From nature’s point of view, flooding and the associated downstream movement of sand and earth are the natural workings of the river forming alluvial valley floodplains. Generally, floodplains offer favorable living conditions for human existence because water is easily available, the land is suitable for cultivation, irrigation is easy and navigation is available. The irony is that due to the very geographic characteristics that are a blessing to their inhabitants, alluvial valley plains are subject to frequent flooding. Without flooding, there could be no sand and earth brought down to enrich the land, so that these areas would not provide favorable conditions for settlement.
Rivers have been a mixed blessing to humankind, and that is why we have tried to tame them with technology. Our efforts, however, have created a new problem: we have been too successful, and have significantly disrupted the cycling of materials by rivers and damaged their ecosystems. Due to enormous dams, many rivers no longer flow naturally. Some rivers have gone dry and many banks are covered with concrete dikes. In short, many rivers have been turned into cold, uninteresting ditches detached from our lives.
In the old days of the Edo era (1603-1868), people knew how to coexist with rivers. For example, by accepting occasional, inevitable flooding, they intentionally allowed the water to overflow in sections where relatively little damage would be done. In this way, overflow was avoided at locations where flooding could collapse the levee and cause great damage. Forest belts were laid along rivers to further reduce flood damage. In these buffer zones, special types of soil were employed as construction material while the surfaces of the levees were also carefully designed. When high water overflowed through it, the forest belt weakened the force of the flow and stopped debris, gravel, and large-particle sand and earth. As a result, the overflow, once out of the forest, contained only fine sediment and was slow moving. Although farmland was submerged, farmers in the Edo Era welcomed a flood if it occurred only about once a decade because the sediments it left enriched their fields.
To cope with possible house flooding, houses with elevated floors were built on mounds and evacuation boats kept available for emergency. Such a combination of measures to minimize flood damage were commonly adopted in many parts of the country in the Edo Era and can be considered a highly-developed form of culture born out of the necessity to live with rivers which people depended upon but which sometimes threatened their existence.
The Engineering Era Dawns
In the middle Meiji Era (1868-1912), modern civil engineering techniques were introduced to Japan, making it possible to attempt to control rivers that could not have been controlled and to develop hitherto undevelopable floodplains. This in turn prompted an explosive increase in the numbers of people moving from relatively safe areas to those more subject to flooding.
Over time, a modern industrial culture evolved that demanded regular, scheduled commuting and production regardless of the fluctuations of the activities of nature. This gives rise to a social attitude intolerant of even small-scale floods. These changes changed people’s perception of natural disasters.
This attitude of zero tolerance for floods came to demand equal protection from floods irrespective of the geographical differences found along differents stretches of rivers. The result was that concrete dikes of identical height and strength were constructed all the way to the upper reaches of streams regardless of the differences in natural conditions. Ironically, this egalitarian approach, when taken to the extreme, brought about a new inequality by increasing the flood flow and thus the damage in the lower reaches. This led to the building of even higher levees to accommodate the design flood flow, which in turn encouraged channelization in the upper reaches, thus creating a vicious cycle of ever increasing the design flood flow up and down the river. We are now reaching the point where it cannot be controlled in the lower reaches. Thus there is a new inequality where some people get flood protection at the expense of others even though great effort has been made to protect all.
Since the mid-Meiji Era, the main flood control measures have been to build high and large levees to contain flood flow in river channels. Thus, many flood-free years followed in some areas where floods had occurred almost yearly in the Edo Era. Accordingly, land development has been conducted based on the assumption that there will be no flood damage. This policy, however, means that hardly any measures have been taken against the possible consequences of the rivers overflowing, causing increased devastation. Furthermore, due to the numerous dams and debris barriers built in the upper reaches, the circulation of sand and earth has been increasingly disrupted, resulting in an insufficient supply of sand and earth downstream. This has contributed to lowering river beds and coastal erosion near the mouths of rivers.
The situation in Japan means that floodplains and their ecosystems have all but disappeared. Rivers are not rivers any more, as they have been deprived of some of the fundamental functions and essential characteristics they once possessed. With their relationship to rivers severed, people cannot continue to develop their culture through intimate interactions with rivers. On the other hand, it is still impossible to control very large floods of magnitudes that may strike every several centuries. We may be certain that such a flood, should it occur, would prove disastrous.
Lessons from the Past
Is there any way to restore the lost functions and characteristics of rivers? There is, provided we are willing to tolerate floods to a certain extent. It would be essential for us to accept the fact that we are part of nature and to put up with some inconveniences as the price to pay. It sometimes seems as though it may be virtually impossible for us to do so as we are so used to modern convenience.
The present situation warrants the revival of the Edo philosophy of river management and the acceptance of a certain level of overflowing. Flood control and river improvement should be based on the knowledge of nature and the limitations of technology, and the willingness to coexist with rivers. If this policy is implemented, flooding would be more frequent in some areas. People would have to recognize the inherent inequality in flood damage due to the differences in geographical location and accept a certain amount of overflow.
House flooring would need to be elevated, the basements water-proofed, and flood damage insurance introduced as practical measures to minimize damage. The construction of retarding basins and permeable pavements should also be promoted along the entire system of each river. At the same time, the maximum size of flood flow to be contained in the channel should be decided while taking measures to make the overflow runs gently and be returned to the channel. Implementation of all the above would require the establishment of a system in which those concerned, however far apart they may live along the same river, can negotiate and iron out the differences of opinions and interests in a civilized manner. Were this plan to be a reality, assistance of the local government’s engineers would be essential. This is where these engineers should devote all their energies and skills.
One feasible way to implement the above policy would be to adaptively apply the flood prevention forest belt popularized in the Edo Era. Fortunately, we already have large levees; flood control would be complete if these belts were laid covering the levees. One of the biggest hurdles would be securing land to create the forest belts. The land for the inner side of the levee could be secured by relocating fallow fields alongside the river; the land for the outer side could also be secured by utilizing part of the height allowance of the levee for handling the planned high water flow. In this way, the current capacity of the channel could still be maintained. (Tall trees outside the levees, if uprooted and washed away, might be caught by bridge piers; some appropriate measures should be taken considering the root depth and the possible flow velocity.)
Considerable attention has been paid to the semi-natural river engineering method–a method that attempts to recreate natural river channels. In this respect, the flood restraining forest belt may be the ultimate semi-natural river engineering method as it provides a corridor of a natural habitat connecting the forest and the sea, making the river more natural, and improving the riverside scenery. It also demonstrates that all the remaining forest belts should be conserved and efforts taken to restore them where they have been lost.
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