Reservoirs, 'sponge city' model could mitigate Hanoi flooding: experts

The historic downpour on May 29, with the highest rain level seen since 1986, flooded over 100 locations in the capital. As the climate crisis progresses, along with a growing population and increasing burden on infrastructure, experts say Hanoi would soon need a solution to effectively deal with floods and inundations.

Tran Duc Ha, head of the Water Supply and Sewerage Research Institute, said constructing reservoirs could be scaled up. Hanoi currently has one trial reservoir of 2,000 cubic meters for Nguyen Khuyen Street. Such a model has already been applied in Hong Kong, he said.

Reservoirs are often used to resolve local inundations. To choose a suitable location, authorities need to determine specific areas prone to flooding, as well as ideal localities to build reservoirs. Those locations could include large grass or soccer fields, he added.

When it rains, the water would flow toward reservoirs instead of flooding the streets.

"The stored water could be used for irrigation, firefighting or be processed into tap water," Ha said.

In Hong Kong, one such reservoir is placed beneath Happy Valley. The reservoir has a capacity of 60,000 cubic meters, is situated underneath five soccer fields and has greatly contributed to Hong Kong's anti-flooding efforts.

Ha said reservoirs and sewage systems should be separated. A sewage system would operate on a much larger scale and be costlier. They are typically connected to treatment plants and direct water toward rivers and lakes.

Rainwater would flow directly to rivers and lakes, while wastewater would be treated first, preventing pollution.

Tokyo has one of the most advanced sewage systems, with its Metropolitan Area Outer Underground Discharge Channel (MAOUDC) considered one of the world’s largest anti-flooding structures.

The system drains water from small- and medium-sized rivers in northern Tokyo, feeding five reservoirs. The water from the reservoirs are pumped through a 6.3-kilometer tunnel towards Edo River at 200 cubic meters per second.

If the water level in Edo River gets too high, MAOUDC would slow down the flow in response.

Nguyen Van Lien, former Minister of Construction, said the most important factor is a space for water to be drained away, using materials that absorb water well, while expanding grassland areas and water bodies.

The method Lien was referring to is known as a "sponge city", an urban construction model for flood management focusing on ecological infrastructure. Instead of draining all the water away, such a model would either retain some of the rainwater for agricultural purposes or treat it instead. Beijing and Shanghai are two examples of using such a model for flood management.

Cities can apply the model simply by planting more trees on rooftops, building more reservoirs, using water-absorbent materials for roads and expanding ponds and lakes.