Geography Reference
In-Depth Information
snow accumulation or heavy rainfall, the widespread use of flat roofs, especially
in large warehouse or retailing operations may be cheaper, but run the risk of col-
lapse under the weight of snow or rain accumulation. One frequent requirement in
areas prone to earthquakes is to improve the integrity of structures by reconstruct-
ing or upgrading buildings to better withstand hazards, as seen recently in much of
the central area of the city of Mashhad in Iran, or in individual buildings such as
the renovation of the historic Kabukiza theatre in Ginza, Tokyo, which opened in,
spring 2013. Maintenance of structures, especially infrastructures, is also important,
even on such minor issues as the frequent blocking of drainage ditches by garbage
or invasive species, which reduces their utility in dealing with heavy rainfall.
Desert or low rainfall areas are often prone to infrequent but intense downpours
that lead to flash floods which transform natural gullies into raging torrents in the
rare rains. These locations in urban areas must be preserved from development. An
alternative, used in Las Vegas, was to construct a series of underground tunnels
to rapidly drain off the excess rain. The city now has over 270 miles of these run-
off tunnels, although unfortunately many have been occupied by homeless people,
which make them especially prone to harm when heavy rainfall occurs. Too much
urbanization has been allowed on flood plains, while bridges, roads and other infra-
structures are often built without due consideration to extreme floods, which mean
they are often destroyed, adding to the costs of reconstruction. Traditionally, a wide
range of measures have been adopted to reduce the risk of floods, especially in areas
subject to winter melts after high snowfall accumulations, or heavier rainfalls, such
as: creating upstream reservoirs to absorb the extra flow; clearing out debris in riv-
ers; blowing up ice obstructions in spring; straightening and dredging rivers in order
to improve channel flow. Another traditional approach has been to increase the size
of embankments, or to make them stronger by using concrete or stone in critical
parts, especially where currents come close to the banks, to reduce the risk of col-
lapse, but there are limits to the utility of such measures, for if they are breached
major flooding will occur. An additional, but very expensive, policy adopted by
coastal cities such as London and Rotterdam has been to build tidal surge barriers
in their estuaries that can be raised when higher tides or storm surges are predicted.
This type of solution is also being developed by the Venice MOSE system in the
three entrance channels to the Venice lagoon and the vulnerable heritage city, which
has been sinking for centuries. These barriers can be raised when high tides or
surges over 110 cm (3.5 ft) occur which will protect the city to some extent. In addi-
tion a series of other flood accommodation measures have been implemented, such
as raising the pavement levels alongside its historic canals.
An alternative practice is to divert flood waters around an affected city. Fig-
ure
9.3
shows the location of a 47 km long artificial channel to the east of the city
of Winnipeg built between 1962-1968, with entry barriers and submersible gates
on the main river. This carries off flood waters, usually in the spring melt from the
Red River, and returns it near Lockport. When completed it was the second largest
earth-moving project in the world at the time, second only to the Panama Canal.
Although costing over $ 63 million at the time, it is estimated to have prevented at
least $10 billion of flood damage. Recognition that its standards of preventing a 1
