Pitter patter, rising water: The story behind recent flash floods in Penang

It is all started with a heavy rain from 2:40 am on 15 September that caused flash floods and uprooted trees in Balik Pulau, Jelutong and Gelugor, disrupting morning traffic and raising concerns over slope and drainage stability. The Penang Fire and Rescue Department’s Assistant Director of Operations, John Sagun Francis, identified the worst-affected areas as Jalan Jelutong near the police station, Jalan Bukit Lama, Jalan Genting, Jalan Sungai Nipah in Balik Pulau and Jalan Kampung Kastam in Gelugor.

The incident highlights broader systemic issues in urban hydrology and climate resilience, particularly in rapidly developing areas like Penang. According to the Malaysian Meteorological Department, Penang and the northern region have been experiencing more frequent and heavier downpours, even if they last only a short time. This trend matches global climate models that warn of more unpredictable and extreme weather as the planet warms.

Studies show that in areas like Penang, the number of intense rain events like those that drop a lot of rain in under an hour, has increased steadily since the 1970s. In fact, over 60% of rain stations in the region have reported higher peaks in short bursts of rainfall. During the monsoon season, rainfall in some northern states is now 20–40% heavier than in past decades, putting added pressure on drainage systems and increasing the risk of flash floods.

Intensifying rainfall increases pressure on vulnerable terrain and infrastructure, often overwhelming poor drainage systems, and saturating soils. This weakens slopes, triggering erosion or landslides, especially in hilly areas lacking proper safeguards. Without urgent upgrades and slope management, extreme weather will keep endangering lives, damaging infrastructure, and straining emergency services.

Is soil the culprit?

While the immediate trigger of the recent incident was intense, short-duration rainfall, geotechnical experts highlight deeper vulnerabilities within Penang’s urban and semi-hilly terrain. The region’s susceptibility to flash floods and slope failures is not just a result of rainfall volume, but also of the underlying soil behaviour under saturated conditions.

In hilly areas such as Balik Pulau, where moderate to steep slopes are underlain by weathered granitic residual soils, the rapid infiltration of rainwater poses a critical challenge. These soil, often silty or sandy with limited natural bonding, are prone to saturation, particularly when vegetation cover is sparse or root systems are shallow. During heavy rain, water infiltrates the vadose (unsaturated) zone, and as it accumulates, it increases the pore water pressure within the soil matrix.

From a geotechnical standpoint, this build-up of pore water pressure reduces the effective stress, which is the force keeping soil particles interlocked. As this “grip” weakens, so does the soil’s shear strength, making slopes more prone to shallow slips, debris flow, or even rotational landslides—particularly along naturally weathered or pre-existing weak layers.

A lesser known but equally important mechanism is the loss of matric suction in partially saturated soils. In dry or semi-dry conditions, suction between soil particles provides extra cohesion, which helps stabilise slopes. However, as saturation increases during intense rainfall, this suction disappears, further lowering the soil’s resistance to movement, especially in fine-grained or clayey residual soils common in the region.

Water is the foe?

The situation is further aggravated when surface drainage is poor or culverts become blocked. Inadequate water discharge capacity means more water remains on the surface, resulting in excessive runoff, soil erosion, and localized flooding—especially at slope toes or in channels where water converges. In such cases, the geotechnical and hydrological failures are interconnected: saturated slopes fail due to internal instability, while blocked runoff routes cause overland flow to rapidly accumulate, overwhelming stormwater systems.

This is particularly evident in low-lying urban zones such as Jelutong and Gelugor, where aged or undersized drainage infrastructure is common. During high-intensity rain events, these systems can no longer accommodate the volume of runoff, leading to flash flooding, submerged roads, and stress on surrounding structures. When drainage backs up at the foot of unstable slopes, the resulting water pressure can further destabilise embankments and retaining walls—turning isolated incidents into cascading urban hazards.

In short, flash floods in urban Penang are not merely surface-level problems; they are deeply tied to subsurface geotechnical processes. As rainfall patterns grow more erratic due to climate change, the interaction between infiltration, pore pressure, slope stability, and drainage capacity must be better understood and managed through integrated planning and resilient infrastructure design.

Future preparedness: Soil and water ready

With hydrometeorological hazards on the rise, civil and geotechnical engineers are urging more frequent, detailed risk assessments. This is especially so in steep, landslide-prone, or flood-vulnerable areas. These assessments should involve subsurface investigations, slope stability analysis, infiltration studies, and drainage evaluations, particularly ahead of and during monsoon seasons.

Crucially, such evaluations must form part of an integrated disaster risk reduction strategy. This includes real-time monitoring systems, such as the installation and monitoring of inclinometers, piezometers rain gauges connected to automated early warning platforms, in parallel with robust community outreach to improve public awareness, evacuation readiness and response coordination.

For the bigger picture, embedding geotechnically sound and sustainability-driven practices into urban planning is essential for building climate-resilient cities. This includes enforcing zoning regulations based on slope stability and flood hazard mapping, restricting, or engineering development on marginal terrain, implementing nature-based solutions like soil bioengineering and reforestation, as ell as adopting low-impact urban drainage designs to manage runoff effectively. When planning decisions are grounded in robust geotechnical principles and guided by ethical engineering practices, urban areas become better equipped to withstand future disasters while safeguarding communities and ecosystems for generations to come.

Are we rain-ready?

As Penang advances toward its smart and sustainable city goals, today’s event serves as a stark reminder that infrastructure resilience must be deeply aligned with environmental realities, recognizing that even short, intense bursts of rainfall can trigger significant impacts in our increasingly volatile climate.

Integrating responsible engineering practices, proactive risk management and community engagement will be essential to safeguarding the city’s future and ensuring truly sustainable urban development.