Every time, I leave the detailed information to wikipedia and other sources (times have changed), but the Honen-ike Dam seems to have left its mark on Japanese dam history, and although it was close to my home town, I had never given it a second thought. If you were asked to name ten famous dams in Japan, they would all be mentioned. I found out that it is also designated as a monumental monument. In what sense are they worthy of special mention?

1) The period when it was built  and technique
2) Structure Type ; the multiple arch type dam
3) The siphon type, flood discharge

According to the Ministry of Agriculture, Forestry and Fisheries (MAFF), the construction was completed in 1930, and it is said to have been a revelation at the time, employing a number of ground-breaking methods. In recent years, it is said that leaks have been observed due to ageing, as it is made of bricks, but the area has been properly renovation work was completed in 1994. I could see and understand the new part in 2023 when I visited. 


 粗い情報はwikipedia などに譲る(ネットが普及したので、時代が変わった、調べ物はみな平等)が、豊稔池ダムはたいてい誰に聞いても日本のダム史に残るものらしく、日本で名作ダムを10基挙げろと言われれば、たいてい皆が取り上げられる。傑作であるという理由は、農水省のWEBにも記されている。竣工が1930年(=昭和5年)で、日本で当時としては画期的な方法をいくつも採用して関係者の度肝を抜いたとのこと。メーソンリーで造られていることもあり、近年は老朽化で漏水が見られたらしいが、そのあたりもちゃんと改修されていて現役。改修工事が平成6年竣工とある。私が訪れた2023にも新しい部分は見て分かった。こういった古い石建造物の趣はたしかに英国を思わせる。調べてみると重文にも指定されているらしい。




1) Location, Imminent Background

I have learnt since I was a child that Kagawa Prefecture ≒ Sanuki is surrounded by mountains and has frequent water shortages due to its topography and climate, and there were often real-time restrictions on tap water intake during the period I lived there.

The Kunita-River flows northwards around the western Kagawa plain called Onohara, and flows into the sea around Kanonji City. According to literature, the Onohara fan area was said to have suffered from water shortages to the extent that it was said to 'burn in the moonlit night'. Plans to dam the Kunita-River, which flows from the Shikoku Mountains, began before the Taisho era (1912-1926), and even a brief history of the project shows that there were repeated droughts and farmers' revolts. In 1924, two years before construction began, there were droughts and peasant revolts, according to records. These records alone provide an analogy to the urgent situation at the time. The completion of the Hounenike Dam and its irrigation in 1930 led to the development of rice, wheat and leaf tobacco cultivation, and the area is still one of the leading onion and lettuce production areas in Japan.

It also states that the dykes were partially undertaken by local residents, including a total of 150,000 farmers in the Taisho era, and were completed in just three years and eight months. Although the times may be different today, I feel that I have been shown a true public works project. This story, as well as the monument on the right bank of the river, shows that this was a long-held dream of the local community. 



 「柞田川」大野原と呼ばれる香川県西部の平野を北に回り込むように、観音寺市のあたりで海にそそぐ、比較的短い河川である。文献によると下流の大野原扇状地では「月夜に焼ける、月の光でさえ水が蒸発する」といわれるほどの水不足地帯であったそう。そのため、開墾がほかの地域に比べて遅れた四国山脈から注ぐ柞田川をせき止める計画は大正時代以前からあり、歴史を軽くなぞるだけでも、度重なる旱魃が挙げられている。着工の二年前、1924年(大正13年)にも、旱魃に加え農民一揆があったと記録にある 。こういった記録をさらうだけでも、当時の切迫した状況を想像できる。それが、1930年の豊稔池ダムとその灌漑の完成により現在でも日本国内でも有数の玉ねぎ、レタス産地となったようだ。


地形をよく見てね。引用;北本 朝展 @ 国立情報学研究所(NII)よりリンク

2) Time and Technology; Multiple-Arch Structual Type

The oldest dam in Japan is Sayama Pond (Osaka, 616?), which is mentioned in the Chronicles of Japan, but the Sanuki region, not to be outdone, is said to have had water problems since ancient times. Mannoike (Kagawa, 704?), also in central Kagawa, is so old that Kukai appears in its history. However, there is no end to the history from this period, so I will briefly describe it by technology in the context of Western engineering technology from the Meiji to Taisho periods, which is a little more modern.

Technical mermarks as follows; One is the water storage technology introduced from the UK. One project was the Honkouchi High Dam (Nagasaki, 1891), which was the first water supply dam built in Japan. Later, an additional Honkouchi Low-Part Dam (Nagasaki, 1903) was built downstream and is still in use today after renovation. In terms of structural form, the oldest arch dam in Japan is the Ominato No. 1 Water Reservoir Weir (Aomori, 1909), which was completed by the Imperial Japanese Navy in 1909. Photographs show that it was built of masonry and designed by an engineer studying at the University of London. Lastly, the materials used. The construction of dams using concrete began in earnest at the end of the Meiji period, and the Nunobiki-Gohonmatsu Dam in Hyogo and the Ueda-ike weir on Awaji Island are said to be representative early structures. Incidentally, instead of being constructed of reinforced concrete as we know it today, the above dam was a composite production of coarse stone and middle filling, and is now also known as the Masonry Dam.

Although the current understanding simply imagines gravity-type reinforced concrete, it is frequently documented in various sources that these materials and construction methods were used in Japan at a time when the use of concrete for large civil engineering structures was late, and  the quality of concrete was poor, and it was expensive. The Honenike-Dam (Tano-no-ike Dike at the time) was built of stone around the perimeter and filled inside with foundation stones and mortar. It is only in the Heisei era that leaks became noticeable, but the aforementioned Gohonmatsu Dam was already leaking water at the time, so this material and construction method was used as an improvement plan, according to the report. A search of the literature shows that the design was basically rationalistic.



技術的メルマークをいくつか記そうと思う。一つは英国から導入された貯水技術である。プロジェクトとしては本河内高部ダム(長崎、1891年(明治24年) )のようで、日本初の水道用ダムが建造された。そのご、下流に本河内低部ダム(長崎、1903年(明治36年))が追加建造され、現在でも改修を経て使用されている。また構造形式的な意味では、日本で最古のアーチ式ダムが、大湊第一水源池堰堤(青森県, 1909)で日本海軍により1909年=明治42年に竣工とある。写真を見るところ、石積みでロンドン大学に留学していた技術者によって設計されたとある。最後に材料である。明治期の終わりにコンクリートを用いたダム建設が本格化をはじめ、兵庫の布引五本松ダム、淡路島の上田池堰堤などが初期の代表的な構造物らしい。ちなみに現在のような鉄筋コンクリート造ではなく、上記のダムは粗石と中詰めによる複合的生産で、現在ではメーソンリーダムとも呼ばれる。


It was basically understood that this form, which is called buttress in English, has a Gothic aspect of construction, i.e. structural rationality was given the highest priority. Reference books also state that the multiple-arch dam form is generally employed when the bedrock on either side of the bank is not strong enough to support the load by itself.


The dam was designed and planned by Midori Matsuura and Nobuo Suzuki, engineers from the Ministry of Agriculture and Forestry, under the guidance of civil engineer Dr Tojiro Sano (the monument says 'advisor'). This form is rare in Japan, with only two surviving. Dr Sano was originally employed by Osaka City and Daido Electric Power Company, and as he was on a return trip to the UK and India, it is easy to imagine that he had a deep knowledge of civil engineering techniques using concrete in various parts of the world at the time. Dr Sano also states in his paper that he used the techniques of Rankine, Delocre and Weggman. 

According to several documents from the Japan Society of Civil Engineers (JSCE), several American multiple-arch style dams were featured in the society at the time (Ikeda, et al., 1998), particularly the Erewa Dam at the northern end of the USA (Tsuji, et al., 2011). Examination of photographs shows that it does indeed have a somewhat similar design form with a continuous flood discharge at the top, called a tenter gate. Incidentally, the original Erewa Dam in the US was completed in 1913 and removed in 2012.

It is also mentioned that engineers had been brought in from the USA for the larger Oi Dam (1924, Gifu), which had been planned some time earlier, so even if it was not a multiple-arch dam, they must have seen and heard about the buttress-style split-anchorage form.



 このダムの建造に際して土木技術者、佐野藤次郎博士の指導の下(碑には顧問とある)、農林省の技師である松浦翠、鈴木信夫が設計・施工計画を行ったとある。この形式は 日本ではレアで、現存するものは2基のみということだ。佐野博士はもともと大阪市や大同電力等で務めていたということらしく、英国およびインドの洋行帰りということもあり、当時のコンクリートを用いた世界各地の土木技術に関して造詣が深かったことは容易に想像できる。また、佐野博士は、自身の論文の中で、Rankine,Delocre,Weggmanの技術を援用したと、明記している。 




Dams likely to have been referred to by Dr Sano/佐野博士が参考にした可能性の高いダムたち

❷Reasonsfor this form of dam design (including analogy) 

Leaving this background aside, when Dr Sano, the design and construction advisor, first visited the site in 1925, he initially planned to design a gravity-type dam, which had already been used elsewhere. As it turned out, the planned site of the Hounenike Dam was unexpectedly hit by bedrock during the excavation of the riverbed, and the design was hastily changed (Ikeda et al., 1998). Now, there are several descriptions of the reasons for the change from the gravity type to the composite retaining wall type

From the above, it can be concluded that this form was derived from the form that was beginning to be adopted in other countries at the time, the presence of intellectuals who had seen and heard about it, and practical reasons on the ground.




❸An Aside

 Given that there are two dams in Japan of the same type, one wonders what the other, Okura Dam (Miyagi Prefecture), is like, but looking at the photographs, they are quite different. The Hounenike example is classic, while the other is a huge two-arch, with a wide span and reminiscent of topographical constraints. The Okura Dam is said to have had a strong bedrock, but the site was quite large, so an artificial weight was placed in the centre.

 In both cases, artificial weights seem to be vulnerable to earthquakes and are not widely used in Japan. However, from my knowledge, there are some civil engineering techniques, such as the anchorage between the North Bisan Seto and South Bisan Seto Bridges, so it must be difficult to build them.

 As for other dams with buttress structures, such as the Marunuma Dam (Tochigi Prefecture), there are examples in Japan of space frames due to the same concrete price problem, but they have not been adopted in recent years, not only because of the difficulty of design and construction, but also because of the problem of deterioration from freezing and thawing.





Now, I will briefly summarise what I have learnt from the multiple-arch form. First, it is sort of self-explanatory if you follow the history of Gothic architecture, etc., but let me reiterate the language. The mechanism of the buttress is an attempt to artificially create even the bearing capacity, thus building a part of the earth or, strictly speaking, an intermediate object between the earth and the artefact, which is a terribly artificial act. The word artificial can also be translated as constructive or architectural. In other words, whatever the conditions of the ground, it is a human-controlled arrangement of a mechanism that can receive and return the stresses of the material and the reaction forces for its function, and keep it stationary. From the designer's point of view, this suggests unlimited buildability and can be a tool to aid his megalomania. As for the arch, it is not an invention here, but has to do with combining small material units - bricks and blocks - that could be made by humans, to make the span fly. In order to escape the yoke of mechanics, this force-line-aware structural form manifests itself time and again, even in form. As the design supervisor says, "We designed them so that the hydraulic forces applied were the same", and here too they functioned to channel the tremendous loads equally over the buttresses, which were divided into smaller sections. These forms are evident in the Roman aqueducts and modern civil engineering structures in the UK, where humans have built massive structures from a series of smaller units.





3) Strange design; flood vents open their mouths in anguish

The design is often described as "reminiscent of an old European castle", but this is not by chance, and is understandable in view of the background of the advisor, Dr Sano. He has been to the West many times and has seen the English and Scottish landscapes in particular. If we follow Dr Sano's other work, we can read more into his intentions. There is no doubt that he had knowledge and interest in style and decoration at the Gohonmatsu-Dam, even though there was no uniformity of order, such as the neoclassical ornamentation that was popular in Britain, the Renaissance essence at the top of the arches and the introduction of Indian-style costumes in the tower house. 

Thus as a result, it is not surprising that the building is reminiscent of an ancient castle. However, they are of course different in terms of function - a dam and a water outlet as opposed to a castle and a window. (I will add to this when I have calmed down a little more).




The Honenike-Dam is a hybrid of a siphon hole and full overflow for the treatment of excess water. There are not many examples in Japan of siphon systems being used for flood discharges. A siphon is a mechanism for transferring liquid from a higher position to a lower position using pipes, and has been widely studied from ancient Egypt until the 17th century. The advantages of using this in dams can be summarised as follows.  

Weaknesses also exist 

It was often decided to use a separate route for the British-style flood discharge from the embankment, and according to British theory, the construction of a conduit route within the embankment should be avoided. By analogy with these principles and Dr Sano's other work at the time, the top of the arch was narrow, making it difficult to install a large flood discharge. Therefore, the basic design relied on this siphon, and a mechanism to release the water from the foundation of the embankment to a distant location with high efficiency was probably considered.

It is also interesting to note that the overflow is allowed from the top of the arch. Basically, excess water overflows the entire top of the embankment and then hydraulically gouges the foundation downstream of the embankment, which is not a good idea. This is not a problem here, as a similar stone platform has been provided at a lower position in the downstream section of the embankment. In some places, the structural strength of the embankment was being attacked by masonry construction at the time, and this system was probably adopted as a supplement to prevent critical collapse in the event of flooding, in addition to the overflow from the top of the embankment. However, as will be discussed later, there may also have been design considerations.







4) Now, what I have Consideration, 2023

Considering the past work of the same design supervisor, this structure is unique, and I wondered why. This curious structure, which appears suddenly in a rural area of Japan, is a good example of the rationale for its design when you examine the circumstances below ground level, and social conditions. The design emerges from a synthesis of mechanics, materials, construction methods and the social conditions of the time. I am not a humanist, but the structures that I have built together with the local community are still very valuable. There is the seriousness of the thirst for water and the sincerity of the engineers, so there is a taut tension even though it is just an object.

Another point that struck me was the different perspectives on civil structure design. I was reminded once again that there is a serious eye for the way in which man-made and natural objects are joined. I have an architecturally educated mindset, so I unconsciously conceptualised the upper-structure and decided what was below ground, whereas civil engineers define the boundaries of their designs by imagining the transmission of forces below ground, bedrock and joints, and the upper-structure emerges as a result of this. At least this buttress seems to have been adapted to the terrain in this way.

And 100 years after its construction, it has been renovated and restored. It's not something that academics are messing around with, it's not something that will be thrown away after a few years, it's necessary, and that says it all.






Well-thought-out coalescing forces, oblique lines of force in riverbeds that could only be stopped by friction, and designs that aid frictional forces by vertical drag. The structually rationale of the form that was determined by a deep consideration of the boundary conditions of the built structure and natural condition. / よく考えられた合力、摩擦で止めるしかなかった河床部に、斜めに入る力線、垂直抗力で摩擦力を助ける設計。形の合理性もわかるが、構造物の境界条件に対する深い考察からきた構造形式