On the eastside of Niiharu Citizen Forest, there is a man-made pond, connected to Umeda River that is a second tributary for Tsurumi River 鶴見川 via On’daRiver 恩田川. The existence of it has been always a mystery for me. The pool is made of concrete walls, although now it is full of aquatic life with muds flown in from the forests. The size of Niiharu Forest gives us a feeling we are standing in a traditional agricultural area. It is not at all true. Just 5 minutes’ walk from the pond, we are surrounded by ordinary suburbia of Megalopolis Tokyo. There is no agriculture needing water reservoir for their business. Then, what’s for that pond? It must have costed lots of money to build such structure. I know it’s not a private property, but of the City. So, that pool is made of taxpayers’ money. There must be a good reason for the existence of the pond. Recently, I learned the raison d’être of it. It has a VERY serious duty to perform, I realized.
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| This is the pond. The forest over there is the eastside of Niiharu Forest. |
If you’re familiar to ordinary towns of Yokohama, you may notice there are lots of signs like this:
The panel says, “Below this building, there is a storm water reservoir.” It is a way of controlling flood often caused by small rivers running in our city. You see? Yokohama is a port town, i.e. we have many small but steep hills falling down to Tokyo Bay. Such geological feature makes the place good for port. At the same time, in Yokohama very tiny water streams go down the ubiquitous small valleys between the steep hills. They may be pequeños, but for a densely populated city can be troublesome enough with torrential rains. As they are small, they can quickly reach to the capacity, and cause floods in downtown. How to control such incidents is always the important issue for ages in Japan.
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| Small Umeda River |
The other day, Prof. Yuji Kishi of Keio University explained to us Kanagawa Forest Instructors the basic method of controlling such flood. Before, when our society was mainly for agriculture, the flat places where water entered easily was used for rice paddies. The sites were good for staple cultivation, and acted as an important buffer for overflow caused by rains and rivers. Many traditional rice cultivars in Japan were resilient for floods so that their quality was not affected much, even after several weeks of inundation in late summer and fall. Those were the days. Now, especially Yokohama and Kawasaki in Metropolitan Tokyo area, the places prone to flood are developed for factories, offices, housings, shopping centers … with millions of people. Meanwhile, rivers won’t change their course when we build our sweet home on their way. When a storm comes, nature wins, and humans face devastation with deluge.
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| The mouth of KatabiraRiver 帷子川 at the Port of Yokohama. Here, we have brackish water, but not yet of sea. You see? Global HDQ of Nissan stands at the lovely waterfront, but if flood occurs, the building does not stand at the pole position. |
The professor pointed out, especially for Tsurumi River (my post on August 2, 2019), its geological condition presents THE serious threats. It’s a very short river; 42.5km in total starts from a spring at ASL170m in Machida City of Tokyo. Direct distance between the origin to its mouth is less than 20km, whereas the river creates 235km2 of catchment area. i.e., The flow is meandering the relatively flat flood plain before ending at Tokyo Bay. No wonder it overflows very often when strong typhoon comes. The flood plain of Tsurumi River is home of huge population larger than the City of Osaka. During the high growth era of the 1960s, regularly 20000 households of Yokohama and Kawasaki suffered a lot with the flood of Tsurumi River. I imagine the situation could be similar to the current problem of people in Dhaka, Bangladesh … In the early 1970s, the mandarins of Japanese government reached the conclusion they could not contain Tsurumi River with a “standard way.” In terms of civil engineering, Professor Kishi continues, the easiest way to avoid inundation in Yokohama and Kawasaki area by Tsurumi River is to cut all the trees in upstream hills, including Niiharu Forest, concrete every slopes for rain water to pass smoothly, and widen the river by removing all the human activities from the flood plain. In this way, rainwater would smoothly flow into the sea. BUT, of course, we cannot take this method in Megalopolis Tokyo.
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| Recent Tsurumi River
near Kamoi Ohashi Bridge 鴨居大橋. People are busy restoring the bank destroyed by Typhoon Hagibis (; my post on January 17, 2020). |
Even though, if we can control the amount of water flowing into the river at least to some extent, we can protect our life. People living along Tsurumi River including municipal governments + the central government gathered their brain to come up with a plan. In 1980, the Integrated Flood Management Policy for Tsurumi River went into the effect. 2020 is its 40th anniversary. The measure is unique in Japan as this is a cross-municipal arrangement where the central government and civil society organizations act as a mediator, or connector, of sorts among 3 regional governments; Machida, Kawasaki, and Yokohama along the river. Prof. Kishi presides TRnet 鶴見川流域ネットワーキング which is a civic organization covering the Tsurmi River itself and sits at the Inter-municipality + National Office Council regarding Tsurumi River Basin 鶴見川流域水協議会.
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| Despite of a substantial
flood last fall, water birds are surviving ... |
The Ministry of Land, Infrastructure, and Transport is executing civil engineering projects to fortify banks, smoothing out the river flow with various measures (including underground aqueducts) to lessen the impact of flood water running down the sea. It also created the huge multipurpose anti-flood pond which is the New YokohamaSports Park with Nissan Stadium for 2002 FIFA World Cup and 2019 Rugby World Cup. (More to this next week.) The inter-municipal arrangement is also engaging in the measures to improve the quality of forests along the river in order to advance forests’ ability to retain rainwater before washing down to Tsurumi River. Each municipality supports civil society activities to maintain the health of forests along Tsurumi River. Yokohama has 12 Citizen Forests in Tsurumi River Basin. Each has Lovers Organization of volunteers that receive financial and technical assistance from the City with Green Tax funding. Lovers of Niiharu Citizen Forest is the largest. We are busy with maintaining our forest to keeping its ability of retaining water. For Niiharu Forest, also the Council for Fun Learning along Umeda River 梅田川水辺の楽校協議会, a member for Niiharu Council together with the Lovers (; my post on April 22, 2016), works closely with TRnet. Machida and Kawasaki too have their own scheme to promote civic activity for the health of forests in Tsurumi River Basin. In addition, municipal governments constructed numerous storm water ponds not only for Tsurumi River but also for the other rivers under their territory. The pond next to Niiharu Forest is one of them.
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| The New Yokohama Sports Park with Nissan Stadium |
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| Umeda River becomes
maddeningly beautiful with Spiraea thunbergii in Spring. Please come in late March to early April for your relaxed walk along the river, next to Niiharu Forest. The Council for Fun Learning along Umeda River is in charge of picking up garbage and mowing in the riverbed. |
I was a little taken aback to the map shown by the professor which showed the location of flood reservoirs along the river. The shape of the basin for Tsurumi River looks like a cute tapir. Then, the dots representing ponds were like polka dots decorating the entire body of the animal, like some object created by Yayoi Kusama. In total there are more than 4900 ponds for flood control of Tsurumi River. Woooooooooow. The professor told us this arrangement was possible because it is for Tsurumi River that runs the richest municipalities in Japan, very near to the offices of the central government. The place has money to mobilize all the relevant actors, national and local governments + civil society, to control the flood. “I think the only place possible to replicate Tsurumi River model would be in Osaka whose budgetary scale could support such project. However, the local governments of Osaka are at odds with the national offices in Tokyo so that mandarins in Tokyo do not like to touch it …” Prof. Kishi told us. That’s sounds depressing. If such coordination is rare even for tiny rivers, what would happen for those rivers flowing internationally? How can people manage natural (or otherwise) disasters caused by their river when your neighbor speaks completely different “language”?
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| A tapir wearing a coat of polka dots |
Prof. Kishi added more in his seminar; with such ferocious downpouring these days, the era we can count on the ability of forests to retain rainwater is over. Yes, healthy forest with enough mulch absorbs droplets from heaven, up to a point. If an extreme shower occurs in a very short period of time like that of Typhoon Hagibis last year, or, if substantial raining continues for unusual period of time like we had last October and November, any forest cannot keep water beneath the system of roots and soils. Once the ground reaches to the max capacity, it gives in to the law of gravity, and causes landslide. Any landslip with lots of living trees causes the max devastation. Debris flow contains not only earth and rocks, but also logs, huge roots, and anything that was once a part of a forest. The impact of such avalanche is far worse. Say, if they reach to the river, mixture of logs and earth destroys bridges far rapidly than the slide only with tumbling rocks. “The arrangement for Tsurumi River is not yet at all adequate to tackle frequent ‘once in 100 years’ typhoons. Considering its extremely urban circumstances, we may have hit the technical limit already.” Hmmmmmmmmm … At least last year, the reservoir attached to Niiharu Forest retained water while Umeda River next to the forest run maddeningly to On’da River. How long it can go that way? At least, we Lovers of Niiharu Citizen Forest are every weekend busy maintaining our forest with its mulch, hoping it retains rain …
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| The collapse |
By the way, this Sunday,
February 24th, from 13:30 at Tsurumi River Watershed Center 鶴見川流域センター next to
Yokohama Sports Center, Prof. Kishi will have a workshop about Tsurumi River
and the New Yokohama Sports Park with Nissan Stadium. RSVP at
Tsurumi River Watershed
Center
Phone: 045-475-1998
FAX: 045-475-1999
The participation fee
is 100 yen for a cup of coffee served there. Anyway, when you find issues in Tsurumi
River, please make a contact with
Keihin River Office, Kanto
Development Bureau
Ministry of Land,
Infrastructure and Transport of Japan
国土交通省 関東地方整備局 京浜河川事務所
2-18-1 Tsurumi-chuoh,
Tsurumi Ward, Yokohama, 230-0051
〒230-0051 神奈川県横浜市鶴見区鶴見中央2-18-1
Phone: 045-503-4000
