adjacent area of the Province of North Holland. In the latter studies, deep seepage was determined using computer models. Such models have also been applied in existing reclaimed lands to facilitate potential adjustments to water management.
To the extent possible, I investigated the seepage rate for each individual drained lake for my dissertation on Water management of the drained lakes in the Netherlands. The collected data showed that seepage ranged from 0 to 25 millimetres per day. However in the vast majority of the drained lakes seepage does not exceed half a millimetre per day. For some large drained lakes, such as the Beemster, the Noordoostpolder, Oostelijk Flevoland, the Wieringermeer, and the Zuidplaspolder, seepage values range between half a millimetre and one millimetre per day. Drained lakes with high seepage are primarily located in the provinces of Overijssel, Utrecht, and South Holland. Examples include the Bethunepolder, the reclaimed lands near Giethoorn, the Horstermeerpolder west of Hilversum, the Koekoekspolder near Kampen, and the polders of Mijdrecht.
