Urbanization, in general, has four immediate repercussions on the hydrological cycle: flooding (e.g. as a result of increased soil sealing), water shortage (e.g. due to rising consumption), changes in the river and groundwater regimes as well as water pollution.
Rapid urbanization has brought about the increasing growth of urban water use especially domestic water consumption, giving rise to tension in urban water supply and demand.
The loss of green cover, solid waste dumping in the water bodies, and construction by filling the lakes and along the watercourse leads to Urban flooding. The growing cities which have more impervious surfaces in place of green cover earlier cannot absorb or manage the rainfall. The continuous flood events also reduce the quality of life & urbanites are prone to health risks from water-borne diseases.
Flooding doesn’t happen only due to extreme rainfall events. Though the increase in the frequency of climate uncertainties like high rainfall and no rainfall is due to global warming. The main reason for flooding in the recent decades is Rapid Urbanization.
Incidences of urban flooding are brought about by the confluence of growing ermeable
The rainwater infiltrates into the ground, flows as surface runoff and evapotranspirates into the atmosphere . Runoff is defined as the portion of the precipitation that makes its way towards rivers or oceans as surface flow or groundwater flow. Urbanization changes a watershed's response to precipitation. The most common effects of urbanization are reduced infiltration and decreased travel time, which significantly increases peak discharges and runoff. For eg: The surface runoff in a forest is 10% whereas the surface runoff of hardscapes such as roads is 90%. As cities grow, buildings and roads are constructed on open spaces and natural ecosystems. Areas that absorbed rainwater are replaced by hard, impermeable surfaces and hence there is a drastic reduction of infiltration of rainfall which increases the volume and speed of runoff.
Change in Landuse patterns of Indian cities
Hyderabad
Bangalore
Delhi
The Increasing demand for land for construction to build more residential & commercial zones to meet the demand of the increasing population in the urban area has led to extensive building activity near lakes, streams, and wetlands. The flood buffer offered by these natural spaces has been lost.
Cities are expanding but water bodies are shrinking. The waterbodies also have to grow in proportion to the increase in built-up areas to match with and to be able to manage the rainwater.
Climate change-driven rainfall variability is increasing with rainfall equivalent to monthly/seasonal averages falling within a few days.
There is a need for Urban planning departments of Central & State Governments to focus on effective flood mitigation in their development agenda and build disaster-resilient infrastructure.
Green Infrastructure
Historically, communities have used gray infrastructure—systems of gutters, pipes, and tunnels—to move stormwater away from where we live to treatment plants or straight to local water bodies. The gray infrastructure in many areas is aging, and its existing capacity to manage large volumes of stormwater is decreasing in areas across the country.
The existing urban development with grey infrastructure needs to be integrated with the new blue-green approach by making use of natural systems to bolster their capacity to manage stormwater and thereby become more resilient.
The parks & green covers reduce or slow down the storm-water runoff while the water bodies hold the rainwater. The hybrid approaches of porous pavements, bioswales, rain gardens, and water retention planters along the streets can be a part of water prudent landscapes.
Instead of flooding the low-lying areas in times of heavy rainfall, the city parks and playgrounds can act as makeshift ponds to hold the runoff.
Green Infrastructure Strategy is a strategically planned network of natural and semi-natural areas with other environmental features designed and managed to deliver a wide range of ecosystem services in both rural and urban settings.
It is impossible for the Governments alone to manage the floods. The communities or developments should manage the rainfall of their site. The building bye-laws have to be stringent and make the developments managed in such a way that only the rainwater to the extent of pre-development run-off is allowed to go into the municipal stormwater drain. By doing this, the load on the municipal stormwater network reduces drastically. By decentralizing, it also becomes easy for the communities to filter and reuse the rainwater. This in turn reduces the freshwater demand.