A debris flow is a liquid landslide with more soil, rock, and organic material than water. It can travel faster than a person can run and can carry boulders as large as a van. Large debris flows can destroy bridges, roads, and houses. Debris flows are caused by a combination of water saturation and flow; loose, heavy earth materials, and steep slopes. Concave slopes, such as gullies or ravines, are particularly susceptible to debris flows because they tend to concentrate surface water runoff and accumulate loose soil and debris. Debris flows can be initiated when small landslides, excess erosion, or failures of man-made or natural dams occur in a mountain stream or gully. By the time a debris flow reaches a creek fan, the volume of debris can amount to tens of thousands cubic metres. Debris flows are most often triggered during intensive periods of rainfall that follow prolonged rainfall that results in wet soils.
Debris flows initiate on steep slopes (initiation zone) and travel rapidly down confined channels (transportation zone), where they can entrain large and heavy debris such as boulders and logs. Once the flow reaches a flatter or less confined area (deposition zone), it will spread out, lose speed and deposit the large debris.
- Develop a household emergency plan that suits your family’s needs and limitations.
- Assign specific safety tasks to family members.
- Practice your household emergency plan until everyone is sure of their role and keep it current by practicing every six months.
- Prepare an emergency kit to be self-sufficient for a minimum of 72 hours, but preferably one week.
- As well as having an emergency kit at home, you should have one in your vehicle and at school or work.
- Plan a family meeting place.
- Plan to have an out-of-area contact where all family members can check in, in case you are separated.
- Take first aid and emergency preparedness workshops and keep your training current.
- Keep your vehicle in good repair and keep your gas tank at least half full.
- Stay in touch with the world around you. Keep informed of impending dangers. When warnings occur, stay tuned to the radio, TV, and internet (including social media) for more information as it becomes available.
- Be familiar with the debris flow hazard and risk associated with your property and understand the debris flow warning classification. Debris flow risk assessment reports can be viewed at the District of North Vancouver.
- If your house is in immediate danger of debris flow damage, protect people first and evacuate. Immediate signs of possible debris flow may include the following:
- Rapid water or slurry flow, where it has not been observed before.
- Irregular or suddenly stopped stream flow.
- Cracking or falling trees, particularly in the absence of strong winds.
- If you identify any of the above conditions, evacuate immediately and phone 9-1-1.
- The most effective way to improve one’s safety is to evacuate once a warning has been issued; however, the decision to evacuate is entirely up to the resident and is based on individual risk tolerance.
- If you decide to evacuate, use caution when crossing creeks and check to ensure that bridges and culverts along the escape route are still in place. Particularly at night, drive slowly as roads could be covered with excess water along with mud and debris from small landslides. A water-based evacuation may be an alternative option.
- If a debris flow has already occurred and your house has not been damaged, it may be prudent to stay in the house and wait for emergency crews to arrive. Do not cross a creek if it has blocked your escape route as there could be additional flow surges.
- One Step at a Time: A Guide to Disaster Recovery has further information on recovery activities.
- Restock any emergency supplies that you might have used.
References & Links
- District of North Vancouver - Understanding Debris Flows
- District of North Vancouver - GEOweb
- Emergency Management B.C. - One Step at a Time: A Guide to Disaster Recovery
- Metro Vancouver