Based on the limited information I have, I would suggest the following dzongkhags to alert and evacuate people settled along river valleys near Lake Lunana:
1. Tashigang dzongkhag: As Lake Lunana is located in Tashigang dzongkhag, any potential outburst from the lake would directly impact areas and villages in Tashigang dzongkhag that are located along the river valleys and tributaries connected to the lake. There is highest risk of flash floods and landslides in such areas.
2. Trashigang dzongkhag: River valleys of Trashigang dzongkhag, especially in the eastern parts, are also fed by streams and tributaries originating from Lake Lunana. Flooding and muddy landslides could damage property and infrastructure in such areas. It is best to evacuate people as a precaution.
3. Samtse dzongkhag: Although Lake Lunana is not within Samtse dzongkhag, some of the river valleys of Samtse dzongkhag in the north are connected to the lake system. There is a possibility of flood waves and increased water discharge impacting these river valleys in case of any lake outburst. It is advisable to alert communities residing in such valleys.
The key reasons to evacuate people from these areas are:
1. Higher risk of flash floods, flooding and landslides due to the proximity to Lake Lunana and connected river system.
2. Limited time to evacuate in case of any sudden lake outburst leading to flooding. It is best to evacuate as a precaution.
3. Damage to life and property. Flooding can cause injuries, deaths as well as devastation of infrastructure and homes.
4. Disruption of supplies and connectivity. Floods can isolate villages by damaging roads, bridges and supply lines.
Early evacuation can help reduce risks to human life and make rescue operations easier. But local authorities should assess the situation properly based on the lake conditions before ordering evacuation.