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Landslides: Mapping/Technical Resources
Introduction
This section provides information about technical resources related to landslide hazards, including the mapping of landslide hazard areas.
 
Much of the information has been gathered from the Oregon Department of Geology and Mineral Industries (DOGAMI) and the Oregon Department of Forestry (ODF), as well as from resources of Department of Land Conservation and Development (DLCD).

Reducing Risks from Landslides: Technical Resources
Risks from landslides, as with any geologic hazard, are a combination of 1) the likelihood of the hazard actually occurring at a given site within a particular timeframe, and 2) the exposure of persons or property to the hazard. Management of the risk can involve reduction of the hazard and/or reduction of the exposure to the hazard.
 
 
Before and after photo of house in Columbia River Gorge
 
A thorough discussion of risks associated with natural hazards of all kinds, and information about reducing those risks, can be found in the DOGAMI's Special Paper 31: Mitigating Geologic Hazards in Oregon: A Technical Reference Manual A summary was also published as Special Paper 32: Geologic Hazards: Reducing Oregon's Losses. Copies of these and other publications and maps about Oregon's geology and geologic hazards may be purchased through the Nature of the Northwest Information Center.
 
Another excellent technical resource is DLCD's Planning for Natural Hazards: Oregon Technical Resource Guide. Chapter 5 of the guide deals specifically with landslide hazards and provides numerous additional technical references.

Landslide Mapping: Characterization of the Hazard
Reduction of landslide risks must include a good understanding of the landslide hazard, including mapping of landslides or landslide hazard areas. DOGAMI Special Paper 31 (see above) describes three distinct levels of landslide mapping:
  • Regional landslide mappingshowing general landslide (or potential landslide) distribution inferred from general features and geology, and used for general policy development and to identify target areas for more detailed mapping.
  • Subdivision or local landslide mappingwhich focuses on local development, such as a subdivisions, and needed information to manage non-site-specific causes of slides such as regional drainage, cumulative runoff, cumulative erosion, and problems associated with lifelines such as roads and buried utility lines.
  • Site-specific mappingused to manage or regulate a hazard and risk unique to a particular site, although site-specific studies alone do not always address cumulative problems or problems that can originate off-site.
 
Mapping of landslide hazards in Oregon began in the early 1970s when DOGAMI mapped existing landslides in much of coastal Oregon (published as Environmental Geology Bulletins).
 
Now, as part of meeting the requirements of Senate Bill 12, DOGAMI and ODF are working together to map debris flow hazard areas on a region-wide scale. These efforts are described briefly below. A more complete discussion of all of these projects can be found in the Report to the 71st Legislative Assembly on the Implementation of 1999 Senate Bill 12 Relating to Public Safety and Rapidly Moving Landslides (pdf), which was submitted by DOGAMI, DLCD, and ODF to the Legislature in December 2000. Technical appendices (pdf) contain additional information.

Identifying Further Review Areas - Status of Mapping Effort
DOGAMI is currently working to refine the ODF debris-flow maps, and to identify "further review areas" as required by Senate Bill 12. Targeting some of the higher hazard areas identified on the ODF maps and where debris flows were known to have occurred, DOGAMI has performed preliminary field investigations throughout western Oregon. These efforts focused primarily on improving the delineation of the down slope run-out areas – the most critical areas in terms of public safety. Findings from those field investigations are being used to develop and evaluate improved methods for GIS modeling of debris flow hazards. Several appropriate models have been identified and are currently being tested.
 
In addition, DOGAMI received funds from the Federal Emergency Management Agency (FEMA) to lead an inventory project for the three major storms of February 1996, November 1996, and December 1996/January 1997. The project consisted of a statewide consolidation of slope failure information, and involved participation from state and federal agencies, local governments and the private sector. The final inventory identified 9,582 total landslide locations (the actual number of landslides is estimated to be many times the documented total). For each documented landslide, up to 15 descriptive items are reported. From this study, DOGAMI found that "over 98 percent of the landslides were recorded in the western portion of the state, mainly in the Coast Range and the Cascade Range, with fewer in the Willamette Valley and the Klamath Mountains. Counties with the highest percentage of total landslides reported are Lane (24 %), Douglas (11 %), Linn (10 %), Clackamas (9 %), Tillamook (9 %), Lincoln (8 %), and Multnomah (7 %)."
 
DOGAMI’s database is an outstanding resource and is being used to further improve and evaluate the debris-flow models. The database and an associated report text are available for purchase through the Nature of the Northwest Information Center). An abstract of "Slope Failures in Oregon, Special Report 34" is available.
 
The database was utilized for another related project at DOGAMI: a study of the relationship between rainfall events and debris flows, which was described in a paper published in the March/April 2000 issue of Oregon Geology (available through the Nature of the Northwest Information Center). That study provides a better understanding of how landscapes and rock types respond to rainfall in western Oregon. Records from the four major storms that hit western Oregon during 1996 and 1997 confirm that the occurrence of many landslides and debris flows can be related to rainfall intensity and duration. The relationships between rainfall intensity and debris flows described in the paper are useful in helping to determine areas where debris flow warning systems are appropriate. A debris-flow hazard warning system has been developed, and a current alert message can be found at the Oregon Department of Forestry.