旭川分流部改修を記念し「永忠堤」と命名しての説明看板設置!―百間川の荒手の新愛称が説明板に正式記載される―

English is here.

(一社)岡山藩郡代 津田永忠顕彰会
会長 小嶋光信

2018年の西日本豪雨で、岡山市内の洪水を間一髪で回避できた旭川の放水路である百間川の功績は大きいものがあります。その百間川の改修を記念して、その取水口付近の岡山市中消防署の駐車場西側に説明看板が設置され、顕彰会が愛称として提案した「永忠堤」の名が掲載されました。

百間川は江戸時代の承応3(1654)年に旭川で起きた大洪水を契機に、岡山城やその城下町を洪水から守るために熊沢蕃山が考案した「川除の法」(荒手と呼ばれる堰を段階的に造って水流を弱め洪水を防ぐ治水方法)を基にして、津田永忠が3段の荒手によって水の勢いを弱めながら百間川へ分流することで旭川の氾濫を防げるよう設計し、1686年に概ね完成したものです。

江戸時代より永年、岡山市を洪水の被害から守ってきました。また、単に防災の面だけでなく、百間川の築堤・整備は日本を代表する大名庭園である後楽園の築庭や沖新田をはじめとする児島湾の大規模な干拓にも深く関わっているのです。

しかし、昭和47(1972)年7月の洪水(家屋浸水戸数約4,300戸、浸水面積3,278ha)や、平成10(1998)年10月の洪水(家屋浸水戸数約 970戸、浸水面積 389ha)を教訓に、将来の洪水被害を守るために改修が計画されていました。

岡山河川事務所さんによると「2118年の梅雨前線の影響で、旭川流域の流域平均総雨量は約324mm(7月5日(木)3:00~8日(日)22:00)を記録し、旭川放水路分流部の上流側に位置する下牧水位観測所で氾濫危険水位を超過しましたが、7月6日(金)には、分流部の「一の荒手」を越流し、洪水を放水路へ分派しました。

今回の豪雨では、旭川放水路がなかった場合に、「岡山市街地(JR岡山駅含む)の約450ha及び約5,050戸の家屋の浸水被害が発生する恐れがありましたが、洪水を分流したことによって、旭川の水位を約1.5m低下させ、洪水を安全に流下させました。」ということで、間一髪で改修が間に合って岡山市中心部は大災害から免れることができました。

もしこの津田永忠の百間川がなかったら、またこの西日本豪雨に改修が間に合わなければ大惨事になるところでした。

思えば20数年前、顕彰会を立ち上げたときに、すでにこの百間川の改修の方式をどのようにするかが議論されており、吉野川第十堰の可動堰化で世の中大騒ぎの時期でもあり、荒手などの歴史遺産として残すか、それとも災害防止で可動堰化するかの判断が迫られているナーバスな時期でした。

当時の河川事務所の幹部の方も個人的に我々の顕彰会に加わり、歴史的な勉強をされて、我々顕彰会も地元の皆さんとともにこの協議に加わったことで、一時は険悪になりそうなこの問題も永忠さんの一石二鳥、三鳥にあやかって「これも、あれも」ということで歴史的遺産の荒手も保存し、防災能力も改善するということで話が進み、可動堰ではなく荒手の修復に国が舵を切ってくれました。

地元と国などの円満な話し合いでスピーディーに工事に入れたことが、今度の西日本豪雨で岡山市中心部の洪水被害を防げたと思っています。
 
なお、「永忠堤」名称を顕彰会が提案した理由は以下の如くです。

津田永忠は、江戸時代前期(元禄時代の頃)に岡山藩主・池田光政公、綱政公の二君に仕えた岡山藩士で、現存する庶民のための学校としては世界最古といわれる閑谷学校の創設により「教育県・岡山」の礎を築き、世界のモンスーン地帯では最大の干拓といわれる1,900町歩の沖新田をはじめとする約2,500町歩に及ぶ大干拓によって「豊かな岡山」と評されることになる大事業を次々と成し遂げた「天下に比類なき土木巧者」である。

彼の没後300年以上を経た今でも、地元・岡山の人々は身近にある彼の功績を語り継ぎ、その功績に感謝し、親しみを込めて「永忠(えいちゅう)さん」と呼んでいる。

彼の事業の中でも特に、沖新田の干拓は、彼の師でもあった熊沢蕃山の反対をはじめ多くの技術上、財政上の障害を自らの才知で乗り越え、彼の卓越した知識と技術力から河口に「唐樋」と、「大水尾」という遊水地を建造することで旭川と吉井川という大河川の間に広大で豊かな農地を生み出したという功績が大きい。

この二大河川の河口付近を干拓すれば洪水の危険が大きいという難問を解決したのが百間川の整備(築堤)にあるといえる。

百間川の整備は、岡山城下を洪水の被害から守るとともに、沖新田等の干拓地の灌漑や排水機能をも兼ね備えた、一石二鳥、一石三鳥ともいえる卓越した治水・土木工事であった。

百間川は、旭川がある一定の水量になると、一の荒手を越流し、まず、大きな石などを落とし、二の荒手で中ぐらいの石などを落とし、流れを弱めながら、三の荒手で穏やかな流れへと変えて旭川の氾濫を下流へ放水させるという工夫が施され、整備されてから今日まで、国の改築も相まって洪水などの水害から地域を守っている優れものといえる。

残念なことに、三の荒手は明治25年の洪水で消失したが、一の荒手、二の荒手は地域住民の要望を国が受け止め、貴重な文化財として現存させ、現在も機能している。

今日まで脈々と続く百間川の機能は、国の的確な改修により一層の効果を発揮し、2018年7月の西日本豪雨でも岡山市内中心部などを洪水の被害から守るなど、今もなお市民とその生活を水害から守っている。

戦国時代に甲斐の守護であった武田信玄が築かせた「竜王川除場」を地元では現在でも「信玄堤」と称えているが、これを東の堤とするならば、西の堤は、岡山にあるこの優れた機能を持つ、稀なる越流堤を「永忠堤」として、これからも永く世に伝えていきたい。

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To commemorate the renovation of the place where the Asahi River branches, an explanatory sign board has been placed
- The new nickname Eichuu-Tsutsumi(Tsutsumi=weir) of the Arate(low dividing dike) of the Hyakken River is officially listed on the explanation board -

※Eichuu…NagatadaTsuda’snickname(A different reading of Nagatada’s name) ※Arate…When the water in the Asahi river gets too high, it will flow over into the Hyakken river. The Hyakken river is separated from the Asahi River by a low “Arate” overflow dike. It diverts excess water from the center of Okayama city. This is how it protects the city. It works like a valve in the sense that it controls the flow of water in the river.

February 6, 2020

Okayama Clan Nagatada Tsuda study group Chairman Mitsunobu Kojima

The construction of the Hyakken River, a side arm of the Asahi River, was a big achievement. It has helped diverting floods from Okayama City. To commemorate the renovation of the Hyakken River, an explanatory sign board was installed on the west side of the parking lot of the Okayama City Central Fire Station, near the entrance, and the name of “Eichuu-Tsutsumi” proposed by the Honoring Society was posted on it.

In the wake of the great Asahi River flood of 1654(Jyoou3) in the Edo Period, based on the flood control method to weaken the water flow and prevent flooding, Nagatada Tsuda designed the Arate with three steps, and it was roughly completed in 1686. With this the power of the Asahi River could be controlled and floods prevented.

For many years since the Edo period, Okayama city has been protected from flood damage. The Hyakken River not only served to prevent disaster, but it also contributed to the creation of the Okayama Kourakuen daimyo garden, and the large₋scale Kojima Bay reclamation, including Okishinden. Both are representative for Japan.
During the flood of the year Showa 47 (1972) about 4,300 houses on 3,278 ha were flooded. In Heisei 10 (1998) about 970 houses and 389 ha were flooded. We learned from these floods and planned to protect the land from future flood damage.
According to The Okayama RiverOffice in 2018, the average total rainfall in the Asahi River basinbetween Thursday, July 5, at 3:00 and Sunday, July 8, at 22:00 was approximately 324 mm, due to the heavy rains.
If there had been no Asahi River drain (Hyakken River) about 450ha and about 5,050 houses in Okayama city (including JR Okayama Station) would be at the risk of flood damage, but the Arate weir system caused the water level to drop by about 1.5m and the water to be safely diverted. So, the center of Okayama City narrowly escaped a catastrophe.

If it had not been for the Hyakken River that Nagatada Tsuda had created, and the fact that the Arate dikes had just been renovated, the heavy rains in Western Japan would have caused a disasterhere.

When we had our first honorary meeting over 20 years ago, we discussed about how to renovate the Hyakken River. At that time building a movable weir at Daijuu on the Yoshino River was big news in Japan. We had to decide whether to leave the Arate as historical heritage, or to replace it with a movable weir to better prevent disasters.

The honor association discussed with local people. Sometimes this was difficult and the mood turned bad. We wanted to preserve the Arates and their historical heritage, but also improve the disaster prevention capacity. Eventually, the country decided to repair the Arates rather than replace them with a movable weir.

I believe that the speedy construction after a harmonious discussion between the local and national governments has prevented flood damage in the center of Okayama Cityfrom the heavy rains of Western Japan.

Below are the reasons why the Honoring Association proposed the name Eichuu-Tsutsumi”.

Nagatada Tsuda was a man from Okayama who served under two generations of feudal lords. First, he worked for Lord Mitsumasa Ikeda and then under his son Lord Tsunamasa. This was in the early Edo period. Nagatada Tsuda laid the foundations of education in Okayama Prefecture by establishing the first school for the children of commoners in the world that still exists today. This was Shizutani School. He was a “civil engineer unparalleled in the world” who accomplished large projects which made Okayama wealthy. He completed projects one after another and reclaimed land to the extent of about 2,500 hectares, including the Okishinden land of about 1,900 hectares. This is said to be the largest reclamation in the global monsoon zone.

More than 300 years after his death, the people of Okayama still talk about and thank him for his achievements. They call him affectionately “Eichuu-san”.

In particular, with the reclamation of Okishinden, he overcame financial obstacles with his own talent. He also faced the opposition of his mentor, Banzan Kumazawa, who was a great expert and had a lot of technical skill. By creating the Ohmio reservoir and the Karahi water gate he reclaimed a vast and rich farmland between the great Asahiand Yoshii rivers.

The construction of the Hyakken river works has reduced the flood risk of the reclaimed land in the vicinity of the river mouths.
Banzan Kumazawa opposed the plans because of the flood risk. However, Nagatada Tsuda created a solution.

Even now, the Hyakken River continues to be an outstanding flood control and civil engineering work. The river, as we say in Japanese, killed two or even three birds with one stone. *
It provided irrigation and drainage for the reclaimed Okishinden land as well as preventing flood damage in the Okayama castle town.

*Explanation An old Japanese saying is “Iseki-nichou”, to kill two birds with one stone. Now, our chairman said “Iseki-sancho”. This time we hit three birds with one stone. That means that this time it was even better than “Iseki-nichou”.

When the Asahi River has an excess amount of water, and stones and branches are being carried downriver, the Ichi no Arate(first Arate)stops the biggest rocks and lets smaller objects go over. The Ni no Arate(second Arate)stops them and only lets middle sized stones and objects pass. When it still existed, the San no Arate(third Arate)used to only let the smallest stones and objects pass. By the time the stream had reached this last Arate, the structure had turned it into a gentle flow.
By releasing the overflow of the Asahi River, the water level is regulated. If we keep maintaining the improvement works in the country, we can keep their excellent protection against floods.

Unfortunately, the San no Arate(third Arate)disappeared in the floods of the year 1892 (Meiji 25), but the Ichi no Arate(first Arate)and Ni no Arate(second Arate)are still functioning and are valuable cultural assets. The government has listened to the demands of the residents.

The Hyakken River, which continues to function today, has become even more effective by the upkeep of the land. It continues to protect lives and property in the center of Okayama from floods. It even prevented flood damage in the heavy rains of Western Japan in July 2018.

Shingen Takeda was the guardian of the Kai area in today’s Yamanashi prefecture during the Sengoku(civil war)period in the 16thcentury. His work is still referred to as the “Shingen-Tsutsumi”(the weirs of Shingen)in his local area. If they are called “Shingen-Tsutsumi”, I think the weirs of Nagatada Tsuda should be known as the “Eichuu-Tsutsumi”, as he is Takeda’s counterpart from Western Japan. I want the world to know about him.