Kathmandu, Sept. 21 -- Two months after amassive debris flowon the Melamchi River knocked out Nepal's most expensive water project, geologists investigating the disaster say the danger will remain for decades to come.

Torrents of boulders and mud paste buried the headwork of theMelamchi Water Supply Projectunder 20m of debris on 15 and 30 July. Waves of mud washed away the highway and bridges connecting the site, submerged settlements, and killed at least 30 people.

The much-delayed$800 million projecthad started test distribution of water from the Langtang National Park througha 26.5km tunnel to Kathmanduresidents. Fortunately, the tunnel had been sealed for inspection a few hours before the flood hit that day, and was saved.

Geologis...