Landslide Reconnaissance Following the December 3, 2007 Storm - Mason County
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Landslide Reconnaissance Following the December 3, 2007 Storm - Mason County 
 

Holiday Beach Landslides

On December 3rd, a hyperconcentrated flow moved through the middle of Holiday Beach, flooding houses and depositing a thick layer of debris through the city. Smaller landslides north and south of the town isolated the community. Hyperconcentrated flows often are refered to as 'mudflows', especially in the media; however, 'mudflow' is often a generic term that does not describe the landslide type. Hyperconcentrated flows are an intermediate landslide, somewhere between a debris flow (or a landslide with more rock than water) and typical sediment loads in flood waters. Hyperconcentrated flows are basically floods with a significantly higher amount of sediment and debris than a normal flood.

Location of events

 

Map 1: Location map of the Holiday Beach landslides. The location is approximate for the hyperconcentrated flow and only maps the extent in Holiday Beach. Click on map to view larger image.

North of Holiday Beach
This small landslide along the road bank slid towards the below house, damaging the roof, but leaving the house structure intact. Luckily, no deaths or injuries were reported from this landslide.

 

Figure 1: The landslide failed along the bank of Highway 101 and into the house below. The landslide did not appear to be over bedrock, but might have been aided by water concentrating from the highway. Click on photo to view larger image.  (photo by Isabelle Sarikhan)

Figure 2: Viewing the house that was damaged by the landslide. The house suffered a low amount of damage from the landslide, which broke a part of the roof. Click on photo to view larger image.  (photo by Isabelle Sarikhan)

Holiday Beach
Holiday Beach was one of the hardest hit communities along Highway 101. A creek flows along the southern part of town and flows near or above some of the houses. The creek probably has had repeated hyperconcentrated flows, depositing material in the channel. Because this creek flows at or near the level of some houses, mitigation is critical to prevent future events. It is highly likely that future events will be as bad as or worse than this event.

 

Figure 3: An aerial photo of Holiday Beach; note the two landslides in the center of the photo and the thick sediment in Holiday Beach. Click on photo to view larger image. (photo by Kelsay Davis)

Figure 4: Looking north along Highway 101 through Holiday Beach. Note that the sediment becomes thicker as you head north. The main channel of the hyperconcentrated flow was tracked north of the creek. Click on photo to view larger image. (photo by Isabelle Sarikhan)

 

Figure 5: The stream that goes through Holiday Beach. Note the sediment on each side of the stream; the left side of the stream probably was moved there. Click on photo to view larger image. (photo by Isabelle Sarikhan)

 

Figure 6: A look at one of the houses near the stream channel. The stream appears to be at or above the house level. The yard of the house is thickly covered by debris from the hyperconcentrated flood. Click on photo to view larger image. (photo by Isabelle Sarikhan)

Figure 7: Looking upstream from the bridge on Highway 101; note that the stream appears to be at or above the house level to the right. Click on photo to view larger image. (photo by Kelsay Davis)

Figure 8: Looking north along Highway 101 from the bridge crossing the creek. The sediment is not very thick here, but becomes thicker farther north. Click on photo to view larger image. (photo by Isabelle Sarikhan)

Figure 9: Looking south towards the creek. The large piles of sediment in the distance was probably dredged or removed from the creek previously, but the remaining sediment was desposited from the hyperconcentrated flow. Click on photo to view larger image. (photo by Isabelle Sarikhan)

 

Figure 10: Looking north from Nancy Street and Highway 101; the sediment was thickest here and the main flow appeared to flow down Nancy Street. Click on photo to view larger image. (photo by Isabelle Sarikhan)

Figure 11: Looking east up Nancy street from Highway 101. The sediment becomes thicker as we walk up Nancy Street. Click on photo to view larger image. (photo by Isabelle Sarikhan)

Figure 12: Looking south from approximately Nancy Street and North Myrtle Street. The main flow of the hyperconcentrated flow probably was diverted from the creek, through this house, and onto Nancy Street. It is uncertain if this flow damaged the garage door of the residence. Click on photo to view larger image. (photo by Isabelle Sarikhan)

Figure 13: A small landslide on a dirt road south of the creek. This small slide appeared to block the road, but was cleared before our investigation.  Click on photo to view larger image. (photo by Isabelle Sarikhan)

Figure 14: South of Holiday Beach a landslide blocked half of the highway. The landslide did not appear to completely block Highway 101, but moved a significant amount of material onto the road. Click on photo to view larger image. (photo by Isabelle Sarikhan)

Figure 15: Looking at the landslide from the toe; compared to other landslides, it did not contain much woody debris and didn't appear to have failed off of impenetrable bedrock. Click on photo to view larger image. (photo by Isabelle Sarikhan)

South of Holiday Beach
South of Holiday Beach along a steep bluff, a landslide failed across Highway 101, damaging a power pole and lightly damaging a house.

Figure 16: South of Holiday Beach a landslide blocked the highway and lightly damaged a house. Click on photo to view larger image. (photo by Kelsay Davis)

Figure 17: South of Holiday Beach looking north on Highway 101; the landslide flowed across Highway 101, moving debris and trees across the road. The landslide is typical of other landslides during this event, shallow soils on bedrock. Click on photo to view larger image. (photo by Isabelle Sarikhan)

Figure 18: DNR geologist Isabelle standing on a log in the landslide. The trees appeared to have shallow roots that were unable to penetrate into the bedrock. Click on photo to view larger image. (photo by Kelsay Davis)

Figure 19: Looking towards a house that appeared to sustain light damage from woody debris in the landslide. Click on photo to view larger image. (photo by Isabelle Sarikhan)

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 Contacts

Stephen Slaughter
Geology & Earth Resources Division, Hazards Geologist
Washington State Department of Natural Resources
360-902-1498
Fax 360-902-1785
stephen.slaughter@dnr.wa.gov

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