1 / 32

Section 3: Debris flow initiation potential in gullies

Section 3: Debris flow initiation potential in gullies. 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

shadi
Download Presentation

Section 3: Debris flow initiation potential in gullies

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Section 3: Debris flow initiation potential in gullies

  2. 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

  3. 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

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

  5. 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 >25m2

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

  7. 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

  8. 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

  9. 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)

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

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

  12. Median angle of entry

  13. Volume of debris into channel

  14. Minimum failure sizes for ChDF • Headwalls: 11 m3 or 33 m2 • Sidewalls: 25 m3 or 50 m2

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

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

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

  18. Gully wall failure potential • Headwalls1)Till slopes: failures >50%2) Colluvial slopes a few failures >60% • Sidewalls1) Till slopes: failures >60%2) Colluvial slopes: failures >70%

  19. Channel gradient (SW only)

  20. 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

  21. GAP 2001: GWFP

  22. GAP 2001: GGPDFI

More Related