Managing Monsoon Stormwater in Bhutan’s Urban Landscapes: A Sustainable Perspective

by Water Research Bhutan, 10 Aug 2025 waterresearchbhutan@gmail.com

Urbanisation increases impervious surfaces by transforming the natural and permeable land to impervious surfaces like roads, parking lots and buildings. These impervious and hard surfaces prevent rainwater from draining naturally into the ground, causing it to run off rapidly into waterways, leading to increased flooding risk in urban areas. In Thimphu alone, the population has expanded by 45% between 2005 (total population of 79,185) and 2017 (total population of 114,551). Likewise, the built-up area in Thimphu city has increased from 5 km² to 13 km² between 2000 and 2019. This expansion has increased the impervious area with the rise in the number of buildings and other infrastructure. The impact of increasing urban expansion in Thimphu is visible as we frequently witness facilities such as roads in Thimphu being flooded, and the drainage system, at times, overwhelmed by the storm runoff during even the normal monsoon rainfall. Studies consistently project that by 2050, flood events currently classified as 100-year events will occur at least twice as often in urban centres, making climate change a significant threat to Thimphu city’s resilience. Hence, with the continuous expansion of urban areas and increasing extreme rainfall events, we will see an increase in the frequency and magnitude of such flooding across urban areas in Bhutan. Here, we conducted first-order combined hydrological and hydraulic modelling using the Hydrologic Engineering Center's (CEIWR-HEC) River Analysis System (HEC-RAS) two-dimensional rain-on-grid model to predict a plausible flood map for Thimphu town. The model indicates that a 24-hour storm with a maximum rainfall of up to 80 mm could inundate an area of approximately 3.4 km² and affect over 1,300 buildings.

Bhutan’s effort to mitigate this urban flooding is largely limited to traditional engineering measures such as constructing gullies, pipes, culverts and channels to move water quickly away from roads and buildings. However, using such engineering measures alone is insufficient, especially in the face of increasingly extreme weather events triggered by extreme climate events. It is urgent that we instead resort to sustainable and resilient solutions by complementing these engineering measures with nature-based solutions (NbS), such as by developing/increasing blue-green infrastructure within urban areas, which uses natural and semi-natural systems to manage water in urban environments. These will not only reduce the flood risk but also promote the protection or restoration of riverine ecosystems and their social and environmental benefits.

Further, ‘blue infrastructure’ includes water-retaining features like ponds, wetlands, swales and detention basins. Green infrastructure includes parks, grassed areas, woodlands, gardens, green roofs and street trees. Together, they help slow down surface water runoff, allow water to infiltrate into the ground and reduce pressure on drainage systems. For example, how much water could be retained on the hills of Buddha Point if all the concrete surfaces were replaced with lawns interspersed with native vegetation? Could the playground at Olakha be redesigned to incorporate more water-wise urban features? Using native vegetation in open spaces is often more suitable for local water regimes compared to imported species, as native plants are better adapted to the climate and soil conditions. These changes could significantly improve stormwater retention, reduce runoff, and enhance the ecological resilience of urban landscapes.

A recent study by engineering students (Lhendrup, et al, 2024, titled - Feasibility Assessment and Implementation of Sustainable Urban Drainage Systems in Phuentsholing Town) from the College of Science and Technology, found that retrofitting Phuentsholing town with nature-based drainage solutions could reduce urban flooding. The research focused on Sustainable Urban Drainage Systems (SUDS), such as permeable pavements and vegetated swales, to improve water absorption and slow stormwater runoff. Their analysis showed these methods could cut surface runoff by over 15%, even in crowded urban spaces, using existing areas like roadside verges and parking lots. The findings suggest SUDS can effectively manage monsoon rains, prevent erosion, and support Bhutan’s sustainability goals without major land changes.

The frequent flooding of roads in densely populated urban areas like Thimphu is a grim reminder that catastrophic floods could occur in the future, especially with the projected increase in extreme rainfall events. While we invest in engineering measures to mitigate this problem, it is not too late to think holistically and incorporate nature-based solutions. These approaches would not only effectively reduce flood risks in urban areas but also improve water quality in rivers and enhance the overall ecosystem.

Figure – Comparing urban growth in Thimphu in 2003 and 2023

Stormwater flooding and sewer line breakages are common occurrences in urban Thimphu, often disrupting daily life and causing significant economic implications. Despite these impacts, the long-term ecological consequences and public health risks associated with such events remain largely unexplored in Bhutan. Recent research conducted by a Bhutanese PhD candidate revealed that the increasing extent of impervious surfaces in Thimphu city is negatively affecting the health of the Thimphu River and its water-dependent ecosystems (Jamtsho et al., 2023). The study also reported the presence of microplastics along the river channel, with their concentration closely linked to the extent of impervious land cover. Other potential risks, such as contaminants washed off roads and sewer overflows, have not yet been investigated.

Neglecting stormwater management is akin to waiting for a disaster to occur, often followed by a reactive cycle of blame among various stakeholders. A proactive approach is essential, as the risks to infrastructure and public health may exceed the capacity of local governments to manage effectively. As new urban developments such as Gelephu Mindfulness City emerge, residents with higher living standards will increasingly demand water-wise urban strategies. These include efficient irrigation systems for private gardens, sustainable maintenance of public green spaces through reticulated irrigation, and protection of freshwater ecosystems by reducing nutrient and pollutant runoff from frequent stormwater overflows—issues already evident in Thimphu.

As a team of water researchers, we recommend prioritizing the collection of localized hydrogeological data to guide the development of tailored local water management strategies. These strategies should incorporate innovative urban water designs inspired by best practices from well-planned cities around the world. A dynamic, water-wise approach to urban development in Bhutan is critical to conserving limited water resources and supporting water-efficient buildings and businesses, public open spaces, urban greening, biodiversity, tree canopy coverage, and urban cooling. Together, these elements can help build climate-resilient communities across Bhutan’s urban centres.

Figure - Shows flood map of Thimphu based on 24-hour rainfall with a maximum of up to 80 mm.

The article is published based on personal experiences and observations by a group of water researchers from Bhutan www.waterresearchbhutan.org 

The article was also published in the National Newspaper of Bhutan (Kuensel) on 30 August 2025 Search result for: "Managing Monsoon Stormwater in Bhutan’s Urban Landscapes: A Sustainable Perspective"