'Urbanisation caused floods in Kerala, K'tka': IIT-B

Researchers at IIT Bombay drew the conclusion on the basis of a sophisticated computer model-based study on the land use and land-use changes analysing the trend.

Rapid urbanisation that led to increased concretisation and loss of forest due to a rise in farming are the two major contributors behind frequent floods in the Nethravathi basin in south Karnataka, researchers at IIT Bombay have demonstrated.

The conclusion was drawn on the basis of a sophisticated computer model-based study on the land use and land-use changes analysing the trend.

While the research was conducted for the Nethravathi basin, its conclusion applies to other basins as well.

“Flood is a consequence to unplanned growth in the river plain. What happened to Kerala and south Karnataka this time can also be blamed to unplanned urbanisation to a large extent. So much of unplanned development took place in the periphery of Periyar river in Kerala damaging the floodplain,” T I Eldho, a professor of civil engineering at Indian Institute of Technology, Bombay told DH.

Earlier this year, Eldho and his colleague Rakesh Kumar Sinha showed heightened flood risk was an outcome of the change in basin characteristics seen along the Nethravathi. Such land use pattern changes increase draining of the rainwater and washing away of the soil downstream in the Netravathi basin, thus compounding the risk.

Studying the changes at five different time periods since 1972 and one projected scenario for 2030, the Indian Institute of Technology Bombay team reported that urbanisation enhanced the risk of floods, adversely impacted groundwater availability and caused loss of agricultural productivity due to erosion.

Netravati originates in the Western Ghats and is the primary source of drinking water for the cities of Mangalore and Bantwal. “Approximately 1.2 million people live in the Netravati basin, and this number is expected to more than double by the year 2030.

The study – published recently in the journal Environmental Earth Sciences - reports a four-time increase in the urban area from about 60 sq km in 1972 to 240 sq km in 2012 and projects it could increase to a whopping 340 sq km by 2030. The rapid urbanisation increases the risk of floods.

Another contributing factor is the loss of forest, or rather the conversion of forest land to farmland. Increasing agriculture is the second major cause for the alterations in basin characteristics. Since 1972, the agricultural area has increased by around 15% until 2012 and is projected to increase by another 24% by 2030. On the contrary, there has been a continuous decrease in the forest area resulting in 18% of deforestation during the same period. The study predicts a further decline of about 26% in the forest area by 2030.

Besides increasing the flood risks, urbanisation also depletes groundwater used for drinking and irrigation because of an increase in watertight concrete cover. Water drains off quickly because of this, reducing the seepage of rainwater into the ground, triggering water scarcity.

“The urban expansion is mainly found at the downstream stretch of the river basin around the city of Mangalore. The maximum change in agriculture is observed in the eastern and lower parts of the basin along the riverbed near urban areas. The impacts of the changes in the urban land are expected to continue till 2030”, said Eldho.

In another unrelated study, Kerala Sasthra Sahitya Parishad, a people's science movement cites government figures to claim Kerala has about 75 lakh families but 85 lakh built-up homes. Every year, 2.7 lakh new buildings or flats are constructed. At the same time, there is a loss of wetlands, from 8.5 lakh hectares in the 1980s to just 1.9 lakh hectares now.

“This kind of heavy rainfall that was seen in 2018 can be expected once in 50 or 100 years. But with proper water and basin management, the scale of damage could be curbed,” said Eldho.

“It is also essential to have an appropriate management plan in areas like Mangalore, Bantwal, and Puttur where the maximum increase in agriculture and urbanisation is expected in the near future,” he added.