Unplanned construction, heavy rain in short spells triggered Shimla landslides
State Disaster Management Authority experts, who studied 16 land subsidence sites in the Himachal Pradesh capital that witnessed 27 deaths in the mid-August rain spell, have suggested proactive field investigation by involving Geological Survey of India
Unplanned construction on hill slopes and open drainage accelerated last month’s landslides in Shimla district that received short but intense rain spells during a prolonged monsoon this year. Twenty-seven people lost their lives in three major landslides at Krishnanagar, Phagli and Shiv Bauri in the state capital from August 14 to 16.
A study by the State Disaster Management Authority (SDMA) found that a combination of factors triggered the over a dozen landslides in Shimla district, including the deposition of thick debris cover on slopes and oversaturated soil strata due to prolonged rain.
The 32-page report found that the rainfall in Shimla this monsoon varied between 221mm and 552mm from April to August and was considerably higher than preceding years. “The wet spell from April to August made the subsoil oversaturated, resulting in debris slides,” it said.
The team of experts, comprised SS Randhawa, the principal scientific officer; Puneet Guleria, state geologist; Sunil Verma, assistant geologist; Neelam Gupta, XEN, design, HPPWD; Basant Rathore, XEN, JSV, Kasumpti; Rajiv Kaundal, assistant engineer, design; Dinesh Thakur, AE, Shimla MC, and naib tehsildar Khem Chand Verma.
The committee studied 16 land subsidence sites in Shimla, including the water tank at Sanjauli, Housing Board Colony, Benmore, Majitha House, Chotta Shimla, Opposite Hotel Himland, Shivpuri MC Road; Krishnanagar, and Lal Kothi Fagli besides the urban development office at Talland.
The report found fractured and linear structural depressions further enhanced the impact, such as at Shiv Bauri. The unplanned construction on slopes and open drainage made the slopes unstable.
The committee suggested field investigations for sites where either the damage is prominent or is geologically active by involving the Geological Survey of India, the nodal agency for landslide management, or any other agency having expertise for mitigation in the Himalayan region.
WHAT LED TO LANDSLIDES
According to the SDMA expert committee’s report, Shimla town suffered widespread destruction due to landslides triggered by heavy rain between August 14 and 16 due to the following reasons:
*Deposition of thick debris cover on fragile slopes
The Himalayan slopes are fragile with the youngest mountain chain still in the building phase. The Indian Plate is moving northward and subducting under the Eurasian Plate, and the Himalayas are still rising at a rate of about 5mm a year, making them geologically active and structurally weak. Geologically, Shimla town is situated over a block formed by the tectonic dislocation of older rocks, overriding younger formations along the Jutogh Thrust..
*Poor disposal of surface water
Over a period, the expansion of Shimla town has resulted in unplanned drainage. The open disposal on the slopes has made them unstable.
*Decreased binding force of aging trees
The uprooting of deodars from almost vertical slopes during heavy rain resulted in debris flow as trees with long lifespans with decreased binding strength with the subsoil surface failed to support the thick soil cover.
*Heavy and continuous rain
The heavy and continuous rain spells was a cause of concern this year. An analysis of meteorological data in Shimla from April to August between 2014 and 2023 shows this year, the town got 221-225mm rainfall in April and 384-552 mm of rain in August. This is considerably higher than the total rainfall in preceding years.
*Fractured nature of metasediments
Metasediments from the Jutogh and Shimla groups of rocks supporting the slopes are fractured in nature, making the slopes fragile.
*Construction on debris cover
Shimla has seen unplanned urbanisation. The slopes that support the built-up environment on the debris cover were unable to withstand the heavy rain.
In metamorphic terrains, rocks lack primary porosity and are permeable along fractures having secondary porosity. The linear depressions in the Himalayas are assumed to be structurally controlled. These fractures and structural depressions are conduits for the infiltration of subsurface water, enhancing the saturation of the substrata.