L. S. F. Kilchis Slide - Geotechnical Report

[ODF-Forestry]

Hello, this is Mark Labhart from the Oregon Dept. of Forestry. I posted a message on this topic awhile back in an effort to provide some information on the Little South Fork Kilchis Slide that occured near the middle of Dec. I'm not sure how much interest the previous posting received but thought there might be some that are interested in the cause still. We have just received the Geotechnical Report from ODF's licensed Geotechnical Engineer. I'll inlcude it in this mail message for those that might be interested. I was particullary interested to know in Dave Michael's professional opinion if the slide originated from the 1997 removal of the alder trees while leaving all the conifer trees. Dave Michael said the following:

"Timber harvest in 1997 removed alder from the slide area as evidenced by aerial photo review and on the ground investigation. The removal of alder from this geologic setting leading to this deep-seated, large, complex landslide seems very unlikely. The rooting depth of the trees removed would have been less than three feet and the failure depth was many times deeper through nearly all of the landslide slip-surface. The effects of forest canopy on the rate of delivery of water to the slope provided by alder trees would have been minimal since they loose their leaves in the winter."

For those that are interested in reading the entire report I have include it also. It is quite technical though. I put this informnation on this web site to provide information to those that are interested in the cause of the slide.

MEMORANDUM

SUBJECT: KILCHIS RIVER SLIDE

TO: MARK LABHART & MIKE BORERLON

FROM: DAVE MICHAEL & JASON HINKLE

DATE: DECEMBER 24, 2003


The purpose of this memo is to document the findings and recommendations for the recent landslide in the Tillamook State Forest, Tillamook District, Kilchis River Basin, Little South Fork Kilchis River (S.E.1/4, Section 23, T.1N., R.9W, W.M.).

On December 15, 2003 Jason Hinkle (Dave Michael was out of state on annual leave) conducted a reconnaissance visit to the site and developed an initial interpretation which he shared with Mark Labhart via E-mail communication. On December 22, 2003, Dave Michael and Jason Hinkle visited the site for the purpose of updating Dave Michael on the landslide event and confirming the initial interpretation. On both field visits the investigation began from the depositional area at the road below the slide then proceeded up the north side of the slide to the top of the slide then down the south side of the slide. We observed the soil, rock, water, and topographic conditions at the site for the purpose of understanding the mechanism of failure and potential for further slope movement. We reviewed aerial photographs of the slide area from 1954, 1961, 1977 & 1996. We made a third visit to the site on Monday December 29, 2003 for the purpose of confirming our confidence in the geologic model and checking for recent secondary slope movements within the slide area.


FINDINGS:

 The slide deposition was divided into two areas. The main deposition was directly below the channel and largely contained on the road and flood plain of the Little South Fork of the Kilchis River, slide deposition reached, but did not cross to the other side of the stream. There was a smaller depositional area just up-stream of the main area that actually extended onto the opposite side of the Little South Fork.

 The landslide originated near the ridge-top and is estimated to be approximately 80 - 120 ft. across widening to 150 - 175 ft. down-slope and tapering to approximately 50 – 75 ft. at the toe. The initiation site is approximately 300 - 400 ft. from the head scarp to the toe. There is a small slide mass remaining in the initiation area.

 The depth of the slide was estimated to be 20 – 30 ft. along the central axis (deepest points of the slide geometry).

 The transport zone or debris track is estimated to be approximately 1,000 – 1,500 ft. The transport zone had both entrainment of natural soil and rock as well as deposition including lateral limit “spill-over” deposition. The question of where there was a net gain or loss in volume of the slide though this zone of transport is problematic.

 We calculate the volume using these rough estimates as 7,000 – 22,000 cubic yards.

 Tension cracks and vertical displacements were observed on blocks on the north side near the top of the initiation area. These blocks appear to have moved slightly between field visits with some possible calving at the toe of the bocks into the landslide scar.

 The head scarp had vegetation in the top 5 – 15 ft. both grasses and moss.

 There is a complex landform that surrounds the slide area composed of older slide scars (escarpments) and hummocky ground (slide blocks) with rock outcrops of less weathered rock protruding typically at the base of these hummocky areas.

 The overall slope is steep ranging from 80 – 90%+ with very small areas of lesser (old slide blocks) and greater slopes (old slide escarpments).

 There are several old cat roads (assumed to be from the oldest logging) that appear to have cut across the slide area.

 There were numerous very well used elk trails throughout the area.

 There were pockets of snow near the top observed on the December 15 visit by Jason Hinkle.

 The rock is of igneous origin, Tillamook Volcanics Formation, and is very deeply weathered with an attitude estimated in the field of N20E20NW. This is a somewhat out of the hill moderate dip. There are prominent rock outcrops throughout the area that appear much less weathered. These rock exposures are common at the toe of the gentler, more hummocky slopes. There was one noted mass plane of separation with an attitude of N50E80NW.

 There are a number of large boulders in the stream within the debris slide path. The boulders have both fresh (blue) and stained (brown) surfaces. There are also older, mossy boulders in this area that predate the slide that were probably deposited by past slide events at this site.

 The waterfall that is approximately 2/3 of the way up the slide scar has moss growing on its surface.

 Aerial photograph review indicates a landslide landform (steep escarpment slopes and hummocky masses) with burned conifer and hardwoods (assumed to be alders) in the draw in 1954 and cat roads cutting across the site in 1961.

 Secondary failures within the slide scar were observed on the December 29 visit.


CONCLUSIONS:

The slide is in the middle of an area that has significant signs of older slide activity, both recent and more ancient. The slides that occur in this geologic setting are small to large scale and typically deep-seated, but rapidly moving, complex (slip-surfaces probably on pre-existing weaknesses) failures. We interpret the site to have a “rock controlled” slope movement history. The prominent, competent rock exposures within this otherwise very deep weathering profile are the main evidence for this interpretation. Normally such a deep weathering profile would not be expected to support such steep slopes without some slope reinforcement. The rock that is less weathered and still intact is effectively holding back portions of the weathered rock and soil above and acts as “key blocks”. These key blocks eventually fracture and rupture, possibly from frost or ice expansion “heave” or just from progressive strain and weathering. When the key block fractures and ruptures there is not enough frictional strength on the surface between partially decomposed rock and the more competent underlying rock mass to maintain stability. At this point, when the pore water pressure of the slope is elevated during the next significant storm event, failure is likely to occur. This is probably an ongoing mechanism of slope movement that might be much like soil creep with these key blocks keeping the movement from developing into a rapid failure until the key blocks themselves fail. The prominent rock exposed at the base of the modern slide mass that is remaining on the slope is evidence of this “rock controlled” slope movement mechanism for this most recent slide event.

The old cat roads on the site can not be ruled out as contributing to the failure in some manner since much of the significant evidence was removed with the landslide. However, there were no indications of significant water flow on the remaining cat roads above and adjacent to the slide area. There might have been some failure of cat road fill slopes that retrogressed and under-cut the slope leading to the larger failure, but this is purely speculative since all evidence would have been destroyed in the landslide process and seems unlikely given the ultimate size and geometry of the slide.

Timber harvest in 1997 removed alder from the slide area as evidenced by aerial photo review and on the ground investigation. The removal of alder from this geologic setting leading to this deep-seated, large, complex landslide seems very unlikely. The rooting depth of the trees removed would have been less than three feet and the failure depth was many times deeper through nearly all of the landslide slip-surface. The effects of forest canopy on the rate of delivery of water to the slope provided by alder trees would have been minimal since they loose their leaves in the winter.

Given the small secondary failure observed on our third visit, as well as the increased cracking in the blocks on the upper north side of the slide, and the general geologic model, we consider the potential for additional slope movement relatively high at this time and in the future. Slope movements within and outside of the recent slide area are likely. Any slope movement within the slide area should be expected to reach the depositional area below even with limited initiation size.


RECOMMENDATIONS:

There may be more sliding in the near future and undoubtedly in the geologic future. There is no practical solution to the failures themselves but we recommend mitigation of impact by removal of the existing slide deposit at the road. It is not necessary to remove all of this deposit, but the portion that is practical and provides an area to catch the possible future slide deposits. This is basically a catch basin or fall out zone approach to mitigation of the impact of possible future events from this slope.

We recommend caution working or even traveling under this slide area in the short term due to the continued movement that we have observed on our visits. The remaining hazard and risk relationship might be viewed as likely but limited for natural resource damage since the volume is somewhat limited. However there is considerable potential energy in any future event even with small initiation size to produce risk of life to any workers in the depositional area.

The slide area itself will undoubtedly re-vegetate with alder but planting conifer on the existing slide mass perhaps as soon as next fall would provide potential large wood for recruitment in the more distant future.

Mulch has already been applied to the depositional area to mitigate rill erosion and associated impacts to water.


David L. Michael R.G., C.E.G. Jason C. Hinkle, G.I.T.
Geotechnical Specialist, NWOA Geotechnical Assistant, NWOA

[ODF-Forestry]

I forgot to mention that I don't monitor this site but wanted to let those know that might read this, if you have nay questions, please free to call me (503-842-2545) if you questions.
Mark

Originally posted by ODF-Forestry:
Hello, this is Mark Labhart from the Oregon Dept. of Forestry. I posted a message on this topic awhile back in an effort to provide some information on the Little South Fork Kilchis Slide that occured near the middle of Dec. I'm not sure how much interest the previous posting received but thought there might be some that are interested in the cause still. We have just received the Geotechnical Report from ODF's licensed Geotechnical Engineer. I'll inlcude it in this mail message for those that might be interested. I was particullary interested to know in Dave Michael's professional opinion if the slide originated from the 1997 removal of the alder trees while leaving all the conifer trees. Dave Michael said the following:

"Timber harvest in 1997 removed alder from the slide area as evidenced by aerial photo review and on the ground investigation. The removal of alder from this geologic setting leading to this deep-seated, large, complex landslide seems very unlikely. The rooting depth of the trees removed would have been less than three feet and the failure depth was many times deeper through nearly all of the landslide slip-surface. The effects of forest canopy on the rate of delivery of water to the slope provided by alder trees would have been minimal since they loose their leaves in the winter."

For those that are interested in reading the entire report I have include it also. It is quite technical though. I put this informnation on this web site to provide information to those that are interested in the cause of the slide.

MEMORANDUM

SUBJECT: KILCHIS RIVER SLIDE

TO: MARK LABHART & MIKE BORERLON

FROM: DAVE MICHAEL & JASON HINKLE

DATE: DECEMBER 24, 2003


The purpose of this memo is to document the findings and recommendations for the recent landslide in the Tillamook State Forest, Tillamook District, Kilchis River Basin, Little South Fork Kilchis River (S.E.1/4, Section 23, T.1N., R.9W, W.M.).

On December 15, 2003 Jason Hinkle (Dave Michael was out of state on annual leave) conducted a reconnaissance visit to the site and developed an initial interpretation which he shared with Mark Labhart via E-mail communication. On December 22, 2003, Dave Michael and Jason Hinkle visited the site for the purpose of updating Dave Michael on the landslide event and confirming the initial interpretation. On both field visits the investigation began from the depositional area at the road below the slide then proceeded up the north side of the slide to the top of the slide then down the south side of the slide. We observed the soil, rock, water, and topographic conditions at the site for the purpose of understanding the mechanism of failure and potential for further slope movement. We reviewed aerial photographs of the slide area from 1954, 1961, 1977 & 1996. We made a third visit to the site on Monday December 29, 2003 for the purpose of confirming our confidence in the geologic model and checking for recent secondary slope movements within the slide area.


FINDINGS:

 The slide deposition was divided into two areas. The main deposition was directly below the channel and largely contained on the road and flood plain of the Little South Fork of the Kilchis River, slide deposition reached, but did not cross to the other side of the stream. There was a smaller depositional area just up-stream of the main area that actually extended onto the opposite side of the Little South Fork.

 The landslide originated near the ridge-top and is estimated to be approximately 80 - 120 ft. across widening to 150 - 175 ft. down-slope and tapering to approximately 50 – 75 ft. at the toe. The initiation site is approximately 300 - 400 ft. from the head scarp to the toe. There is a small slide mass remaining in the initiation area.

 The depth of the slide was estimated to be 20 – 30 ft. along the central axis (deepest points of the slide geometry).

 The transport zone or debris track is estimated to be approximately 1,000 – 1,500 ft. The transport zone had both entrainment of natural soil and rock as well as deposition including lateral limit “spill-over” deposition. The question of where there was a net gain or loss in volume of the slide though this zone of transport is problematic.

 We calculate the volume using these rough estimates as 7,000 – 22,000 cubic yards.

 Tension cracks and vertical displacements were observed on blocks on the north side near the top of the initiation area. These blocks appear to have moved slightly between field visits with some possible calving at the toe of the bocks into the landslide scar.

 The head scarp had vegetation in the top 5 – 15 ft. both grasses and moss.

 There is a complex landform that surrounds the slide area composed of older slide scars (escarpments) and hummocky ground (slide blocks) with rock outcrops of less weathered rock protruding typically at the base of these hummocky areas.

 The overall slope is steep ranging from 80 – 90%+ with very small areas of lesser (old slide blocks) and greater slopes (old slide escarpments).

 There are several old cat roads (assumed to be from the oldest logging) that appear to have cut across the slide area.

 There were numerous very well used elk trails throughout the area.

 There were pockets of snow near the top observed on the December 15 visit by Jason Hinkle.

 The rock is of igneous origin, Tillamook Volcanics Formation, and is very deeply weathered with an attitude estimated in the field of N20E20NW. This is a somewhat out of the hill moderate dip. There are prominent rock outcrops throughout the area that appear much less weathered. These rock exposures are common at the toe of the gentler, more hummocky slopes. There was one noted mass plane of separation with an attitude of N50E80NW.

 There are a number of large boulders in the stream within the debris slide path. The boulders have both fresh (blue) and stained (brown) surfaces. There are also older, mossy boulders in this area that predate the slide that were probably deposited by past slide events at this site.

 The waterfall that is approximately 2/3 of the way up the slide scar has moss growing on its surface.

 Aerial photograph review indicates a landslide landform (steep escarpment slopes and hummocky masses) with burned conifer and hardwoods (assumed to be alders) in the draw in 1954 and cat roads cutting across the site in 1961.

 Secondary failures within the slide scar were observed on the December 29 visit.


CONCLUSIONS:

The slide is in the middle of an area that has significant signs of older slide activity, both recent and more ancient. The slides that occur in this geologic setting are small to large scale and typically deep-seated, but rapidly moving, complex (slip-surfaces probably on pre-existing weaknesses) failures. We interpret the site to have a “rock controlled” slope movement history. The prominent, competent rock exposures within this otherwise very deep weathering profile are the main evidence for this interpretation. Normally such a deep weathering profile would not be expected to support such steep slopes without some slope reinforcement. The rock that is less weathered and still intact is effectively holding back portions of the weathered rock and soil above and acts as “key blocks”. These key blocks eventually fracture and rupture, possibly from frost or ice expansion “heave” or just from progressive strain and weathering. When the key block fractures and ruptures there is not enough frictional strength on the surface between partially decomposed rock and the more competent underlying rock mass to maintain stability. At this point, when the pore water pressure of the slope is elevated during the next significant storm event, failure is likely to occur. This is probably an ongoing mechanism of slope movement that might be much like soil creep with these key blocks keeping the movement from developing into a rapid failure until the key blocks themselves fail. The prominent rock exposed at the base of the modern slide mass that is remaining on the slope is evidence of this “rock controlled” slope movement mechanism for this most recent slide event.

The old cat roads on the site can not be ruled out as contributing to the failure in some manner since much of the significant evidence was removed with the landslide. However, there were no indications of significant water flow on the remaining cat roads above and adjacent to the slide area. There might have been some failure of cat road fill slopes that retrogressed and under-cut the slope leading to the larger failure, but this is purely speculative since all evidence would have been destroyed in the landslide process and seems unlikely given the ultimate size and geometry of the slide.

Timber harvest in 1997 removed alder from the slide area as evidenced by aerial photo review and on the ground investigation. The removal of alder from this geologic setting leading to this deep-seated, large, complex landslide seems very unlikely. The rooting depth of the trees removed would have been less than three feet and the failure depth was many times deeper through nearly all of the landslide slip-surface. The effects of forest canopy on the rate of delivery of water to the slope provided by alder trees would have been minimal since they loose their leaves in the winter.

Given the small secondary failure observed on our third visit, as well as the increased cracking in the blocks on the upper north side of the slide, and the general geologic model, we consider the potential for additional slope movement relatively high at this time and in the future. Slope movements within and outside of the recent slide area are likely. Any slope movement within the slide area should be expected to reach the depositional area below even with limited initiation size.


RECOMMENDATIONS:

There may be more sliding in the near future and undoubtedly in the geologic future. There is no practical solution to the failures themselves but we recommend mitigation of impact by removal of the existing slide deposit at the road. It is not necessary to remove all of this deposit, but the portion that is practical and provides an area to catch the possible future slide deposits. This is basically a catch basin or fall out zone approach to mitigation of the impact of possible future events from this slope.

We recommend caution working or even traveling under this slide area in the short term due to the continued movement that we have observed on our visits. The remaining hazard and risk relationship might be viewed as likely but limited for natural resource damage since the volume is somewhat limited. However there is considerable potential energy in any future event even with small initiation size to produce risk of life to any workers in the depositional area.

The slide area itself will undoubtedly re-vegetate with alder but planting conifer on the existing slide mass perhaps as soon as next fall would provide potential large wood for recruitment in the more distant future.

Mulch has already been applied to the depositional area to mitigate rill erosion and associated impacts to water.


David L. Michael R.G., C.E.G. Jason C. Hinkle, G.I.T.
Geotechnical Specialist, NWOA Geotechnical Assistant, NWOA [/QUOTE]

[BUGLEMAN]

Thanks Mark for your reply in the other thread regarding the L.S.F.K. slide. I will tip my hat to the gusto with which you have responded. I have learned much from this dialogue.

My point as I am sure you have gotten is that slopes like this should never be logged. Technically I tip my hat off to you and it seems like you have enough brass to run with the big dogs. I just would like to see less politics determaining the cut and see the environment and recreation come closer to the forefront of our TSF management. Thanks for you detailed responses. The state needs employees like you.

Thanks,

Kevin Beckman