Understanding the complex relationship between disappearing first-order streams, urbanization, and floods

Urbanization is happening at a rapid pace in India with a surge seen in the past few decades. According to a real estate sector report,75% of the country’s GDP will come from urban centers by 2030. Bengaluru, is one of the largest cities in India with an area of 741 sq. km. The city has seen a growth rate of over 5% per year coupled with a multifold rise in urbanization in the past three decades. Statistics show that between 2005 and 2016, the area of Bengaluru covered by concrete has increased from 17% to 40%. This has caused the city's drainage capacity to reduce from 70% to just 10%. This rapid pace of development and increase in the impervious area has reduced the city's ability to absorb rainfall, leading to flash floods during heavy rainfall events.

In 2017, the city received over 9 inches of rainfall in a single day, causing widespread flooding, discomfort to the citizens and property damage. Bengaluru Development Authority estimated the cost of the damages caused by the floods was around ₹13,000 crore ($1.86 billion USD). Urban flood events like the above cited event are amplified by the disappearance of streams of first-order to third-order in the city. Streams, particularly first-order streams, play a critical role in managing runoff and maintaining the hydrologic balance which in turn leads to increased runoff and reduced infiltration, which can cause flash floods during heavy rainfall events. Furthermore, studies have shown that with the serious effects of climate change, there is a significant change in the timing and intensity of the monsoons in the Indian subcontinent, causing longer dry spells with increased high-intensity shorter duration rainfall events.

At Clear Water Dynamics (CWD), we have developed an algorithm and a specific strategy to deal with these increasing urban floods making the best use of historical and present topography. The study encapsulates high-density network mapping, with fine-scale watershed delineation and 1D-2D flood modeling tools in PCSWMM along with gridded data provided by India Meteorological Dept.

With robust hydrological model simulations for a wide range of rainfall events, taking the historical and current first-order to third-order streams into consideration, the tool is extremely effective in capturing sensitive flood zones. The outcomes of the study could serve as a template for all cities that are following the footsteps of Bengaluru, in managing their waterways for sustainable development which solves very expensive and avoidable urban flood problems.

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