為什麼東京淹水後，積水可以很快排掉？關鍵不只是下水道，而是整套地下分洪系統

很多人會發現，日本東京及其周邊地區在遭遇短時間強降雨時，部分地區雖然也會出現嚴重積水，但洪水退得往往相當快。這並不是因為東京的普通下水道「特別大」，而是日本長期建立了一套從河川、地下隧道、蓄洪設施到大型抽水站相互配合的防洪體系。其中最具代表性的工程，就是位於埼玉縣春日部市的首都圈外郭放水路。

這座工程經常被稱為「地下神殿」。真正令人驚嘆的其實不只是地下巨大的59根柱子，而是它背後完整的流域洪水分流概念。需要特別說明的是，首都圈外郭放水路並不是直接把東京市中心的積水抽走，而是主要負責降低東京東北側中川、綾瀨川等河川流域的洪水風險。由於這些河川與東京首都圈高度連結，因此對整個都市圈的防洪安全具有重要作用。

整套系統可以簡單理解成「中小型河川→越流堰→地下豎井→地下隧洞→第1立坑→調壓水槽→大型抽水站→江戶川」。當暴雨導致河流水位迅速上升，超過設定的警戒水位後，部分洪水便會透過越流堰自動進入地下系統。洪水首先流入巨大的豎井，再經由地下隧洞輸送，最後集中到抽水設施，由大型抽水機將洪水排入江戶川。

這套設計最重要的地方，就是在洪水還沒有把原本的河川完全壓垮之前，先把部分洪峰分流出去。因此它並不是等城市已經被淹沒後才開始排水，而是在河川水位快速上升的過程中，就利用地下通道提前「搶走」一部分洪水，降低原有河道承受的壓力。

整個工程於1993年開始施工，2002年開始部分通水，2006年全線投入運轉，總投資規模約30億美元。地下主隧洞全長約6.3公里，內徑約10公尺，埋深約50公尺，相當於在地下打造了一條巨型人工洪水通道。其中，第1立坑是整套系統的重要節點之一。它的直徑約31.6公尺，開挖深度約76.5公尺。它主要負責承接由6.3公里地下隧洞輸送過來的洪水，讓地下隧洞與調壓水槽在水力上連通，同時在水流狀態轉換過程中提供一定的緩衝空間。此外，第1立坑在工程建設期間也曾作為盾構施工的工作井，完工後則可以提供檢修、檢查及清淤等作業通道。

而最著名的「地下神殿」，其實就是調壓水槽的一部分。巨大的空間與密集排列的混凝土柱，看起來像一座地下宮殿，但它真正的用途是處理和調節大量洪水，使水流能夠更加穩定地進入後方的抽水設施。這些巨大柱體並不是單純為了視覺效果，而是地下大型水利工程結構的重要組成部分。最後，洪水會進入大型抽水站。當地下系統累積大量洪水後，抽水設備便將水抽送至江戶川，再由江戶川向下游排放。換句話說，這套系統並不是讓洪水「消失」，而是將原本集中在低窪城市與中小型河川中的洪水，暫時導入地下、進行分流與調節，再以巨大的抽水能力轉移到具有更大輸水能力的江戶川。

因此，東京部分地區在暴雨後能夠較快退水，背後依靠的並不是某一條超大型排水管，而是一整套多層次的防洪工程。一般降雨由城市排水系統處理；當雨量進一步增加，河川與排水渠道承擔更大的水量；如果洪水規模繼續上升，地下分洪隧道、調壓水槽與大型抽水站便開始發揮作用。不同層級的工程彼此配合，形成一個完整的洪水管理系統。

這也是日本都市防洪思維與單純「把下水道做大」最大的不同。真正有效的防洪並不是幻想讓所有洪水都不要進入城市，而是在極端降雨發生時，想辦法把洪水分流、暫存、調節，再快速輸送到可以承受更大水量的河川。所以，當人們看到東京部分地區在暴雨過後積水迅速下降時，真正值得注意的並不是「日本下水道有多神奇」，而是背後長期累積的流域治理、地下分洪、抽水設備、河川整治、洪水監測與都市規劃。首都圈外郭放水路只是其中最具代表性的案例，而「地下神殿」真正厲害的地方，也從來不是外觀，而是它能在洪峰來臨時，把原本可能集中爆發的洪水迅速重新分配到整個流域的防洪系統之中。

Why Does Tokyo Drain Floodwater So Quickly?

When heavy rain causes urban flooding in Tokyo, the water can sometimes appear to disappear surprisingly quickly. The reason is not simply that Tokyo has larger sewers. Japan has invested in a sophisticated flood-control network that combines underground diversion tunnels, massive shafts, pumping stations and river systems to temporarily store and redirect excess water.

One of the most famous examples is the Metropolitan Area Outer Underground Discharge Channel, located in Kasukabe, Saitama Prefecture, northeast of Tokyo. Often called the “Underground Temple” because of its enormous underground columns, the facility is, from an engineering perspective, much more than an impressive underground structure. It is a large-scale watershed flood-diversion system designed primarily to protect the Nakagawa and Ayasegawa river basins in the northeastern Tokyo metropolitan area, rather than directly draining central Tokyo.

The system can essentially be understood as a chain: smaller rivers → overflow weirs → vertical shafts → underground tunnels → No. 1 shaft → surge tank → large pumping station → Edogawa River. When intense rainfall causes water levels in surrounding rivers to exceed predetermined levels, excess water enters the system through overflow weirs. It is then diverted into five enormous vertical shafts and transported through an approximately 6.3-kilometer underground tunnel. Eventually, the water reaches the pumping station, where it is discharged into the Edogawa River. By removing part of the flood peak from the original rivers, the system reduces the risk of flooding in downstream residential areas.

Construction began in 1993, partial operation started in 2002, and the entire system became operational in 2006. The project required an investment of roughly US$3 billion and represents one of the most ambitious urban flood-control projects in Japan.

The underground tunnel itself is approximately 6.3 kilometers long, with an internal diameter of around 10 meters and a depth of roughly 50 meters below ground. At this depth, the tunnel functions as an enormous underground water-transfer route rather than simply a conventional drainage pipe.

The No. 1 vertical shaft is another critical component. It is approximately 31.6 meters in diameter and 76.5 meters deep. Its primary purpose is to receive floodwater transported through the 6.3-kilometer underground tunnel and connect the tunnel hydraulically with the surge tank. It also provides a degree of buffering as the flow changes from the underground tunnel system into the pumping facility. In addition, the shaft served as a working shaft during tunnel construction and provides access for inspection, maintenance and sediment removal.

The most spectacular part of the facility is the enormous surge tank, where rows of massive concrete columns support the ceiling. These columns are not merely architectural decoration. They are part of a huge underground hydraulic structure designed to control and stabilize the movement of floodwater before it reaches the pumping station.

The final step is handled by the pumping system. Once the diverted floodwater reaches the underground facility, enormous pumps lift and discharge it into the Edogawa River. In other words, the system does not make floodwater magically disappear. It temporarily diverts, stores, controls and pumps the water into a larger river capable of carrying the additional flow.

This is also why comparing Tokyo with cities that rely primarily on ordinary storm drains can be misleading. Tokyo's flood protection is based on multiple layers of infrastructure. Drainage pipes handle ordinary rainfall, rivers and channels carry larger flows, while underground diversion tunnels and reservoirs provide additional capacity during extreme storms. When all these systems work together, floodwater can be removed from vulnerable urban areas much faster than would be possible through surface drainage alone.

The key to the system is therefore not simply the giant underground shafts or the famous “underground temple.” Its real strength lies in moving floodwater away from where it poses the greatest danger before the original rivers reach catastrophic water levels.

This approach reflects a broader principle of Japanese flood-control engineering: rather than attempting to prevent every drop of floodwater from entering cities, engineers build systems capable of diverting, temporarily storing and rapidly discharging excess water. That combination of watershed planning, underground infrastructure, pumping capacity, monitoring and river management is a major reason why some heavily urbanized areas around Tokyo can recover from major rainfall events remarkably quickly.