Holding Back the Rain

When the rain has nowhere to go

Faced with extreme weather in recent years and torrential rains that can bring 200 millimeters of rain in a single hour, have you ever wondered where all the water in a city goes after a storm?

In the agricultural era, farmland itself provided water storage. Fields could once hold roughly 20 centimeters of water for a day or two without harm. With urbanization, however, buildings and concrete cover the ground, leaving water nowhere to collect and causing it to converge more quickly on drainage systems. Flood control has consequently become an issue every city must confront, explains Chiu Peng-hao, chief engineer of the Department of Water Resources, Taoyuan City Government.

Chiu Peng-hao explains that a detention basin temporarily stores excess rainwater, reducing the volume discharged downstream and easing pressure on the downstream drainage system.

“Every waterway has its own temperament and physical characteristics, so each calls for a different solution,” says Chiu. But as cities have developed, much of the land along their waterways has already been put to use. Widening a channel often means land acquisition and massive construction costs. Given these realities, detention basins have become one important tool for urban flood control.

A city’s “room for water”

Located in Taoyuan District near the rear of Taoyuan Railway Station, the Dashulin Detention Basin hardly looks like a flood-control facility. Aboveground, its two grass-covered depressions and surrounding paths resemble an urban park on ordinary days. Belowground, the interplay of light and reflected water makes the floodwater storage space feel like a hidden world unto itself.

Basins A and C (top two photos) and basin B (bottom photo) can hold a combined 118,000 cubic meters of rainwater.

The Dashulin project was designed to address flooding along Dawan Ditch, a tributary of Dongmen Creek, and the nearby Xipo Drainage Channel. Three parcels of public land happened to be available nearby. Adapting the project to local conditions, the city’s Department of Water Resources planned basins A, B, and C there, using diversion and storage to temporarily “hold back” excess rainwater for the city.

Basins A and C are grassed, sunken spaces that resemble urban parks in ordinary days and hold water during the rainy season.

Basins A and C are open-air, sunken spaces where residents walk and relax on ordinary days; during heavy rain, they become holding ponds. Basin B occupies an irregular site, so engineers extended the space underground, excavating vertically to a depth of nine meters to receive excess water from upstream during heavy rain. Once downstream levels subside, 12 pump units discharge the stored water. Draining basin B from full to empty takes just six hours.

Basin B, known as the “underground palace,” has 12 pump units that discharge stored water. It takes just six hours to drain the basin from full to empty.

For the three basins to work together, water must be transferred among them. They are linked by 2.2-meter-diameter culverts beneath the roads, allowing water to move between the basins according to water levels and predetermined routes.

(Left) The three basins are linked by 2.2-meter-diameter culverts beneath the roads, allowing water to be transferred among them.

(Right) The design of the overflow weirs is critical to the basins’ regulating function.

The overflow weirs at the edges of basins A and C are especially important for controlling water levels and directing flow. Engineers must calculate in advance how water will enter and where it will go.

How do you sink a palace into the ground?

Putting such a “palace” underground, however, was not as easy as it might sound.

Chiu asks us to picture basin B as a giant box buried in the ground. But the groundwater would exert upward buoyant force on the box, so engineers had to add structural weight to hold it firmly in place.

The engineers proposed leaving the floor unsealed, a design made possible by Taoyuan’s geology. The area’s strata were formed by alluvial deposits from the ancient Dahan River: over time, large quantities of cobbles, gravel, and clay carried from upstream accumulated to create a relatively impermeable layer. Rather than sealing the basin floor, the engineers retained this natural stratum, allowing groundwater to slowly seep away and reducing the upward force it exerts on the structure.

With its floor left unsealed, the basin exposes the site’s natural cobbles and gravel, along with concrete blocks installed during construction to support heavy-duty shoring and scaffolding.

“This approach takes the geology into account, and it can only be used in Taoyuan and Taichung,” says Chiu, with a note of pride in his voice.

During construction, extensive heavy-duty shoring and scaffolding had to be erected. Because these supports could not stand securely on the cobbles and gravel, 225 concrete blocks were laid out in orderly rows on the basin floor. From a distance, they looked like pieces of “tofu.” Originally installed simply as bases for the construction scaffolding, the blocks were left in place after completion and became part of the space’s landscape.

More Than Just a Photo Op 

Today, the “underground palace” at the Dashulin Detention Basin has unexpectedly become a popular social-media photo spot, with reservation slots often filling as soon as they open online.

“We are actually a flood-control facility; becoming a tourist attraction was a happy accident,” Chiu says with a wry smile. For the Department of Water Resources, however, the unexpected popularity has created a rare opportunity for public education.

Light and shadow give the underground palace a distinctive atmosphere, making it a popular destination.

Curiosity draws visitors into the detention basin, giving them a chance to see another side of urban flood control. A detention basin is not a “flood-control temple” whose construction puts an end to urban flooding once and for all. Its real job is to take in water when a storm hits, lower the flood peak, buy time, and ease pressure on the downstream drainage system.

The basin is also a reminder that every engineering project has limits. What cities truly need to learn is not how to make water disappear, but how to coexist with it.