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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