The background to the rapid increase in population in the basin is the High Economic Growth around 1955-73 (*2). The Tsurumi River basin is adjacent to central Tokyo, and during this period, transport networks (rail and road) crossed various parts of the basin and connected it to the central Tokyo, making it convenient for commuters. This led to a 2.7- fold increase in the basin's population in the 17 years between 1958 and 1975, and the development of residential areas led to a rapid increase in the urbanisation rate (the proportion of urban areas in the basin) from 10% to 60%.
Urbanisation has a negative impact on flood control from two perspectives. First, new development of frequently flooded land (low-lying areas) increases the number of people and properties that may be affected. Secondly, new pavements and embankments on upstream farmlands and vacant lands increase the amount of water that runs off downstream into existing urban areas and allow water to reach them faster, as rainfall is no longer able to infiltrate and the previously inundated water has nowhere to go. In the Tsurumi River basin, flooding also became more severe due to these factors.
In fact, in both 1958 and 1966, during the period of the High Economic Growth, some 20,000 houses were inundated by the floods respectively. Even though the Tsurumi River was designated as an A-class river in 1967 and then the national government started river improvement work to improve safety, subsequent analysis of observation data on the 1976 flood showed that both the size and speed of the floods became severe. The adverse effects of urbanisation were manifesting themselves faster than the river improvement efforts.
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Recommendations based on the Tsurumi River initiative
In line with these points, the following recommendations are considered important when considering and promoting measures in river basins where urbanisation is a concern.
First, it is necessary to take measures to control run-off and limit the spread of damage before urbanisation progresses (or at least before it has completed). It is also important to consider the balance between upstream and downstream areas and to think on a basin-wide scale.
Secondly, it is necessary to continue coordination, discussion and dialogue with enthusiasm beyond the boundaries of national and local authorities. In this process, it is important to explain in a simple manner to be a common understanding that urbanisation may contribute to flood damage, as well as to encourage more enthusiastic discussions among those in charge on any issues that are expected to have an effect on flood control. It is also useful to have academics take the lead in facilitating discussions.
It is also important to have systems to support such discussions and dialogue. Systems for restricting development and guiding land use need to be appropriately prepared.
It is also important, of course, that improvement works by the national government etc. to improve the level of safety and gain the trust of the people and the localities in the basin.
Lastly, the importance of showing the 'effects' of these measures to the people should be highlighted as well. It is important to show how much safety has been improved as a result of the measures taken, so that people can feel the effects of the measures. It is useful not only to show how much the amount of water that can flow through the river improvement works, but also to analyse the data to show how much less damage there was this time compared to past floods, for example, as mentioned above. Simulating how higher river levels would have been if no measures had been taken in the basin is another useful way of communicating the effectiveness. In Japan, in addition to cost-effectiveness analyses in advance for various river improvement works, a simulation is carried out and published every time a flood occurs, showing that the flood would have been even worse this time if those measures had not been taken. The interest of residents in flood control measures increases immediately after a flood, and we believe that by taking advantage of this timing and communicating the effects of the measures, the importance of the measures can be further realised.
SUZUKI Takashi, Advisor for Disaster Risk Management,
Ministry of Agriculture and Rural Development, Vietnam
