15 August 2016

The Dzongu landslide: overtopped but not drained

Posted by Dave Petley

The Dzongu landslide

As expected the Kanaka River has now overtopped the Dzongu landslide in Sikkim, and water is flowing freely.  The Darjeeling Times published this image yesterday of the landslide and the flow of water across the natural spillway:

Dzongu landslide

Darjeeling Times image of the Dzongu landslide in Sikkim, showing the aftermath of the overtopping event


The good news is the channel appears to be eroding laterally rather than vertically at the time of this image, and that there also appears to be large boulders in the channel that may be inhibiting basal erosion.  However, it is really important to stress that the danger is not over at this site. The dam could still enter a failure mode through piping, through erosion of the downward face or through basal erosion.  The dam has yet to be tested in a high flow event (and this is the monsoon season).  The site needs detailed assessment and monitoring, and all should be on their guard.

Meanwhile, the lack of a breach event creates another crisis as the presence of a large landslide and a lake mean that access to communities upstream is impacted.   On Facebook, Bijoy Gurung has written an account of the effects that is worth a read.  Part of his account is as follows:

One word that encapsulates Upper Dzongu and its inhabitants following the Saturday collapse of an entire hill side upon Kanaka river which dammed its flow besides Mantam, 4th Mile and spawned a 300 m lake. The sole road link has been washed away while the 4th Mile suspension bridge is somewhere beneath the nearly 70 m deep lake … Some villagers of Upper Dzongu are trekking through three dense forest hills for almost 4 to 5 hours to reach Lower Lingthem where a motorable road exists leading to Passingdong and then towards Mangan or Gangtok.

It is beyond question for the old and children to make this punishing trek. It is beyond imagination to consider carrying goods or rations on bare backs towards the isolated villages in Upper Dzongu. Soon, the stock of food grains and essential items in the Upper Dzongu villages are going to run out. The old and ill people are going to need medical attention and medicines. Power supply is snapped and the last call from Upper Dzongu ended this morning as the mobile battery has gone dead.

Kalimpong News has an image of the scale of the landslide:

Dzongu landslide

Kalimpong News image of the Dzongu landslide