The natural valleys of Bangalore

Ask any water expert of geologist and he is more likely to say that Bangalore has four major natural valleys located within the water shed of two principal river basins, the Arkavathy to the west and South Pennar to the East.

The Bangalore plateau is made up of  the four valleys which take off from north and fall gradually towards the south. Of the four valleys, the three main are Vrishabhavathi, Koramangala and Chellaghatta and all of them run in a north to the south direction and divide Bangalore into three distinct and separate drainage zones.

The fourth valley is Hebbal and this runs from north of the ridge and continues in the north easterly direction. These are the four valleys but what is not all that well-known is that there are five other valleys and they too play a vital role in the natural drainage system of Bangalore.

The other lesser known five valleys, which  are minor in nature, are Marathalli to the East, Arkavathy and Kethamaranahalli to the North West and Kathriguppa and Tavarekere to the south.

All these five minor valleys lie outside the tributary area of the  major valleys-Hebbal, Chellaghatta, Koramangala and Vrishabhavati and and they drain independently.

The nine valleys together form a unique and natural drainage system and both excess rain water and sewage flow down the city.

However, almost all the natural channels-feeder, storm water, drains- have either been encroached upon or blocked in places and this had led to the reverse flow of water into the city and not outwards, leading to flooding of roads, footpaths and localities.

What has compounded the inability of the natural valleys to drain off the water is the almost century old underground drainage system of  the City. It was way back in 1922 that Bangalore got underground drainage and this commenced from the pete areas of Bangalore.

The underground drainage system received a massive support when the then Bangalore City Corporation (BCC) in 1950 began an extensive and comprehensive programme. Till 1964, it was the BCC that took care of water supply and sewerage. That year, the State Government decided to set up a separate agency to handle water supply, sanitation and sewerage and formed the Bangalore Water Supply and Sewerage Board (BWSSB).

Since then, the BWSSB has been in charge of water and sewerage for Bangalore City.

The BWSSB has set up three main sewage treatment plants- Vrishabavati, Koramangala-Chellaghatta and Hebbal- to deal with recycling of water. Two additional mini-plants have been built near Madiwala and Kempambudi

As far as water goes, the BWSSB supplies approximately 900 million litres (238 million gallons) of water to Bangalore every day. The is as against a demand of 1.3 billion litres.

Coming back to the drainage system, the lakes and tanks in the four main valleys of  Hebbal, Koramangala, Chellaghatta and Vrishabhavati were a vital clog of nature’s  drain. Apart from draining excess water, they also helped in supplying water and keeping the city cool.

These lakes formed the cascading chain system right from Nandi Hills and continued all the way down to Kanakapura, Ramanagar-Chennapatna, Hoskote, Anekal. The interconnectivity of the water bodies within the valleys is really unique in a city which has no major source of water nearby.

The slope of the valleys allowed rain water to drain from the municipal limits of Bangalore within five hours. The percolation into the ground was minimal. All this changed in the 1980s when prolonged drought led to the drying of tanks and lakes and the State Government itself  encouraged the formation of layouts on tanks and lake beds. In all, 43 tanks made way for residential areas and layouts such as HRBR Layout, HSR Layout, Sarakki, Katriguppe.

The valleys too were blocked at several places and the free flow of water ended abruptly, leading to water backing up in Bangalore, leading to urban flooding and massive rain damage to life and property. The sewerage lines here are interconnected by

several tanks.