Introduction

Hong Kong’s high population density, intensive urbanisation and complex geography render the city highly vulnerable to flooding. As a global financial hub, infrastructure disruptions trigger significant ripple effects, making robust flood management vital for economic stability and Hong Kong’s strategic role in national development.

In alignment with national directives, including the National 14th Five-Year Plan and the Guangdong-Hong Kong-Macao Greater Bay Area (GBA) collaborative framework, Hong Kong is advancing towards a “resilient city” through an “adaptation–resilience–management” framework. As the country enters the 15th Five-Year Plan, national goals to build a modern, digital and resilient infrastructure further reinforce Hong Kong’s commitment to integrating with and serving overall national development, while consolidating and enhancing its status as an international financial, shipping and trade centre. This article reviews Hong Kong’s innovative practices and provides a technical roadmap for optimising flood prevention and urban resilience.

Characteristics of flood disasters in Hong Kong

1. Frequent and intense extreme rainfall

Hong Kong is among the world’s “wettest cities”, with an annual rainfall of about 2,400 mm driven by monsoons and tropical cyclones. Rainfall is concentrated between May and September, with rainstorms exceeding 100 mm occurring, on average, more than three times a year. These events exhibit sudden onset, mark significant spatial variability and short warning lead times. For instance, hourly rainfall exceeded 150 mm on 7 September 2023, while rainfall in excess of 100 mm was recorded in the North District in August 2025; both events caused widespread traffic disruption. Such extreme rainfall underscores the critical need for accurate flood forecasting and rapid emergency response to effectively tackle flooding.

2. Steep hillside topography
Approximately 70% of Hong Kong comprises mountainous terrain, creating a hydrological system with high runoff coefficients and short concentration paths. Intense rainfall can generate flash floods which rapidly discharge into foothill urban areas. Located at the Pearl River Estuary, Hong Kong is also vulnerable to compound events in which extreme rainfall coincides with astronomical tides and storm surges, forming a complex land-sea coupled disaster. This geographical configuration necessitates a holistic “mountain-urban-sea” framework for flood resilience.

3. High population density and highly developed urban areas
Intensive urbanisation and high imperviousness rate accelerate the conversion of rainfall into runoff, generating peak flow rates that far exceeding natural conditions. Older drainage systems in districts such as Mong Kok, Sham Shui Po, and Sheung Wan struggle with modern drainage requirements as urban expansion has progressively reduced natural retention areas. Events such as Super Typhoon Mangkhut and the rainstorm on 7 September, 2023 demonstrated how disruptions to transport infrastructure can threaten Hong Kong’s role as an international financial center. Consequently, flood prevention measures must ensure the uninterrupted operation of the city’s core functions.

Concept for Hong Kong flood prevention

1. Situational context and challenges
(i) Current flood risk conditions
Under global climate change, flood risk in Hong Kong are intensifying.. The Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) states that global temperatures rose approximately 1.1°C in 2011–2020, a trend projected to continue. The Hong Kong Observatory (HKO) data show annual rainfall increased by about 2.3 mm per year between 1884 and 2024. Under the medium (SSP2-4.5) and very high (SSP5-8.5) scenarios, maximum daily rainfall for 2081–2100 could rise by 16.0% to 28.0% above the 1995–2014 baseline. Observational peaks are escalating: the highest one-hour rainfall reached 88.4 mm in 1886, 145.5 mm in 2008, and a record 158.1 mm during the 7 September, 2023 rainstorm. This upward momentum increasingly challenges current urban design standards.

Long-term sea level rise caused by glaciers melting also imposes additional stress on Hong Kong’s drainage systems. HKO observations over the past 70 years show Victoria Harbor sea levels have increased by 32 mm per decade, reducing gravity-driven drainage efficiency due to tidal backwater effects. In addition, typhoon intensity has increased. Super Typhoon Mangkhut (2018) was among the strongest on record, followed by Saola (2023), Yagi (2024), and Ragasa (2025). When landfalls
coincide with high astronomical tides, the severity of flooding escalates significantly. Historical storm surges have demonstrated the rise of sea water levels, increasing inundation risks. Combined with more frequent extreme rainfall, these marine stresses further exacerbate Hong Kong’s flood risk.

(ii) Challenges in addressing Hong Kong’s flood disasters
With the continuing development of the GBA, Hong Kong’s role as an international financial and innovation hub has becoming increasingly important. Urban expansion has caused built-up areas encroaching upon farmland and fish ponds that previously functioned as critical flood regulation areas. This reduces infiltration capacity and increases runoff coefficients. Simultaneously, older drainage networks, built to outdated standards and constrained by limited underground space, are struggling to cope with changing hydrological conditions, leaving older districts increasingly vulnerable.

As a core engine of the GBA, Hong Kong’s operational efficiency is of utmost importance. Maintaining socioeconomic stability in a high-density urban environment requires a flood prevention system that goes beyond traditional engineering standards. The Hong Kong 2030+ Plan promotes the vision of a resilient, eco-friendly metropolis, reflecting growing public expectations regarding ecological quality and environmental comfort. Consequently, Hong Kong’s flood prevention framework
must evolve beyond the conventional drainage infrastructures towards a holistic approach encompassing aquatic ecosystem restoration and enhancement of public space to support long-term sustainable development.

2. Integrated flood management strategy
Driven by global warming and intensive urbanisation, Hong Kong’s flood risk has intensified. Conventional Engineering solutions alone are no longer sufficient. In response, Hong Kong has adopted integrated flood management strategies based on the principles of Adaptation, Resilience and Management, together with a Progressive Adaptive Approach to cope with the potentially increasing flood risks arising from climate change. Hong Kong’s current flood protection standards are summarised in Table 1. For many years, the city has adopted a “three-pronged flood prevention strategy” comprising:-

(i) Upstream interception: Drainage tunnels intercept runoff for direct discharge to the sea, bypassing densely populated urban areas. Hong Kong currently operates four major drainage tunnels—the Kai Tak Transfer Scheme, Hong Kong West Drainage Tunnel, Lai Chi Kok Transfer Scheme, and Tsuen Wan Drainage Tunnels—with a combined length of 21 kilometers;

(ii) Midstream storage: Underground stormwater storage tanks temporarily retain runoff to attenuate peak flows, relieving pressure on downstream drainage networks; and

(iii) Downstream drainage improvement: Drainage Conveyance Capacity is enhanced. River channels are widened, deepened or straightened. Between 1990 and 2010, the Drainage Services Department (DSD) completed approximately 100 km of river training in Northern New Territories, resolving regional flooding.

Fig 1. Illustration of the “Three-pronged Flood Prevention Strategy”

Table 1. Hong Kong’s flood protection standards

Innovative practices of flood management in Hong Kong

1. Adaptation: A systematic forward-looking approach
“Adaptation”, as the system's long-term foundation, aims to address uncertainties associated with complex climate change. It is guided by the following key principles: first, accommodating the nature such as topography and tidal patterns; second, reducing flooding impact by balancing flood protection measures with urban development needs; and third, preparing for worsening conditions through a progressive adaptive approach that employs evolutionary standards and phased management aligned with future climate scenarios.

At the planning level, Hong Kong has been implementing forward-looking strategies. The DSD’s Stormwater Drainage Manual (5th Edition, 2018) incorporated climate change effect into the design rainfall parameters for the first time, which enables infrastructure planning beyond existing extremes while reserving capacity for future risk mitigation within the engineering framework.

In terms of infrastructure, these principles are manifested at different scales. At the upstream scale, accommodating natural hydrology involves the use of upstream interception tunnels that leverage elevation for gravity-driven discharge, avoiding reliance on high-energy pumping. For example, the Hong Kong West Drainage Tunnel specifically deploys the supercritical vortex intakes design, to intercept mountain torrents and dissipate hydraulic energy within confined spaces before discharging to the sea at Cyberport. Reducing flooding impact involves deploying composite stormwater storage facilities in densely developed districts, such as Happy Valley, Tai Hang Tung, Sheung Wan, On Sau Road, and Anderson Road. These tanks, with a total capacity of 247,000 m³, serve as peak-attenuation reservoirs during rainstorms while simultaneously integrating with recreational spaces, achieving efficient “single-site, multiple use” outcomes. Preparing for worsening conditions involves strengthening standards and adopting phased management approach, including design provisions that enable the drainage infrastructure to be upgraded in timely and cost-effective manner as needed.

Overall, the essence of Hong Kong’s “adaptation” system lies in shifting from the conventional flood prevention approach to an integrated flood management strategy, providing a sustainable framework for subsequent enhancement.

2. Resilience: Enhancing the scale of urban elasticity
Building upon existing flood prevention measures, Hong Kong explores an elastic water body management system based on the objectives of “Flood Protection, Water Friendliness, Water Conservation, and Ecology”, enabling diversified functions under different operating scenarios. For the “Flood Protection and Water Friendliness” principle, Hong Kong transforms single-purpose drainage into multifunctional systems. For instance, the Revitalisation of Tsui Ping River involved deepening riverbed, removing a pedestrian ramp obstruction, and installing flood walls. A smart water gate rises and falls in response to tidal conditions to stabilise water levels. Coupled with riverside walkways, the channel contains floodwaters within banks
while allowing waterfront access and improving river ecology. Water-friendly platforms provide accessible nearwater spaces, showing a flexible land-use approach where flood protection infrastructure also delivers public and environmental benefits.

For the “Water Conservation and Ecology” principle, Hong Kong integrates stormwater management into its water supply. The Inter-reservoirs Transfer Tunnel connects the Kowloon Byewash and Lower Shing Mun Reservoirs, diverting runoff to increase annual water yield by 3.4 million m³. Meanwhile, the ecological restoration of Kai Tak Nullah incorporates naturalised riverbeds, aquatic vegetation, and eco-shorelines to bolster habitat and ecological selfpurification. Collectively, these initiatives transform water bodies from single-purpose discharge channels into multifunctional infrastructures capable of switching between discharge, recreation, water resource storage, and ecological enhancement.

3. Management: Leveraging smart technology
(i) Strengthening emergency response
During extreme rainfall, the Emergency Control Center (ECC) of the DSD provides centralised coordination through the Hydrological Information System to monitor real-time hydrological conditions and hazard reports. The ECC deploys emergency response teams via an intelligent operational platform, creating an integrated chain of “monitoring, command, and execution”. Prior to heavy rainfall events, the DSD deploys up to 200 emergency response teams at flood-prone locations and implements a “just-in-time clearance” mechanism to mitigate the risk of localised flooding caused by drainage blockages.

(ii) Advancing technological innovation
To enhance risk identification, Hong Kong has established a multi-source, model-driven intelligent early warning system. Over 300 hydrological and video monitoring stations collect territorial-wide data via Internet of Things (IoT) devices and cloud-based platforms. The DSD collaborated with the Pearl River Water Resources Research Institute to develop systems such as the “Flood Alert System Tomorrow (FAST)”, which provides precise water depth warnings at critical locations within one minute of detecting changes. The DSD has also developed the “Mosaic Model Map (M³)” flood risk visualisation engine, which cross-references approximately 3,000 hydraulic modelling results to forecast and generate high-resolution flood risk maps every six minutes. As a technological advancement, the “Explainable AI (XAI)” flood prediction model developed with the South China University of Technology, applies state-of-art technology to simulate potential flood risks for the subsequent hour.
The application of Digital Twin technology at the Yuen Long Pumping Station achieves dynamic operational management. Powerful pumping robots are deployed for high-risk desilting works in difficult site conditions, significantly enhancing emergency-response efficiency.

(iii) Enhancing public awareness of flood prevention
Through educational programmes and publicengagement initiatives, public awareness of flood risks has been reinforced. Moreover, establishing mechanisms for building community resilience and effective risk communication enables the shift to proactive flood management approach, thus enhancing the community’s overall resilience against extreme weather events.

4. Effectiveness of protection against extreme rainfall
Recent extreme rainstorms have served as stress tests for Hong Kong’s “adaptation-resilience-management” framework. With the enhanced flood management adopting the holistic adaptation, resilience and management approach, the consecutive black rainstorms in 2025 revealed the effectiveness of the system’s drainage capacity and recovery performance. The well-established drainage infrastructure including interception tunnels, storage tanks and enhanced conveyance network effectively attenuated peak flows, while resilience measures such as installation of flood barriers at flood-prone locations further reduced localised flooding impacts. In terms of management, the “Mosaic Model Map (M³)” and “Flood Alert System Tomorrow (FAST)” enabled the early deployment of response teams, while high-powered pumping robots handled flooding situations efficiently and safely. The 2025 events resulted in reduced flooded areas and faster restoration of social functions, with most locations recovering within one to two hours via ECC operations. This reflects a clear evolution from conventional drainage infrastructure to an integrated flood management strategy to ensure stable protection and rapid recovery under
increasingly extreme climate scenarios.

Way forward

Looking ahead, climate change will continue to places increasing demands on Hong Kong’s flood prevention capabilities. As intensifying storms challenge traditional engineering approaches, further transformation beyond the current framework will be required. Hong Kong should capitalise on its position as an international innovation hub by implementing the “Arrangement between the Ministry of Water Resources and the HKSAR Government on the Management and Co-operation of Water-related Affairs” while leveraging regional collaboration and technological innovation to realise “Intelligentisation, Synergisation and Value Realisation”.

1. Strengthening design to establish a secure and intelligent flood resilience system
Future efforts should focus on building a secure and intelligent flood prevention system. This entails developing new flood control planning to support the Hong Kong 2030+ Plan objectives of “enhancing livability, embracing economic opportunities and challenges, and creating development capacity”.

Strengthening cooperation with the Mainland remains pivotal. This includes advancing rainfall monitoring through satellites, radar, and automatic monitoring station networks to establish a GBA joint flood prevention mechanism. Overcoming data sharing barriers is essential for improving intelligent flood prediction through big data and Explainable AI (XAI). In addition, integrating forecasting, warning, simulation, and planning into a unified platform can enhance the effectiveness of emergency mobilisation.

2. Leveraging the integrated functions and multifaceted value of water bodies to establish a new paradigm for sustainable development
To advance the development of a livable city, flood prevention projects should closely integrate with urban public spaces and ecological landscapes, as demonstrated into projects such as Tsui Ping River and Kai Tak River. Transforming infrastructures into vibrant, and water-friendly corridors supports a sustainable development model centred on “Flood Control, Water Friendliness, Water Conservation, and Ecology”. Leveraging these integrated functions allows Hong Kong to pioneer a multi-objective approach to flood management.

3. Driving innovation to establish a model hub
Hong Kong should leverage its resources and capacity as an international innovation hub to drive advances in water conservancy innovation. By integrating large AI model technology with advanced early-warning systems, the city can translate its technological strengths into effective capabilities for disaster prevention and mitigation.

About the authors:

1. W C MOK, the Drainage Services Department, HKSAR Government
2. YANG Fang, Key Laboratory of Water Security Guarantee in Guangdong-Hong Kong-Macao Greater Bay Area of Ministry of Water Resources, Pearl River Water Resources Research Institute of the Pearl River Water Resources Commission
3. J H W LEE, Key Laboratory of River Basin Digital Twinning of Ministry of Water Resources, Macau University of Science and Technology