debris flow warning system for public use CRHNet October 2010

1 m Debris Flow Initiation

Britannia Beach, BC fatalities

The Classic Fan

6

We know where debris flows occur, but do we know when they occur? Are they possible to forecast?

10 PM Pacific Standard Time (PST) on October 15, PM Pacific Standard Time (PST) on October 16, PM Pacific Standard Time (PST) on October 17, PM Pacific Standard Time (PST) on October 18, 2003.

Vancouver

The discriminant classification functions (2009 version) CS LS = A 4W I 48h A 2d CS NL = A 4W I 48h A 2d 4 week antecedent rainfall (saturates soils) 48 hour rainfall intensity (during storm) 2 day antecedent rainfall (raises porewater pressures) IF CS L > CS NL debris flows are likely IF CS L < CS NL debris flows are unlikely

Warning LevelMessage No watch Due to current and forecasted weather conditions, it is very unlikely that debris flows will occur in the North Shore Mountains. Watch 1 (conditions improving) Due to current and forecasted weather conditions, it is unlikely that debris flows will occur in the North Shore Mountains, and it is unlikely that the Warning level will be reached. Watch 2 (conditions deteriorating) Due to current and forecasted weather conditions, it is unlikely that debris flows will occur in the North Shore Mountains, but it is possible that the Warning level will be reached. Warning Due to current and forecasted weather conditions, it is likely that debris flows will occur in the North Shore Mountains. Severe Warning Due to current and forecasted weather conditions, it is very likely that debris flows will occur in the North Shore Mountains.

Winter ’08 - ’09

How is the system used? By the public via website and phone line October – April must be combined with knowledge about risks at specific sites and personal risk tolerance By District staff – inspections, worker safety, fire department By neighbouring municipalities and jurisdictions By our consultants – calibration based on actual events

Implementation Considerations Not 100% accurate and cannot predict the exact location of debris flows nor their total number. On average, 5 warnings per year and only one to two debris flow-triggering storms will occur (warning  debris flows) At any given site, the probability of it having a debris flow may be very low, but increases by an order of magnitude for severe warnings versus warnings The only way to increase personal safety is to evacuate

Debris flow warning system graph

Summary A debris flow warning system can be an effective way to reduce risk It is not fail-safe nor 100% accurate (system redundancies can be built in) The DNV system is based on sound science and robust statistics Public education is the key for successful implementation and risk reduction