Ditching or draining

In a previous contribution to Flevolands Geheugen, I wrote about Water management of a polder. In short, water management is achieved through a field drainage system, a main drainage system, and a discharge system. Irrigation can also be applied to promote crop germination or growth, or to bridge dry periods.

The field drainage system drains excess water over or through the topsoil to the main drainage system and also determines the desired groundwater level in the plots. In the Netherlands, this system can consist of trenches and/or collector drains, or subsurface drainage, depending on the topsoil in the polder. Trenches and/or collector drains are generally applied for the drainage of peat polders, and subsurface drainage is applied for the drainage of clay polders.

Originally, the layout of the polders in the Netherlands was primarily based on agricultural land use. Especially since World War II, urban development and the creation of nature and recreation areas have also taken place in the polders. In this article, I will discuss agricultural land use.

Peat polders are primarily reclaimed marshlands. Excess water is drained through trenches in the land to the collector drains, or directly from the plots to the collector drains. Collector drains are used to create fields consisting of multiple plots separated by trenches. Direct drainage to the collector drains involves very narrow plots.

In these polders, the goal is to keep groundwater levels as high as possible, as deeper groundwater levels result in faster oxidation and shrinkage of the peat, with all its related consequences. This includes the drying out of wooden pile heads of foundations under buildings and constructions and the emission of CO₂ and other elements. On the other hand, drainage must be sufficient to allow livestock to graze on these lands and facilitate agricultural use. In recent years, there has been considerable discussion and research regarding water management in these polders, particularly in connection with the negative climate impacts caused by oxidation of the peat.

The clay polders primarily concern the lands gained on the sea and the drained lakes. In the clay polders, there is virtually no oxidation, but subsidence certainly occurs during reclamation. However, this is a finite process, which, with the exception of a few areas in Polder Flevoland (Eastern Flevoland and Southern Flevoland), has now come to a halt.

In clay polders, deeper drainage is applied, which is usually achieved through subsurface drainage. In the past, this drainage often consisted of clay pipes, but nowadays, almost exclusively of corrugated PVC pipes, with or without an envelope are used. The subsurface drains discharge into collector drains along the long sides of the plots. Consequently, the plots are much larger than in peat polders.

For centuries, this method has been used to drain agricultural areas in the polders in the Netherlands, achieving excellent results. It will remain crucial to continue to implement proper drainage in the future, as it directly impacts agricultural opportunities.