The natural water cycle is impacted by buildings and sealed surfaces generating stormwater and altering natural water flow. This stormwater from urban centers is loaded with pollutants often serving as a conduit transferring the pollutant into natural water bodies. Stormwater causing local flooding in Thimphu city during heavy rainfall events is not uncommon. Thus, a Water Sensitive Urban Design (WSUD) design of buildings, subdivisions, and works is required to minimize the impact of development on the surrounding environment and waterways. WSUD involves treating and reducing stormwater flows, increasing soil moisture, urban greening, and providing an alternative water source. This article explores the effects of Thimphu’s urban development on the riparian ecosystem along the Wangchu River. The health of this water-associated ecosystem is critical for buffering the negative effects of urbanization on water quality and the quality of the river.
One of the most common features of urbanization is the replacement of natural land
covers with impervious cover (IC). IC refers to rigid structures such as buildings, roads, parking lots, recreational areas, and sidewalks that hinder natural water infiltration. Numerous studies within the scientific community have investigated the impacts of IC on rivers or stream ecosystems, but surprisingly there is limited consensus on the impacts of IC on urban riparian vegetation and its soil. The physical environmental variation and temporal effect of climate significantly influence the magnitude of the effect of IC on the critical riparian ecosystem. Thus, a group of water experts led by a Ph.D. scholar conducted a literature review and summarized IC’s status and potential effects on water-associated ecosystems within Thimphu city.
The once-terraced paddy field of Thimphu was identified to become a city in 1961, and
development activities accelerated since then. It is evident from a 1984 photograph of a part of Thimphu valley that the riparian zone at that time was distinct and relatively intact, with riverine floodplain, upland vegetation, and paddy fields dominating land cover. Most of them are now replaced by IC. Further, traditional houses, riparian vegetation, marshlands, and paddy fields gradually wanned with the emergence of IC in the form of modern buildings, roads, parking lots, and recreational spaces. IC seals the land surface, removes natural vegetation, and led to riparian zone fragmentation causing biogeochemical changes which would eventually degrade the water bodies and water-associated ecosystems. The extent of the urban riparian zone in Thimphu city is typically narrow, given the topographic features of the valley. Any small modification has the potential to cause irreversible damage, and may completely remove the riparian zone.
Limited research and development – ICs are disconnecting and isolating riparian zones from upland vegetation, forcing them to function independently. The functional capacity of isolated riparian zones is yet to be known. Besides scanty environmental assessment reports, as of today, not a single research and exploration has been undertaken in understanding the relationship between urban centers and riparian ecosystems in Bhutan. Such studies would be useful in urban design, infrastructure development, and flood management along the main rivers.
Unknown threats on biodiversity – The compounded effects of increased IC coverage, the introduction of exotic plant species, and their population explosion then homogenize riparian vegetation communities. Increased IC exacerbates community homogenization by creating hydrologic conditions favorable for exotic and upland species to dominate. These events reduce local and regional biodiversity negatively affecting the overall ecosystem functioning. Urban habitats are typically hotspots of cultivated and exotic plants due to the influx of imported commodities contaminated by exotic seeds. ICs potentially promote alien or exotic invasions into the riparian zone by providing suitable pathways for seed dispersals. IC also increases light availability for exotic plants allowing them to thrive.
Risks to soil structure and fertility – The structural and functional integrity of riparian soil is equally threatened consistently by the emergence of ICs. ICs alter the chemical properties of soil through increased pH and calcium concentration due to the dissociation of their constituents through weathering of calcium compounds. ICs also significantly alter other basic elemental cycles through changes in Carbon, Nitrogen, soil organic matter, salinity, phosphorus, and other major ions. Changes in chemical components lead to changes in physical properties, such as porosity, density, dispersion, and compactness. IC is also associated with the accumulation of pollutants in the soil. For example, Mónok et al. (2021) observed that urban soil close to IC contained a higher concentration of toxic heavy metals. Heavy metals can reduce or alter soil fungi and microbial communities making urban riparian soils to reduce their holding capacity of key nutrients like nitrates. Increased release of nutrients like nitrates into our water system could trigger the growth of microorganisms reducing overall water quality.
Unknown threats from emerging pollutants – Another emerging impact of urbanization on riparian soil is microplastic (MP) contamination. MP is a plastic granule less than 5 mm. Nematollahi et al. (2022) found that urban soils contained more MPs than industrial soils. While the presence of MP in soil and its impacts are gaining more attention from scientific communities, no research has been conducted to investigate the relationship between IC and the accumulation of MP in urban riparian soil. Since IC is heavily involved in the kinetics of surface runoff and municipal wastes, it will be worth investigating the correlation between these variables in urban riparian soil.
Most urban centers in Bhutan are situated along river valleys thus it is essential that a local-scale study to decipher the response of riparian vegetation to urbanization is critical. Lack of knowledge of the local urban riparian zone makes it vulnerable to destruction through inappropriate urban landscaping and anthropogenic activities stemming from urbanization. Such events may lead to the destruction and removal of the urban riparian zone before it is even known or its values appreciated and can seriously affect the support it provides to flora and fauna in urban landscapes. Such risks are evident and soaring in Thimphu and other towns in Bhutan due to rapid urban growth and expansion along the valley systems. For example, Thimphu city expanded by 36.28% between 2001 and 2017. Such a trend may spread across the country as Bhutan plans to develop at least one new township in each of its 20 districts.
Riparian vegetation mitigates floods and stabilizes and protects river banks. The system also filters the incoming polluted water and thus enhances water quality which is critical for both ecological and economic reasons. The riparian areas also provide a cooling effect onto urban heat islands thereby improving the liveability. Thus, a sufficient understanding of the impact of IC on the urban riparian zone is critical for urban development in Bhutan.
The article is published based on personal experiences and observations by a group of water researchers from Bhutan. The group can be contacted at waterresearchbhutan@gmail.com