- Over 1,400 major landslide-prone sites identified after Cyclone Ditwah
- 16,848 houses in high-risk zones must be relocated
- Saturated slopes face fresh danger as inter-monsoon rains set in
Nearly a year after Cyclone Ditwah tore through the island, the hills of the upcountry still bear its scars. And with the second inter-monsoon season now under way, those living on its slopes are being warned that the danger has not passed.
When Ditwah made landfall in late November last year, more than 300 millimetres of rain fell within 24 hours in some areas, setting off landslides across the tea-growing districts of Badulla and Nuwara Eliya and well beyond.
Described as the worst natural disaster to strike Sri Lanka in two decades, it claimed hundreds of lives and affected well over a million people. Even after the cyclone moved away towards India, geologists cautioned that mountain slopes remained saturated and prone to further failure.
That warning still holds. Cyclone Ditwah’s rains weakened hillsides that had stood firm for generations, and its aftermath exposed just how much of the central hill country now sits on unstable ground. Add the ongoing monsoonal rains and the increasingly erratic weather that scientists link to a warming climate, and the upcountry finds itself more vulnerable to landslides than at any time in recent memory.
“Intense or prolonged rainfall is one of the main factors that trigger landslides in Sri Lanka, particularly in the central hill country,” said National Building Research Institute (NBRI) Landslide Research and Risk Management Division Senior Geologist Laksiri Indrathilaka, speaking to The Sunday Morning. “When the soil becomes saturated, it gets heavier and the slope loses its stability. That is when landslides and earth slips occur.”
The NBRI, formerly the National Building Research Organisation (NBRO), came into being following the enactment of the National Building Research Institute Act No.20 of 2025 by Parliament.
A self-income-generating semi-governmental institution functioning under the Ministry of Defence, it serves as the Government’s research and technical arm, responsible for landslide risk management, geotechnical engineering, environmental monitoring, human settlements planning, building materials research, and engineering project management.
A map redrawn by disaster
The true scale of the threat emerged only after Cyclone Ditwah. Comprehensive studies carried out in the wake of the cyclone identified more than 1,400 major landslide-prone locations, with the Kandy District recording the highest number.
Within these locations stand 56,463 buildings. Of them, 16,848 houses lie in high-risk zones and must be relocated to safer ground.
“Where schools, hospitals, roads, and other important structures have come up in landslide-prone locations, those sites must be compulsorily mitigated,” Indrathilaka said.
The transport network has not been spared either. Around 400 landslide-prone locations along the road network and nearly 40 along the upcountry railway line have been earmarked for mitigation following the cyclone. After evaluating each site, the NBRI has informed residents whether they live in a high-, medium-, or low-risk zone.
No new districts have been added to the official list. Fourteen districts, namely Badulla, Nuwara Eliya, Kandy, Matale, Kegalle, Ratnapura, Kalutara, Galle, Matara, Hambantota, Kurunegala, Monaragala, Gampaha, and Colombo, remain demarcated as landslide-prone. Together, the potentially vulnerable land across these districts is estimated at between 19,500 and 20,000 square kilometres, close to 30% of the country’s total land area.
Rain refuses to follow a pattern
Sri Lanka ordinarily experiences four rainy seasons: the southwest and northeast monsoons, and two inter-monsoon periods from March to April and from October to November. It is the current inter-monsoon season that has geologists on alert, arriving as it does on soils that have barely had a chance to recover.
What makes this period particularly treacherous is its unpredictability. “During the inter-monsoon period, it is not possible to accurately predict the exact locations that will receive rainfall,” Indrathilaka explained. “Rainfall patterns change from day to day. An area that receives rain today may stay dry tomorrow, while another area may get heavy rainfall.”
He offered an example. “On one day, rain may fall in the Badulla, Nuwara Eliya, Kandy, or Matale Districts, but not on the next. The following day, showers or thundershowers can be expected somewhere else. Most of the rain tends to come in the evening hours.”
Across the central hill country, fairly heavy falls of about 75 millimetres and heavy falls exceeding 100 millimetres are possible within the warning areas.
Indrathilaka added that the persisting El Niño condition had altered rain patterns significantly. “Sometimes there are dry spells, and then on certain occasions there can be intense rainfall exceeding 100 millimetres, which can suddenly stop,” he said.
Such abrupt swings between dry and drenched are precisely what climate scientists expect to become more common as the planet warms. For a hill country already weakened by Cyclone Ditwah, every sudden downpour now carries greater weight.
How a slope gives way
The mechanics of a landslide are deceptively simple. Heavy or prolonged rain soaks into the ground, adding weight and reducing the soil’s ability to hold together. Water seeping into cracks loosens rocks and debris on steep slopes, while erosion eats away at their base.
The consequences, however, can be devastating. Roadside cuttings, railway embankments, and slopes near homes can collapse without warning. Houses, bridges, utility lines, and farmland may be damaged or buried. Falling rocks and earth slips can block roads and railway tracks, disrupting public transport and essential services, while rapid run-off can flood low-lying areas downstream.
“People living in landslide-prone areas may face displacement, property damage, injuries, and even loss of life,” Indrathilaka warned.
The NBRI urges residents in vulnerable areas to watch for signs that a slope may be on the move. These include:
- Cracks in the ground, or in the walls, floors, and foundations of buildings and in roads
- Subsidence or sinking of land
- Rock falls, loose boulders, and unstable rock formations
- Water seeping out of slopes, newly emerging springs, or muddy water
- Poor drainage and water collecting on slopes
- Trees, electricity poles, fences, or retaining walls beginning to tilt
- Scars of earlier landslides or evidence of past ground movement
Human activity is often part of the problem. Cutting into slopes for construction and altering natural drainage paths can turn a stable hillside into a hazard.
Mapping the danger
The NBRI identifies landslide-prone areas through two complementary approaches.
The first, an indirect method, is landslide hazard zonation mapping. Scientists study geology, slope steepness, soil characteristics, drainage patterns, and land use to grade areas according to their susceptibility. The resulting maps, which also mark past landslides, subsidence, and rock falls, guide land-use planning and development decisions.
The second, a direct method, relies on field investigations and rainfall monitoring. NBRI officers visit sites to look for unstable slopes, ground cracks, soil movement, and changes in drainage. Rain gauges installed across the central hill country record rainfall, which is then compared against established warning thresholds.
“We assess slope stability by combining field inspections, geological and geotechnical investigations, ground-crack observations, and rainfall monitoring,” Indrathilaka said. “These assessments are particularly important in the central hill country and other landslide-prone districts, where prolonged rainfall can weaken soil and rock formations and increase the likelihood of slope failure.”
Probabilistic hazard maps add a further layer. By estimating the likelihood of a landslide at a given location, they help authorities see where people live in relation to unstable slopes, and whether settlements fall within high-, medium-, or low-hazard zones.
Yellow, amber, red
Based on forecasts from the Department of Meteorology, the NBRI issues three levels of landslide warning to residents in the central hill country and neighbouring districts: Level 1 (yellow) for low risk, Level 2 (amber) for medium risk, and Level 3 (red), which calls for evacuation.
The organisation runs a 24-hour early-warning service throughout the year, monitoring rainfall and vulnerable sites and coordinating with the relevant authorities to alert residents and advise them on precautions, including evacuation when instructed.
Moving families out of harm’s way
Where the risk cannot be adequately reduced through mitigation alone, the only safe option is to move. The NBRI provides technical assessments and recommendations to support the relocation of families from high-risk areas.
Priority is given to households whose homes have already been damaged by landslides, those living near the upper part of an unstable slope, and those whose houses lie along a landslide path or in an area where debris may accumulate.
According to Indrathilaka, the Government’s relocation programme is progressing without hindrance and a large number of locations have already been identified. It runs on two tracks. Under group resettlement, suitable land identified by the respective divisional secretariats is distributed among affected families to build new homes. Under individual resettlement, beneficiaries identify land themselves, which is then allocated to them by the Government.
So far, 6,677 land plots have been submitted to the NBRI and proposals under the individual resettlement programme continue to come in.
Funding the watch
Although the NBRI generates part of its own income, it relies on significant State support for its landslide work.
An annual allocation of Rs. 20 million goes towards upgrading and updating landslide hazard zonation maps. Following the cyclone and the extensive damage it caused, the Government allocated a further Rs. 299 million for disaster-related research, along with Rs. 50 million to meet the organisation’s operational expenses.
Living with the hills
For thousands of upcountry families, the hills are home, livelihood, and heritage. Cyclone Ditwah showed how quickly that ground can turn against them. As the inter-monsoon rains return and the weather grows ever less predictable, vigilance, early warning, and, where necessary, relocation are no longer optional.
The slopes, it seems, have little margin left to give.