Tuesday, April 22, 2014

Landslide Discussion with Whatcom County Council

Whatcom County Council wants to start a dialog on landslide hazards and I was asked to make a presentation. The following are the images from the presentation with just a few notes. John Thompson, geologist with Whatcom County Public Works River and Flood and Natural Resources will present some information as well. A strength Whatcom County has had has been staff geologists that have played a role in reducing hazard risks.
I decided to start the talk with a brief overview of the Oso/Hazel Landslide with some emphasis on the fact that this slide risk was well known by geologists and was foreseeable.

Snohomish County Geohazard Map

LiDAR of Oso area landslides showing age relationships of past large slides

State DNR checked the slide after a storm event in 2009
Most comparable Whatcom County slide to Hazel is the Clay Banks

Recent river blocking slide at Clay Banks

Block of clay showing run out extended beyond current river

LiDAR of Clay Banks and Nooksack River impingement

Aerial of Clay Banks area

This small collapse of an old logging road traveled over one mile

Bark stripped off tree at upper end of failure

Slide mobilized large boulders and wood in debris flow

Home damaged by debris flow/flood

Boulders, logs and mud from debris flow

Small shallow slide in big leaf maple dominated forest

Damaged home from slide

Another slide down small stream

Directly aimed at home
Lifted from Dart's paper on rock failure in New Zealand

Rock failure above the Skagit River in Whatcom County

Damaged truck and dock

Swift Creek Landslide

Fracture scarp on upper Swift Creek Landslide

Bald Mountain Slide in Canyon Creek watershed includes most of the forested slope
Picture taken from a vantage point on another deep-seated  landslide

LiDAR of lower Canyon Creek

Bedrock deep-seated failure in Jones Creek above the town of Acme

Upper part of slide with trees on slide

Assessing the passage of a debris flow on Jones Creek

DEM of massive Church Mountain Landslide
Slide is approximately 2,000 years old
Town of Glacier is built on slide

Bedrock landslide on unknown age near Kendal

Staring point of conversation

LiDAR and property lots from Jefferson County Parcel Search web page

Potential landslide risk hazard maps

Screen shot

A popular quote for geohazard types

Sunday, April 20, 2014

Notes from the West Side of Marrowstone

I had a project on the southwest shore of Marrowstone Island near the upper, south end of Scow Bay. The beach here is primarily gravel at the surface but with a mix of sand and silts under the gravel as well. Except for the upper beach the tidelands here are crunchy with various tidal sea life too the point of influence where I walked.

barnacle encrusted beach gravel

Great heron sharing the beach with me

The protected nature of the bay is such that the gravel remains undisturbed for long periods and as such becomes encrusted with barnacles and muscles and the underlying sediment is full of clams as well as sand dollars.

The shoreline bluff is primarily glacial drift. The discontinuity of a thin bed of cross-bedded sands formed a rather continuous line along the shore bluff perhaps a brief period when subsurface water flowed under this part of the ice sheet.

A very red skinned madrone at the top of the beach and base of the bluff

The above spectacular madrone was one of several in a stand and like many large healthy madrones despite the shoreline position, this madrone was rooted into a midden of shells and charcoal.

Shell fragments, charcoal and fire cracked andesite cobble

Layer of clam shells. Just a note - When I see these locations I look only.

The shore is not entirely glacial. Outcrops of sandstone rise above the beach level for a brief section of the shore.

Barnacles encrust the bedrock, but the odd concretions that dominate this formation are present although no where near as spectacular as the concretions on the east side of the island marrowstone-island-geology-trip-to-see-concretions 

The bedrock outcrops on the island are oddly limited to the shoreline only - as far as I know and as far as geologic maps of the island show. At all locations the bedrock is covered with drift and does tot extend to the tops of the bluffs. There is likely some sort of normal fault cutting across the island as the south half of the island has bedrock outcrops and bedrock has been encountered in well borings while depth to bedrock on the north half of the island has not been reached even with relatively deep wells.    

Saturday, April 19, 2014

Trillium Season

Trillium is an April flower and its brief appearance of large while flowers in the otherwise dark forest floor marks the change over from bare branches to the return of the western Washington jungle. The flower always just beats the leafing out of the red alder and big leaf maple.

Friday, April 18, 2014

Church Mountain and Sanford Pasture Landslides

LiDAR imagery does not extend all the way up to Glacier, Washington. As noted in my last post, the Church Mountain Landslide fills the valley floor of the North Fork Nooksack River near Glacier.
DEM (digital elevation model) of Church Mountain Landslide
Summit of Church Mountain is on the upper left
Lumpy area in lower center filling the valley is the thicker part of the slide deposit
The slide deposit extends nearly all the way to the left edge of the image
Note the change in image sharpness with LiDAR DEM on the left half

The Church Mountain slide is absolutely huge. Not to take anything away from the Church Mountain landslide, but the petty side of me has to point out that the Church Mountain landslide is not the largest landslide in the United States or even Washington State. The Sanford Pasture Landslide west of Naches, Washington is substantially larger. A very small secondary portion of this landslide collapsed a few years ago and blocked the Naches River. 
DEM of Sanford Pasture Landslide west of Naches

USGS topo map of Sanford Landslide

Aerial of Landslide

A final note: Sanford is not the Washington State champion; there is an even a bigger landslide south of Wenatchee.