Culvert - Ankyo

A water ditch that is underground and cannot be seen from the outside. It is the opposite of an open or open channel, which is a waterway where the running water surface is visible, and is a general term for under drains, collecting channels, and closed conduits, which have different purposes and structures. A drainage culvert is a trench dug in the ground a little less than one meter deep, with an absorption culvert laid at the bottom and backfilled. It is used to absorb and remove excess water from the soil, to control the soil moisture of farmland and sports grounds at an appropriate level, or to cause a downward infiltration water flow into the soil to wash away salt in the soil. In recent years, with the spread of irrigation agriculture, the destruction of soil resources due to salt accumulation has become a global problem, and the latter role is being given increasing importance. A simple culvert is a drainage culvert made from rough piles of brushwood and gravel, and its history dates back to the ruins of Ur (southern Iraq) from about 3000 BC. However, today most culverts are complete culverts made from clay pipes or perforated plastic pipes, or materialless culverts (also called mole culverts) made by simply drilling holes in the ground with a boring machine, and continuous burial machines for long, flexible plastic pipes have become widespread.

A water collection culvert is a horizontal well that is buried to take in relatively shallow groundwater, such as river underflow, for use as water, and structurally it is not very different from a drainage culvert. Qanat (underground waterway), famous as a water source facility for oases in the Middle East, is nothing but a very large example of this.

A water culvert is a waterway that is installed underground for various purposes. A typical example is a sewer pipe culvert, but recently there have been many examples of culverts being used to cover drainage ditches along city streets and to widen the streets.

[Masahiko Tomita]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

地下に設けられていて外からは見えない水溝(すいこう)。流水面が見える水路を開渠または明渠とよぶのに対する語で、目的、構造の異なる排水暗渠under drain、集水暗渠collecting channel、通水暗渠closed conduitの総称。排水暗渠は、地面に深さ1メートル弱の溝を掘り、底に吸水渠を敷設して埋め戻したもので、土壌中の過剰水を吸収排除して、農地、グラウンドなどの土壌水分を適正値に制御するために、あるいは、下向きに土中浸透水流を生じさせて土壌中の塩分を洗い流すために用いられる。近年灌漑(かんがい)農業の普及によって塩類蓄積による土壌資源の破壊が世界的に問題になってきて、後者の役割がとみに重視されている。吸水渠を粗朶(そだ)、石礫(せきれき)などの粗積みで形成したものを簡易暗渠といい、紀元前約3000年のウル遺跡（イラク南部）にさかのぼる歴史をもつが、現在は土管、多孔プラスチック管などによる完全暗渠や、穿孔(せんこう)機を用いて土中に孔を穿(うが)つのみの無材暗渠（モグラ暗渠ともいう）が主であり、長尺可撓(かとう)プラスチック管の連続埋設機が普及している。

　集水暗渠は、河川伏流水など比較的浅い地下水を用水として取水するために埋設される横形式の井戸で、構造的には排水暗渠と大差はない。中近東でオアシスの水源施設として名高いカナート（地下水路）は、これのきわめて大きなものにほかならない。

　通水暗渠は、種々の目的で地下に設けられる水路である。下水管渠が代表的なものであるが、最近は都市内の街路沿いの排水溝に蓋(ふた)をして暗渠とし、街路の拡幅にあてる例も多い。

［冨田正彦］

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