Bart Schultz
For protection against flooding, there is often a sea dike and where necessary there are also river dikes. The water management system usually consists of an irrigation system and a drainage system. This makes the construction and maintenance of these polders relatively expensive. The big advantage in Malaysia, however, is that due to the favourable climatic conditions, with good water management, three harvests per year are possible. This means that the yields per year can be higher than under temperate climatic conditions. There are also a number of polders with mainly oil palm plantations. There, the yields are apparently such that these polders can be managed properly.
The irrigation system in the polders is usually fed by water from the adjacent river. Malaysia is located in the humid tropics, so there are only very exceptional cases of water shortage in the dry season. However, the drainage system must have a large capacity, because in the wet season – the wet monsoon – the polders can experience quite extreme showers.
The advantage of most polders is that they are located directly on or close to the coast and that there is a considerable tide at sea and in the estuaries. This allows for good drainage at low tide. This drainage usually takes place by means of discharge sluices with flap gates. When necessary, these flap gates open automatically when the outside water level is lower than the water level in the drainage system of the polder. This means that relatively cheap drainage is achieved.
However, there is a special story with the oil palm plantations. If these plantations are established on clay soil, there is usually not much going on and the polders can be managed properly as described above. However, if the plantations are on peat soil, which is often the case, then there is rapid oxidation and subsidence of the soil.
In the humid tropical climate zone, this oxidation and subsidence generally occurs at a rate of 10-15 centimetres per year. If there is a thick peat layer, the soils will continue to subside at this rate. In many of these polders, this means that after about twenty to forty years, drainage through the discharge sluices is no longer possible. Drainage must then be by pumping, which is a very expensive affair under the prevailing climatic conditions with 2.0-2.5 metres of rain per year. In a number of cases, the areas are then abandoned. What remains is a kind of battlefield with crooked or even fallen palm trees. It will take many years, if not centuries, before these areas have recovered naturally.
I have several times been to Malaysia over the years and have visited several of the polders. They have often become very prosperous areas with beautiful rice fields. The oil palm plantations on the clay soils also generally look excellent. However, the oil palm plantations on the peat soils clearly show that such a development in other peat areas can better be prevented.
