1. Section 3: Debris flow initiation potential in gullies

5. Debris flow initiation in gullies A slope failure (landslide) starts on the headwall, sidewall, or outside of the channel The failure mass enters the gully channel, channel sediment starts to move - a channelized debris flow About 98% of Coastal B.C. debris flows result from slope failures

7. Debris flow initiation study: objectives To better define the factors that affect debris flow initiation in gullies To develop more accurate methods of identifying gully reaches prone to debris flow initiation

8. Study areas Vancouver Island, north of Nitinat Lake Vancouver Island, south of Nitinat Lake Mainland Coast near Squamish Queen Charlotte Islands

9. Data collection - site selection Within an area, we chose gullies that: 1) Were logged 5 - 15 years ago 2) Had at least one slope failure 3) Had reasonable access In each gully, inventoried slope failures >25m 2

10. Data collection - predictor variables Headwall or sidewall location Gully wall slope angle Gully wall slope distance Channel gradient Terrain type Soil drainage

11. Data collection - predictor variables con’t Surficial material depth and soil depth Initial slope failure dimensions Volume of debris delivered to channel Original slope gradient Failure plane slope Angle of entry

12. Response ChDF - the initial slope failure resulted in a channelized debris flow NochDF - the initial slope failure did not result in a channelized debris flow

13. Analytical methods Univariate analysis Logistic regression - uses continuous, ordinal and nominal variables combined Logistic regression ideal for a binomial response (either a debris flow initiated, or it did not)

14. Results Number of gullies assessed: 144 ChDF: 75 NoChDF: 211

15. Headwalls vs. Sidewalls Headwalls: 66% ChDF (39 of 59 failures) Sidewalls: 16% ChDF (37 of 227 failures)

18. Median angle of entry

19. Volume of debris into channel

20. Minimum failure sizes for ChDF Headwalls: 11 m 3 or 33 m 2 Sidewalls: 25 m 3 or 50 m 2

23. What about the GAP criteria? Gully wall slope angle and surficial material Gully wall slope distance and channel gradient

24. GWSA and surficial material 1: C and/or C/R. 2: M 3: C & M

25. GWSA and surficial material 1: C and/or C/R. 2: M 3: C & M

26. Gully wall failure potential Headwalls 1) Till slopes: failures >50% 2) Colluvial slopes a few failures >60% Sidewalls 1) Till slopes: failures >60% 2) Colluvial slopes: failures >70%

27. Channel gradient (SW only)

30. Does the 1995 DFIP method work? Gully wall slope angle and surficial material - good, needs tweaking Gully geometry potential for debris flow initiation - fairly good, needs tweaking No recognition of differences in headwalls vs. sidewalls

31. GAP 2001: GWFP

32. GAP 2001: GGPDFI