Protocol for Shallow-Landslide Susceptibility Mapping

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PROTOCOL FOR SHALLOW-LANDSLIDE SUSCEPTIBILITY MAPPING

by William J. Burns, Ian P. Madin, and Katherine A. Mickelson

SPECIAL PAPER 45

2012

G E O L O G Y F A N O D T N M I E N M E T R

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A L

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R E S OREGON DEPARTMENT OF GEOLOGY AND MINERAL INDUSTRIES O

1937 NOTICE The Oregon Department of Geology and Mineral Industries is publishing this paper because the subject matter is consistent with the mission of the Department. This paper describes a method for preparing regional estimates of shallow-landslide susceptibility, and these methods may not be suitable for site-specific purposes. Landslides hazards for a specific site are best assessed through geotechnical or engineering geological studies by qualified practitioners.

Oregon Department of Geology and Mineral Industries Special Paper 45 Published in conformance with ORS 516.030

For copies of this publication or other information about Oregon’s geology and natural resources, contact:

Nature of the Northwest Information Center 800 NE Oregon Street #28, Suite 965 Portland, Oregon 97232 (971) 673-2331 http://www.NatureNW.org

For additional information: Administrative Offices 800 NE Oregon Street #28, Suite 965 Portland, OR 97232 Telephone (971) 673-1555 Fax (971) 673-1562 http://www.oregongeology.com http://egov.oregon.gov/DOGAMI/ State of Oregon Department of Geology and Mineral Industries Vicki S. McConnell, State Geologist

Special Paper 45

PROTOCOL FOR SHALLOW-LANDSLIDE SUSCEPTIBILITY MAPPING

William J. Burns1, Ian P. Madin1, and Katherine A. Mickelson1

G E O L O G Y F A N O D T N M I E N M E T R

R A

A L

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1937

2012

1Oregon Department of Geology and Mineral Industries, 800 NE Oregon Street, #28, Suite 965, Portland, OR 97232; Protocol for Shallow-Landslide Susceptibility Mapping

TABLE OF CONTENTS 1.0 EXECUTIVE SUMMARY ...... 1 2.0 INTRODUCTION ...... 1 2.1 Shallow Landslides Defined ...... 2 3.0 METHODOLOGY ...... 5 3.1 Overview ...... 5 3.2 Extraction of Landslide Inventory ...... 5 3.3 Calculation of the Factor of Safety ...... 8 3.3.1 Geology—Geotechnical Material Properties ...... 9 3.3.2 Groundwater Height above Failure Surface ...... 10 3.3.3 Depth to Failure Surface ...... 11 3.3.4 Slope Angle ...... 11 3.4 Creation of the Factor of Safety (FOS) Class Map ...... 12 3.5 FOS Class Map Filtering and Clipping ...... 14 3.5.1 Need for Filter ...... 14 3.5.2 Methods Tested ...... 14 3.5.3 Selecting Appropriate Focal Relief and Focal Range Values ...... 17 3.5.3 Clipping the FOS Class Map ...... 22 3.6 Buffers ...... 23 3.6.1 Landslide Head Scarp Buffer ...... 23 3.6.2 Factor of Safety Buffer ...... 24 3.7 Final Susceptibility Map ...... 26 4.0 SHALLOW LANDSLIDE SUSCEPTIBILITY MAP TEMPLATE ...... 27 5.0 LIMITATIONS OF MAPS PRODUCED USING THIS PROTOCOL ...... 28 6.0 POTENTIAL USES OF SHALLOW LANDSLIDE SUSCEPTIBILITY MAPS ...... 28 7.0 ACKNOWLEDGMENTS ...... 29 8.0 REFERENCES ...... 29 9.0 APPENDIX A—FACTOR OF SAFETY CALCULATOR ...... 31

LIST OF TABLES

Table 1. Example landslide inventory geodatabase table ...... 7 Table 2. General soil and rock material properties ...... 10 Table 3. Example Factor of Safety values calculated for various slopes and values of z, using parameters for residual soil on basalt .11 Table 4. Sample Factor of Safety calculations from spreadsheet for geologic unit landslide deposits of Table 2 ...... 12 Table 5. Reclassification scheme to convert individual unit Factor of Safety rasters to susceptibility zones ...... 13 Table 6. Maximum slope values corresponding to focal relief range values ...... 16 Table 7. Results of parametric spatial statistical analysis of the comparing raw and clipped focal relief maps to the landslide databases ...... 20 Table 8. Summary of factors contributing to the final susceptibility map ...... 26

ii Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

LIST OF FIGURES

Figure 1. Risk diagram displaying overlap of landslide hazard and vulnerable population...... 1 Figure 2. Common types of landslides...... 3 Figure 3. Diagram of a slump-earth flow showing common features...... 4 Figure 4. Example of shallow and deep-landslides...... 4 Figure 5. Example of a DOGAMI landslide inventory map ...... 6 Figure 6. Infinite slope analysis: diagram shows parameters for one DEM grid cell and the Factor of Safety equation. . . . . 8 Figure 7. Example of a geologic-material properties map...... 9 Figure 8. Example of a slope map created from the lidar-derived bare earth DEM...... 11 Figure 9. Example of dialog box for Esri Spatial Analyst Reclassify tool...... 13 Figure 10. Example of dialog box for Esri Spatial Analyst Mosaic To New Raster tool...... 13 Figure 11. Photo of slope on residential street in east Portland...... 14 Figure 12. Sample from the Silverton study area used to test alternative methods for reducing the impact of low steep slopes on susceptibility zones...... 15 Figure 13. Example focal relief analysis on cell using a 15 ft2 (4.5 m2) neighborhood...... 17 Figure 14. Map of area used to test focal range values...... 18 Figure 15. Raw Factor of Safety zone map...... 19 Figure 16. Factor of Safety class map for the test area clipped using a focal range value of 4 ft (1.2 m)...... 21 Figure 17. Example of dialog box for Esri Spatial Analyst Focal Statistics tool...... 22 Figure 18. Diagram of the 2H:1V head scarp buffer...... 23 Figure 19. Diagram of the 2H:1V buffer applied to all Factor of Safety less than 1.5...... 24 Figure 20. Example of dialog box for Esri Spatial Analyst Expand tool used to create Factor of Safety buffer map...... 25 Figure 21. Examples of head scarps and Factor of Safety buffers...... 25 Figure 22. Example shallow-landslide susceptibility map...... 27

Oregon Department of Geology and Mineral Industries Special Paper 45 iii Protocol for Shallow-Landslide Susceptibility Mapping

iv Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

1.0 EXECUTIVE SUMMARY

Landslides are one of the most significant natural haz- DOGAMI Special Paper 42 [Burns and Madin, 2009a]) ards in Oregon and cause millions of dollars in damage with hazard zones derived from a Factor of Safety (FOS) annually. Identifying areas susceptible to future land- map and buffers. slides is a critical step in reducing landslide risk. This This protocol also includes a map template for pro- paper describes a standardized procedure for develop- ducing a standardized shallow-landslide susceptibility ing shallow-landslide susceptibility maps. This proce- map at a scale of 1:8,000, tiled by quarters of U.S. Geo- dure is being used by the Oregon Department of Geol- logical Survey (USGS) 7.5 minute quadrangles. ogy and Mineral Industries (DOGAMI) to produce By identifying areas prone to future damaging land- standardized shallow-landslide susceptibility maps for slides, this protocol and products produced by follow- areas of Oregon. ing this protocol can be used to help Oregon communi- The shallow-landslide susceptibility map proto- ties become more resilient to the impacts of landslide col combines an inventory of existing landslides (see hazards.

2.0 INTRODUCTION

Worldwide, landslides cause billions of dollars in property damage and thousands of deaths every year (Hong and others, 2007). In the United States landslides cause Expanding an average of 25–50 deaths and over $2 billion in eco- Landslide Landslide Vulnerable nomic losses annually (Turner and Schuster, 1996; Hazard Risk Population Spiker and Gori, 2003). Climate, geology, and topography combine to make Oregon a landslide-prone state, with landslide losses exceeding $100 million in direct damage during severe winter storms (Wang and others, 2002). Landslides are also a chronic hazard in Oregon, with annual average maintenance and repair costs for Figure 1. Risk diagram displaying the overlap of the landslide hazard and the vulnerable population (modified after Wood, 2007). landslides in the state estimated at over $10 million (Wang and others, 2002). The growing Oregon population inevitably pushes development onto landslide- tic morphology of landslides with great completeness, prone slopes, adding to population and infrastructure accuracy, and precision, even in heavily forested areas. at risk (Figure 1). Mitigating the risk starts with having In 2007, DOGAMI began to collect high-resolution accurate, detailed, and comprehensive landslide hazard lidar topographic data over large swaths of Oregon. maps, including landslide inventory and susceptibility With lidar data currently available for over 85% of the maps. state population, DOGAMI has begun to systemati- In 2005, DOGAMI began a collaborative landslide cally map landslides in Oregon, guided by DOGAMI research program with the U.S. Geological Survey Special Paper 42 ( SP-42), Protocol for Inventory Map- (USGS) Landslide Hazards Program to identify and ping of Landslide Deposits from Light Detection and understand landslide hazards in Oregon. The key ini- Ranging (Lidar) Imagery (Burns and Madin, 2009a). A tial finding was the importance of high-resolution complete SP-42 landslide inventory results in an Arc- lidar-derived digital elevation models (DEMs) for both GIS-format geodatabase of landslide data that include inventory of existing landslides and modeling of land- landslide type, size, scarp height, estimated depth to slide susceptibility areas (Burns, 2007). We use lidar failure plane, and confidence of identification. DEMs with a resolution of 1 m to map the characteris-

Oregon Department of Geology and Mineral Industries Special Paper 45 1 Protocol for Shallow-Landslide Susceptibility Mapping

With an accurate inventory in hand, the next step 2.1 Shallow Landslides Defined in a complete landslide hazard mapping program is to develop susceptibility maps for common types of The term landslide includes a wide range of gravity- landslides. This paper describes a protocol for using driven downslope movements of material that all have detailed landslide inventory data collected under the different speeds, sizes, frequencies of movements and SP-42 guidelines, lidar topographic data, and geotech- triggering conditions, and very different resulting haz- nical data to produce a standardized shallow-landslide ards. Landslide types differentiated in SP-42 include susceptibility map. Coupled with the inventory map, falls, topples, slides, spreads, and flows as illustrated in the shallow-landslide susceptibility map can provide Figure 2 and Figure 3. residents, local government, and developers with criti- As the name implies, shallow landslides involve cal information for reducing landslide risk through movement of a relatively thin layer of slope material planning and engineering. The protocol is intended and have a shallow failure plane (Figure 4). Shallow to standardize and streamline DOGAMI’s efforts to landslides in Oregon are typically slumps, translational create shallow-landslide susceptibility maps in Oregon. slides, earth flows, or complex combinations of these It is also intended to serve as a guide for others who are types. In order to classify landslides as shallow, we need interested in producing standardized shallow-landslide to know or estimate the depth to the failure plane, and susceptibility maps and to help end users of these maps define a depth threshold to separate shallow landslides understand how the maps were created. To this end, from deep landslides. SP-42 details a method for cal- we have included a detailed description of DOGAMI’s culating the estimated depth of failure using informa- shallow-landslide susceptibility mapping procedure tion about scarp height and adjacent slope angles, so and a template for standardized 1:8,000-scale shallow- that landslide inventories developed according to that landslide susceptibility maps based on quarters of protocol will include values for depth to failure plane. USGS 7.5 minute quadrangles. There is no widely accepted boundary value between DOGAMI intends to use this protocol and template deep and shallow landslides. We have selected 4.6 m to complete standardized shallow-landslide suscepti- (15 ft) as the boundary between shallow and deep-seat- bility maps for as much of Oregon as funding and staff ed landslides based on the combination of several fac- allow. By following and referencing this paper, maps tors and results from other studies discussed in detail can be made more quickly and consistently, and other in SP-42. parties can make maps that conform to this standard. This study was funded in part by the U.S. Geolog- ic Survey (USGS) Landslide Hazards Program under Cooperative Agreement #05CRGR0002.

2 Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

Falls are near-vertical, rapid movements of masses of materials, such as rocks or boulders. The rock debris sometimes accumulates as talus at the base of a cliff.

Topples are distinguished by forward rotation about some pivotal point, below or low in the mass.

Slides are downslope movement of soil or rock on a surface of rupture (failure plane or shear-zone). • Rotational slides move along a surface of rupture that is curved and concave. • Translational slides displace along a planar or undulating surface of rupture, sliding out over the original ground surface.

Spreads are commonly triggered by earthquakes, which can cause liquefaction of an underlying layer and extension and subsidence of commonly cohesive materials overlying liquefied layers.

Channelized Debris Flows commonly start on a steep, concave slope as a small slide or earth flow into a channel. As this mixture of landslide debris and water flows down the channel, it pick ups more debris, water, and speed, and deposits in a fan at the outlet of the channel.

Earth Flows commonly have a characteristic “hourglass” shape. The slope material liquefies and runs out, forming a bowl or depression at the head.

Complex landslides are combinations of two or more types. A common complex landslide is a slump-earth flow, which usually exhibit slump features in the upper region and earth flow features near the toe.

Figure 2. Block diagrams and detailed descriptions of some of the most common types of landslides (modified from Highland [2004] and Varnes [1978]).

Oregon Department of Geology and Mineral Industries Special Paper 45 3 Protocol for Shallow-Landslide Susceptibility Mapping

Tension cracks (crown cracks) Internal scarps Crown

Flank (minor scarp)

Compression ridge (transverse ridge) Head scarp

Failure plane Toe (surface of rupture) Landslide deposit (main body)

Figure 3. Block diagram of a slump-earth flow showing common features (modified from Highland [2004] and Varnes [1978]).

Upper-surficial soils or highly weathered bedrock

Weathered to unweathered bedrock

Figure 4. Examples of shallow and deep landslides.

4 Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

3.0 METHODOLOGY

3.1 Overview The final susceptibility zones are displayed using a standardized map template. Each of these steps is The method we use to identify areas susceptible to shal- described in sections 3.2 through 3.7. We use Esri low landslides combines 1) areas of mapped shallow ArcView® software and the 3D Analyst™ and ArcGIS® landslides extracted from an SP-42 inventory with 2) Spatial Analyst™ extensions for the GIS portion of this calculated factors of safety (FOS). These two contribut- procedure. Unless otherwise specified, all rasters used ing factors are then filtered and buffered, and the result- have a cell size of 3 ft (1 m), which matches the native ing four contributing factors are assigned to high, mod- resolution of lidar data collected by DOGAMI. erate, or low susceptibility zones. The four contributing factors (maps; see below) that are combined to create 3.2 Extraction of Landslide Inventory the final shallow-landslide susceptibility map are: Note: To aid in understanding the sequence of steps required to • Inventory zone map: mapped shallow-landslides complete the protocol, a small graphical “progress bar” is provided from an SP-42 inventory for each step. Yellow bar indicates current step. • FOS class map: map of Factor of Safety classes Extract landslide Deal with Factor Apply buffers to Combine con- inventory data of Safety (FOS) landslide data tributing factors (high, moderate, and low) and FOS to create map • Head scarp buffer map: map of SP-42 inventory head scarp buffers The landslide inventory used in the shallow-landslide • FOS buffer map: map of moderate and high FOS susceptibility protocol was developed following SP-42 Class buffers standards. An example SP-42 map is shown in Figure 5, and a sample of landslide attribute data from this map/ geodatabase is shown in Table 1. All shallow-seated landslide deposit polygons (except channelized debris flow deposits; i.e., fans) and their head scarp polygons are queried from the inventory database and converted to a raster map with a value of 3 (high-susceptibility zone) for the polygons. This is theinventory zone map. Black bar indicates completed step: Extract landslide Deal with Factor Apply buffers to Combine con- inventory data of Safety (FOS) landslide data tributing factors and FOS to create map  Inventory Zone map

Oregon Department of Geology and Mineral Industries Special Paper 45 5 Protocol for Shallow-Landslide Susceptibility Mapping

G E O L O G Y F A N O D IMS-26 T N M E I N M E T R R A Landslide Inventory Map of the Northwest Quarter of the Oregon City Quadrangle, INTERPRETIVE MAP SERIES A L P STATE OF OREGON I E

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S DEPARTMENT OF GEOLOGY AND MINERAL INDUSTRIES O

I Landslide Inventory Map of the Northwest Quarter of the R

E VICKI S. MCCONNELL, STATE GEOLOGIST S O Clackamas County, Oregon Oregon City Quadrangle, Clackamas County, Oregon

1937 2009 by William J. Burns and Ian P. Madin Partial funding provided by the U.S. Geologic Survey,

000 000 000 122°37'30"W 530 532 534 122°33'45"W Landslide Hazards Program (Grant 00320340) 45°22'30"N 45°22'30"N EXPLANATION This map is an inventory of existing landslides in this quarter quadrangle. The landslide inventory is one of the essential data layers EFL used to delineate regional landslide susceptibility. This landslide inventory is not regulatory, and revisions can happen when new information regarding landslides is found or when future (new) landslides occur. Therefore, it is possible that landslides within the ES-R + EFL ES-R + EFL mapped area were not identified or occurred after the map was prepared. This inventory map was prepared by following the Protocol for Inventory Mapping of Landslide Deposits from Light Detection and DFL DFL Ranging (Lidar) Imagery developed by Burns and Madin (2009). The three primary tasks included compilation of previously mapped EFL landslides (including review of DOGAMI Special Paper 34 [Hofmeister, 2000] and the Statewide Landslide Information Layer for Oregon, release 1 [Burns and others, 2008]), lidar-based morphologic mapping of landslide features, and review of aerial photographs. ES-R + EFL Clackamas River Interstate 205 Landslides identified by these methods were digitally compiled into a GIS database at varying scales. The recommended map scale for these data is 1:8,000, as displayed on this map. Each landslide was also attributed with classifications for activity, depth of failure, movement type, and confidence of interpretation. The landslide data are displayed on top of a base map that consists of an aerial photograph (orthorectified) overlaid on the lidar-derived hillshade image.

ES-R + EFL This landslide inventory map is intended to provide users with basic information regarding landslides within the quarter quadrangle. The geologic, terrain, and climatic conditions that led to landslides in the past may provide clues to the locations and conditions of Highway 213 future landslides, and it is intended that this map will provide useful information to develop regional landslide susceptibility maps, to guide site-specific investigations for future developments, and to assist in regional planning and mitigation of existing landslides. DFL

Highway 99E Willamette Drive (Highway -43) LANDSLIDE CLASSIFICATION EFL ES-R + EFL Each landslide shown on this map has been classified according to a number of specific characteristics identified at the time recorded in 4000 4000 the GIS database. The classification scheme was developed by the Oregon Department of Geology and Mineral Industries (Burns and

502 502 Madin, 2009). Several significant landslide characteristics recorded in the database are portrayed with symbology on this map. The DFL EFL ES-R + EFL specific characteristics shown for each landslide are the activity of landsliding, landslide features, deep or shallow failure, type of landslide movement, and confidence of landslide interpretation. These landslide characteristics are determined primarily on the basis of RS-T EFL geomorphic features, or landforms, observed for each landslide. The symbology used to display these characteristics is explained below. EFL EFL EFL LANDSLIDE ACTIVITY: Each landslide has been classified according to the relative age of last movement. This map display uses Holcomb Boulevard color to show the activity. EFL EFL WEST LINN EFL HISTORIC and/or ACTIVE (movement less than 150 years ago): The landslide appears to have moved within EFL EFL historic time or is currently moving (active). DFL EFL EFL EFL PREHISTORIC or ANCIENT (movement greater than 150 years ago): Landslide features are slightly eroded and there is no evidence of historic movement. In some cases, the observed landslide features have been greatly eroded and/or covered with deposits that result in smoothed and subdued morphology.

LANDSLIDE FEATURES: Because of the high resolution of the lidar-derived topographic data, some additional landslide features EFL were identified. These include: EFL DFL HEAD SCARP ZONE and FLANK ZONE(S): The head scarp or uppermost scarp, which in many cases exposes EFL the primary failure plane (surface of rupture), and flanks or shear zones. EFL EFL EFL EFL EFL DFL tte River Redland Road EFL HEAD SCARP LINE and INTERNAL SCARP LINES: Uppermost extent of the head scarp and internal scarps EFL within the body of the landslide. Hatching is in the down-dropped direction. EFL EFL ES-R EFL Willame DFL EFL DEPTH OF FAILURE: The depth of landslide failure was estimated from scarp height. Failures less than 4.5 m (15 ft) deep are EFL EFL classified as shallow seated and failures greater than 4.5 m (15 ft) deep are classified as deep seated. EFL

EFL ES-R + SHALLOW-SEATED LANDSLIDE: Estimated failure plane depth is less than 4.5 m (15 ft). EFL EFL DFL EFL EFL ES-R + DEEP-SEATED LANDSLIDE: Estimated failure plane depth is greater than 4.5 m (15 ft). EFL EFL EFL EFL

EFL

A CONFIDENCE OF INTERPRETATION: Each landslide should be classified according to the confidence that the mapper assigns

EFL b based on the likelihood that the landslide actually exists. Landslides are mapped on the basis of characteristic morphology, and the

EFL e r confidence of the interpretation is based on how clearly visible that morphology is. As a landslide ages, weathering (primarily through

n erosion) degrades the characteristic morphologies produced by landsliding. With time, landslide morphologies may become so subtle

EFL EFL e t that they resemble morphologies produced by geologic processes and conditions unrelated to landsliding. h OREGON CITY EFL y Landslides may have several different types of morphologies associated with them, and we define confidence through a simple point C system (see table below) associated with these features. The point system is based on a ranking of four primary landslide features with r a ranking of 0 to 10 points per feature. For example, if during mapping, the head scarp and toe of a landslide were identifiable and Newell Creek e ES-R + clearly visible, the mapper would apply 10 points for the head scarp and 10 points for the toe, equaling 20 points, which would be EFL EFL e EFL k associated with a moderate confidence of identification. Interstate 205 EFL EFL DFL The visual display of this landslide characteristic is through the use of different line styles as shown below. 7th Street EFL EFL EFL EFL EFL EFL Washington Street EFL Landslide Feature Points EFL DFL HIGH CONFIDENCE (≥30 points) EFL Head scarp 0-10 EFL DFL EFL Flanks 0-10 MODERATE CONFIDENCE (11-29 points) EFL EFL Toe 0-10

Internal scarps, sag ponds, compression ridges, etc. 0-10* EFL ES-T EFL LOW CONFIDENCE (�10 points) EFL EFL EFL EFL *Applied only once so that total points do not exceed 40. RS-R EFL EFL ES-R EFL EFL EFL 2000 EFL EFL 2000 EFL CLASSIFICATION OF MOVEMENT: Each landslide was classified with the type of landslide movement. There are five types of

502 EFL EFL EFL 502 landslide movement: slide, flow, fall, topple, and spread. These movement types are combined with material type to form the landslide EFL classification. Not all combinations are common in nature, and not all are present in this quadrangle. EFL DFL DFL EFL EFL ES-R + EFL EFL RS-R+EFL EFL – Earth Flow – Abbreviation for class of slope movement. The table below displays the types (Varnes, 1978). RS-R + EFL DFL EFL EFL EFL Generalized diagrams (some modified from Highland, 2004) showing types of movement are displayed below the DFL table. EFL EFL DFL ES-R + ES-R + EFL EFL Type of Material ES-R + EFL RS-T RS-R+ Type of EFL EFL Movement EFL EFL ES-T EFL EFL Rock Debris Soil EFL EFL Fall RF rock fall DF debris fall EF earth fall EFL Topple RT rock topple DT debris topple ET earth topple EFL EFL EFL Slide-rotational RS-R rock slide-rotational DS-R debris slide-rotational ES-R earth slide-rotational RS-R + EFL EFL EFL EFL Slide-transitional RS-T rock slide-transitional DS-T debris slide-transitional ES-T earth slide-transitional EFL Lateral spread RSP rock spread DSP debris spread ESP earth spread EFL Flow RFL rock flow DFL debris flow EFL earth flow EFL DFL ES-R + EFL Complex C complex or combinations of two or more types (for example, ES-R + EFL) RS-R + EFL DFL EFL RS-R+ ES-R + DFL EFL EFL Falls are near-vertical, rapid movements of masses of materials, such as rocks or boulders. The rock DFL DFL debris sometimes accumulates as talus at the base of a cliff. EFL DFL RS-R +

EFL DFL Topples are distinguished by forward rotation about some pivotal point, below or low in the mass. Molalla Avenue EFL RS-R+EFL Highway 2 Holly Lane ES-R + RS-R+ ES-R + DFL Slides are downslope movements of soil or rock on a surface of rupture (failure plane or shear zone). EFL EFL EFL ES-R +

EFL 13 EFL RS-R+EFL RS-R+EFL Rotational slides move along a surface of rupture that is curved and concave. EFL

Linn Translational slides displace along a planar or undulating surface of rupture, sliding out over the EFL original ground surface. Avenue DFL C

DFL EFL EFL Spreads are commonly triggered by earthquakes, which can cause liquefaction of an underlying layer and extension and subsidence of commonly cohesive materials overlying liquefied layers. EFL

DFL Initiation Transport EFL DFL RS-R+ RS-R+EFL EFL Channelized Debris Flows commonly start on a steep, concave slope as a small slide or earth flow into a EFL channel. As this mixture of landslide debris and water flows down the channel, the mixture picks up more Warner Milne Road debris, water, and speed, and deposits in a fan at the outlet of the channel. Deposition RS-R+ EFL RS-R+EFL Earth Flows commonly have a characteristic “hourglass” shape. The slope material liquefies and runs out, forming a bowl or depression at the head.

Complex landslides are combinations of two or more types. An example of a common complex landslide is a slump-earth flow, which usually exhibits slump features in the upper region and earth flow features near the toe. 0000 0000 502 502

LIMITATIONS

This landslide inventory was developed with the best available data, using the protocol of Burns and Madin (2009). However there are inherent limitations as discussed below. These limitations underscore that this map is designed for regional applications and should not Beavercreek Road be used as an alternative to site-specific studies in critical areas. 1. Every effort has been made to ensure the accuracy of the GIS and tabular database, but it is not feasible to completely verify all original input data. 2. Burns and Madin (2009) recommend a protocol to develop landslide inventories that is based on four primary tasks: 1) interpretation of lidar-derived topographic data, 2) compilation and review of previously mapped landslides, 3) review of historic air photos, and 4) limited field checking. These tasks can affect the level of detail and accuracy of the landslide inventory. We expect the lidar data quality to improve in the future, which will likely result in the identification of more landslides with greater accuracy and confidence. Due to time limitations some previously mapped landslides have likely been missed. In some locations, historic air photos may not be available. Because field work is time consuming and therefore expensive, field checking may be extensive in some locations and very limited in other locations.

Central Point Road 3. The lidar-based mapping is a “snapshot” view of the current landscape that may change as new information regarding landslides becomes available and as new landslides occur. 4. Because of the resolution of the lidar data and air photos, landslides that are smaller than 100 square meters (1,075 square feet) may not be identified. Some small landslides were included if they were reported by a local governmental agency, a site- specific study, a regional study report, or a local area landslide expert, and are found to be accurately located by the mapper. 5. Even with high-quality lidar-derived topographic data, it is possible that some existing landslides will be missed, overlooked, or misinterpreted by the map author. This database and map were prepared in accordance with a published protocol (Burns and Madin, 2009) and were reviewed to minimize these problems. 6. Earthwork related to development on hillsides can remove the geomorphic expressions of past landsliding. This can result in landslides being missed in the inventory. Earthwork on hillsides can also create geomorphic expressions that mimic past landsliding; for example, a cut and fill can look like a landslide scarp and toe. This limitation can sometimes be addressed by viewing aerial photographs that predate development in the area being mapped. Therefore, to ensure that past landslides have been adequately identified, if a landslide was identified on the predevelopment air photos, it was included in the landslide inventory, whether or not surface expression was located in the lidar-derived mapping. 7. Some landslides have been mitigated. Because it is not feasible to collect detailed site-specific information on every landslide, for example if it has been mitigated and what level of mitigation was implemented, mitigation has been omitted. Again, because of these limitations this map is intended for regional purposes only and cannot replace site-specific investigations. However, the map can serve as a useful tool for estimating the regional landslide hazard and as a starting place for future detailed landslide site-specific maps. Please contact DOGAMI if errors and/or omissions are found so that they can be corrected in future versions of this map. Meyers Road

Leland Road

ACKNOWLEDGMENTS Highway 213 We thank the people at the U.S. Geological Survey Landslide Hazard Program who contributed to the protocol and map template through discussions and suggestions, especially Jeff Coe, who provided a detailed review that significantly improved the protocol used to map this quarter quadrangle. We also thank DOGAMI staff who helped with this project through technical assistance, review, and general support, especially Rob Witter, Yumei Wang, and Deb Schueller.

REFERENCES ES-R RS-R + EFL Burns, W. J., and Madin, I. P., 2009, Protocol for inventory mapping of landslide deposits from light detection and ranging (lidar) RS-R + EFL imagery: Portland, Oreg., Oregon Department of Geology and Mineral Industries Special Paper 42, 30 p. Burns, W. J., Madin, I. P., and Ma, L., 2008, Statewide landslide information layer for Oregon (SLIDO), release 1: Portland, Oreg., Oregon Department of Geology and Mineral Industries Digital Data Series SLIDO-1, 45 p., 1 pl., scale 1:500,000. Highland, L., compiler, 2004, Landslide types and processes, U.S. Geological Survey Fact Sheet 2004-3072 (ver. 1.1), 4 p. Hofmeister, R. J., 2000, Slope failures in Oregon: GIS inventory for three 1996/97 storm events: Portland, Oreg., Oregon Department of Geology and Mineral Industries Special Paper 34, 20 p. Varnes, D. J., 1978, Slope movement types and processes, in Schuster, R. L., and Krizek, R. J., eds., Landslides—Analysis and control: RS-R + EFL Washington, D. C., Transportation Research Board Special Report 176, p. 11–33. RS-R + EFL 8000 RS-R + EFL 8000 RS-R + 501 501 EFL LOCATION MAP RS-R +

EFL

Æ ¤£30 Ä Washougal ¨¦§5 EFL 213 RS-R + Hillsboro Linnton Mount Tabor RS-R + EFL ¤£30BY Camas ¨¦§84

EFL 45°18'45"N Portland

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122°37'30"W 530000 532000 534000 122°33'45"W 8 Ä

Æ Ä Multnomah

Æ 217 Ä

Washington 224 ¤£26

Lake Oswego Gladstone Damascus Sandy Æ

Scholls Beaverton Ä Ä Æ

Ä Æ Æ

Cartography by William J. Burns, Oregon Department of Geology and Mineral Industries 210 43 Ä 213 212

Outside agency review by Nancy Kraushaar, City of Oregon City 99W Ä

Æ Base Map: SCALE 1:8,000 Ä 16.5 ° 224 ¨¦§205

Lidar-derived elevation data are from The City of Oregon City, 2005. Digital elevation

5 Estacada Æ ¨¦§ Ä

model (DEM) consists of a 3-foot square elevation grid that was converted into a 1,300 650 0 1,300 2,600 3,900 5,200 Canby Redland

Newberg Sherwood Oregon City 211

Ä Æ Æ

hillshade image with sun angle at 315 degrees at a 45 degree angle from horizontal. Feet Ä Æ Yamhill 551 Ä For copies of this publication contact:

The DEM is multiplied by 5 (vertical exaggeration) to enhance slope areas. IMPORTANT NOTICE 219

Clackamas 224 Æ

Ä Nature of the Northwest Information Center

0.25 0.125 0 0.25 0.5 0.75 1 551 Æ Orthophoto is from Oregon Geospatial Enterprise Office, 2005 and consists of 2005 This map depicts an inventory of existing landslides based on Ä 800 NE Oregon Street, #5, Ste. 177 213 TRUENORTH Portland, Oregon 97232 orthophoto draped over DEM with transparency. Miles published and unpublished reports and interpretation of

Marion telephone (503) 872-2750

MAGNETIC NORTH

topography derived from lidar data and air photos. The inventory Molalla Colton Elwood Æ

OREGON SaintÄ Paul Woodburn Yoder http://www.naturenw.org

Æ Projection: North American Datum 1983, UTM zone 10 north. was created following the protocol defined by Burns and Madin 219 Ä Æ APPROXIMATE MEAN Ä 0.25 0.125 0 0.25 0.5 0.75 1 211 DECLINATION, 2006 (2009). This map cannot serve as a substitute for site-specific 99E Software: ESRI ArcMap 9.3, Adobe Illustrator CS2. Kilometers investigations by qualified practitioners. Site-specific data may give results that differ from those shown on this map. U.S. Geological Survey 7.5-minute quadrangles and counties are Source File: Rocks\Publications\IMS-26\IMS-26.mxd. labeled. This map extent shown as gray rectangle.

Figure 5. (a) Example of a DOGAMI landslide inventory map for the Oregon City quadrangle (DOGAMI IMS-26, actual map plate dimensions are 36 × 42 inches [Burns and Madin, 2009b]). The template includes explanation of the symbology used on the map including landslide activity, features, depth of failure, confidence of interpretation, and classification of movement. (b) An area in the northern portion of the map with many shallow landslides. (c) An area in the central portion of the map with generally large deep landslides.

6 Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

Table 1. Example landslide inventory geodatabase table* (Burns and Madin, 2009b).

OBJECTID SHAPE_Leng SHAPE_Area UNIQUE_ID TYPE_MOVE MOVE_CLASS MOVE_CODE CONFIDENCE AGE 976 665.28829393800 14955.92269280000 Oregon_City_17 Flow Debris Flow DFL Moderate (11-29) Historic (<150yrs) 977 368.74027446700 5455.59656431000 Oregon_City_170 Flow Debris Flow DFL High (=>30) Historic (<150yrs) 978 542.66829747200 10992.82005570000 Oregon_City_171 Flow Debris Flow DFL High (=>30) Historic (<150yrs) 979 725.48317371700 15815.09065630000 Oregon_City_172 Flow Debris Flow DFL High (=>30) Historic (<150yrs) 980 635.83947679300 25833.35565330000 Oregon_City_173 Flow Earth Flow EFL High (=>30) Historic (<150yrs) 981 1295.76075306000 78588.00239690000 Oregon_City_174 Flow Earth Flow EFL High (=>30) Historic (<150yrs) 982 2745.38388612000 385769.04703800000 Oregon_City_175 Slide Complex RS-R+EFL High (=>30) Pre-Historic (>150yrs) 983 1476.62269809000 144910.93313100000 Oregon_City_176 Slide Complex-Earth Slide- ES-R+EFL Moderate (11-29) Pre-Historic (>150yrs) Rotational+Earth Flow 984 671.66180282100 29819.61018390000 Oregon_City_177 Flow Earth Flow EFL Moderate (11-29) Pre-Historic (>150yrs) 985 2067.76691857000 240197.09559000000 Oregon_City_178 Slide Complex-Earth Slide- ES-R+EFL High (=>30) Historic (<150yrs) Rotational+Earth Flow 986 6363.46488433000 1636933.77794000000 Oregon_City_179 Slide Complex RS-R+EFL Moderate (11-29) Pre-Historic (>150yrs) 987 441.31665802900 8700.82205880000 Oregon_City_18 Flow Debris Flow DFL Moderate (11-29) Historic (<150yrs) 988 605.04255044600 22854.23258460000 Oregon_City_181 Flow Earth Flow EFL Moderate (11-29) Historic (<150yrs) 989 1128.71921665000 40219.89008570000 Oregon_City_182 Flow Debris Flow DFL High (=>30) Historic (<150yrs) 990 4537.60120344000 966447.88196900000 Oregon_City_183 Slide Complex RS-R+EFL High (=>30) Pre-Historic (>150yrs) 991 3665.52714980000 625881.42477300000 Oregon_City_184 Slide Complex RS-R+EFL High (=>30) Historic (<150yrs) 992 3643.05658106000 713317.28781100000 Oregon_City_185 Slide Complex RS-R+EFL High (=>30) Pre-Historic (>150yrs) 993 465.52123138400 9138.63555378000 Oregon_City_186 Flow Debris Flow DFL High (=>30) Historic (<150yrs) 994 764.21787525400 23121.38473620000 Oregon_City_187 Flow Earth Flow EFL High (=>30) Historic (<150yrs) 995 743.76388204600 24203.91041030000 Oregon_City_188 Flow Earth Flow EFL High (=>30) Historic (<150yrs) 996DATE_MOVE686.49151977800NAME13266.47853570000GEOL Oregon_City_189SLOPE HS_HEIGHTFlow DebrisFAN_HEIGHT Flow FAIL_DEPTHDFL DEEP_SHALHigh (=>30) HS_IS1Historic (<150yrs) IS1_IS2 997 332.15409540200 4132.58629138000Tgsb 10.00000000000Oregon_City_19 0.00000000000Flow Debris29.00000000000 Flow 0.00000000000DFL Moderate (11-29)0.00000000000Historic (<150yrs)0.00000000000 998 461.59889696300 8481.95429193000Tt 10.00000000000Oregon_City_190 0.00000000000Flow Debris8.00000000000 Flow 0.00000000000DFL High (=>30) 0.00000000000Historic (<150yrs)0.00000000000 1127 2036.46268954000 262121.41459900000Tt 10.00000000000Oregon_City_180 0.00000000000Slide Complex-Earth14.00000000000 Slide- 0.00000000000ES-R+EFL Moderate (11-29)0.00000000000Pre-Historic (>150yrs)0.00000000000 Rotational+Earth Flow Tt 10.00000000000 0.00000000000 13.00000000000 0.00000000000 0.00000000000 0.00000000000 Tt 22.00000000000 15.00000000000 0.00000000000 13.90890000000 Shallow 0.00000000000 0.00000000000 Tt 22.00000000000 17.00000000000 0.00000000000 15.76340000000 Deep 0.00000000000 0.00000000000 Mountain View Cemetary Tt 20.00000000000 75.00000000000 0.00000000000 70.48170000000 Deep 110.00000000000 125.00000000000 Landslide Tt 20.00000000000 30.00000000000 0.00000000000 28.19270000000 Deep 80.00000000000 0.00000000000 Tt 20.00000000000 17.00000000000 0.00000000000 15.97590000000 Deep 0.00000000000 0.00000000000 Highway 213 Landslide Tt 20.00000000000 45.00000000000 0.00000000000 42.28900000000 Deep 0.00000000000 0.00000000000 Highway 213 Landslide Tt 20.00000000000 25.00000000000 0.00000000000 23.49390000000 Deep 120.00000000000 0.00000000000 Tgsb 10.00000000000 0.00000000000 5.00000000000 0.00000000000 0.00000000000 0.00000000000 Highway 213 Landslide Tt 22.00000000000 14.00000000000 0.00000000000 12.98160000000 Shallow 0.00000000000 0.00000000000 Tt 10.00000000000 0.00000000000 10.00000000000 0.00000000000 0.00000000000 0.00000000000 Tt 20.00000000000 65.00000000000 0.00000000000 61.08420000000 Deep 80.00000000000 80.00000000000 12/1/2005 Newell Creek Apartments Tt 20.00000000000 80.00000000000 0.00000000000 75.18050000000 Deep 250.00000000000 0.00000000000 Landslide Tt 20.00000000000 40.00000000000 0.00000000000 37.59020000000 Deep 250.00000000000 175.00000000000 IS2_IS3 Tt IS3_IS410.00000000000 HD_AVE0.00000000000 DIRECT10.00000000000 0.00000000000AREA VOL 0.00000000000QUADNAME0.00000000000Text 0.00000000000 Tt0.0000000000030.000000000000.0000000000015.0000000000045.000000000000.00000000000 14955.9000000000012.99250000000 Shallow144573.000000000000.00000000000Oregon_City0.00000000000 0.00000000000 Tt0.0000000000030.000000000000.0000000000020.0000000000090.000000000000.00000000000 5455.5700000000017.32330000000 Deep14548.200000000000.00000000000Oregon_City0.00000000000 0.00000000000 0.00000000000 0.00000000000 90.00000000000 10992.80000000000 51299.60000000000 Oregon_City Tt 10.00000000000 0.00000000000 8.00000000000 0.00000000000 0.00000000000 0.00000000000 0.00000000000 0.00000000000 0.00000000000 292.50000000000 15815.00000000000 68531.80000000000 Oregon_City 0.00000000000 0.00000000000 Qff0.0000000000015.000000000000.000000000000.0000000000090.000000000006.00000000000 25833.300000000000.00000000000 359312.000000000000.00000000000Oregon_City 0.00000000000 Tt0.0000000000010.000000000000.000000000000.00000000000112.500000000009.00000000000 78587.700000000000.00000000000 1238810.000000000000.00000000000Oregon_City0.00000000000 Highway 213 Landslide0.00000000000 Tt0.0000000000020.00000000000118.0000000000015.0000000000090.000000000000.00000000000 385768.0000000000014.09630000000 Shallow27189600.00000000000150.00000000000Oregon_City0.00000000000 0.00000000000 0.00000000000 80.00000000000 360.00000000000 144910.00000000000 4085410.00000000000 Oregon_City 0.00000000000 0.00000000000 0.00000000000 67.50000000000 29819.50000000000 476392.00000000000 Oregon_City 0.00000000000 0.00000000000 0.00000000000 247.50000000000 240196.00000000000 10157700.00000000000 Oregon_City 0.00000000000 0.00000000000 120.00000000000 225.00000000000 1111760.00000000000 26119500.00000000000 Oregon_City 0.00000000000 0.00000000000 0.00000000000 45.00000000000 8700.79000000000 14501.30000000000 Oregon_City 0.00000000000 0.00000000000 0.00000000000 360.00000000000 22854.10000000000 296684.00000000000 Oregon_City 0.00000000000 0.00000000000 0.00000000000 292.50000000000 40219.70000000000 134066.00000000000 Oregon_City 125.00000000000 0.00000000000 95.00000000000 315.00000000000 966444.00000000000 59034400.00000000000 Oregon_City 0.00000000000 0.00000000000 250.00000000000 90.00000000000 625879.00000000000 47053900.00000000000 Oregon_City 0.00000000000 0.00000000000 212.00000000000 315.00000000000 713314.00000000000 26813700.00000000000 Oregon_City 0.00000000000 0.00000000000 0.00000000000 337.50000000000 9138.60000000000 30462.00000000000 Oregon_City 0.00000000000 0.00000000000 0.00000000000 67.50000000000 23121.30000000000 300403.00000000000 Oregon_City 0.00000000000 0.00000000000 0.00000000000 337.50000000000 24203.80000000000 419290.00000000000 Oregon_City 0.00000000000 0.00000000000 0.00000000000 292.50000000000 13266.40000000000 35377.10000000000 Oregon_City 0.00000000000 0.00000000000 0.00000000000 45.00000000000 4132.57000000000 8265.14000000000 Oregon_City 0.00000000000 0.00000000000 0.00000000000 90.00000000000 8481.92000000000 25445.80000000000 Oregon_City 0.00000000000 0.00000000000 150.00000000000 225.00000000000 262120.00000000000 3694940.00000000000 Oregon_City

*This table has been split into three sections to show all the fields.

Oregon Department of Geology and Mineral Industries Special Paper 45 7 Protocol for Shallow-Landslide Susceptibility Mapping

3.3 Calculation of the Factor of Safety A FOS > 1 would theoretically be a stable slope

Extract landslide Deal with Factor Apply buffers to Combine con- because the shear resistance (or strength) would be inventory data of Safety (FOS) landslide data tributing factors greater than the shear stress. A FOS < 1 would theoreti- and FOS to create map cally be an unstable slope because the stress would be  Step 1: calculate greater than the shear strength. A critically stable slope Inventory Zone FOS map would have a FOS = 1. Because it is impossible to know all conditions present within a slope, most geotechni- The mechanics of slope stability can be divided into cal engineers and engineering geologists recommend two forces: driving forces and resisting forces. These that slopes with a FOS < 1.5 be considered potentially two forces oppose each other, and the state of stability unstable (Turner and Schuster, 1996; Cornforth, 2005). (limit-equilibrium analyses) existing in a slope can be Furthermore, in the State of Oregon Building Code thought of as their ratio: (Oregon Residential Specialty Code, R404.5, 2008) a resisting forces FOS of 1.5 is commonly required for design of slopes and slope structures such as retaining walls. driving forces Software packages commonly used in GIS analyses When the material properties and geometry of a to estimate the stability of slopes represented in data slope are examined, this simplified ratio becomes an grids by calculating the FOS of each grid cell include equation called the Factor of Safety (FOS) against land- SINMAP (Stability Index MAPping; Pack and others, sliding and is defined as (Cornforth, 2005): 1999), SHALSTAB (Montgomery and Dietrich, 1994), total available shear resistance LISA (Level I Stability Analysis; Denning, 1994), TRIGRS (Transient Rainfall Infiltration and Grid- Factor of Safety (FOS) = shear force needed for static equilibrium Based Regional Slope-Stability Analysis; Baum and others, 2002). These programs calculate slope stability using an infinite slope equation analysis (Harp and others, 2006). For this protocol, we also use the infinite slope FOS equation, as illustrated in Figure 6, to calcu-

α Ground Material Properties y c’ = Cohesion (effective) Surface φ ‘ = Angle of Internal Friction (effective) x γ = Soil Density (unit weight) γ t w = Groundwater Density (unit weight) Other Variables t = Depth to Failure Surface h m = Groundwater Depth Ratio α = Slope Angle (degrees) x = Horizontal Grid Distance (on DEM) y = Vertical Grid Distance (on DEM) Failure Surface

c’ tanφ’ m γw tanφ’ Factor of Safety (FOS) = + - γ t sinα tanα γ tanα

Figure 6. Infinite slope analysis: diagram shows parameters for one digital elevation model (DEM) grid cell and the Factor of Safety (FOS) equation (Harp and others, 2006).

8 Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

late the FOS for every grid cell in a map area (Harp and 3.3.1 Geology—Geotechnical Material Properties others, 2006). Because the use of the infinite slope equation for The geotechnical material properties needed for the regional stability analysis is limited to a grid type analy- infinite slope analysis are cohesion, angle of internal sis, the results are a calculated FOS for each individual friction, and soil density. Because regional maps of grid cell. This type of analysis does not consider the these properties are not available, we substitute the potential impact of adjacent slopes or three-dimension- best available digital geologic map (e.g., Figure 7) and al effects. Therefore, a conservative approach that tends link the geologic units to material properties by either to underestimate the FOS is used in most steps to cal- collecting local measurements if available or consulting culate the FOS. The limitations and results of this type the literature for data from similar units. Because the of approach are discussed in section 5 of this report. material properties can vary within a particular geolog- In order to calculate the FOS throughout any area, ic unit we choose values that are conservative, that is, several datasets are necessary and are discussed in values near or at the low end of the range that result in detail in sections 3.3.1–3.3.4: lower FOS values to account for some of this variability. • Geology—geotechnical material properties • Groundwater height above failure surface • Depth to failure surface • Slope angle

Qls (landslides) Qal (recent alluvium) QTb (basalt) Tt (mudstone) Tcr (basalt)

Meters 0 250 500 1,000 Feet 0 900 1,800 3,600

Figure 7. Example of a geologic-material properties map. Map is of a portion of the Oregon City quadrangle. Each geologic unit on the map has associated material properties shown in Table 2 (modified from Madin, 2009).

Oregon Department of Geology and Mineral Industries Special Paper 45 9 Protocol for Shallow-Landslide Susceptibility Mapping

Because site-specific material properties are not 3.3.2 Groundwater Height above Failure Surface available throughout the state, a table of general values related to different types of common geologic forma- The height of the groundwater table above the failure tions in Oregon is provided (Table 2). In areas where surface varies widely from place to place and from more detailed or more accurate material properties are season to season. We choose the most conservative available, those values should supersede the general case of complete saturation, in which the groundwater values given in Table 2. table is at the ground surface, and h in the FOS equation We convert the vector-based digital geologic map for is equal to z* (Figure 6). In areas where more detailed the study area to a raster, with numeric values that code or more accurate groundwater data are available, those for each unit (e.g., the raster geologic unit codes [Geol- values should supersede the conservative complete sat- Code field] for the area in Figure 7 would have geologic uration value used in this protocol. unit Qls = 1, Qal = 3, QTb = 6, Tt = 7, and Tcr = 6 so that we can perform GIS raster calculations with each geologic unit as a variable for preparing the FOS map *. *In this paper, depth to failure surface is denoted by z (section 3.4). rather than by the t of Harp and others (2006).

Table 2. General soil and rock material properties (Harp and others, 2006; Cornforth, 2005; Denning, 1994). Common Angle of Unit Weight Common Unit or Common Raster Internal Cohesion (c ) (saturated) Lithology Formation Unit Value Friction (φ) Slope Slope Description Name Label GeolCode (degrees) (kPa) (lb/ft2) (kN/m3) (lb/ft3) FOS>1.5 FOS>1.25 Sheared landslide Qls — 10 0 0 19 122 3.0 4.0 landslide debris (silts, clays, sands) Shearing landslide, Qls, Qc 1 28 0 0 19 122 9.5 11.5 mainly along colluvium deep failure plane Sand, silt, artificial fill Fill, Qf 2 30 0 0 19 122 10.5 12.5 gravel, debris mixtures Silt, sand Quaternary Qal, Qff, 3 30 0 0 19 122 10.5 12.5 alluvium, Ql loess Sand, gravel, Quaternary Qal, Qcf 4 34 0 0 19 122 12.0 14.5 boulders alluvium, gravel fan Sand, silt, glacial till Qva, Qt 5 34 10 209 19 122 16.5 19.5 clay, gravel Silty clay Columbia Tcr 6 28 24 501 19 122 20.0 24.0 with River Basalt boulders Silty sand, Troutdale Tt 7 30 10 209 19 122 14.5 17.5 sandy silt, Formation silty gravel

10 Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

3.3.3 Depth to Failure Surface

We have defined shallow landslides as having failure Table 3. Example Factor of Safety (FOS) values planes that are ≤ 15 ft (4.6 m) deep. Several lidar-based calculated for various slopes and values of z, using parameters for residual soil on basalt from Table 2. landslide inventories have been created over the past several years for Oregon including the City of Silver- Depth to Failure Factor of ton (Burns and Mickelson, 2012), City of Oregon City, Slope Plane (z) Safety (FOS) and the City of Astoria areas (Burns and Mickelson, 24 5 2.8 2010a,b). The shallow landslides mapped in each of 24 10 1.7 these three studies have these mean depth to failure 24 15 1.3 plane values: Silverton = 7.9 ft (2.4 m), Oregon City = 29 5 2.4 10.0 ft (3 m), and Astoria = 10.3 ft (10.1 m). Also, the 29 10 1.4 shallow landslide dataset (LS-1) used in this study has a 29 15 1.1 mean of 9.1 ft (2.8 m). In cases where the material has cohesion, using 15 ft (4.6 m) for the z value (depth to failure plane) in the FOS equation results in the most conservative values, as illustrated in Table 3. Because we have chosen to make h = z, in cases where the soil has no cohesion the depth to failure plane term is not a factor in the equation (Cornforth, 2005). Therefore we use 15 ft (4.6 m) for both z and h in all cases.

3.3.4 Slope Angle

This protocol requires the use of a high-resolution lidar-derived bare-earth DEM to create a map of slope Slope Angle (degrees) angles (in degrees) for each grid cell (Figure 8). We use 0 - 10 the Slope tool from the Esri 3D Analyst or Spatial Ana- 10 - 20 lyst extension with default settings to create the slope 20 - 30 map for analysis. 30 - 40 40 + Extract landslide Deal with Factor Apply buffers to Combine con- inventory data of Safety (FOS) landslide data tributing factors Meters and FOS to create map 0 250 500 1,000 Feet  FOS calculated Inventory Zone 0 900 1,800 3,600 map Figure 8. Example of a slope map created from the lidar-derived bare earth digital elevation model (DEM). Map is of a portion of the Oregon City quadrangle. For display, slope angles have been grouped into five slope categories.

Oregon Department of Geology and Mineral Industries Special Paper 45 11 Protocol for Shallow-Landslide Susceptibility Mapping

Table 4. Sample Factor of Safety (FOS) calculations from 3.4 Creation of the Factor of spreadsheet for geologic unit landslide deposits (GeolCode Safety (FOS) Class Map Qls =1) of Table 2. Threshold values marked in yellow.

Extract landslide Deal with Factor Apply buffers to Combine con- Slope Factor of inventory data of Safety (FOS) landslide data tributing factors and FOS to create map (Degrees) Safety (FOS) 1.0 14.48  Step 2: create FOS 1.5 9.65 Inventory Zone class map map 2.0 7.24 2.5 5.79 3.0 4.82 Our approach to creating a GIS-based FOS class map 3.5 4.13 using the parameters described in section 3.3 involves 4.0 3.61 carrying out the FOS calculations in a spreadsheet to 4.5 3.21 5.0 2.89 Low FOS class determine threshold slope values for each geologic 5.5 2.63 (FOS >1.5, slope <9.5°) unit. These threshold slope values are then used to 6.0 2.41 define high, moderate, and low FOS classes (Table 4). 6.5 2.22 7.0 2.06 For each geologic unit we query the slope map for the 7.5 1.92 values that correspond to each class, then reclassify 8.0 1.8 and mosaic together all the query rasters to produce 8.5 1.69 the complete map. 9.0 1.60 9.5 1.51 We use a spreadsheet (provided as Appendix A) to 10.0 1.43 Moderate FOS class calculate the FOS for each geologic unit in the map area 10.5 1.36 (FOS 1.25–1.5, slope by using the appropriate material property values and 11.0 1.3 9.5°–11.5°) slope angles in increments of 0.5 degrees. 11.5 1.24 12.0 1.19 From the calculated FOS values in the spreadsheet, 12.5 1.14 we pick the slope angles that produce FOS values clos- 13.0 1.09 High FOS class est to 1.5 and 1.25. A FOS value of 1.5 is considered 13.5 1.05 (FOS <1.25, slope >11.5°) 14.0 1.01 by the geologic and engineering communities as stable 14.5 0.98 and a FOS value of 1.25 is considered moderately stable. These are the threshold slope angles for that particular geologic unit and are used to build GIS queries to pro- this process for each geologic unit in the area. To com- duce the FOS class map. In order to account for vari- pile all of the geology/critical slope value queries into ability in geotechnical parameters, we group the FOS a map, we first reclassify (by using the Esri Spatial in classes. We use the raster calculator function in the Analyst Reclassify tool, Figure 9) the individual FOS Esri Spatial Analyst toolbox to build two queries for class rasters following the scheme in Table 5, so that each geologic unit in the map area. One query is for all each raster now represents an FOS-based susceptibil- areas with slopes between the threshold values (Mod- ity zone. We then combine all of the reclassified FOS erate class), and the other query is for slopes above the rasters (using the Esri Spatial Analyst Mosaic To New high (FOS = 1.25) threshold value (High class). So, for Raster tool, Figure 10) to produce the final OSF class example, to select the appropriate values to go with map, a single grid divided into areas of high suscepti- the calculated values displayed in Table 4 (we used the bility = 3, moderate susceptibility = 2, and low suscep- GeolCode 1 for unit Qls), we build the following que- tibility = NoData. ries to apply to the geology and slope rasters: Extract landslide Deal with Factor Apply buffers to Combine con- Query 1 (Moderate class): GeolCode = 1 AND Slope >= 9.5 AND inventory data of Safety (FOS) landslide data tributing factors Slope <= 11.5 and FOS to create map  FOS class map created Query 2 (High class): GeolCode = 1 AND Slope > 11.5 Inventory Zone map

The resulting rasters have values of 1 where the condition is true and 0 where it is false. We then repeat

12 Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

Figure 9. Example of dialog box for the Esri Spatial Analyst Reclassify tool.

Table 5. Reclassification scheme to convert individual unit Factor of Safety (FOS) rasters to susceptibility zones. FOS Old New Susceptibility ≤ 1.25 0 NoData — ≤ 1.25 1 3 High 1.25 – 1.5 0 NoData — 1.25 – 1.5 1 2 Moderate

Figure 10. Example of dialog box for the Esri Spatial Analyst Mosaic To New Raster tool used to create the raw susceptibility zone raster. Cell type can be changed to 2_Bit as there are only two values (2 and 3).

Oregon Department of Geology and Mineral Industries Special Paper 45 13 Protocol for Shallow-Landslide Susceptibility Mapping

3.5 FOS Class Map Filtering and Clipping slope angles and can be extensive in many developed

Extract landslide Deal with Factor Apply buffers to Combine con- areas, particularly when the buffering steps (described inventory data of Safety (FOS) landslide data tributing factors in section 3.6) expand these falsely classified hazard and FOS to create map zones over significant parts of the map.  Step 3. filter and Inventory Zone clip FOS class map map 3.5.2 Methods Tested

3.5.1 Need for Filter We examined eight GIS approaches to reduce the overprediction of susceptibility introduced by the FOS cal- The bare-earth lidar DEMs available for Oregon typi- culation. We selected a study area including flat agri- cally have a raster cell size of 3 ft2 (1 m2). When the FOS cultural areas and steep slope areas near Silverton, class map (section 3.4) is prepared using a slope map Oregon, for which we had complete lidar coverage and with such high resolution, many areas with shallow- a complete SP-42 landslide inventory. The goal was to landslide susceptibility are falsely classified as having remove areas of high or moderate susceptibility associ- moderate or high susceptibility. This occurs because ated with low relief steep slopes without reducing our many fine-scale topographic features are represented success in identifying areas with significant shallow- in the lidar DEM that do not have sufficient vertical landslide susceptibility. The methods we examined or lateral extent to pose a significant shallow landslide included: hazard. The lidar DEM resolves features like ditches, • Native grid—FOS class map made with native small retaining walls, road cuts, and other steep slopes 3 ft2 (1 m2) DEM converted to native 3 ft2 (1 m2) having low vertical relief (Figure 11) that are common slope grid (Figure 12A). in developed landscapes. These features return low FOS • Resized slope grid—FOS class map made with values (high susceptibility class) because of their steep native 3 ft2 (1 m2) DEM converted to native 3 ft2 (1

Figure 11. Photo looking down a residential street in east Portland that displays low vertical relief but steep slopes. Elevation change of slope is roughly 3 feet (1 m). Photo location is shown in Figure 14 and Figure 15 (green dot).

14 Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

Figure 12. Sample from the Silverton study area used to test alternative methods for reducing the impact of low steep slopes on susceptibility zones. For comparison, note areas with potential for overgeneralization (small ovals) and areas with low-relief steep slopes that should not be included (large ovals).

Oregon Department of Geology and Mineral Industries Special Paper 45 15 Protocol for Shallow-Landslide Susceptibility Mapping

m2) slope grid. This slope grid was then resampled Each of the eight resulting susceptibility maps (A–H, (grid sized) to 15 ft2 (4.6 m2) cells (Figure 12B). Figure 12) was visually evaluated for its success at cap- • Resized DEM grid—FOS class map made with turing the landslides in the inventory with moderate native 3 ft2 (1 m2) DEM resampled to 15 ft2 (4.6 and high hazard zones and its ability to remove region- m2). This resampled DEM was then converted to ally flat areas from the moderate and high hazard zones a 15 ft2 (4.6 m2) slope grid (Figure 12C). thereby reducing those areas incorrectly identified as • Smoothed DEM grid coarse—FOS class map susceptible. made with native 3 ft2 (1 m2) DEM that was Thefocal relief approach (Figure 12H) was judged to smoothed with Spatial Analyst Focal Statistics produce the best result because it substantially reduces tool (neighborhood analysis; Figure 13A) by the number of moderate and high susceptibility classes applying the mean value of a 60 ft2 (18 m2) neigh- in flat areas, such as plowed fields, while capturing vir- borhood. This smoothed DEM was then convert- tually all of the mapped landslides (compare Figure 12A ed to a slope grid (Figure 12D). and Figure 12H). This method works through analysis • Smoothed DEM grid medium—FOS class of the neighborhood surrounding a single raster cell map made with native 3 ft2 (1 m2) DEM that (Figure 13A). The range of elevation change (relief) is was smoothed with Spatial Analyst Focal Statis- calculated for the entire neighborhood and that range tics tool (neighborhood analysis; Figure 13A) by value is assigned to the cell. Then the next cell is exam- applying the mean value of a 30 ft2 (9 m2) neigh- ined and so on. The two ideal results are: borhood. This smoothed DEM was then convert- 1. The entire neighborhood is flat with no elevation ed to a slope grid (Figure 12E). change (Figure 13B) and a low focal range value • Smoothed DEM grid fine—FOS class map made and thus unlikely to be susceptible to shallow- with native 3 ft2 (1 m2) DEM that was smoothed seated landslides. with Spatial Analyst Focal Statistics tool (neigh- 2. The neighborhood is nearly all flat, except the ele- borhood analysis; Figure 13A) by applying the vation change which takes place between 2 adja- mean value of a 15 ft2 (4.6 m2) neighborhood. This cent raster cells in a single step (Figure 13D). Low smoothed DEM was then converted to a slope values of focal range will have low susceptibility grid (Figure 12F). because the total height of the slope is not suf- • Smoothed slope grid—FOS class map made with ficient to generate a significant landslide. 3 ft2 (1 m2) slope grid derived from native 3 ft2 (1 m2) DEM and then filtered (smoothed) with Spa- However, another possibility is that the entire neigh- tial Analyst Focal Statistics tool (neighborhood borhood is continuously sloping (Figure 13C) and the analysis; Figure 13A) using the mean value of a 15 elevation change is equal to the focal range. Larger ft2 (4.6 m2) neighborhood (Figure 12G). values of focal range correspond to steeper slopes (see • Focal relief—FOS class map made with native Table 6), which are more susceptible to shallow land- 3 ft2 (1 m2) DEM converted to native 3 ft2 (1 m2) slides. slope grid. Portions of the FOS class map were then clipped (removed) with the focal relief grid. Table 6. Maximum slope values corresponding to different focal relief range values. The focal relief grid was derived by filtering the native 3 ft2 (1 m2) DEM with Spatial Analyst Focal Focal Relief Resultant Maximum Statistics tool (neighborhood analysis, Figure Range Values Slope Angle (Degrees) 13A) using the range of values in a 15 ft2 (4.6 m2) 3 ft (1.0 m) 11.5 neighborhood. Areas with a range of values less 4 ft (1.2 m) 15 than 5 ft (1.5 m) (in other words, less than 5 ft of 5 ft (1.5 m) 18.5 vertical relief across the 15 ft2 neighborhood) were 6 ft (1.8 m) 22 clipped from the FOS class map (Figure 12H).

16 Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

A. Neigborhood urban area (Figure 14). In order to examine how well 4.5m (15ft) each of the focal relief values performed, we ran a parametric spatial statistical analysis with three landslide databases. The first database (LS-1) consists of 1,517

4.5m shallow landslides identified spatially by polygons (15ft) (Burns, 2009a,b; Madin, 2009; Madin and others, 2008; Madin and Niewendorp, 2008; DOGAMI unpublished data) and created following the protocol outlined in SP-42 (Burns and Madin, 2009a). This database was examined in two ways: 1) as an aggregated area of all B. Ideal result #1: No elevation change the landslide polygons, and 2) as individual landslide 0ft 0ft polygons. The second landslide database (LS-2) consists of 649 shallow landslides that occurred during severe storms C. Ideal result #2: Single step in 1996-1997 and are identified spatially as points 4ft (Burns and others, 1998); 309 of these point records 0ft were converted to polygons (Drazba, 2008) using lidar DEMs and the original field notes from Burns and

D. Continuous slope others (1998). 4ft The third database (LS-3) consists of the pre-failure slope angles measured at each of the 1,517 shallow land- 0ft slide sites in the LS-1 database discussed above. Each slope measurement was taken adjacent to the landslide; Figure 13. (A) Focal relief analysis on a cell (red) using a 15 ft2 (4.5 this slope represents the likely slope angle before each 2 m ) neighborhood. Ideal results for various topography are (B) no landslide failed and changed the slope. elevation change and (C) single step. Another possibility is (D) continuous slope. To test the effects of the various focal ranges, we created a test FOS class map of the study area and applied buffers (described in section 3.6) to produce the raw FOS class map shown in Figure 15. Areas of high (red) Clipping areas out of the FOS class map that have and moderate (orange) susceptibility cover much of the low values of focal relief range therefore removes (1) map, not only in the steep, landslide-prone Portland flat areas, (2) areas with continuous but gentle slopes, Hills that traverse the study area to the northwest but or (3) areas with steep slopes but low relief (i.e., a near- also in flat residential neighborhoods (see inset, Figure vertical step). The third case targets exactly the kind of 15) where there is little significant hazard. Although features that we intend to reduce, whereas the first case this map is very effective at capturing identified land- has no impact and the second case impacts the FOS slide sites from all three databases (Table 6), it does so class map only if the focal range value is large, corre- at the cost of including 68% of the study area—includ- sponding to steeper slopes. Therefore it is important ing obviously flat areas—in the high and moderate to select the correct value of focal range to use as the hazard zones. threshold for clipping the FOS class map. We then prepared four FOS class maps by clipping the raw FOS class map using the focal range raster and 3.5.3 Selecting Appropriate Focal range values of 3, 4, 5, and 6 ft (approximately 1, 1.2, 1.5, Relief and Focal Range Values and 2 m), then applying buffers the same way as for the unclipped map. We compared how well each of these To test the effects of various values of focal relief, we maps captured the sites from the three landslide data- selected an area where we had multiple landslide inven- bases, as well as the sacrifice in terms of the amount of tory databases and abundant lidar data—roughly 370 the map area included in high and moderate zones. The mi2 (1,000 km2) centered on the Portland, Oregon, results of the analysis are summarized in Table 7.

Oregon Department of Geology and Mineral Industries Special Paper 45 17 Protocol for Shallow-Landslide Susceptibility Mapping

Figure 14. Map of the area used to test focal range values. The three landslide databases (LS-1, LS-2, and LS-3) are displayed on the map.

18 Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

Figure 15. Raw Factor of Safety (FOS) zone map (red = high, orange = moderate, gray = low) for the test area. Black outlines are mapped landslides developed following SP-42 (Burns and Madin, 2009a). Green dot in inset is the Figure 11 photo location.

Oregon Department of Geology and Mineral Industries Special Paper 45 19 Protocol for Shallow-Landslide Susceptibility Mapping

Table 7. Results of parametric spatial statistical analysis of the comparing raw and clipped focal relief maps to the landslide databases. Raw Factor Focal Relief, % of Safety (FOS), % 3 ft (0.9 m) 4 ft (1.2 m) 5 ft (1.5 m) 6 ft (1.8 m) Percent of study area in moderate or high zone 68 52 42 34 28 (includes overprediction of impact of low steep slopes on susceptibility zones) Lidar based landslide inventory (LS-1) Percent of aggregated landslide inventory 98 95 90 82 73 within moderate or high hazard zone Percent of each individual inventoried landslide polygon touching (minimum 9 grid cells) 99.9 99.9 99.9 99.8 99.8 moderate or high hazard zone Field based landslide inventory (LS-2) Percent of landslide inventory points within 92 91 89 87 84 moderate or high hazard zone Percent of 1996-1997 landslide inventory 99 99 99 99 99 polygons within moderate or high hazard zone Pre-landslide slope angle database (LS-3) Percent of slopes included that are higher than the minimum pre-failure slope angle from the 100 96 89 81 59 inventory (5 degrees)

We selected the focal range value of 4 ft (1.2 m) as the appropriate value to use, based on its very high capture rate for all three landslide databases coupled with a sub- stantial reduction in the overprediction of the amount of the map area included in the hazard zones. This choice is further supported by the observation that the Oregon Residential Specialty Code (chapter 4, section R404.1.3; 2008) states that “landscape” type retaining walls can be constructed if the wall supports less than 48 inches (4 ft or 1.2 m). In other words, undesigned cuts and fills (with “landscape” type walls) are allowed as long as they do not exceed 4 ft (1.2 m) in height and have a flat (non-sloping) backfill. Most, if not all cities and counties have adopted the state building code. Because one of DOGAMI’s goals is for city and county governments to use these maps as part of their local building code regulations, we think it is appropriate to match the existing allowable height of 4 ft with the focal height of 4 ft, which effectively removes these noncon- sequential slopes from the hazard zone. An example of the finalFOS class map for the test area using the 4 ft (1.2 m) focal range value is shown in Figure 16.

20 Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

Figure 16. Factor of Safety (FOS) class map for the test area clipped using a focal range value of 4 ft (1.2 m) (red high, orange moderate, gray low). Black outlines are mapped landslides developed following SP-42. Green dot in inset is the photo location in Figure 11. Also see Figure 12H (focal relief) example.

Oregon Department of Geology and Mineral Industries Special Paper 45 21 Protocol for Shallow-Landslide Susceptibility Mapping

3.5.3 Clipping the FOS Class Map

To clip the raw FOS class map using the focal relief, we first use the Esri Spatial Analyst Focal Statistics tool using a 15 ft2 (4.6 m2) neighborhood and the range sta- tistic type (Figure 17). We then use the raster calculator to select all values ≤ 4 ft, which produces a clipping raster that has a value of 0 where the focal range is ≤ 4 and a value of 1 when the focal range is > 4. We then multiply the raw FOS map by the clipping raster, which makes all FOS values in areas with a focal range ≤ 4 equal to 0 and preserves the FOS values in areas with focal range > 4. The resultant raster is the clippedOS F class map.

Extract landslide Deal with Factor Apply buffers to Combine con- inventory data of Safety (FOS) landslide data tributing factors and FOS to create map   Inventory Zone Clipped FOS map class map

Figure 17. Example of dialog box for the Esri Spatial Analyst Focal Statistics tool.

22 Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

3.6 Buffers

One disadvantage of a slope stability analysis using a the proximity to the head scarp clearly increases the raster or grid-based infinite slope equation is that the susceptibility. analysis looks at each raster cell independently. The To account for the increase in susceptibility of the FOS is calculated in the same way regardless of where area above head scarps, we apply a 2 horizontal to 1 the cell falls on a slope or where it sits in relation to vertical ratio (2H:1V) head scarp buffer Figure( 18B). important topographic features or changes. Because The 2H:1V ratio is commonly used in geotechnical the location of a cell can have an important impact engineering because the slope angle of a 2H:1V slope on the landslide susceptibility, we have developed two is equal to 26 degrees (Figure 18A). This is important buffers: 1) a head scarp buffer to address the elevated because most natural, unfailed (non-landslide) geolog- hazard around existing landslides and 2) a FOS buffer ic units have an angle of internal friction or equivalent to address the impact of slope position on hazard zones. shear strength of at least 26 degrees. To create the head scarp buffers, we query all of the 3.6.1 Landslide Head Scarp Buffer shallow landslide head scarp polygons from the land-

Extract landslide Deal with Factor Apply buffers to Combine con- slide inventory and apply a buffer of 30 ft (9 m), which inventory data of Safety (FOS) landslide data tributing factors is twice the depth of failure (15 ft) we have defined for and FOS to create map shallow landslides. We then convert the buffer poly-   apply landslide gons into a raster in which the polygon areas have a Inventory Zone Clipped FOS head scarp buffer map class map value of 3 (high susceptibility zone). This is the head scarp buffer map. Most landslides tend to have a steep head scarp above the failed mass. The head scarp area will commonly fail Extract landslide Deal with Factor Apply buffers to Combine con- inventory data of Safety (FOS) landslide data tributing factors retrogressively or a separate landslide will form above and FOS to create map the head scarp, because of the loss of resisting forces    Inventory Zone Clipped FOS Head directly adjacent and below the head scarp (Figure 18C, map class map scarp area labeled V and outlined in red). In these instances buffer the cell-based FOS map returns a low hazard value for map the flat areas adjacent to and above the head scarp, but

C. Block Diagram A. 2 Horizontal : 1 Vertical Diagram B. Cross-Section (profile)

2H:1V Head Scarp Buffer (orange) 2H:1V Head Scarp Buffer Head Scarp (red outline) 2H : 1V Height (V) 26 H ori zontal ( H ) V erti cal ( V ) Head Scarp Height (V) Distance H = V

Figure 18. Diagram of the 2H:1V head scarp buffer (red on block diagram).

Oregon Department of Geology and Mineral Industries Special Paper 45 23 Protocol for Shallow-Landslide Susceptibility Mapping

3.6.2 Factor of Safety Buffer

Extract landslide Deal with Factor Apply buffers to Combine con- inventory data of Safety (FOS) landslide data tributing factors foot of the slope, we apply a 30 ft (9 m) buffer (twice the and FOS to create map defined depth to failure for shallow landslides) for all    apply Factor of areas with a calculated FOS ≤ 1.5. We create the buffer Inventory Zone Clipped FOS Head Safety buffer map class map scarp by using the Esri Spatial Analyst Expand tool (Figure buffer map 20) on the clipped FOS class map. The output from this step is then reclassified, setting values of 2 and 3 equal The FOS values we calculate with the infinite slope to 2. This results in theFOS buffer map, which has equation are derived for each cell in the map area in values of 2 (moderate susceptibility) or NoData (Figure isolation. This means that the FOS values do not take 21). into account where on a particular slope the cell lies. As shown in the block diagram in Figure 19, there is a dif- Extract landslide Deal with Factor Apply buffers to Combine con- inventory data of Safety (FOS) landslide data tributing factors ference between a cell with an FOS value > 1.5 (stable) and FOS to create map that is located immediately adjacent to the zone of FOS     Inventory Zone Clipped FOS Head FOS values < 1.5 (potentially unstable), and one that is far map class map scarp buffer from the edge of a steep slope. Because landslides that buffer map originate on the steep slope may extend back into the map flat area above the slope or out on to the flat area at the

2H:1V FOS Buffer (orange) 2H:1V FOS Buffer (orange) FOS <1.5 (purple) FOS <1.5 (purple)

FOS > 1.5

2H:1V FOS Buffer 30ft

FOS < 1.5

FOS > 1.5

2H:1V FOS Buffer 30ft

Map View Block-Diagram View

Figure 19. Diagram of the 2H:1V buffer (orange) applied to all Factor of Safety (FOS) less than 1.5 (purple).

24 Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

Figure 20. Example of dialog box for the Esri Spatial Analyst Expand tool used to create Factor of Safety (FOS) buffer map. Number of cells = 10 corresponds to a 30-ft . (9 m) buffer when used with a 3-ft cell grid.

FOS Between 1.5 - 1.25

River FOS < 1.25

Steep High Susceptibility River Banks Moderate Susceptibility

Low Susceptibility Flat Agricultural Field Top of Bank

Feet 0 437.5 875 ¹ Meters 0 130 260 A Flat Agricultural Field

River

B - FOS after focal mask C - Final susceptibility zones Figure 21. Example of head scarps and Factor of Safety (FOS) buffers. (A) Result of landslide inventory displaying the top of head scarps above the top of the locally steep slope (river bank) area. (B) clipped FOS zone map. (C) clipped FOS zone map with head scarp and FOS buffers added.

Oregon Department of Geology and Mineral Industries Special Paper 45 25 Protocol for Shallow-Landslide Susceptibility Mapping

3.7 Final Susceptibility Map The layers are combined using the Mosaic To New

Extract landslide Deal with Factor Apply buffers to Combine con- Raster tool (Figure 10) using FIRST for the Mosaic inventory data of Safety (FOS) landslide data tributing factors Operator method, with the rasters ordered first to last and FOS to create map as follows:     Inventory Zone Clipped FOS Head FOS 1. Inventory Zone map map class map scarp buffer 2. Head Scarp Buffer map buffer map map 3. Clipped FOS Zone map 4. FOS Buffer map To create the final shallow-landslide susceptibility map, we combine the four layers we created: The result is the final shallow-landslide suscep- • Inventory zone map (values are 3 or High) tibilityp ma (see Figure 22). The contributions to the • Clipped FOS class map (values are 3 or High final map from the layers are summarized in Table 8. and 2 or Moderate) • Head scarp buffer map (values are 3 or High) Extract landslide Deal with Factor Apply buffers to Combine con- inventory data of Safety (FOS) landslide data tributing factors • FOS buffer map (values are 2 or Moderate) and FOS to create map      Inventory Zone Clipped FOS Head FOS shallow-landslide map class map scarp buffer susceptibility buffer map map map

Table 8. Summary of factors contributing to the final shallow-landslide susceptibility map.

Final Susceptibility Zones Contributing Factors High Moderate Low

 Factor of Safety (FOS) less than 1.25 1.25 –1.5 greater than 1.5 Landslide Deposits and Head Scarps included — — Buffers 2H:1V (head scarps) 2H:1V (FOS less than 1.5) —

Map layers that contribute to the mosaicked final map:

Extract landslide Deal with Factor Apply buffers to Combine con- inventory data of Safety (FOS) landslide data tributing factors and FOS to create map

    Inventory Zone map Head Scarp Buffer map Inventory Clipped Head scarp FOS shallow-landslide zone map + FOS zone map + buffer map + buffer map = susceptibility Clipped FOS Zone map    (High)  (Moderate) map FOS Buffer map

26 Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

4.0 SHALLOW LANDSLIDE SUSCEPTIBILITY MAP TEMPLATE

A map template (one quarter of a USGS 7.5-minute dardized collar elements, a simplified description of quadrangle at 1:8,000 scale) was developed as part of this methods, disclaimers, and legend elements. An exam- protocol to provide a consistent display of the results of ple is shown at reduced scale in Figure 21. the susceptibility mapping. The template includes stan-

G E O L O G Y F A N O D T N M I E N M E T R

R A A Open-File Report O-12-05 L P STATE OF OREGON

I E

D U

N Shallow-Landslide Susceptibility Map of the City of Silverton, S

O DEPARTMENT OF GEOLOGY AND MINERAL INDUSTRIES

R Regional Landslide Hazard Maps of the City of Silverton,

R E S O VICKI S. MCCONNELL, STATE GEOLOGIST Marion County, Oregon

1937 Marion County, Oregon by William J. Burns and Katherine A. Mickelson 2012 Partial funding provided by the City of Silverton (IA No. 41460-11242008)

PLATE 2

EXPLANATION This map is a shallow-landslide susceptibility map of a portion of this quadrangle. The shallow-landslide susceptibility map identifies landslide prone areas within the region. This susceptibility map is not regulatory, and revisions can happen when new information regarding factors that affect landslide susceptibility is found or future (new) landslides occur. Therefore, it is possible those areas susceptible to shallow-landslides within the map were not identified or that the condition that lead to such susceptibility developed after the map was prepared. Albiqua Creek On the basis of several factors and past studies (described in detail in Burns and Madin, 2009), a value (or depth) of 4.5 m (15 ft) is used to divide shallow from deep-landslides. This susceptibility map was prepared by combination of three factors: 1) calculated factor of safety (FOS), 2) landslide inventory data, and 3) buffers of the previous two factors. The factor of safety was calculated using conservative values such as a water table at the ground surface. The landslide inventory data were taken from the complimentary inventory map. The combinations of these factors comprise the relative susceptibility hazard zones: High, Moderate, and Low. The landslide data are displayed on top of a base map that consists of an aerial photograph (orthorectified) overlain on the lidar-derived digital elevation model. For additional detail on how this map was developed see Burns and others (2012). This susceptibility map is intended to provide users with relative hazard information regarding shallow-seated landslide susceptibility within the quadrangle. The map is not intended to replace site-specific engineering geologic and geotechnical investigations. It is intended that this map will provide useful information to guide regional and site-specific investigations for future developments, assist in regional planning, and to reduce risk in areas where moderate and high hazards intersect vulnerable population.

SHALLOW-LANDSLIDE SUSCEPTIBILITY CLASSIFICATION

Each landslide susceptibility hazard zone shown on this map has been developed according to a number of specific factors. The classification scheme was developed by the Oregon Department of Geology and Mineral Industries (Burns and others, 2012). The symbology used to display these hazard zones is explained below.

HWY 214 (Hillsboro-Silverton Highway) Landslide Susceptibility Zones: This map uses color to show the relative degree of hazard. Each zone is a combination of several factors.

HIGH: High susceptibility to shallow landslides.

MODERATE: Moderate susceptibility to shallow landslides.

LOW: Low susceptibility to shallow landslides.

Hazard Zone Matrix Table

Final Hazard Zone Contributing Factors* High Moderate Low

Factor of Safety (FOS) less than 1.25 1.25 - 1.5 greater than 1.5 Meridian Rd. 1 Hobart Rd. 2 Landslide InventoryDeposits & Head Scarps included — — 3 Buffers 2H:1V (head scarps) 2H:1V (FOS less than1.5) — *See explanation of corresponding contributing factors below. 1 Factor of Safety Map Factor of Safety (FOS) Map: The mechanics of slope stability can be divided into two forces: driving forces and resisting forces. These forces are a function of the material properties and the geometry of the slope. These two forces oppose each other, and slope stability can be thought of as their ratio.

Factor of Resisting Forces = Safety Driving Forces A FOS > 1 would theoretically be a stable slope because the shear strength would be greater than the shear stress. A FOS < 1 would Webb Lake theoretically be an unstable slope because the actual shear stress would be greater than the shear strength. A critically stable slope would have a FOS = 1. Because of the inability to know all the conditions present within a slope, most geotechnical engineers and engineering geologist recommend that slopes with a factor of safety less than 1.5 be considered potentially unstable (Turner and Schuster, 1996; Cornforth, 2005). Hobart Rd. NE The factor of safety was calculated using the infinite slope equation with conservative parameters. Saturation condition were used so that a “worst case” scenario could be evaluated. Because of limitations related to a grid type analysis, isolated areas with small (less than 4 feet high) elevation change were removed using a standardized process (Burns and others, 2011). This map uses color to show the change in the factor of safety across the map as explained below. EXPLANATION

Factor of Safety less than 1.25 Factor of Safety between 1.25 and 1.5 Factor of Safety greater than 1.5

13 (Cascade Highway) Y 2 JamesSt. HW 2 Landslide Inventory

Pine St. Shallow-Landslide Deposits and Head Scarps Inventory Map: This map is an inventory of existing shallow-landslides in this quadrangle. This inventory map was prepared by compiling all previously mapped landslides from published and unpublished geologic and landslide mapping, lidar-based geomorphic analysis, and review of aerial photographs. Each shallow-landslide was also attributed with classifications for activity, depth of failure, movement type, and confidence of interpretation. The Protocol for Inventory Mapping of Landslide Deposits from Lidar Imagery (Burns and Madin, 2009) was Silver Creek developed with input from many sources, along with years of experience. This map uses color to show different landslide features across the map as explained below.

First St. EXPLANATION

Landslide Deposits

E Main St. Landslide Head Scarps

Jersey St.

Evans Valley Rd.

Water St.

HWY 213

W Main St.

Eureka Ave.

Ike Mooney Rd. 3 Buffers for Head Scarps and Factor of Safety Less Than 1.5

2H:1V Head Scarp Buffer Buffer for Head Scarps: This buffer was 2H:1V Head Scarp Buffer Head Scarp Height (V) applied to all head scarps from the landslide inventory. The buffer consists of a 2:1 horizontal to vertical distance (2H:1V). This buffer is different for each head scarp and is Head Scarp Height (V) dependent on head scarp height. For example, a head scarp height of 2 m (6 ft) has a 2H:1V buffer equal to 4 m (12 ft)(Highland, 2004).

Cross-Section (profile) Block Diagram

2H:1V Factor of Safety Buffer = 9 m (30 ft) Buffer for Factor of Safety Less Than 1.5: This buffer was applied to all areas with a calculated FOS less than 1.5. The buffer 2 times V = 2H consists of a 2:1 horizontal to vertical distance Depth Horizontal (H) (2H:1V). The maximum depth for shallow- V = 4.5 m (15 ft) Vertical (V) seated landslides is 4.5 m (15 ft), the 2H:1V buffer equals 9 m (30 ft).

Cross-Section (profile) 2H:1V Diagram

Silver Creek LIMITATIONS

The shallow-landslide susceptibility protocol was developed with input from many sources, along with years of experience. Several limitations are worth noting and underscore that this hazard map is useful for regional applications but should not be used as an Pettit alternative to site-specific studies in critical areas.

Reservoir Quall Rd. NE 1) Every effort has been made to ensure the accuracy of the GIS and tabular database, but it is not feasible to completely verify all of the original input data. 2) The shallow-landslide susceptibility maps are based on three primary sources: a) landslide inventory, b) calculated factor of safety, c) buffers. Factors that can affect the level of detail and accuracy of the final susceptibility map include: a. Limitations of the landslide inventory, which are discussed in the Special Paper 42 (Burns and Madin, 2009). Edison Rd. NE b. The infinite slope factor of safety calculations are done on one individual grid cell at a time without regard for the adjacent grids. The results sometimes underestimate or overestimate the level of stability for a certain area. We eer Dr. developed buffers for areas with low factors of safety to try to counter the tendency to underestimate susceptibility. We developed the focal relief method to try to reduce the problem of overestimation of susceptibility due to steep

Pion slopes with low relief. However, the overestimation and underestimation of susceptible areas is still likely in some isolated areas. c. The factor of safety calculations are strongly influenced by the accuracy and resolution of the input data for material properties, depth to failure surface, depth to groundwater, and slope angle. The first three of these inputs are usually estimates (material properties) or conservative limiting cases (depth to failure surface and groundwater), and local conditions may vary substantially from the values used to make these maps. 3) The susceptibility maps are based on the topographic and landslide inventory data available as of the date of publication. Future changes in topography or the occurrence of new landslides may render this map locally inaccurate. 4) The lidar-based digital elevation model does not distinguish elevation changes that may be due to the construction of structures like retaining walls. Because it would require extensive GIS and field work to locate all of these existing structures and remove them or adjust the material properties in the model, they have been included as a conservative approach and therefore must be examined on a site-specific basis. HWY 214 5) Some landslides in the inventory may have been mitigated, reducing their level of susceptibility. Because it is not feasible to collect detailed site-specific information on every landslide, potential mitigation has been ignored. Because of these limitations this map is intended for regional purposes only and cannot replace site-specific investigations. However, the map can serve as a useful tool for estimating the regional landslide hazard and as a starting place for future detailed site-specific maps. Please contact DOGAMI if errors and/or omissions are found so that they can be corrected in future versions of this map. Silver Creek Reservoir REFERENCES

Vict Burns, W. J., and Madin, I. P., 2009, Protocol for inventory mapping of landslide deposits from light detection and ranging (lidar) or Poin imagery: Oregon Department of Geology and Mineral Industries Special Paper 42, 30 p., geodatabase template.

Burns, W.J., Madin, I.P., and Mickelson, K.A., 2012, Protocol for shallow-landslide susceptibility mapping: Oregon Department of Geology and Mineral Industries Special Paper 45, 32 p. Rd.t

Cornforth, D. H., 2005, Landslides in practice: Investigation, analysis, and remedial/preventative options in soils: Hoboken, N.J., John Wiley and Sons, Inc., 596 p. AREA NOT MAPPED Turner, A.K., and Schuster, R.L., eds., 1996, Landslides: Investigation and mitigation: Transportation Research Board, National Research Council Special Report 247, 673 p.

LOCATION MAP

Dundee Newberg Sherwood Canby Oregon City Redland

Dayton Saint Paul Woodburn Yoder Molalla Colton

Cartography by William J. Burns, Oregon Department of Geology and Mineral Industries Mission Bottom Gervais Silverton Scotts Mills Wilhoit Fernwood SCALE 1:8,000 Outside agency review by Rich Barstad, City of Silverton Base Map: 16. 5°

Lidar-derived elevation data are from the Oregon Lidar Consotrium, 2007. Digital elevation 1,300 650 0 1,300 2,600 3,900 5,200 model (DEM) consists of a 3-foot square elevation grid that was converted into a Feet hillshade image with sun angle at 315 degrees at a 45 degree angle from horizontal. Salem West The DEM is multiplied by 5 (vertical exaggeration) to enhance slope areas. Salem East Stayton NE Drake Crossing Elk Prairie Gawley Creek 0.25 0.125 0 0.25 0.5 0.75 1 IMPORTANT NOTICE: Orthophoto is from Oregon Geospatial Enterprise Office, 2005 and consists of 2005 Miles TRUE NORTH For copies of this publication contact: orthophoto draped over DEM with transparency. MAGNETIC NORTH This map depicts existing landslide susceptibility zones on the basis Nature of the Northwest Information Center of limited data. The susceptibility zones were created following the APPROXIMATE MEAN 0.25 0.125 0 0.25 0.5 0.75 1 Sidney 800 NE Oregon Street, #28, Ste. 965 Projection: North American Datum 1983, UTM zone 10 north. methods in the accompanying report (Burns and others, 2012). This Turner Stayton Stout Mountain Lyons Mill City North DECLINATION, 2006 Kilometers OREGON Portland, Oregon 97232 map cannot serve as a substitute for site-specific investigations by telephone (971) 673-2331 Software: MapInfo Professional 8.0, Esri ArcMap 9.2, Adobe Illustrator CS2. qualified practitioners. Site-specific data may give results that http://www.NatureNW.org differ from those shown on this map. Source File: Rocks\Publications\Silverton.mxd. U.S. Geological Survey 7.5-minute quadrangles and counties are labeled. This map extent shown as gray rectangle.

Figure 22. Example shallow-landslide susceptibility map for Silverton area.

Oregon Department of Geology and Mineral Industries Special Paper 45 27 Protocol for Shallow-Landslide Susceptibility Mapping

5.0 LIMITATIONS OF MAPS PRODUCED USING THIS PROTOCOL

Limitations of the input data and modeling methods c. Factor of safety calculations are strongly influ- we use to make these maps are such that the maps are enced by the accuracy and resolution of the not suitable to answer site specific questions. The maps input data for material properties, depth to should be used only for regional or community-scale failure surface, depth to groundwater, and purposes. The following list of specific limitations is slope angle. The first three of these inputs included as part of the standard material on the sus- are usually estimates (material properties) or ceptibility map template (Figure 22). conservative limiting cases (depth to failure 1. Every effort has been made to ensure the accuracy surface and groundwater); local conditions of the GIS and tabular database, but it is not feasible may vary substantially from the values used to completely verify all original input data. to make these maps. 2. The shallow-landslide susceptibility maps are based 4. The susceptibility maps are based on topographic on three primary sources: a) landslide inventory, b) and landslide inventory data available as of the date calculated Factor of Safety, and c) buffers. Factors of publication. Changes in topography or the occur- that can affect the level of detail and accuracy of the rence of new landslides may render this map locally final susceptibility map include: inaccurate. a. Limitations of the landslide inventory, which are 5. The lidar-based digital elevation model does not discussed in SP-42 (Burns and Madin, 2009a). distinguish elevation changes that may be due to b. The infinite slope Factor of Safety calculations the construction of structures like retaining walls. are done on one individual grid cell at a time Because it would require extensive GIS and field without regard to adjacent grids. The results work to locate all existing structures and remove sometimes underestimate or overestimate the them or adjust their material properties in the level of stability for a certain area. We devel- model, the structures have been included as a con- oped buffers for areas with low factors of safety servative approach and therefore must be examined to try to counter the tendency to underesti- on a site-specific basis. mate susceptibility. We developed the focal 6. Some landslides in the inventory may have been relief method to try to reduce the problem of mitigated, thereby reducing their level of suscepti- overestimation of susceptibility due to steep bility. Because it is not feasible to collect detailed slopes with low relief. However, overestima- site-specific information on every landslide, poten- tion and underestimation of susceptible areas tial mitigation has been ignored. is still likely in some isolated areas. 6.0 POTENTIAL USES OF SHALLOW LANDSLIDE SUSCEPTIBILITY MAPS

Shallow-seated landslide susceptibility maps created • Estimation of potential losses from specific using this protocol are intended to provide regional and hazard events (before or after a disaster hits) community-scale information to help avoid or properly • Prioritization of mitigation measures to reduce engineer development in susceptible zones, plan for future losses landslide disasters, and mitigate landslide risk for exist- • Development of City development regulation ing development in susceptible areas. For example, the ordinances maps can aid in: • Issuance of building permits or proposed grading • Identification of very high hazard areas permit conditions • Identification of areas for possible buyouts in • Public works planning and operations life-threatening hazard areas • Regional risk-reduction planning and activities • Evaluation of environmental and sustainability • Neighborhood-scale risk-reduction activities issues • Emergency management • Identification of vulnerable areas that may • Public awareness campaigns require planning considerations

28 Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

7.0 ACKNOWLEDGMENTS

Funding for this project was provided by the State of Godt and Ed Harp, who provided a detailed review that Oregon and by the U.S. Geologic Survey Landslide improved this paper. We would also like to thank Erica Hazards Program under Cooperative Agreement Koss from the City of Portland, who provided a review #05CRGR0002. that improved this paper. We thank the people at the USGS Landslide Hazards We would like to thank DOGAMI staff who helped Program who contributed to this protocol through work on this project through technical assistance, discussions and suggestions including Bill Schulz, Rex review, and general assistance, especially Vicki McCo- Baum, Jeff Coe, Jon McKenna and, especially, Jonathan nnell, Yumei Wang, and Deb Schueller.

8.0 REFERENCES

Baum, R. L., Savage, W. Z., and Godt, J. W., 2002, TRI- Burns, W. J., and Mickelson, K. A., 2010a, Landslide GRS—a Fortran program for transient rainfall inventory maps for the Astoria quadrangle, Clat- infiltration and grid-based regional slope-stability sop, County, Oregon: Oregon Department of analysis: U.S. Geological Survey Open-File Report Geology and Mineral Industries Interpretive Map 02-0424, 27 p., 2 app. Web: http://pubs.usgs.gov/ IMS-31, 4 pl., scale 1:8,000. of/2002/ofr-02-424/ Burns, W. J., and Mickelson, K. A., 2010b, Landslide Burns, W. J., 2007, Comparison of remote sensing data- inventory maps for the Oregon City quadrangle, sets for the establishment of a landslide mapping Clackamas, County, Oregon: Oregon Department protocol in Oregon: AEG Special Publication 23, of Geology and Mineral Industries Interpretive Vail, Colo., Conference Presentations, 1st North Map IMS-30, Plates 1, 2, 3, scale 1:8,000. American Landslide Conference. Burns, W. J., and Mickelson, K. A., 2012, Regional land- Burns, W. J., 2009a, Landslide inventory map of the slide hazard maps of the City of Silverton, Marion southwest quarter of the Beaverton quadrangle, County, Oregon: Oregon, Oregon Department of Washington County, Oregon: Oregon Depart- Geology and Mineral Industries, 21 p., 3 pls. ment of Geology and Mineral Industries Interpre- Burns, S. F., Burns, W. J., James, D. H., and Hinkle, J. tive Map IMS-27, scale 1:8,000. C., 1998, Landslides in the Portland, Oregon, met- Burns, W. J., 2009b, Landslide inventory maps for ropolitan area resulting from the storm of Febru- the Canby quadrangle, Clackamas, Marion, and ary 1996: inventory map, database, and evaluation: Washington Counties, Oregon: Oregon Depart- Portland State University, Department of Geology, ment of Geology and Mineral Industries Interpre- report to Portland Metro, contract 905828, 68 p. tive Map IMS-29, 4 pl., scale 1:8,000. Cornforth, D. H., 2005, Landslides in practice: Inves- Burns, W. J., and Madin, I. P., 2009a, Landslide proto- tigation, analysis, and remedial/preventative col for inventory mapping of landslide deposits options in soils: Hoboken, N. J., John Wiley and from light detection and ranging (lidar) imagery: Sons, p. 596. Oregon Department of Geology and Mineral Denning, C., 1994, Fundamental stress-stain relation- Industries Special Paper 42, 30 p., geodatabase ships, in Hall, D .E., Long, M. T., and Remboldt, M. template. D., eds., Slope stability reference guide for Nation- Burns, W. J., and Madin, I. P., 2009b, Landslide inven- al Forests in the United States: Washington, D. C., tory map of the northwest quarter of the Oregon United States Department of Agriculture, Forest City quadrangle, Clackamas County, Oregon: Service Publication EM-7170-13, v. II, p. 331–343. Oregon Department of Geology and Mineral Web: http://www.fs.fed.us/rm/pubs_other/wo_ Industries Interpretive Map IMS-26, scale 1:8,000. em7170_13.html

Oregon Department of Geology and Mineral Industries Special Paper 45 29 Protocol for Shallow-Landslide Susceptibility Mapping

Drazba, M. C., 2008, Landslide susceptibility map for Montgomery, D. R., and Dietrich, W. E., 1994, A physi- shallow landslides for the West Hills of Portland, cally based model for the topographic control on Oregon, using GIS and lidar: Portland State Uni- shallow landsliding: Water Resources Research, v. versity, Department of Geology, M.S. thesis, 158 p. 30, no. 4, p. 1153–1171. Harp, E. L., Michael, J. A., and Laprade, W. T., 2006, Oregon Residential Specialty Code, R404.5, 2008, Shallow-landslide hazard map of Seattle, Wash- Web: http://www2.iccsafe.org/states/oregon/08_ ington: U.S. Geological Survey Open-File Report Residential/08Res_Frameset.html 2006-1139, p. 20. Web: http://pubs.usgs.gov/ Pack, R. T., Tarboton, D. G., and Goodwin, C. N., 1999, of/2006/1139/ GIS-based landslide susceptibility mapping with Highland, L., compiler, 2004, Landslide types and pro- SINMAP, in Bay, J. A., ed., Proceedings of the 34th cesses, U.S. Geological Survey Fact Sheet 2004- symposium on Engineering Geology and Geo- 3072 (ver. 1.1), 4 p. Web: http://pubs.usgs.gov/fs/ technical Engineering, p. 210–231. old.2004/3072/ Spiker, E. C., and Gori, P., 2003, National landslide haz- Hong, Y., Adler, R. F., and Huffman, G. J., 2007, Satellite ards mitigation strategy — a framework for loss remote sensing for global landslide monitoring: reduction: U.S. Geological Survey Circular 1244, Eos, v. 88, no. 37. 56 p. Web: http://pubs.usgs.gov/circ/c1244/ Madin, I. P., 2009, Geologic map of the Oregon City 7.5′ Turner, A. K., and Schuster, R. L., eds., 1996, Land- quadrangle, Clackamas County, Oregon: Oregon slides: investigation and mitigation: Washington, Department of Geology and Mineral Indus- D.C., National Research Council, Transportation tries Geologic Map Series GMS-119, 46 p., scale Research Board Special Report 247, 673 p. 1:24,000. Varnes, D. J., 1978, Slope movement types and pro- Madin, I. P., and Niewendorp, C. A., 2008, Prelimi- cesses, in Schuster, R. L., and Krizek, R. J., eds., nary geologic map of the Dixie Mountain 7.5′ Landslides analysis and control: Washington, quadrangle, Multnomah and Washington Coun- D.C., National Research Council, Transportation ties, Oregon: Oregon Department of Geology and Research Board Special Report 176, p. 11–33. Mineral Industries Open-File Report O-08-07, 43 Wang, Y., Summers, R. D., and Hofmeister, R. J., 2002, p., scale 1:24,000. Landslide loss estimation pilot project in Oregon: Madin, I. P., Ma, L., and Niewendorp, C. A., 2008, Pre- Oregon Department of Geology and Mineral liminary geologic map of the Linnton 7.5′ quad- Industries Open-File Report O-02-05, 23 p. rangle, Multnomah and Washington Counties, Oregon: Oregon Department of Geology and Mineral Industries Open-File Report O-08-06, 35 p., scale 1:24,000.

30 Oregon Department of Geology and Mineral Industries Special Paper 45 Protocol for Shallow-Landslide Susceptibility Mapping

9.0 APPENDIX A—FACTOR OF SAFETY CALCULATOR

The information shown in this appendix is also available on the CD-ROM in Adobe Acrobat PDF file and Micro- soft Excel spreadsheet file formats. Formulas in the equation below from Harp and others (2006) refer to cells in the Excel spreadsheet.

Effective cohesion c’ 0 lb/ft2 First portion of equation C$1/(C$3*C$5*SIN(C10*PI()/180)) Effective internal Second portion of ’ 28 (TAN(C$2*PI()/180))/(TAN(C10*PI()/180)) friction φ equation Unit weight (soil) γ 122 lb/ft3 Third portion of equation (C$7*C$4*TAN(C$2*PI()/180))/(C$3*TAN(C10*PI()/180))

3 Unit weight (water) γw 64 lb/ft Factor of Safety D10+E10-F10 Depth to failure t 15.0 ft surface Proportion of slope m 1.0 thickness saturated see table Slope α below deg.

Slope Factor of Slope Factor of Slope Factor of (α) 1 2 3 Safety (α) 1 2 3 Safety (α) 1 2 3 Safety 1.0 0.00 30.46 15.98 14.48 15.0 0.00 1.98 1.04 0.94 30.0 0.00 0.92 0.48 0.44 1.5 0.00 20.31 10.65 9.65 15.5 0.00 1.92 1.01 0.91 30.5 0.00 0.90 0.47 0.43 2.0 0.00 15.23 7.99 7.24 16.0 0.00 1.85 0.97 0.88 31.0 0.00 0.88 0.46 0.42 2.5 0.00 12.18 6.39 5.79 16.5 0.00 1.80 0.94 0.85 31.5 0.00 0.87 0.46 0.41 3.0 0.00 10.15 5.32 4.82 17.0 0.00 1.74 0.91 0.83 32.0 0.00 0.85 0.45 0.40 3.5 0.00 8.69 4.56 4.13 17.5 0.00 1.69 0.88 0.80 32.5 0.00 0.83 0.44 0.40 4.0 0.00 7.60 3.99 3.61 18.0 0.00 1.64 0.86 0.78 33.0 0.00 0.82 0.43 0.39 4.5 0.00 6.76 3.54 3.21 18.5 0.00 1.59 0.83 0.76 33.5 0.00 0.80 0.42 0.38 5.0 0.00 6.08 3.19 2.89 19.0 0.00 1.54 0.81 0.73 34.0 0.00 0.79 0.41 0.37 5.5 0.00 5.52 2.90 2.63 19.5 0.00 1.50 0.79 0.71 34.5 0.00 0.77 0.41 0.37 6.0 0.00 5.06 2.65 2.41 20.0 0.00 1.46 0.77 0.69 35.0 0.00 0.76 0.40 0.36 6.5 0.00 4.67 2.45 2.22 20.5 0.00 1.42 0.75 0.68 35.5 0.00 0.75 0.39 0.35 7.0 0.00 4.33 2.27 2.06 21.0 0.00 1.39 0.73 0.66 36.0 0.00 0.73 0.38 0.35 7.5 0.00 4.04 2.12 1.92 21.5 0.00 1.35 0.71 0.64 36.5 0.00 0.72 0.38 0.34 8.0 0.00 3.78 1.98 1.80 22.0 0.00 1.32 0.69 0.63 37.0 0.00 0.71 0.37 0.34 8.5 0.00 3.56 1.87 1.69 22.5 0.00 1.28 0.67 0.61 37.5 0.00 0.69 0.36 0.33 9.0 0.00 3.36 1.76 1.60 23.0 0.00 1.25 0.66 0.60 38.0 0.00 0.68 0.36 0.32 9.5 0.00 3.18 1.67 1.51 23.5 0.00 1.22 0.64 0.58 38.5 0.00 0.67 0.35 0.32 10.0 0.00 3.02 1.58 1.43 24.0 0.00 1.19 0.63 0.57 39.0 0.00 0.66 0.34 0.31 10.5 0.00 2.87 1.50 1.36 24.5 0.00 1.17 0.61 0.55 39.5 0.00 0.65 0.34 0.31 11.0 0.00 2.74 1.43 1.30 25.0 0.00 1.14 0.60 0.54 40.0 0.00 0.63 0.33 0.30 11.5 0.00 2.61 1.37 1.24 25.5 0.00 1.11 0.58 0.53 40.5 0.00 0.62 0.33 0.30 12.0 0.00 2.50 1.31 1.19 26.0 0.00 1.09 0.57 0.52 41.0 0.00 0.61 0.32 0.29 12.5 0.00 2.40 1.26 1.14 26.5 0.00 1.07 0.56 0.51 41.5 0.00 0.60 0.32 0.29 13.0 0.00 2.30 1.21 1.09 27.0 0.00 1.04 0.55 0.50 42.0 0.00 0.59 0.31 0.28 13.5 0.00 2.21 1.16 1.05 27.5 0.00 1.02 0.54 0.49 42.5 0.00 0.58 0.30 0.28 14.0 0.00 2.13 1.12 1.01 28.0 0.00 1.00 0.52 0.48 43.0 0.00 0.57 0.30 0.27 14.5 0.00 2.06 1.08 0.98 28.5 0.00 0.98 0.51 0.47 43.5 0.00 0.56 0.29 0.27 29.0 0.00 0.96 0.50 0.46 44.0 0.00 0.55 0.29 0.26 29.5 0.00 0.94 0.49 0.45 44.5 0.00 0.54 0.28 0.26 (continued on next page)

Oregon Department of Geology and Mineral Industries Special Paper 45 31 Protocol for Shallow-Landslide Susceptibility Mapping

(continued from previous page)

Slope Factor of Slope Factor of Slope Factor of (α) 1 2 3 Safety (α) 1 2 3 Safety (α) 1 2 3 Safety 45.0 0.00 0.53 0.28 0.25 60.0 0.00 0.31 0.16 0.15 75.0 0.00 0.14 0.07 0.07 45.5 0.00 0.52 0.27 0.25 60.5 0.00 0.30 0.16 0.14 75.5 0.00 0.14 0.07 0.07 46.0 0.00 0.51 0.27 0.24 61.0 0.00 0.29 0.15 0.14 76.0 0.00 0.13 0.07 0.06 46.5 0.00 0.50 0.26 0.24 61.5 0.00 0.29 0.15 0.14 76.5 0.00 0.13 0.07 0.06 47.0 0.00 0.50 0.26 0.24 62.0 0.00 0.28 0.15 0.13 77.0 0.00 0.12 0.06 0.06 47.5 0.00 0.49 0.26 0.23 62.5 0.00 0.28 0.15 0.13 77.5 0.00 0.12 0.06 0.06 48.0 0.00 0.48 0.25 0.23 63.0 0.00 0.27 0.14 0.13 78.0 0.00 0.11 0.06 0.05 48.5 0.00 0.47 0.25 0.22 63.5 0.00 0.27 0.14 0.13 78.5 0.00 0.11 0.06 0.05 49.0 0.00 0.46 0.24 0.22 64.0 0.00 0.26 0.14 0.12 79.0 0.00 0.10 0.05 0.05 49.5 0.00 0.45 0.24 0.22 64.5 0.00 0.25 0.13 0.12 79.5 0.00 0.10 0.05 0.05 50.0 0.00 0.45 0.23 0.21 65.0 0.00 0.25 0.13 0.12 80.0 0.00 0.09 0.05 0.04 50.5 0.00 0.44 0.23 0.21 65.5 0.00 0.24 0.13 0.12 80.5 0.00 0.09 0.05 0.04 51.0 0.00 0.43 0.23 0.20 66.0 0.00 0.24 0.12 0.11 81.0 0.00 0.08 0.04 0.04 51.5 0.00 0.42 0.22 0.20 66.5 0.00 0.23 0.12 0.11 81.5 0.00 0.08 0.04 0.04 52.0 0.00 0.42 0.22 0.20 67.0 0.00 0.23 0.12 0.11 82.0 0.00 0.07 0.04 0.04 52.5 0.00 0.41 0.21 0.19 67.5 0.00 0.22 0.12 0.10 82.5 0.00 0.07 0.04 0.03 53.0 0.00 0.40 0.21 0.19 68.0 0.00 0.21 0.11 0.10 83.0 0.00 0.07 0.03 0.03 53.5 0.00 0.39 0.21 0.19 68.5 0.00 0.21 0.11 0.10 83.5 0.00 0.06 0.03 0.03 54.0 0.00 0.39 0.20 0.18 69.0 0.00 0.20 0.11 0.10 84.0 0.00 0.06 0.03 0.03 54.5 0.00 0.38 0.20 0.18 69.5 0.00 0.20 0.10 0.09 84.5 0.00 0.05 0.03 0.02 55.0 0.00 0.37 0.20 0.18 70.0 0.00 0.19 0.10 0.09 85.0 0.00 0.05 0.02 0.02 55.5 0.00 0.37 0.19 0.17 70.5 0.00 0.19 0.10 0.09 85.5 0.00 0.04 0.02 0.02 56.0 0.00 0.36 0.19 0.17 71.0 0.00 0.18 0.10 0.09 86.0 0.00 0.04 0.02 0.02 56.5 0.00 0.35 0.18 0.17 71.5 0.00 0.18 0.09 0.08 86.5 0.00 0.03 0.02 0.02 57.0 0.00 0.35 0.18 0.16 72.0 0.00 0.17 0.09 0.08 87.0 0.00 0.03 0.01 0.01 57.5 0.00 0.34 0.18 0.16 72.5 0.00 0.17 0.09 0.08 87.5 0.00 0.02 0.01 0.01 58.0 0.00 0.33 0.17 0.16 73.0 0.00 0.16 0.09 0.08 88.0 0.00 0.02 0.01 0.01 58.5 0.00 0.33 0.17 0.15 73.5 0.00 0.16 0.08 0.07 88.5 0.00 0.01 0.01 0.01 59.0 0.00 0.32 0.17 0.15 74.0 0.00 0.15 0.08 0.07 89.0 0.00 0.01 0.00 0.00 59.5 0.00 0.31 0.16 0.15 74.5 0.00 0.15 0.08 0.07 89.5 0.00 0.00 0.00 0.00 90.0 0.00 0.00 0.00 0.00

32 Oregon Department of Geology and Mineral Industries Special Paper 45