In Hong Kong, the rainy season generally starts in April. In order to further reduce flood risks during rainstorms, the Drainage Services Department (DSD) has introduced the “just-in-time clearance” arrangement this year. It has also adopted new technologies in using a new remote-controlled desilting robot for silt clearing works at box culverts to enhance the efficiency of desilting works. This time, I have invited a colleague from the DSD to brief us about the operation and functions of the new robot, and on how it works differently from the traditional desilting method.

Preventing silt accumulation from affecting the drainage capacity

Hong Kong faces an average rainfall of about 2 400 millimetres a year, one of the highest among cities in the Pacific Rim. According to Mr POON Tin-yau, an engineer of the DSD, when stormwater is discharged into the sea through box culverts, the washed-off sand, stones and dust will accumulate gradually at the drains to form silt, which will in turn affect the drainage capacity and may lead to flooding in the most serious cases. To avoid the above situation, the department inspects the box culverts on a regular basis and arranges the desilting works if necessary to ensure that the drains are functioning properly.

Operating as a vacuum cleaner

Early this year, a new remote-controlled desilting robot was introduced into the DSD. The DSD conducted a pilot test on the use of the robot for desilting works at the box culverts in Sham Shui Po and Tsuen Wan with its functions monitored.

The robot will be lifted up with a crane and sent into the box culvert concerned through its opening. With the help of closed-circuit television and sonic survey, the operator can then observe the conditions inside the box culvert and remotely operate the robot for desilting from his workstation. Mr POON Tin-yau says that the robot, measuring approximately 3 metres in length, and 1.5 metres in both width and height, works similarly to a vacuum cleaner. Once the silt is sucked by the robot, it will be pumped to a temporary silt container on the ground through a tube connected to the robot. The silt will be transported to a landfill only after dewatering.

Enhancing work safety

According to the traditional desilting method, workers need to go into the box culverts for installation and operation of desilting devices. Given that box culverts are confined spaces, workers working inside will face certain safety risks. The traditional method also requires interception of water flow in the culverts to allow workers to work in an environment without water flowing through, which means the work is limited mostly to dry seasons. On the contrary, the remote-controlled desilting robot can take over diving tasks to spare workers from going into confined and submerged space of the box culverts. Apart from enhancing work safety, the use of the robot allows desilting works in rainy seasons, which in turn will expedite the progress of such works, lower the costs and significantly improve the desilting efficiency.

Implementation of the “just-in-time clearance” arrangement

Furthermore, the DSD had analysed more than 200 flooding cases between 2017 and 2019, finding that more than 60 percent of them were due to blockage of drains by litter, fallen leaves or other washouts carried by surface runoff. This year, the department will implement the “just-in-time clearance” arrangement. Before the onset of a rainstorm, staff will be deployed to inspect about 200 drain locations in the territory which are susceptible to blockage by litter, fallen leaves or the like, and will immediately arrange for clearance if necessary. The department will also send staff to inspect and clear all major drainage intakes and river channels to prevent blockage after a rainstorm or when a typhoon signal is about to be lowered so as to prepare for the challenges of further rainstorms.

Constructing more underground stormwater storage tanks

Apart from strengthening the responsive management measures before and after rainstorms, the DSD will continue to press ahead with its flood prevention strategy, which includes constructing more underground stormwater storage tanks to collect and temporarily store excessive rainwater during rainstorms, thus reducing the loading at downstream drains and the consequential flood risks. At present, six locations are under planning, including Shek Kip Mei Park, Tai Hang Tung Recreation Ground (extension), the Urban Council Centenary Garden in Tsim Sha Tsui, as well as Sau Nga Road Playground, Kwun Tong Ferry Pier Square and Hoi Bun Road Park in Kwun Tong District.

I hope that various departments will continue to take precautions to proactively eliminate the risks brought by rainy seasons and give the highest priority to public safety.