In recent years, extreme rainfall and related disasters, including floods and landslides, have led to significant property damage and loss of life globally. Southeast Asia (SEA) is particularly impacted by these rainfall-driven events. This study reviews research development and approaches to understand the current status of monsoonal extreme rainfall in SEA, with the importance of the impacts of natural and anthropogenic factors. Natural factors, including the individual and combined effects of various climatic phenomena, such as Madden–Julian Oscillation (MJO), El Niño–Southern Oscillation (ENSO) and cold surges (CSs), have significant impacts on rainfall patterns. Anthropogenic factors, including emissions and changes in land use, also play a crucial role in producing extremes. This review identifies key challenges, such as the uncertainty in both available rainfall datasets and climate models, emphasising the needs for climate model improvement and better adaptation to complex regional climatic and geographical environments. The findings enhance understanding and response strategies to extreme rainfall events and mitigate the associated negative impacts.

Growing urbanization has increased impermeable surfaces, raising and polluting stormwater runoff, and exacerbating the risk of urban flooding. Effective stormwater management is essential to curb sedimentation, minimize pollution, and mitigate urban flooding. This systematic literature review from the Web of Science and Scopus between January 2000 and June 2024 presents hydrodynamic separation (HDS) technologies. It sheds light on the significant issues that urban water management faces. HDS is classified into four categories: screening, filtration, settling, and flotation, based on the treatment mechanisms. The results show a shift from traditional standalone physical separations to multi-stage hybrid treatment processes with nature-based solutions. The great advantage of these approaches is that they combine different separation mechanisms and integrate ecological sustainability to manage urban stormwater better. The findings showed that future research will examine hybrid AI-assisted separation technologies, biochar-enhanced filtration, and green infrastructure systems. When adopting an integrated approach, the treatment system will perform like natural processes to remove pollutants effectively with better monitoring and controls. These technologies are intended to fill existing research voids, especially in removing biological contaminants and new pollutants (e.g., microplastics and pharmaceutical substances). In the long term, these technologies will help to enforce Sustainable Development Goals (SDGs) and orient urban areas in developing countries towards meeting the circular economy objective.

An extreme rainfall event that occurred from 16 to 18 December 2021 along the coastal regions of Peninsular Malaysia (PM) caused widespread flooding and substantial socioeconomic impacts. This study investigates the mechanisms leading to this event, focusing on the roles of climatic phenomena and local terrains. Two atmospheric interactions play key roles in triggering the event. Firstly, a strong cold surge (CS) associated with the East Asian winter monsoon (EAWM) interacted with the easterly surge over the southern South China Sea, leading to the formation of Borneo vortex. Secondly, a strong northeasterly and CS largely contributed to enhancing and transporting the vortex towards the PM and across the Titiwangsa mountain ranges. The phase change of the Indian Ocean Dipole (IOD) facilitated the eastward propagation of the vortex. Sumatra and PM terrains significantly modulated vortex evolution and moisture convergence over the Strait of Malacca. These findings are analyzed to shed light on interactions between large-scale climate drivers and localized terrain in generating extreme rainfall, emphasizing the necessity of multi-scale analysis for model accuracy.

Increased extreme rainfall brings huge socio-economic damages to society and draws public attention. Study examines the impacts of cold surges (CS), easterly surges (ES), and El Niño–Southern Oscillation (ENSO) on rainfall in Peninsular Malaysia (PM) during the Northeast monsoons (NEM). Based on data from 1991 to 2020, CS and ES occur on average 28 and 48 days per NEM season, respectively, with weak surges being the most frequent. Monthly distributions show CS frequency peaking in December, while ES peaks in January. Moreover, ENSO phases significantly modulate surge activity. CS occurs most frequently during La Niña years, facilitated by a stronger Siberian high and enhanced pressure gradients, whereas ES is most frequent during neutral years. Rainfall patterns are also influenced by ENSO, with concentrated rainfall over the Borneo coastline and Eastern Peninsular Malaysia during La Niña, and distinct distributions during El Niño and neutral years. Heavy rainfall events are closely associated with CS occurrence and peak in December, regardless of ENSO phase. These findings highlight the combined roles of cold surges, easterly surges, and ENSO in driving rainfall variability over PM during NEM.

Urban catchments increasingly face water-related challenges driven by rapid development and climate change, while conventional stormwater management is often insufficient for removing gross pollutants and sediments. This study evaluates an integrated treatment approach combining continuous deflective separation (CDS) technology with nature-based solutions (NbS)—including biochar, green zeolite, and floating wetlands planted with Vetiver grass—implemented at a commercial urban site in Malaysia. An on-site Internet of Things (IoT)-based monitoring system tracked pH, turbidity, electrical conductivity (EC), dissolved oxygen (DO), total dissolved solids (TDS), and temperature. Additional parameters, including biological oxygen demand (BOD), chemical oxygen demand (COD), nitrate (NO3−), total suspended solids (TSS), heavy metals (Pb, Cu, Zn), and nutrients (N, P), were analysed at the accredited laboratory of the National Water Research Institute of Malaysia (NAHRIM). Results indicate that biochar and green zeolite enhance system performance by adsorbing micro- and near-nano-sized particles and nutrients, preventing clogging, and achieving removal efficiencies of 57% for TDS, 90% for TSS, 78% for NO3−, 50% for oil and grease, 80% for BOD, and 57% for COD. Floating wetlands further improved removal of ammoniacal nitrogen (NH3-N) and phosphate (PO4³−), key eutrophication contributors. The study also demonstrates circularity: harvested Vetiver is processed into economic products, while residual biomass is converted into new biochar via pyrolysis to replace saturated biochar, which is then reused as compost. The integrated CDS-NbS system improves stormwater quality, supports reuse, contributes to carbon sequestration, and aligns with the United Nations sustainable development goals (SDGs).