STORMWATER MANAGEMENT FOR SUSTAINABLE URBAN WATER USE

							                            STORMWATER                  MANAGEMENT
                      FOR SUSTAINABLE URBAN WATER USE

                                         SHOICHI        FUJITA

   Professor, Department of Civil and Environmental Engineering, Nagaoka University of Technology,
                       kamitomioka1603-1, Nagaoka, Niigata, 940-2188 Japan
          (Tel: 81-258-47-9662, Fax: 81-258-47-9662, e-mail: sfujita@vos.nagaokaut.ac.jp )


   Stormwater management including infiltration, known internationally as Best Management Practices
(BMPs), has been employed in many countries in the world since 1980’s.
    Many cities in Japan are also eagerly applying stormwater infiltration, but many others are not yet.
Because of so many reports from those cities eager to perform infiltration, it appears that stormwater
infiltration is widely and generally implemented all over Japan. Many local governments are always
hesitating to ask people to install soakaways and small detention basins in private property. Furthermore,
they are not enthusiastic to install infiltration and storage facilities in public spaces.
       On the contrary, the city of Koganei, a Tokyo suburb with a population of 110,000 and an area of
1,133 ha, is a representative example. As of March 31, 2005, there are as many as 48,935 soakaways and
infiltration trenches as long as 38 km, which may be the highest installation density in the world.




                            Figure 1. Infiltration in sewer system of Tokyo

      In the case of the experimental sewer system (ESS) in the western part of Tokyo, the
Nerima Ward, where such a system was installed is 1,434 hectares. More than 34,000
soakaways, 220 kilometers of infiltration trenches, 70 kilometers of infiltration curbs, and so
much of 500,000 m2 of permeable pavement were provided in the urban area in the 13 years
from 1982 to 1994.
  Nerima is the largest area of stormwater infiltration facilities installed by sewerage works in Japan.

     Apart from the above, a part of Setagaya, Tokyo, is served by a separate sewer system, where
4,700 soakaways have been installed in the area of 2,200 ha these ten years. That means about 500


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soakaways are being installed annually. The number of soakaways looks too small in the large area of
Tokyo.
      Meanwhile, replacement of the old sewer pipes is under way in the central part of Tokyo.
Soakaways and infiltration trenches are going to be installed in the areas where the new sewer pipes are
to be provided, but the pace is extremely slow.

      The Japanese Government revised the cabinet order in April 2001, authorizing permeable
pavement as an official road structure. This is expected to provide an opportunity for further
dissemination of permeable pavement for roads all over Japan.

   Tokyo has been suffering from serious flood problems for a long time. Other cities are
almost in the similar situation. Frequent flood damage is occurred by increased volume of
stormwater runoff and regional torrential rainstorms resulted from the progress of urbanization.
   An underground shopping center in Fukuoka, and the cellar of a private home was flooded
by heavy rainfall in 1999. Both resulted in the death of people. Moreover, more than 3,000
houses were flooded at that time. These accidents awoke the people.
   Therefore, flood control in urban areas was brought to everyone’s attention. Then, the
improvements of the flood control in Japan have been rapidly expanding and developing in
recent years.
   Under the current situation in no assurance of the sufficient sewer and river flow capacity
in urbanized areas, stormwater runoff control by installing stormwater storage/infiltration
facilities is essential for cities to maintain sustainable growth. Many cities in Japan
established its general rules for stormwater management to enforce citywide stormwater
runoff control measures.
    In addition to flood prevention, stormwater runoff control can contribute to creating a
clean water environment and a sound water cycle. Returning rainwater to the earth through
stormwater infiltration facilities can be expected to restore a water cycle, providing spring
water in urban areas, for example.

   Stormwater management technology, including artificial infiltration of stormwater is
expected not only for the flood control measure in the city area, but also, as the combined
overflow control, Also, the technology to infiltrate stormwater is considered effective in
recharging groundwater, restoring spring water, maintaining the natural flow of rivers in cities,
ensuring a comfortable environment in water front spaces, recovering and preserving the
ecosystem, maintaining water for disaster prevention, facilitating heat island control measures,
stimulation of tree growth, preventing ground subsidence, and the natural recurrence of water
circulation, etc.

   The main objective of stormwater infiltration in Japan was for the first the reduction of
storm runoff. Many people have become aware of the effects of runoff infiltration. In the
meantime, the groundwater increasing effect has come to be viewed as more important than
runoff control. It seems likely that this change came about due to recent increased
international awareness of global environmental issues.
   The challenges Japan faces and the concrete achievements Japan makes may be an interesting
example for other countries. International exchange in the field of urban storm management is
increasing in importance.



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