Issues

Laos is a mountainous country with approximately 65% of its population living in rural areas. Villages and small towns are located along the main road arteries, many of which are vulnerable to closure and damage from floods and landslides during the wet season. 2018 was a particularly wet year in Laos, with several rain gauges recording their maximum 24-hr rainfalls since records began. In parts of the mountainous NE of the country, bridges were overtopped and river scour caused major damage to riverside properties. Landslides cause multiple blockages to roads and the scars of shallow debris slides, debris avalanches and gully erosion were conspicuous on many hillsides. Large, deep-seated, first-time landslides were less common, and the one illustrated was by far the largest observed by the authors during their fieldwork in 2018. This landslide is reported locally to have taken place during the night of 10 August, with the debris coming to rest within 7m of a school compound in Sonkhoua village, Houaphan Province. The underlying rock type appears to be predominantly Silurian to Lower Devonian siltstone, possible with some interbedded fine-grained sandstone. Without ground survey or recent satellite imagery, it is not possible to determine the volume of this landslide, but it is estimated to be perhaps as much as 400 000 m³. The photograph shows the landslide debris extending into the Nam Neun River. According to local accounts, on 23 August 2018 the road bridge over this river, close to Sonkhoua village, was overtopped and several roadside properties had their foundations undermined by river scour, and are now abandoned. The nearest rain gauge is located in Meung Kham, approximately 50 km to the south. This rain gauge recorded 110 mm and 105 mm of rain respectively on 19 July and 16 August, representing the 5^th and 6^th highest 24-hr rainfalls since records began in 1999. However, it was likely to have been the effects of more localized intense rainfall, not recorded by the rain gauge, that triggered the landslide, and more prolonged rainfall that resulted in the river flooding.

Photograph by Gareth Hearn