A spell of good rains has brought the city to its knees with many lakes overflowing and others filled to the brim. Encroachments on lake beds, Storm Water Drains and interconnecting rajakaluves have disrupted water flow gradients and patterns, often leading to floods.
Notably, lakes are full despite the weatherman saying that the rainfall is not unprecedented. One of the reasons is silt. The storage capacity of lakes has dwindled. Take for instance Bellandur lake whose storage capacity is estimated to be 8,400 million litres. However, today it doesn’t hold that much water. This was revealed in May in a bathymetric survey by the Indian Institute of Science. The survey estimated silt sedimentation at 7.3 million cubic metres.
The 1970s map showed that the maximum depth of the lake was 10 metres; today, it is less than one metre deep at many places.
This is the case with most lakes. Barring a few individual restoration projects, there has not been a large-scale attempt to desilt lakes in over three decades, say experts. However, in the absence of a survey, there is no data on the extent of silt in lakes.
“Records show that the city's tanks put together had a storage capacity of 35 TMC. Over the last few decades, the city is estimated to have received 15 TMC of rainwater annually. If we are seeing floods, it means that the city is unable to handle even 15 TMC,” says Prof. T.V. Ramachandra, Centre for Ecological Sciences, IISc.
This is a cumulative effect of silt in lakes and encroachment of lake beds resulting in dwindling storage capacity, apart from encroachments and diversions of Storm Water Drains.
Role of urban sprawlAnalysis of spatial data from the National Remote Sensing Centre (NSRC) by the World Resources Institute has revealed that the Bangalore Metropolitan Area (BMA) added 593.98 sq ft of built-up area on ground every minute between 2006 and 2012. Most of it in southeast Bengaluru between Bannerghatta Road, Hosur Road and Sarjapur Road. The exponential increase in the urban sprawl in southeast Bengaluru over the last decade and these parts of the city being the worst hit by floods is not entirely coincidental, say experts. The increase in concrete surface has reduced the quantity of water seeping into the ground, leading to floods.
Half the SWDs encroachedThe recent floods at Kodichikkanahalli was traced back to the missing interlinking channels between Madiwala lake and its upstream lakes. The natural rajakaluve between the two lakes has been encroached and attempts to remove the encroachers has hit a legal hurdle.
A survey by BBMP and Department of Survey Settlement and Land Records has revealed 1,923 encroachments. Of this, the BBMP claims to have removed 822 encroachments.
But more significantly, the survey revealed that of the 857 km long SWD network, more than 400 km has been encroached and rendered ineffective for water flow.
Sewage is the problemDespite its name, it is clear that Storm Water Drains primarily carry sewage. Experts believe the resultant piling of sludge has reduced the carrying capacity of these drains.
BWSSB officials said pipelines connecting HSR Layout with the Bellandur Sewage Treatment Plant (STP) will be built in two months. With this linkage, nearly 150 million litres of waste water from the area will flow directly through 2,000mm pipes towards STPs instead of drains. However, in the Koramangala-Challaghatta valley, a major portion of work on STPs is pending.
But even once all the STPs are ready and the pipelines are in place, BWSSB will continue to face a ‘huge challenge’ convincing households that have come up alongside Storm Water Drains not to dump sewage and garbage.
