Storms and Flooding: Drainage Systems and Treatment Plants that Protect Cities

The video footage has gone viral across the web: wave after wave of brown, foaming water bursting through the doors of a Hong Kong hotel. The surges were so strong that they swept people off their feet in the lobby of the Fullerton Ocean Park Hotel on Hong Kong Island in the city’s southwest.

It was the most vivid testament of the strength of Super Typhoon Ragasa, the biggest typhoon in Asia and in the world so far this year.

After causing death and destruction in Taiwan and elsewhere, the typhoon raged past Hong Kong, lashing it with severe storms and winds of up to 220 kilometres per hour as it headed towards mainland China.

It sent more than 100 people to the hospital, and hundreds more sought refuge; trees were felled, buildings swayed, and neighborhoods flooded. It also forced the cancellation of hundreds of flights, stranding tens of thousands of travellers.

As it later approached Guangzhou, the largest city in southern China, millions of people were evacuated as schools and businesses closed. The economic damage is estimated to be in the billions.

The flooding caused by the heavy rains and surging ocean waves, among other consequences of the typhoon’s passage, not only confirmed the growing danger posed by more frequent storms in a warming climate, but also highlighted the urgency for coastal cities like Hong Kong to prepare for them. It is particularly exposed to them because it is a densely populated city with its myriad of buildings located either on the coastline or up steep hills.

 Hong Kong already invests billions of Hong Kong dollars to mitigate future damages caused by these severe storms, even more so since 2023 when a so-called once-in-500-years storm struck, with record downpours causing a massive water disaster. In Wong Tai Sin, Kowloon, the lower ground floor of the Temple Mall North was completely submerged.

The city also has emergency response teams to unclog drains during storms, sensors that monitor water levels in areas prone to water flooding, drainage inlests, roadside gullies and water drainage channels. At the Anderson Road Quarry site, the city has its first flood retention lake, while reservoirs optimize water storage in places like Kowloon.

Hong Kong’s Drainage Services Department, which overseas more than 2,000 kilometres of stormwater drains, has more than 10 major stormwater drainage improvement projects underway. There are also nine stormwater storage schemes and improvements to more than 50 kilometres of drainage channels.

The aim is to keep people and property as safe as possible, and allow for the city to return to its daily activity as soon as the storm has passed.

In all of these investments, deployed one way or another by cities across the globe, there is also a health safety element. Many have combined sewage systems, whose age and size make them inadequate to serve the growing demands of these burgeoning urban centres. Therefore, there is the real risk of disease.

Severe storms, which are becoming more frequent and abundant, often overwhelm these systems, forcing utilities to dump untreated water and sewage, or combine sewage overflow (CSO), into nearby lakes and rivers.

Washington, D.C. is one of those cities expanding their sewage treatment plant and drainage systems. It had Webuild excavate the Anacostia River Tunnel (ART), one of five designed to help the water utility better protect the Anacostia and Potomac rivers.

Nearly four kilometres long, it acts as a storage for stormwater, releasing it to the treatment plant when necessary. The ART can hold more than 38 million gallons of CSO during a given storm, reducing the discharge of polluted and untreated water into rivers by 98%.

In Cleveland, where discharges of sewage water has led to bacteria, pathogens and other pollutants harming the quality of the water in Lake Erie, Webuild developed the Dugway Storage Tunnel. It stores and conveys CSO from ancillary collection and relief sewers. It can store up to 370 million gallons every year and hold them until the rains subside and the treatment plant has the capacity to treat them.

There are similar wastewater treatment plants even in the Middle East, a region that is not usually associated with heavy rainfall.

In Abu Dhabi, the Strategic Tunnel Enhancement Programme oversaw the excavation of a tunnel tens of kilometres long to collect sewage by means of gravity and direct it to a treatment plant in Al Wathba.

Its excavation involved the simultaneous use of five tunnel-boring machines, led by Webuild, now a world leader in the hydro sector.