Experts from Nepal Landslide Society and Nepal Engineering College visit Landslide disaster along Naubise – Mugling – Narayanghat– Hetauda – Naubise road corridors

The heavy and torrential rain with cloud burst during 28-29 June 2003 (14-15 Asadh 2060) had inflicted severe damages to infrastructures in general and roads in particular in the Trisuli River catchment area of the Central Nepal. The Naubise – Mugling section of Prithivi Highway, large part Mugling – Narayanghat road, Narayanghat – Hetauda section of Mahendra Highway and part of Tribhuvan Highway had been severely damaged. Hundreds of large and small landslides occurred along these roads. Among them, Mugling – Narayanghat road was most severely damaged causing loss of hundreds of million rupees and blocking movement of good and people to the capital city for more than a week.

 In order to study, document and to find out the mechanism and causes of such extensive and intensive damages along the road, a team of experts from Nepal Landslide Society (NELS) and Nepal Engineering College (NEC) visited the area during 16-18 August 2003. The team consisted of Mr. Achyuta N. Bhandary and Mr. Jagadiswar Shrestha from NELS and Messers Ramesh Rajbhandari and Roshan Khadka from NEC.

The team inspected almost all landslides along the severely damaged road circuit. However, due to the limited time available only cursory inspection of slides and preliminary identification of causes were possible. Almost all the landslides were photographed and documented briefly. Some typical slides were selected for future detailed study.

A brief inspection showed that main causes of the landslides were, apart from the heavy rain, high slope of the terrain, heterogeneous geology reflected on weak and fractured bed rock, intense weathering of rocks, intense vehicular movement, deforestation due to settlements and intensive agriculture along the road alignment, haphazard construction of buildings and mitigative measures along the road and local hydrological conditions.

Geologically, within the area of the Lesser Himalaya, rocks of Benighat, Nourpul and Kunchha formations were noted to be most prone to landslides. Prime cause of slides in these rocks seem to be intense fracturing, and alteration of weak rocks such as slates and hard and massive rocks such as quartzites and dolomites along the road corridor. In Siwaliks or Churia range, low degree of cohesion of sediments seems to be the main cause. In granites, highly jointed systems were the prime cause of rockslide.

Major types of landslides observed were rock slide, shallow linear slide, debris flow, debris slides, subsidence, bank cutting.  Major threats to roads are from bank cutting, debris flow and subsidence. Damages to roads due to bank cutting and deposition of debris from gullies formed primarily by rock and overburden slides are the main causes of road blocking. Retaining walls were found to be better than tetra poles.