Sewage Works in Osaka, Japan

Sewage Works Project Program

Sewerage projects in Osaka City focus on "Flood Control", "Water Quality Enhancement" and "Promotion of Sewerage Resource Recycling" because its policy target of sewerage activities is to make the city more attractive and safer in order to facilitate one of the city’s master concept, "Make Osaka an attractive city to live and visit".
Under those policies, we work on "Flood control" "Combined Sewer Overflow control" and "Reconstruction and renewal of sewerage facilities" as urgent and important tasks and we also implement "Improving sewerage service efficiency" and " Disaster prevention measures".

Configuration of Osaka City

(1) Flood Control Projects to Provide Safer Living Environment

Osaka city has been implementing countermeasures against 60mm/hour rainfall which is estimated once 10 years, however, inundation still occurs when we have concentrated heavy rain.
Therefore, we pursue the construction of major trunk sewers such as Yodo Grand Floodway as well as construction and expansion of pumping facilities as drastic countermeasures against flooding. In addition, as a lot of time is required for construction of such a big scale stormwater discharge facility, we also pursue localized flood control measures.

Flooding Area

The area where stormwater cannot be discharged by gravity flow

The area where stormwater cannot be discharged by gravity flow Osaka plain is an alluvial plain which is the result of the sediment deposition from Yodo river. 90 % of the city area is prone to flooding by heavy rain and stormwater has to be drained by pumps. In addition, most stormwater cannot seep into the ground due to high pavement ratio. Stormwater exceeding the capacity of sewers may flow into the sewer networks and caused inundation.
Thus, flood control is one of the most important roles of the sewerage system.

Locations of Major Trunk Sewers

Ground Level Diagram of Osaka City

Flood Control Measures

In Osaka City, almost all the city area is now served by sewerage system. However, the recent rapid urbanization has decreased the amount of farmland and vacant lots which formerly served to retain stormwater.
This has led to a significant increase in the stormwater runoff rate, causing even areas equipped with a sewerage system to flood during heavy rainfall.
To address this problem, a fact-finding investigation has been conducted to obtain data concerning stormwater runoff coefficient and other values. Using a result obtained, stormwater runoff rate was calculated with Brick’s formula, based on which the existing factors to be considered in planning were being reviewed (runoff coefficient: 0.5 - 0.6 → 0.7 - 0.9, average ground gradient: 1‰ → 5 - 15‰) so as to meet the requirements of heavy rainfall (60 mm per hour) which occurs approximately once every 10 years.
Thus developed new design values have being utilized to construct stormwater drainage facilities including large-scale trunk sewers and pumping stations, which has reduced extensive flooding.
Stormwater runoff control facilities such as stormwater reservoirs have also been constructed to reduce stormwater runoff volume.

Major Construction Projects

The city of Osaka launched the Naniwa Grand Floodway construction project in 1985 as a drastic flood control measure for Hirano ward and neighboring areas located in the southeastern part of the city.
In the trunk sewer construction, the shield tunneling method was employed for underground tunnel excavation to minimize obstruction to traffic.
The flood control facility consists of a 12.2 km long trunk sewer (8.5 km trunk sewer section plus 3.7 km quasi-trunk sewer section) with a maximum inside diameter of 6.5 m and Suminoe Pumping Station, with a design drainage capacity of 75m3/s commenced full-scale operation on April 1, 2000.
The Naniwa Grand Floodway, which has a storage capacity of approximately 300,000 m3, also contributes to combined sewer system improvement because it conveys stored stormwater to Hirano Sewage Treatment Plant and is able to reduce the pollution load discharged.
Rain water on the rooftop of Suminoe pumping station is drained into the artificial stream in the recreational garden. The open spaces above the facilities are also used to provide a playground and other recreational spots where residents can spend enjoyable time.

Naniwa Grand Floodway

Suminoe Pumping Station

Cross Section of the Suminoe Pumping Station

Location of Naniwa Grand Floodway

Tosabori-Tsumori Trunk Sewer

The west-central section of Osaka City, an area which serves as the nucleus of commercial activities, is the place where the city launched its first sewage works and installed sewerage system in 1937.
This large-scale trunk sewer, with a total length of 6.7 km and maximum inside diameter of 6.25 m, is comprised of the main trunk sewer running north to south 5.4 km and connected quasi-trunk sewers stretching over 1.3 km. The shield tunneling method was employed for constructing the trunk sewers in order to minimize obstruction to traffic. Stormwater collected via the trunk sewer is discharged into the Kizu River, which is close to Osaka Bay, from the pumping station (pumping capacity: 89 m3/s) constructed at the Tsumori Sewage Treatment Plant.
The trunk sewer began full operation on Apr.1.2003.

Location of Tosabori-Tsumori Trunk Sewer

Tosabori-Tsumori Trunk Sewer

Cross Section of the Tsumori Pumping Station

Pump Station

Storm-water Pump

Yodo Grand Floodway

The construction of the Yodo Grand Floodway was planned as a drastic flood control measure for the northern area of the Yodo River. This large trunk sewer will be total 22.5 km in length and 7.5 m in maximum inside diameter, exceeding the size of Naniwa Grand Floodway.
This Grand Floodway consists of two trunk sewers in order to provide flexibility in handling concentrated heavy rain including sudden showers. It is so designed that excess stormwater in other trunk sewers overflows into the Grand Floodway.
At the end of the Grand Floodway facing Osaka Bay, it is planned to construct a new pumping station with a drainage capability of 105 m3/sec.
The whole construction of the Floodway employs the shield tunneling method to minimize obstruction to traffic.
Construction was started in 1991 and the completed section has already been utilized since September in 1997 as a temporary stormwater reservoir (storage capacity: 145,000 m3, March in 2007).

Location of Yodo Grand Floodway

Yodo Grand Floodway

Yodo Grand Floodway under construction

Localized Flood Control Measures

Construction of a large stormwater drainage facility is time-consuming and it may not completely prevent extremely localized floods from occurring in low-lying areas.
Accordingly, the city of Osaka is taking localized flood control measures including development of a rainfall information system, construction of stormwater reservoirs in such public spaces as parks and schools, and construction of local drainage pumping facilities.

Localized Flood Control

The subsidy for household rain water store tanks

The city of Osaka established "The subsidy for household rain water store tanks" as one of the flood control measures. This is to encourage citizens to take part in the city’s flood control program and save tap water use.

Rain Water Store Tanks

Rainfall Information System

A rainfall radar is installed on the roof of a 200m-high building (ORC 1 Plaza in Minato ward) to minimize urban flood damage caused by concentrated heavy rain and typhoons. It has the system which can catch the localized real time rainfall information in areas around Osaka City and helps the effective operation of stormwater drainage pumps in the concentrated heavy rain event.
The information is provided to citizens as "Rainfall information in Osaka City" via facsimile or the Internet. In addition, "Disaster Prevention Map" is also provided so that citizens can get the information of inundation prediction and evacuation.

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