Flooding of urban areas is of a recent origin. The shrinking of open spaces and indiscriminate paving activity carried out in urban areas by both the state and the society are the key contributory factors for flooding in urban areas. Rainfall, remaining the same over time, does not find a place either above the soil or below to get collected and thus results in floods after excessive rain. Chennai has demonstrated that making use of floodwaters to recharge the groundwater helps to improve water quality of ground water and mitigate flooding.
Groundwater source is like a bank and its extraction is similar to withdrawing money. Groundwater can be compared to making deposits in the bank and failure to do so would lead to depletion of the source or saline ingress in coastal areas. Ground water is more important in urban areas more than in rural areas. Most of the cities not only in India but the world over depends on the groundwater source to meet their need for water. Therefore, it is vital that there are measures in place to sustain the ground water.
The annual rainfall over India, for example, is around 1170 mm and occurs during short spells of high intensity. This results in either flooding of the area or very little or no infiltration into the soil. Often, in coastal cities the water is wasted ending up flowing into the sea. This situation is similar in other countries such as Sri Lanka, Bangladesh and Pakistan among SAARC countries and Japan etc.
In any coastal city with sandy beaches, which receives a reasonable amount of rainfall, an area of width 2 kilometers from the coastline and with the alluvium extending up to ten meters and above for the entire length of the coast is bound to have a freshwater aquifer. The reason being the area consisting of sandy or other permeable soil allowing the rainwater to seep in. This aquifer is very sensitive and if over-exploited would lead to intrusion of sea water into it, which causes a permanent and almost irreversible damage to it. This will have to be prevented rather than cured and is best done by allowing rainwater to percolate into the soil either naturally or by artificial means.
A survey conducted two years ago as a result of an initiative to recharge ground water in Chennai by the Rain Centre revealed changes to ground water quality. This initiative implemented in complying with a government enactment had resulted in rising of the ground water level from 6 to 8 meters and improvement of the ground water quality.
This phenomenon results due to the less harmful particulars in rainwater being able to dilute the groundwater of its excess salts and the presence of metals. Areas where the groundwater contains metals like iron and arsenic and injurious salts like Fluoride, ground water recharging will dilute them and even leech them out. This was evident in a few areas in Chennai, near the sea and where groundwater had become saline. The Total Dissolved Salts (TDS) was shown less than 500 ppm and the groundwater had even became potable. Another huge benefit of ground water recharging is the contribution to flood mitigation. A classic example is the Chennai city. The city did not experience the negative effects of severe floods in 2020 as in the previous years despite the excess rainfall that it received.