Urbanisation eats away Bengaluru's lakes

Urban and periurban water landscapes of Bengaluru

Urbanisation has greatly altered the structure of the urban and peri urban landscapes in Bengaluru, known as the city of lakes since ancient times. The lakes in Bengaluru were artificially constructed or man made and are interconnected through storm water drains which carry excess water from higher to lower elevations. Over time, these lakes have provided various ecological services and supported local livelihoods by providing water for irrigation, silt for farmlands, grass or fodder, drinking and cleaning water, and fishing areas. 

At present, many of them are used by urban communities for walking, boating, yoga, bird watching, and cultural events, but they also play an important role such as flood control, groundwater recharge, wastewater treatment, lowering the ambient temperature and biodiversity conservation.

Increasing water demand and deteriorating lakes

Over the last decades the water demand in Bengaluru has increased, however the integration of the water bodies into the supply and reuse has not happened informs this paper titled 'Urbanization and seasonal effects on the lake ecosystem dynamics from 2002 to 2022 in Bengaluru, South India' published in Environmental Challenges.

Water scarcity has led to a digging spree of private borewells that drill down to a depth of 400 m. A 100 km long trench has also now been made to extract water resources of the conflict ridden Cauvery-Arkavathy river basins to meet the rising water demand of the city. The surface and groundwater resources in the city are getting contaminated increasing the threat to the health of residents in Bengaluru.. 

The condition of lakes in the city is extremely deplorable. As high as 98 percent of lakes in Bengaluru are encroached upon while almost 90 percent of the lakes are polluted due to release of untreated sewage, industrial effluents, and the dumping of solid wastes and building debris into the water. Despite this pollution, local farmers use the lake waters to generate fodder for their cattle and to irrigate their fields. 

The threatened lake ecosystems

Macrophytes (plant that grows in or near the water and is either emergent, submergent or floating) and bacterial-algal communities of phytoplankton play an important role in the lake ecosystem as they help to purify water. However,  a quarter of the Bengaluru lakes are fully covered by macrophytes that under normal conditions serve numerous ecological functions like sequestration of organic carbon and nutrients, stabilisation of sediments and shorelines and can be used as indicators of water quality, However, their overgrowth can be indicative of  lake degradation.

Macrophytes include floating water hyacinth (Eichhornia crassipes (Mart.) Solms) and rooted alligator weed (Alternanthera philoxeroides (Mart.) Griseb.), nut grass (Cyperus rotundus L.), and narrow leaf cuttail (Typha angustifolia L.) along with Chlorella algae and a Cyanophycea bacterial population.  Rapid growth of macrophyte cover leads to reduction in algal activity creating high oxygen demand in lake ecosystems.

The study

The study aims at exploring the effect of urbanisation on wetland environments by looking at the seasonal and temporal dynamics of vegetation in the lakes, proportion of built-up areas surrounding different lake types and the extent of drying up of lake shores due to siltation.

Urban lakes: Dodda Bommasandra and Bellandur 

In Dodda Bommasandra, there has been an increase in macrophytes due to the rise in runoff flow in recent years, resulting in higher water and nutrient content. The increased macrophyte growth, along with the rise in water runoff, has resulted in a decrease in dried-up area inside the lake. 

In Bellandur, there has been a rise in macrophytes due to changes in the lake’s surroundings. The western lake side has become partly silted over time, creating a more favourable environment for perennial grasses to thrive. The area covered by algae has significantly decreased despite the expansion of the water area as the water is retained for a very short period in the lake due to silt after which it overflows to nearby areas, preventing algae from thriving. 

Bellandur Lake has thus experienced an increase in dried land over the last two decades due to large-scale conversion of its watershed area into residential and commercial layouts, altering the hydrological regime and increasing silt movement into the catchment area inside the lake or low-lying areas. Furthermore, ongoing unplanned construction activities throughout the year has resulted in debris being dumped in the lake. 

Peri urban lakes – Attur and Puttenhalli 

Attur Lake has been repeatedly drained or blocked and  the macrophyte area in the lake has increased while dried land has declined due to rejuvenation activities such as dredging that leads to accumulation of water at the eastern lake side. 

Puttenhalli Lake was declared a Bird Conservation Reserve on 5 June 2014. The treatment plant at its eastern corner has been insufficient to treat the heavy inflow of sewage water resulting in an increase in the water surface area, macrophytes and a decrease in the algae area, and a slight increase in the dried land due to continuous siltation process near the run-off inlet at the western side. 

Rural lakes – Vrishabhavathi and Chudahalli

Chudahalli is located in the center of the stream, Vrishabhavathi is situated at its base where the water flow has substantially slowed down. Chudahalli Lake lies downstream of the south-west watershed catchment area which leads to increased flow of water and thus vegetation, including macrophytes, do not remain in the water for extended periods. As the water surface increases, the extent of dried land decreases. 

Vrishabhavathi Lake functions as a water reservoir than as a traditional lake. It receives municipal and industrial wastes from Bengaluru, resulting in the reservoir carrying high loads of nutrients and contaminants. The surface water area is relatively stable even if the reservoir lacks tight bunds or walls on the eastern side, allowing water to flow freely in that direction. This reservoir contains perennial grasses and water hyacinths. Due to the inflow of additional nutrients, the macrophyte population has increased and covered most of the water surface. The area of dried land seemed to remain stable in this reservoir because it receives municipal and industrial wastewater throughout the year.

Built up areas around the lakes

Dodda Bommasandra has the highest built-up areas within the buffer zone with an increase from 47 percent in 2004 to 60 percent in 2014. For Bellandur lake, the built-up area within the 300 m buffer zone has increased from 13 percent in 2005 to 23 percent in 2013, before decreasing to 15 percent in 2020.

Attur shows a steep rise (2  percent to 33 percent) in the built-up area from 2004 to 2014, and a further increase to 38 percent from 2014 to 2021. There has been a rapid increase in built-up area from 6.5 percent to 16.0 percent from 2015 to 2019 in Vrishabhavathi lake while the built-up area around Chudahalli Lake has remained contstant over the years. 

The study findings

• The density of built-up areas around lakes decreases along the urban-rural gradients
• However, the increase of built-up areas in the buffer zone of the lakes does not always positively correlate with the wet surface area of the lakes. This can be due to inflow of sewage water from other areas, or blockage and siltation.
• Rainfall was positively correlated with the wet surface area (that is macrophytes, algae, and water) in comparatively dry lakes while air temperature was negatively correlated with the wet surface area except for the Vrishabhavathi Lake.

The study underlines the importance of understanding lake vegetation dynamics due to urbanisation at periphery and shows that each lake has its own distinct characteristics, whereby the quality and quantity of sewage inflow interacts with the effects of peripheral built-up in the lake buffer zone and beyond.