RESILIENT CITY TOYAMA - 1 -

Flood Control and Flood Risk Management Ø  Toyama is located between the Sea of Japan and the crest of the Northern

Japan Alps on an alluvial plain formed by the Jinzu and Joganji rivers. Ø  With steep gradients over loose volcanic soils, both rivers often caused flooding. Ø  Flood control and flood risk management are critical challenges for Toyama.

国土交通省富山河川 国道事務所提供

Toyama Bay

Northern Japan Alps

RESILIENT CITY TOYAMA - 2 -

Mountain Sabo Dams In 1858 a magnitude 7 earthquake in the mountains above Toyama caused Mt. Otombi and Mt. Kotombi to collapse, damming the Joganji River in the Tateyama Caldera with 410 million cubic meters of debris. Heavy rains overwhelmed the earthquake dam, causing a devastating flood which destroyed large portions of Toyama downstream. The Shiraiwa sabo dam was constructed in 1939 to prevent further debris laden flooding. About 200 million cu meters of sediment still remains in the watershed and sabo dam construction continues.

Mt. Otombi

Mt. Kotombi

Tateyama Caldera

Shiraiwa Sabo Dam

Joganji River Watershed

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Straightening the Jinzu River Downtown The Jinzu River flows directly through downtown Toyama and into Toyama Bay. The basin area is about 2720 km2. and the length of the main stream is about 120km. With its headwaters at 1626 meters, the Jinzu has a bed gradient of about 1/20 in the mountains. Torrential waters from the mountain come to a more gentle gradient of about 1/250 on the plain, but the course of the river is winding, causing frequent flooding. A major bend in the river which had caused frequent downtown flooding was straightened between 1901 and 1920s.

Jinzu River at present

Bend of Jinzu River in Downtown Toyama

early1900s

Old River Bed

Straightened Jinzu River

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Increased Frequency of Torrential Rain

Ø  In recent years the frequency of torrential rains has increased nationwide. The amount of rainfall per hour can reach 100mm.

Ø  Climate change has contributed to these increased torrential rains which cause flooding of both medium and small rivers and overwhelms the urban drainage in the city center.

Flood in City Center August, 2010

1976-1985 AVG10.7

1986-1995 AVG12.4

1996-2005 AVG16.9

2006-2015 AVG18.0

■Number of rainfall events exceeding 80mm per hour nationwide

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Diversifying Rainwater Catchment Reservoirs

Ø  River channel improvements take a long time so it is important to quickly develop “rainwater adjustment ponds” to control the inflow volume of rainwater into rivers.

Ø  Various rainwater catchment methods are adopted to fit local conditions. 雨天時の貯留状況

Storage at school rainwater ground gutters

Underground rainwater storage facility Aboveground storage facility

Storage in paddy fields and

agricultural canals

Normal period During heavy rain

Water way

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Rainwater Overflow Storage Tunnel in City Center

To alleviate future flooding damage in the city center, a 1,069 meter rainwater storage tunnel with a diameter of 5 meters will be completed by March, 2018.

松 川県庁

護国神社逓信病院

Heiwa St.

Suzu

kake

St.

Diameter: 5.0 5.4m Length 1,069m

Excess Rainwater Storage Tunnel

Connection to existing storage pipes

Diameter: 0.6m Length 626m

Matsu River

This pipe helps prevent sewage inflow into the Matsu River.

Castle Park

City Hall

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Flood and Inundation Hazard Map Ø  The new flood and inundation hazard map informs citizens about the predicted

range of rainwater runoff inundation and depth, and identifies evacuation sites. Ø  Distributing these maps to citizens helps strengthen preventative flood damage

measures and develops better voluntary evacuation behavior during flooding.

Inundation hazard map Flood hazard map

Inundation range and depth

These hazard maps are based on information

provided by the national and prefectural

governments, and research institutes. Evacuation sites

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Comprehensive Flood Control/Flood Risk Management

Ø  As the factors affecting flooding change, such as rapid urbanization and increased torrential rainfall, it is insufficient to respond only with river modification.

Ø  A diverse set of flood control measures involving construction, IT and “soft” approaches are vital and should be integrated into a comprehensive plan.

River improvement

Areas along the rivers

Mitigating damage

-Sabo works -Embankment improvement -Anti erosion measures

- Storage in paddy fields - Storage at school grounds -Underground storage facility -Rainwater overflow storage tunnel

- Developing and distributing hazard maps - Providing clear information to citizens - Training voluntary disaster prevention organizations - Implementing evacuation drills

Construction Measures Soft Measures

Resilient Toyama

IT Measures

Efficient management

-Establish an integrated lifeline platform -Rapid Warning Radar flood response system