Monday, October 29, 2018

Shore Bluffs, Sediment and Beaches: One Example

The composition of beach sediment is dictated by the source material for the beach as well as wave energy. A common beach type in western Washington is gravel and cobble beaches with scattered boulders.

East shore of Lopez Island

The sediment source is from an eroding bluff that extends to the south.

Glacial erratic boulder embedded in a poorly sorted glacial till

The sediment source for this beach is glacial till. The silt and clay content is much greater than the gravel, cobble and boulder content, but the fine grained silt and clay is readily transported off the beach by wave action.

Typically I would call this bluff a "feeder bluff". That is erosion of the bluff provides sediment to the beach, and hence, feeds the beach. MacLennan and others (2013) came up with a mapping scheme to identify and map feeder bluffs.

This particular bluff was mapped as a transport zone, not a feeder bluff. In this case the erosion rate is very slow due to the very moderate wave energy combined with the very hard resistant glacial till. The fact that much of the slope is vegetated and in many areas tree covered further suggests that this shore bluff is providing very little sediment input. Another possibility is that the toe of the slope did not exhibit the erosion shown above when the shore reach was mapped.

Another mapping scheme might be to call this a slow feeder bluff. For rapid erosion bluffs that send lots of sediment to the beach, MacLennan and others (2013) use the term "feeder bluff exceptional", but for slow erosion sites like that shown above they use transport zone. That is a reach where sediment is transported along the beach with very minimal input from the bluff.

Shore reach designations can be important in regards to shoreline regulations. Feeder bluffs in particular are important; if the sediment source from a feeder bluff is stopped, the beach will become starved and the beach will disappear - a loss for recreation and habitat. Hence, the effort to identify and map these shore forms.   

Sunday, October 28, 2018

Rubus ursinus

There are various plants that present field work challenges in western Washington. Rubus ursinus or trailing blackberry is a low key plant that has temporarily flustered me while navigating through brush. 

Rubus ursinus set as a couple of trip wires on a moss covered log

High stepping and an awareness that a wire like vine may be wrapped around an ankle is a key strategy

The sharped barbs are not long, but they are very effective at shredding skin

The blackberry is a native and does produce tasty fruits. They are far less problematic compared to thick Himalayan blackberry thickets or rose thickets. 

Sunday, October 21, 2018

Forest Blocking Debris Flows

Debris flows and debris floods area a significant hazard on alluvial fans.  Forested alluvial fans can significantly reduce the debris flow and flood hazards. A forest stand of stout trees can act as a stationary rake preventing large wood debris from spreading out on the fan surface. I have been to several alluvial fans where homes were saved by the presence of forest on either side of the stream on the fan, and I have seen homes destroyed that likely would not have been impacted if the forest had not been removed. 

The debris flow path shown above came off of a very steep mountain slope. The debris flow initiated as a small shallow slide about 1,500 feet in elevation above this point. The slide mixing with creek water flushed out the accumulation of large wood within the steep channel and delivered the pile of logs with mud and water to the base of the mountain.

Forest along the side of the stream kept the wood debris as well as the stream confined to the existing stream route.

From a land use geology hazard policy, leaving forest stands on some alluvial fans can be a form of mitigation, but also a restraint on property owners from cutting trees and thereby increasing the risk to neighboring properties. I have recommended leaving trees on several development projects. At least one county has utilized restrictive covenants on permits based on those recommendations. 

Saturday, October 20, 2018

Ground Fog During the Dry Spell

Northwest Washington has had a nice stretch of sunny weather. The air has cooled quickly in the evening along the ground with frost in places. But overhead, even in the morning it has been warmer. I noted sharp swings in temperature on even modest rises in the land while heading out on my various ventures.

The mix of clear sky and ground fog made for nice scenery.

Heading towards the Chuckanuts across the Samish Flats

Cattle on Lopez Island

Baker to the left of large cottonwood/eagle tree and Twin Sisters to the right 
Fish and Wildlife land in the foreground

Lummi Peak 

Thick ground fog 

Another band of fog forming over field and spilling across road

Western Washington should turn back to cool and wet again. Nice scenery as well - just different.

Wednesday, October 10, 2018

Orcas Landing Feeder Bluff

I visit Orcas Landing on a regular basis as it is the location of the ferry landing for Orcas Island.

For shoreline wonks there is an example of an armored feeder bluff just to the east of the grocery store.

Feeder buffs are eroding shoreline bluffs that provide sediment (feed) to beaches. By lining the feeder bluff with a rockery armor at the top of the beach, erosion of the bluff is stopped. The result is a well vegetated bluff as well as a protected road at the top of the bluff. The unarmored bluff further to the east (right in the picture above) has eroded with the subsequent loss of vegetation. The downside of armoring feeder bluffs is that the beach becomes starved of sediment and non armored areas may see a substantial increase in erosion. The loss of sediment supply from erosion will also lead to the loss of the beach. 

Friday, October 5, 2018

Inkstain and Walla Walla's Mill Creek

J. Fleck at Inkstain revisited Mill Creek in Walla Walla. I am a regular reader of Fleck's blog on western water issues. He is southwest U.S. centric, but his college days were spent in Walla Walla and he began his career there. Mill Creek and the other waters of the Walla Walla Valley fit the typical challenges of western water challenges.

I have a couple of old posts from my own observations of Mill Creek (walla-walla-1931-flood and  mill-creek-and-walla-walla).

Like Fleck, I have my own memories of a younger me associated with Mill Creek and Walla Walla. I clocked my fastest mile time during a race in Walla Walla. I was never able to improve that personal best - maybe it was the water from Mill Creek that made the difference.