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Open-File Report O-12-06, Landslide Hazard and Risk Study of the U.S. Highway 30 (Oregon State Highway 92) Corridor, Clatsop and Columbia Counties, Oregon, by Katherine A. Mickelson and William J. Burns. 105 p. PDF report, 4 PDF thematic map plates, scale 1:24,000; inset maps 1:8,000. 42 x 38 inches. Esri ArcGIS v. 10 geodatabases.

Order publication CD-ROM, $15, from Nature of the Northwest.

Deliverables of this study include the following:

Executive Summary

Landslides and debris flows are common in the Oregon Coast Range due to the combination of high precipitation, steep slopes, and landslide-prone geologic units. Cutting through the northern Coast Range, the U.S. Highway 30 (Oregon State Highway 92) corridor is prone to slope instability. In December 2007, a series of powerful storms produced heavy rainfall causing landslides and severe flooding. Due to the severe damage caused by these storms, the President of the United States issued a disaster declaration that allowed FEMA Hazard Grant funding to become available under FEMA DR-1733-OR. In September, 2010, the Oregon Department of Geology and Mineral Industries (DOGAMI) entered an intergovernmental agreement with Oregon Emergency Management (contract no. DR-1733-OR-14-F) to perform regional landslide hazard evaluation along the Highway 30 corridor in Clatsop and Columbia Counties. The primary purpose of this project was to provide detailed information about landslide hazards and assets at risk in this area.

On the basis of the tasks detailed in the original proposal, the five main objectives of this project were to:

The completed landslide inventory maps a total of 588 landslides within the 90 mi2 study area with 288 landslide deposits classified as deep and 140 as shallow. Also mapped on the lidar were 150 debris flow deposits and 10 rock fall deposits. Over half (380) of the landslides are classified as historic, having moved in the last 150 years. Of these historic landslides, 80 have recorded dates of movement in the period 1930 to 2011.

Landslides occur on slopes ranging from 10 degrees to 62 degrees, with a mean estimated pre-slide slope of 33 degrees. Depth to slip surfaces for shallow landslides range from 0.5 ft to 15 ft, with an average of 10 ft; depth to slip surfaces for deep landslides range from 15 ft to 369 ft, with an average of 45 ft. The landslide deposits are highly variable in size. The smallest covers an area of approximately 75 ft2, while the largest deposit covers an area over 177,000,000 ft2 (4,000 acres). The Wauna landslide, over 5 miles long, is situated near the communities of Wauna and Westport and is the second largest landslide found. Highway 30 and one of the main transmission lines cut across the massive body of the landslide (Figure 1).

Debris flow fans account for a quarter of the landslide failures mapped. Historic accounts indicate that the area has been plagued by catastrophic debris flow events since the 1800s, most notably along Highway 30. Five major debris flows along the highway have damaged residences between 1914 and 2007. Along Highway 30, a number of structures exist on debris flow fan deposits, causing property and people to be vulnerable to these potentially catastrophic events (Figure 2).

This study indicates that the Highway 30 corridor in Columbia and Clatsop Counties is at significant risk from landslide hazards. Landslides cover 25% of the study area, and 33% of the City of Clatskanie is covered by large, deep landslides. The large number of people and structures residing on these deposits highlights the potential danger present and shows the need for public awareness on landslide hazards.

The landslide susceptibility maps show that this area is highly susceptible to deep and shallow landslides. The high-susceptibility zone for shallow landslides covers 15% and the moderate susceptibility zone covers 32% of the study area. For deep landslides, the high-susceptibility zone covers 28% of the study area and the moderate susceptibility zone covers 15%. Most low-susceptibility zones for both shallow and deep landslides are restricted to the floodplain of the Columbia River.

Exposure and HAZUS-MH (FEMA, 2011) based risk analyses were used to estimate potential losses and damages from landslide hazards. The HAZUS-MH software program allows the user to estimate potential losses from earthquake-induced landslides. The results of these analyses showed that residential buildings are the most exposed asset. Primary infrastructure, mainly roads and electric transmission lines, is also at risk. Sixty-eight percent of the electric transmission lines and 57% of the transmission towers are currently routed on landslide deposits, making the entire system vulnerable. Highway corridors also are exposed, with 76% at risk from shallow landslides. A road closure in this area can have a potentially large economic impact because 6,000 vehicles travel these routes per day. The results from the risk analysis allow planners and first responders to understand where resources should be directed.

The results of this study include this report, a detailed landslide inventory including pre-historic, historic, and active landslides, and a set of susceptibility maps identifying areas at risk for landslides.

Plate 1, Project Overview Map:

City of Silverton landslide inventory map

 

Plate 2, Landslide Inventory Map:

City of Silverton shallow-landslide susceptibility map

 

Plate 3, Shallow-Landslide Susceptibility Map:

City of Silverton deep-landslide susceptibility map

Plate 4, Deep-Landslide Susceptibility Map:

City of Silverton deep-landslide susceptibility map