Bart Schultz
Horseshoe-shaped polders are polders that are protected from flooding by dikes on the upstream side and along the adjacent rivers. On the downstream side there is, or originally was, no dike, because this was not necessary at least initially due to the slope in the landscape. In this way, the dikes created the shape of a horseshoe. The excess water can or could flow freely to the river on the downstream side. On the upstream side there is one or more inlets for irrigation water, which makes more or less excellent water management possible.
Since a certain degree of subsidence has also occurred in the polders in the Irrawaddy Delta after reclamation and there is a gradual rise in sea level, a dike has now also been constructed on the downstream side in a number of the polders, usually with a drainage sluice in it.
A danger to this type of polder is formed by cyclones, during which water from the sea is pushed into the river mouths and thus also into the downstream side of the polders. This happened, for example, in 2008 during cyclone Nargis. This cyclone caused 140,000 casualties and 770,000 hectares of rice fields were damaged. The damage was caused by flooding and the ingress of salt water.
In the polders that are completely surrounded by dikes, drainage takes place under gravity. The water level in the drains fluctuates depending on the discharge of excess rainfall, river water levels and management of the discharge sluices. During the wet season, rainwater usually accumulates in the polders. Where possible, excess water is discharged via the discharge sluices to the adjacent river. During the dry season, the water level in the canals in the polders is usually lower than the water level in the adjacent rivers. In the coastal zone, these levels are influenced by the tidal fluctuation.
When salinization problems may occur, the sluices must be operated in such a way that the salty river water cannot enter the polders. Therefore, there are two types of gates in the sluices: automatic flap gates on the river side to prevent river water from flowing into the polder, and vertical gates on the polder side to regulate the storage of fresh water and to discharge excess water when necessary. The gates in the polders in the Irrawaddy Delta are operated according to a simple rule. That is, the gates open on May 15 and the gates close in the second half of September. The gates usually open automatically due to the difference in water level.
As mentioned above, a certain degree of subsidence is also taking place in the polders in the Irrawaddy Delta. This can be caused mainly in the peatlands by oxidation and subsidence of the topsoil, but especially in the urban areas by deep extraction of groundwater, which can cause the overlying clay layers in particular to subside. For example, in the capital Rangoon, a subsidence of two to eleven centimetres per year has been measured. This is considerably faster than the current sea level rise, which is in the order of magnitude of 0.3 centimetres per year.
