Bart Schultz
In most cases, however, drainage must still be provided for. This may be the case, on the one hand, to prevent the accumulation of salts in the root zone and, on the other, to achieve the desired open water level or groundwater level during the wet season and the harvest. However, rice fields in lowland areas are in many cases located in polders, because not only drainage must be provided for, but also protection against flooding. In such cases, we are indeed speaking about polders.
Two years ago, during a workshop of the Working Group on Sustainable Development of Tidal Areas of the International Commission on Irrigation and Drainage (ICID), a Japanese participant gave a presentation on Tambo dams in rice fields in Niigata Prefecture on the west coast of Japan. This presentation caught my attention, as it was a very interesting example of integrated water management.
On the downstream side of a rice field, there is usually an adjustable spillway to regulate the water level in the rice field on the one hand, and to drain excess water on the other. In its most primitive form, such a spillway can simply consist of a depression in the clay dike surrounding the rice field, varying in height depending on the desired water level. There are also more advanced versions in the form of sluice gates and movable spillways.
When implemented as a Tambo Dam, a function is essentially added at the level of the entire irrigation system in the form of flood wave reduction. This is achieved by the joint operation of the spillways within the system in such a way that a flood wave is spread out as much as possible, thereby minimizing nuisance or flooding downstream. When this is done jointly across various irrigation systems in a river basin, the application therefore affects the flood wave throughout the entire river basin area.
Such a form of integrated water management therefore requires not only good technical management, but also good coordination between the farmers involved and the water management organization at the system, or even river basin level. In Japan, they are certainly capable of achieving this.
The concept of Tambo dams was first implemented in Japan in 2002 in a small village in Niigata. Because urbanization in Japan is taking place primarily in the downstream sections of rivers, on the one hand, and because climate change is causing more extreme rainfall - resulting in multiple floods in recent decades - the use of Tambo dams has gradually increased since then, particularly in areas that have suffered from flood damage.
Following the coming into force in 2021 of the River Basin Flood Protection Act, the use of Tambo dams became National policy. Simultaneously with the implementation of the Act, the National government launched the River Basin Improvement Programme. Under this programme, flood protection projects are to be realized in 109 river basins, a number of which are to be implemented using Tambo dams. By 2023, 55 of these projects using Tambo dams had already been completed or were in an advanced stage.
The authors of the article presented during the workshop had studied the effect of the application of Tambo dams in the Niigata Plain, finding a significant reduction in the high water peak, which confirmed the effectiveness of the method. It can be expected that Tambo dams will be applied on a much larger scale in the coming years, leading to a significant reduction in flooding and the associated damage. A very interesting example of integrated water management.
