Floods refer to the partial or complete inundation from rapid accumulation or run-off of water, resulting in damage to property and loss of biotic elements (including humans). Floods are caused due to natural (high-intensity rainfall) or human activities involving blockages in the drainage network and an increase in paved surfaces (rapid urbanisation), leading to a rise in the overflow with a large quantum of water.
Urban flooding is a consequence of increased impermeable catchments resulting in higher catchment yield in a shorter duration, and flood peaks sometimes reach up to three times. Thus, flooding occurs quickly due to faster flow times (in minutes). Causal factors include loss of pervious area in urbanising landscapes, inadequate drainage systems, blockade due to indiscriminate disposal of solid waste and building debris, encroachment of stormwater drains, loss of interconnectivity among lakes, housing in floodplains and natural drainage, and loss of natural flood-storages sites.
Bengaluru has been experiencing unprecedented rapid urbanisation and sprawl due to an erroneous developmental path with impetus on industrialisation, completely ignoring the ecology and livelihood of people. This concentrated growth has resulted in the increase in population and consequent pressure on infrastructure, and natural resources, which is ultimately giving rise to a plethora of serious challenges such as climate change, enhanced greenhouse gases emissions, lack of appropriate and adequate infrastructure, traffic congestion, and lack of basic amenities (electricity, water, and sanitation) in many localities, etc. There has been an increase of 1055% in paved surfaces during the past five decades with implications on the natural resources (88% decline in vegetation cover and 79% decline in water bodies), and local climate (an increase of ~2 to 2.5 ºC during the last decade) necessitating appropriate strategies to make the Bengaluru liveable.
Frequent flooding even during normal rainfall in Bengaluru is a consequence of the increase in impervious area with the high-density urban development in the catchment and decline in wetlands and vegetation. The flooding is exacerbated by narrowing and concretising of stormwater drains, lack of appropriate drainage maintenance works with the changes in enhanced run-offs, the encroachment and filling in the floodplain on the waterways, obstruction by the sewer pipes and manholes and relevant structures.
The causal factors in Bengaluru include a significant narrowing of the waterways and filling in of the floodplain by illegal developments meaning a loss of loss of interconnectivity among lakes due to the encroachment of drains or dumping of municipal solid wastes, construction and demolition waste. There is also the encroachment of flood plains and wetlands, construction in valley zones, flood plains and lake beds and denotifying lakes- under the guise of ‘dead lakes’. No lake can be dead as it does the job of groundwater recharge. Narrowing and concretising storm water drains impair hydrological functions of the natural drains.
The remedial measures to mitigate the impacts of flooding primarily depend on the ecological management of storm water drains and wetlands.
This entails restoring interconnectivity among lakes by removing all encroachments and solid waste dumping. What must also be done is that valley zones and buffer regions of wetlands must be protected. The narrowing and concretising of drains must stop immediately, and the riparian vegetation in flood plains must be ensures, which will help in retaining and recharge of water. It is this vegetation in the drain that takes the load during peak monsoon, and there is no need to concretise the channel. Vegetation both allows recharge and treats the water through bioremediation.
Experts must think sensibly about these issues with holistic knowledge rather than fragmented, narrow sector thought. There must be a move to decongest Bangalore, and shift major installations to other districts. They must also protect open spaces like lakes and parks, and the perhaps the creation of a mini forest of native species, of 2 hectares each in every ward to enhance the recharge potential of the landscape. Environmental clearance must also be awarded only as per the norms of the Environment Protection Act (2016), Wetlands (Conservation and Management) Rules, 2016, Solid Waste Management Rules 2016, Air act 1981, Water (Prevention of Pollution) Act, 1974.
(Dr TV Ramachandra, is convener, Environmental Information System, Indian Institute of Science, Bangalore)