‘Landslides don’t happen all of a sudden, change in slope geometry and land use pattern aggravates it’

The July 19 landslide in Irshalwadi that claimed more than 80 lives highlighted the ecological threat in the Western Ghats. What could be the causes behind the catastrophe?
We have already visited the landslide-affected areas and are also preparing to carry out a survey to study the slopes of the hills and the overall Geological components of that region. We can’t comment on what caused the landslide immediately, but a report based on our findings may reflect the probable causes. However, imagery shows that there are agricultural fields on the foothills in Irshalwadi. Through our survey, we will check the drainage pattern and how the entire drainage system works there. We will also try to find out if minor landslide incidents took place before, so that we get a broader understanding about what exactly triggered the July 19 incident.

Mumbai has more than 290 locations, which includes hillocks and ridges that are prone to landslides. What is the root cause of a country’s financial capital being susceptible towards such a natural hazard?
In 2017-18, we (GSI) on the request of Brihanmumbai Municipal Corporation carried out a vulnerability assessment study of 249 landslide-prone locations which have human habitation in the slopes, following which 46 locations were labelled as highly sensitive in terms of risk. Post 1950s, Mumbai saw rapid industrialization and development. During this period, people would extract stones and aggregates through mining and quarrying from these hills and this would in turn create a depression or wide open space over there where people would eventually go and settle. Besides, people also gradually moved towards foot hills and encroached the upslope in some areas by cutting the natural hill slopes. Most of the mining work was carried out through drilling and blasting methods. Whenever there is such blast being carried out, the hills get damaged, as the blast makes the outer hill slopes hollow from inside up to certain extent, opens the rock joints and loosen its inherent strength. Now, during monsoon, water starts to percolate inside the already disturbed slope and overburden the soil, making it loose and if there is heavy rain for 3-4 days straight, the situation aggravates and landslides take place.

How can future catastrophes be averted?
For the planning purpose, our recommendations to the state government were to frame a Land-use-Policy and a Hill Area Development Policy that would include a set of directives on what could be the different types of civil and development works that should be permitted in hilly areas of Mumbai. Besides this, we have already submitted a list of vulnerable landslide-prone locations of Mumbai. The local authorities can refer to it and arrange for temporary accommodation for the residents living there during the course of any adverse weather situation. This could save several lives during any incident.

Mumbai is also witnessing several infrastructure projects which involve creation of underground tunnels. For example, the Mumbai Coastal Road Project and upcoming Metro Rail network has a series of tunnels. Will these pose any threat to the city in any way during the near future?
As per my knowledge, almost all the tunnels for the above-mentioned projects are being constructed by using Tunnel Boring Machines (TBM). This rules out the chances of catastrophe incidents in future since the process of using a TBM doesn’t affect the geological balance as the cutting of the tunnel is being carried out on the basis of scientific calculation. On the contrary, creating tunnels through blasting disturbs the balance and prepares the slope for landslide prone incidents in the future, which may get triggered in the case of an excessive rainfall.

Are landslides related to climate change?
Over the past few years, Mumbai has witnessed a drastic change in rainfall pattern. Which means the overall amount of seasonal rainfall has remained the same, but the adverse weather pattern has increased. For example, three-four years back, if 500 mm of rain was expected in 30 days, then the amount of rainfall recorded during these periods would be uniform. However, over the past couple of years, we have seen that for one or two days, the city records 100-200 mm of rains, while during the remaining days only 20 mm-30 mm of rains are recorded. Since rainfall during these couple of days becomes intense, it triggers the vulnerable locations immediately. During our survey in 2017-18, we had marked four areas in eastern suburbs of Mumbai and had told the local authorities that these places may see landslides during a heavy rainfall event and in 2021, landslides were reported from two out of these four places.

What could be some of the pre-emptive measures that local authorities can adapt to avert loss of lives and properties?
Sustainable modes of development are the only way forward, since tampering with nature will have adverse effects. Therefore, scientific study and a sustainable approach should be taken up whenever development projects are being taken up. There needs to be proper land usage policy in place, which would restrict man- made activities like settlement in slopes of hills. Also, whenever a highway or tunnel is being made through a hillslope or mountain, then a proper method of maintaining the slopes of hills should also be implemented. Also, proper sewage lines should be created if settlements are being made in slopes of hills or else the natural flow of water would result in surface run-off.