What a 1,000-Year-Old Drainage System Teaches Us About Flood Risk

Ganzhou lies at the confluence of the Gong and Zhang Rivers, which merge to form the Gan River. This geographical setting historically exposed the city to seasonal flooding during the rainy season. Annual rainfall in the region often exceeds 1,500 mm, with approximately 64% of runoff occurring between April and September, creating significant pressure on urban drainage.

To address these challenges, city planners during the Song Dynasty constructed the Fushougou drainage system to serve the southeastern and northwestern sections of ancient Ganzhou. The system covered approximately 3 km² and included more than 12 km of underground drainage pipelines, working in coordination with an approximately 8 km-long city wall.

Historical sources attribute the expansion and refinement of the system to Liu Yi, a water management specialist during the Northern Song Dynasty, who improved the connectivity and capacity of the drainage network.

The system relied entirely on gravity-driven flow, utilizing the natural terrain gradient of the city rather than mechanical pumping. By integrating drainage channels, hydraulic control structures, and surface water bodies, Fushougou functioned as a comprehensive water management network that managed stormwater, urban wastewater, and river flooding simultaneously.

Three key components formed the backbone of the system.

1. Brick-lined drainage channels and pipelines

The primary infrastructure consisted of arched brick-lined drainage channels, designed for both structural stability and hydraulic efficiency. Larger channels reached approximately 0.9–1.0 m in width and 1.6–1.8 m in height, while smaller channels were covered with stone slabs.

One notable feature of the system is the depth of the pipelines, which in many locations extend roughly 2 meters below ground level. The deeper placement increased storage capacity within the network and reduced the risk of surface flooding during heavy rainfall.

Hydraulic performance was also enhanced through relatively steep channel gradients. For example, recent studies have identified a slope of approximately 4.25% near one of the system’s discharge structures. Such gradients produce relatively high flow velocities that help flush sediments and debris through the system, reducing blockages and minimizing long-term maintenance requirements.

In some sections, culverts narrow near discharge points, increasing flow velocity and helping to facilitate efficient outflow through the system’s outlets. These design characteristics demonstrate a strong understanding of hydraulic principles and the importance of self-cleansing flow velocities in urban drainage systems.

2. Water windows: Passive hydraulic control structures

The drainage network discharged through twelve outlets located along the city wall, known as “water windows.”

These structures functioned similarly to modern hydraulic check valves. Under normal river conditions, stormwater from the city could flow outward through the windows into surrounding rivers. In some locations, accelerated flow from narrowed culverts helped push the windows open.

However, when river levels rose during flood events, the increased external water pressure forced the openings closed, preventing floodwaters from flowing back into the city.

This passive hydraulic mechanism allowed the system to automatically respond to changing river levels without mechanical components or manual intervention. Such gravity-based and pressure-responsive controls represent a simple yet effective method for preventing backflow in flood-prone urban environments.

3. Interconnected storage ponds

Complementing the underground drainage network was a system of interconnected ponds located within and around the city. Historical accounts suggest that the system was linked to over one hundred ponds, forming a distributed storage network.

These ponds served multiple functions, including agricultural irrigation, aquaculture, and stormwater retention. Fish farming was commonly practiced, and nutrient-rich sediments accumulated in the ponds were reused in agriculture, forming a localized ecological cycle.

During heavy rainfall, the ponds acted as temporary storage basins, capturing excess runoff and reducing peak flows within the drainage system. Water could then be gradually released after storm events, helping to moderate downstream flood risks.

From a contemporary perspective, these ponds function similarly to decentralized stormwater detention and retention systems, which are widely used in modern sponge city designs. In addition to their hydraulic role, the ponds also supported microclimate regulation and aquatic ecosystems, demonstrating how water infrastructure can simultaneously deliver environmental and social benefits.

Historical records indicate that by the late 1960s approximately 12.6 km of the Fushougou drainage network was still functioning and serving an estimated 100,000 residents in Ganzhou’s old urban district.

Rapid urban expansion during the twentieth century led to the loss of many original components of the system. Today, only about 1 km of the original drainage network, along with two ponds and two water windows, remain intact.

Despite this partial loss, the surviving elements of the system continue to illustrate the durability of its design. The longevity of the infrastructure highlights several characteristics that contributed to its resilience:

• Gravity-driven drainage, eliminating reliance on mechanical pumps
• High hydraulic capacity relative to the scale of the historic city
• Self-cleansing flow velocities that reduce sediment accumulation
• Distributed storage through ponds, which buffer peak stormwater flows
• Integration with natural terrain gradients

Together, these features demonstrate how urban water infrastructure designed around natural hydrology and passive hydraulic processes can remain functional for centuries.

Although the historical context of Fushougou differs from that of modern megacities, several design principles remain relevant for contemporary urban flood risk management.

1. Integrating natural and engineered infrastructure

The Fushougou system demonstrates how engineered drainage can be combined with natural landscape features such as ponds and terrain gradients. This hybrid approach aligns with current nature-based solution strategies, which seek to enhance the role of natural systems in urban water management.

2. Multifunctional water infrastructure

The system simultaneously supported stormwater drainage, wastewater conveyance, irrigation, and aquaculture. Such multifunctionality is increasingly recognized as an important characteristic of sustainable urban infrastructure, allowing water systems to deliver environmental, economic, and social benefits.

3. Passive hydraulic design

The water windows illustrate how simple hydraulic mechanisms, including pressure-responsive closures and flow-accelerating channel design, can provide reliable flood protection without energy inputs or complex mechanical systems.

4. Designing for longevity and maintainability

The steep gradients and self-cleansing flow conditions of the drainage network reduced sediment accumulation and maintenance demands. Designing systems that can maintain hydraulic performance with minimal intervention is an important consideration for long-term infrastructure sustainability.