





	One of the essential roles of Sewerage Systems is stormwater
	drainage. Efficient stormwater drainage control is needed to
	secure Citizen's life and property against flooding and
	contribute to the sound urban development.
	Please refer to
	http://www.sbmc.or.jp/english/051026/Major_Objectives_of_ Sewerage_Systems.htm

	In Japan, "Development of Safety City for Heavy Rain" is one
	of important target outcomes in "Infrastructure Intensive
	Development Plan" formulated by Ministry of Land Infrastructure
	and Transport.
	Please refer to
	http://www.sbmc.or.jp/english/infrastructure/infrastructure.htm

	In recent year, total amount of stormwater runoff has been
	increasing because of reduction of permeable area by urban
	development. Before urbanization, there is wide permeable area
	such as farming land and forest, but after urbanization,
	rainwater permeable area has decreased as a result of increase
	of roads area, construction of buildings, etc. Then, most of
	rainwater dose not penetrate into the ground, and total amount
	of stormwater runoff has increased. However, the flowing
	capacity of existing rivers or channels is usually constant
	and increased amount of stormwater runoff has caused the
	flooding in urban area. (Fig-1)

	Before Urbanization (Wide permeable area like farm land, forest, etc.)		After Urbanization (Reduction of permeable area as a result of increaseof building, houses, roads, etc.)


	Much Rainwater Infiltration into Ground Less Stormwater Runoff		Little Rainwater Infiltration into Ground Much Stormwater Runoff


	1945 (Before Urbanization)		1988 (After Urbanization)

	Fig.1 Occurrence of Flooding in Urban Area by Increase of Stormwater Runoff Amount

		By urbanization, flooding has often occurred in the
	developed area where there had been no flooding. To cope
	with this kind of flooding in urban area,folowing counter
	measures have been implemented.

	1)	Construction of Stormwater Drainage Facilities such
		as Trunk Stormwater Pipe and Pumping Facilities
		To make up for shortage of flowing capacity of existing
	stream or river for increased stormwater runoff amount,
	rainwater drainage pipe has been constructed. Usually,
	there is little space for drainage facility in urban area,
	these pipes are constructed under ground. As collected
	rainwater in the pipe is transported by gravity, at the end
	of the pipe, pumping facility is constructed to lift the
	rainwater and discharge it to the main river or ocean.
	In Osaka City, Naniwa Grand Floodway construction project
	consisting of Suminoe Pumping Station with a capacity of
	75m³/s and 12.2 km long trunk sewer with maximum inside
	diameter of 6.5m was completed in 1999, and commenced
	full-scale operation in April, 2000.



	Fig.2 Naniwa Grand Floodway Project in Osaka City


	Fig-3 Construction of large diameter stormwater pipe

	2)	Construction of Rainwater Storage Facilities
		In rain event, some part of stormwater is stored
	in storage facilities to prevent the flooding. Stored
	rainwater is discharged to the river after rain event.
	Rainwater storage facility has an effect to reduce peak
	flow by storage, though total stormwater runoff amount
	is not reduced. There are two types of storage facilities,
	On-site type and Off-site type. In the case of On-site
	type, rainwater is stored under the park or playground,
	and in the case of Off-site type such as stormwater
	reservoir for flood control, stormwater tank, and stormwater
	storage pipe, rainwater is collected, transported and stored
	at other location.


	In Dry Weather	In Rain Event

	Fig.4 Stromwater Reservoir for Flood Control

	3)	Constriction of Stormwater Infiltration Facilities
		By installing stormwater infiltration facilities such as
	infiltration inlet, infiltration trench and permeable pavement,
	part of rainwater penetrates into the ground, and total storwater
	runoff can be reduced to prevent the flooding.


	Conventional type inlet and pipe	Infiltration Pipe and Inlet made from porous
		concree

	Fig.5 Example of Stromwater Infiltration Facilities

	In addition to these constructions of facilities, following
	measures are also quite important.

	1)	Provision of information to the residents
		It is desired that hazard map indicating the danger area
		for flooding and degree of flood risk, means of escape,
		etc. are informed to the residents in advance.

	2)	Information sharing
		Relating information should be shared with river
		administrator, responsible person of underground facilities
		like subway, mall, etc.

	3)	Rapid and efficient management
		Rapid and effective management by installing of rainfall
		radar and rainfall information system, remote control of
		pumping facilities, etc. are also conducted in Japan.


		Fig-7 Installed Rainfall Radar


	Fig.6 Rader installed on the top of 200m-high	Fig.8 Real Time Precipitation Scope
	building in Osaka City

	In Osaka City, a rainfall radar has been installed on the
	roof of a 200m-hgh building in the city to observe rainfall
	on a real time basis to minimize flood damage. The observed
	data are transmitted online to Wastewater Treatment Plants,
	Pumping Stations, and other facilities to control
	facilities and respond accurately to citizen's inquiries.
	"Rainfall Information Service" has been implemented since
	1999, and criticizens can directly access radar information
	on rainfall via facsimile or the Internet.
	(Data and photos from Leaflet "Sewage Works in Osaka"
	published by Osaka City)

	In order to cope with flooding in urban area, provision
	of information to the residents, information sharing,
	and rapid and efficient management are important in
	addition to the construction of stormwater drainage,
	storage, and infiltration facilities, and comprehensive
	countermeasures for flooding in urban area should be
	established in cooperation with local communities
	and residents.