Shallow Landslide Susceptibility Map of the Southeast ... · FOSel shaste n 25oq r1.ou at l S...
1
NW Laidlaw Rd CI TY OF P ORTLAND Johnson Creek NW Thompson Rd NW Skyline Blvd NW Cornell Rd CI TY OF Be a ve rt on HWY 26 HWY 217 122 °45 ' 0 " W 122 °45 ' 0 " W 122 °48 ' 45 " W 122 °48 ' 45 " W 45 °33 ' 45 " N 45 °33 ' 45 " N 45 °30 ' 0 " N 45 °30 ' 0 " N 516 000 516 000 518 000 518 000 504 0000 504 0000 504 2000 504 2000 504 4000 504 4000 Shallow Landslide Susceptibility Map of the Southeast Quarter of the Linnton Quadrangle, Washington and Multnomah Counties, Oregon SCAL E 1 : 8 , 000 2015 0 . 25 0 0 . 25 0 . 5 0 . 75 1 0 . 125 Kilometers 0 . 25 0 0 . 25 0 . 5 0 . 75 1 0 . 125 Miles 1 , 300 0 1 , 300 2 , 600 3 , 900 5 , 200 650 Feet O R E G O N D E P A R T M E N T O F G E O L O G Y A N D M I N E R A L I N D U S T R I E S 1 9 3 7 Base map for plates in th is publication : Lidar data from DOGAMI Lidar Data Quadrangle LDQ - 2009 - 45122E7 - Linn ton , Digital elevation model ( DEM) consists of a 3 - foot- square elevation gr id that w as converted into a h ilsh ade image with sun angle at 315 degrees at a 60 - degree angle from h or izon tal . The DEM w as multiplied by 5 ( vertical exaggeration ) to enhance slope areas. 2009 orth oph oto imagery is from Oregon Geospatial Enterpr ise Office and is draped over the h ilsh ade image with transparency. Proj ection : North Amer ican Datum 1983 , UTM zone 10 North . Softw are: Esr i ArcMap 10 , Adobe I lustrator CS2 . Source F ile: Proj ect\W A County Area 93\P late. mxd For copies of th is publication con tact : Nature of the North w est In formation Cen ter 800 NE Oregon Street , #28 , Ste. 965 Portland, Oregon 97232 telephone ( 971 ) 673 - 2331 h ttp: / / www. naturenw . org Factor of Safety Landslide Deposits and Head Scarps EX P LANAT I ON La ndsl i de De posi t s Head Scar ps Factor of Safety (FOS) Map: The mech an ics of slope stability can be divided in to tw o forces: dr iving forces and resisting forces. These forces are a function of the mater ial properties and the geometry of the slope. These tw o forces oppose each oth er , and slope stability can be th ough t of as th eir ratio. Factor of Safety R esisting Forces Dr iving Forces = Landslide Deposits and Head Scarps Inventory Map: Th is map is an inven tory of al existinglandslides in th is area. This inven tory map w as prepared by compiling al previously mapped landslides from publish ed and unpublish ed geologic and landslide mapping, lidar - based geomorph ic analysis, and review of aer ial ph otograph s. Each landslide w as also attr ibuted with classifications for activity, depth of failure, movemen t type, and con fidence of in terpretation . The inven tory w as created by using the protocol developed by Burns and Madin ( 2009 ) . Th is map uses color to sh ow differen t landslide features across the map as explained below . The sh alowlandslides were extractedfrom the inven tory andusedto create th e sh alow - landslide susceptibility map as shown above in the Hazard Zone Matr ix. A FOS > 1 is th eoreticaly a stable slope because the sh ear strength is greater than the sh ear stress. A FOS < 1 is th eoreticaly an unstable slope because the sh ear stress is greater than the sh ear strength . A cr iticaly stable slope h as a FOS = 1 . Because of the inability to know al the conditions presen t w ith in a slope, most geotech n ical engineers and engineer ing geologists recommend that slopes w ith a FOS < 1 . 5 beconsidered poten tialy unstable ( Turner and Sch uster , 1996 ; Corn forth , 2005 ) . The FOS w as calculated using the in fin ite slope equation w ithconservative parameters. Saturatedconditions were usedso that a worst case scenar io could beevaluated. Because of limitations related to a gr id type analysis, isolated areas w ithsmal ( less than 4 ft ( 1 . 2 m) h igh ) elevation change were removed using a standardized process ( Burns and oth ers, 2012 ) . Th is map uses color to show the change in the factor of safety across the map as explained below . EX P LANAT I ON FOS g r eat er t ha n o r e qual t o 1 . 5 FOS bet ween 1 . 25 a nd 1 . 5 FOS l ess t ha n o r equal t o 1 . 25 S ha l l ow- La ndsl i de De posi t s by W iliam J . Burns and Kath er ine A . Mickelson EXPLANATION Th is sh alow landslide susceptibility map iden tifies landslide- prone areas that are defined folow ing the protocol of Burns and oth ers ( 2012 ) . On the basis of several factors and past studies ( descr ibed in detail by Burns and Madin [2009 ]) , a depth of 15 ft ( 4 . 5 m) is used to divide sh alowfrom deep landslides. Th is susceptibility map w as prepared by combin ing three factors: 1 ) calculated factor of safety ( FOS ) , 2 ) landslide inven tory data, and 3 ) buffers applied to the previous tw o factors. The FOS w as calculated using conservative values such as having a w ater table at the ground sur face. The landslide inven tory data were taken from the corresponding inven tory map. The combinations of these factors compr ise the relative susceptibility hazard zones: h igh , moderate, and low as shown by the Susceptibility Hazard Zone Matr ix below . The landslide susceptibility data are displayed on top of a base map th at consists of an aer ial ph otograph ( orth orectified) over lain on the lidar - der ived digital elevation model . For additional detail on how th is map w as developed see Burns and oth ers ( 2012 ) . PLATE 1 Landslide Susceptibility of the Southeast Quarter of the Linnton Quadrangle, Washington and Multnomah Counties, Oregon SHALLOW LANDSLIDE SUSCEPTIBILITY CLASSIFICATION Shallow Landslide Susceptibility Hazard Zone Matrix Each landslide susceptibility hazard zone shown on th is map h as been developed according to a number of specific factors. The classification sch eme w as developed by the Oregon Departmen t of Geology and Mineral I ndustr ies ( Burns and oth ers, 2012 ) . The symbology used to display these hazard zones is explained below . Shallow Landslide Susceptibility Zones: Th is map uses color to show the relative degree of h azard. Each zone is a combination of several factors ( see Hazard Zone Matr ix, below ) . HIGH: High susceptibility to sh alow landslides. MODERATE: Moderate susceptibility to sh alow landslides. LOW: Low susceptibility to sh alow landslides. REFERENCES LIMITATIONS Buffers for Head Scarps and Factor of Safety Less Than 1.5 Buffer for Head Scarps: Th is buffer w as applied to al h ead scarps from the landslide inven tory. Th ebuffer consists of a 2 : 1 h or izon tal to vertical distance ( 2H: 1V) . This buffer is differen t for eachh ead scarp and is dependen ton h ead scarp h eigh t . For example, a h ead scarp h eigh t of 6 ft ( 2 m) h as a 2H: 1V buffer equal to 12 ft ( 4 m) . Buffer for Factor of Safety Less Than 1.5: Th is buffer w as applied to al areas with a calculated FOS less than 1 . 5 . The buffer consistsofa 2 : 1 h or izon tal tovertical distance ( 2H: 1V) . For example, ifth e maximum depth for sh alow landslides is 15 ft ( 4 . 5 m) , th en the 2H: 1Vbuffer w ould equal 30 ft ( 9 m) . Burns, W . J . , and Madin , I. P . , 2009 , Protocol for inven tory mapping of landslide deposits from ligh t detection and ranging ( lidar ) imagery: Oregon Departmen t of Geology and Mineral I ndustr ies Special Paper 42 , 30 p. Burns, W . J . , Madin , I. P . , and Mickelson , K. A . , 2012 , Protocol for sh alow - landslide susceptibility mapping: Oregon Departmen t of Geology and Mineral I ndustr ies Special Paper 45 , 32 p. Corn forth , D. H. , 2005 , L andslides in practice: I nvestigation , analysis, and remedial / preven tative options in soils: Hoboken , New Jersey, John W iley and Sons, Inc. , 596 p. High land, L. , compiler , 2004 , L andslide types and processes: U. S . Geological Survey Fact Sheet 2004 - 3072 ( ver . 1 . 1 ) , 4 p. Turner , A . K. , and Sch uster , R. L. , eds. , 1996 , L andslides: I nvestigation and mitigation : Transportation Research Board, National Research Council , Special R eport 247 , 673 p. Several limitations are w orth noting and include the folow ing. 1 ) Every effort h as been made to ensure the accuracy of the GI S and tabular database, but it is not feasible to completely ver ify al of the or iginal input data. 2 ) The sh alow landslide susceptibility maps are based on three pr imary componen ts: a) calculated factor of safety, b) landslide inven tory, and c) buffers. Factors th at can affect the level of detail and accuracy of the final susceptibility map include the folow ing: a) Factor of safety calculations are strongly in fluenced by the accuracy and resolution of the input data for mater ial properties, depth to failure sur face, depth to groundw ater , and slope angle. The first three of these inputs are usualy estimates ( mater ial properties) or conservative limiting cases ( depth to failure sur face andgroundw ater ) , andlocal conditions may vary substan tialy from the estimated values used to make these maps. b) L imitations of the landslide inven tory, w h ich w as discussed by Burns and Madin ( 2009 ) . c) In fin ite slope factor of safety calculations are done on one gr id cel at a time w ith out regard for the adj acen t gr ids. The results may underestimate or overestimate the level of stability for a certain area. W e developed buffers for areas w ithlow factors of safety to try to coun ter the tendency to underestimate susceptibility. W e developed the focal relief method to try to reduce the problem of overestimation of susceptibility due to steep slopes w ithlow relief. How ever , the overestimation and underestimation of susceptible areas is stil likely in some isolated areas. 3 ) The susceptibility maps are based on the topograph ic and landslide inven tory data available as of the date of publication . Future new landslides may render th is map localy inaccurate. 4 ) The lidar - based digital elevation model does not distinguishelevation ch anges that may be due to the construction of structures like retain ing w als. Because it w ould require extensive GI S and field work to locate al of these existing structures and remove th em or adj ust the mater ial properties in the model , such features have been included as a conservative approachand th erefore must be examined on a site- specific basis. 5 ) Some landslides in the inven tory may have been mitigated, th ereby reducing th eir level of susceptibility. Because it is not feasible to colect detailed site- specific in formation on every landslide, poten tial mitigation h as been ignored. LOCATION MAP Sc ho l l s L i nnt on Ca nby Po r t l a nd H i l l sbo r o Dunde e G l adst one Beav er t on Ne wbe r g Va ncouv er S he r wood Lau r el wood Mount Tabo r Fo r est Gr ov e Or egon C i t y Lake Oswego Sauv i e I sl a nd OREGON U . S. Ge o l og i c al Su r v ey 7 . 5 - mi nu t e quad r a ng l es a nd c ount i es ar e l abel ed . Thi s map ex t e nt shown as g r ay r ec t a ng l e. IMPORTANT NOTICE Th is product is for in formational purposes and may not have been prepared for or be suitable for legal , engineer ing, or surveying purposes. U sers of th is in formation sh ould review or consult the pr imary data and in formation sources to ascertain the usability of the in formation . Th is publication cannot substitute for site- specific investigations by qualified practitioners. S ite- specific data may give results that differ from the results shown in the publication . See the accompanying text report for more details on the limitations of the meth ods and data used to prepare th is publication . * See ex pl a na t i on o f c o r r espond i ng c ont r i bu t i ng fa c t o r s bel ow . * High Moderate Low l ess t ha n 1 . 25 1 . 25 - 1 . 5 g r eat er t ha n 1 . 5 i nc l ude d 2H : 1V ( he a d sc ar ps ) 2H : 1V ( FOS l ess t ha n1 . 5 ) Landsl i de Depos i t s & Head Sc ar ps ( S ha l l ow) Fac t o r o f S a fe t y ( FOS) Bu ffe r s Final Hazard Zone Contributing Factors 1 1 2 2 3 3 STATE OF OREGON DEPARTMENT OF GEOLOGY AND MINERAL INDUSTRIES www. OregonGeology. org Ian P . Madin , In ter im State Geologist The proj ect descr ibed in th is publication w as supported in part by W ash ington County, Oregon , I ntergovernmental Agreement 14 - 0641 . Cartograph y by W iliam J . Burns, Oregon Departmen t of Geology and Mineral I ndustr ies. Th is map also benefited from in ternal review and commen ts by DOGAMI geologist Th omas J . W iley.
Transcript of Shallow Landslide Susceptibility Map of the Southeast ... · FOSel shaste n 25oq r1.ou at l S...
NW Laidlaw Rd
CITY OFP ORTLAND
Johnson Creek
NW Thompson RdNW Skyline Blvd
NW Cornell Rd
CITY OFBe a ve rton
HWY 26
HWY 217
122°45'0"W
122°45'0"W
122°48'45"W
122°48'45"W
45°33'45"N 45°33'45"N
45°30'0"N 45°30'0"N
516000
516000
518000
518000
50400
00
50400
00
50420
00
50420
00
50440
00
50440
00
Shallow Landslide Susceptibility Map of the Southeast Quarter of the Linnton Quadrangle, Washington and Multnomah Counties, Oregon
SCALE1:8,000
2015
0.25 0 0.25 0.5 0.75 10.125Kilometers
0.25 0 0.25 0.5 0.75 10.125Miles
1,300 0 1,300 2,600 3,900 5,200650Feet
OREG
ONDE
P AR TM
E NT O F G E O L O G Y A N D M I NE RALI NDUSTRIES
1937
Base map for plates in th is publication:
Lidar data from DOGAMI Lidar Data Q uadrangle LDQ -2009-45122E7-Linnton,Digital elevation model (DEM) consists of a 3-foot-square elevation grid th at w as converted into a h illsh ade image w ith sun angle at 315 degrees at a 60-degree angle from h orizontal.Th e DEM w as multiplied by 5 (vertical exaggeration) to enh ance slope areas.
2009 orth oph oto imagery is from Oregon Geospatial Enterprise Office andis draped over th e h illsh ade image w ith transparency.
Projection: North American Datum 1983, U TM zone 10 North .
Softw are: Esri ArcMap 10, Adobe Illustrator CS2.
Source File: Project\W A County Area 93\Plate.mxd
For copies of th is publication contact:Nature of th e North w est Information Center800 NE Oregon Street, #28, Ste. 965
Portland, Oregon 97232teleph one (971) 673-2331h ttp://w w w .naturenw .org
Factor of Safety
Landslide Deposits and Head Scarps
EX P LANATION
La nds lide De pos its
He a d S carps
Factor of Safety (FOS) Map: Th emech anics of slope stability can be dividedinto tw o forces: driving forces and resistingforces. Th ese forces are a function of th ematerial properties and th e geometry of th eslope. Th ese tw o forces oppose each oth er,and slope stability can be th ough t of as th eirratio.Factor ofSafety
R esisting ForcesDriving Forces=
Landslide Deposits and Head ScarpsInventory Map: Th is map is an inventory ofall existing landslides in th is area. Th isinventory map w as prepared by compiling allpreviously mapped landslides from publish edand unpublish ed geologic and landslidemapping, lidar-based geomorph ic analysis,and review of aerial ph otograph s. Eachlandslide w as also attributed w ithclassifications for activity, depth of failure,movement type, and confidence ofinterpretation. Th e inventory w as created byusing th e protocol developed by Burns andMadin (2009). Th is map uses color to sh owdifferent landslide features across th e map asexplained below . Th e sh allow landslides w ereextracted from th e inventory and used tocreate th e sh allow -landslide susceptibilitymap as sh ow n above in th e Hazard Z oneMatrix.
A FOS > 1 is th eoretically a stable slopebecause th e sh ear strength is greater th anth e sh ear stress. A FOS < 1 is th eoreticallyan unstable slope because th e sh ear stress isgreater th an th e sh ear strength . A criticallystable slope h as a FOS = 1. Because of th einability to know all th e conditions presentw ith in a slope, most geotech nical engineersand engineering geologists recommend th atslopes w ith a FOS < 1.5 be consideredpotentially unstable (Turner and Sch uster,1996; Cornforth , 2005).
Th e FOS w as calculated using th e infiniteslope equation w ith conservative parameters.Saturated conditions w ere used so th at a“w orst case” scenario could be evaluated.Because of limitations related to a grid typeanalysis, isolated areas w ith small (less th an4 ft (1.2 m) h igh ) elevation ch ange w ereremoved using a standardized process (Burnsand oth ers, 2012).
Th is map uses color to sh ow th e ch ange in th efactor of safety across th e map as explainedbelow .
EX P LANATION
FOS gre ater tha n or e qua l to 1.5 FOS b e twe e n 1.25 a nd 1.5FOS le s s tha n or e qua l to 1.25
S ha llow-La nds lide De pos its
by W illiam J. Burns and Kath erine A. Mickelson
EXPLANATIONTh is sh allow landslide susceptibility map identifies landslide-prone areas th at are defined follow ing th e protocol of Burns and oth ers(2012).
On th e basis of several factors and past studies (described in detail by Burns and Madin [2009]), a depth of 15 ft (4.5 m) is used todivide sh allow from deep landslides. Th is susceptibility map w as prepared by combining th ree factors: 1) calculated factor of safety(FOS), 2) landslide inventory data, and 3) buffers applied to th e previous tw o factors. Th e FOS w as calculated using conservativevalues such as h aving a w ater table at th e ground surface. Th e landslide inventory data w ere taken from th e correspondinginventory map. Th e combinations of th ese factors comprise th e relative susceptibility h azard zones: h igh , moderate, and low as sh ow nby th e Susceptibility Hazard Z one Matrix below . Th e landslide susceptibility data are displayed on top of a base map th at consists ofan aerial ph otograph (orth orectified) overlain on th e lidar-derived digital elevation model. For additional detail on h ow th is map w asdeveloped see Burns and oth ers (2012).
PLATE 1Landslide Susceptibility of the Southeast Quarter of the Linnton Quadrangle,
Washington and Multnomah Counties, Oregon
SHALLOW LANDSLIDE SUSCEPTIBILITY CLASSIFICATION
Shallow Landslide Susceptibility Hazard Zone Matrix
Each landslide susceptibility h azard zone sh ow n on th is map h as been developed according to a number of specific factors. Th eclassification sch eme w as developed by th e Oregon Department of Geology and Mineral Industries (Burns and oth ers, 2012). Th esymbology used to display th ese h azard zones is explained below .
Shallow Landslide Susceptibility Zones: Th is map uses color to sh ow th e relative degree of h azard. Each zone is a combination ofseveral factors (see Hazard Z one Matrix, below ).
HIGH:High susceptibility to sh allow landslides.
MODERATE:Moderate susceptibility to sh allow landslides.
LOW: Low susceptibility to sh allow landslides.
REFERENCES
LIMITATIONS
Buffers for Head Scarps and Factor of Safety Less Than 1.5 Buffer for Head Scarps: Th is buffer w asapplied to all h ead scarps from th e landslideinventory. Th e buffer consists of a 2:1h orizontal to vertical distance (2H:1V ). Th isbuffer is different for each h ead scarp and isdependent on h ead scarp h eigh t. Forexample, a h ead scarp h eigh t of 6 ft (2 m) h asa 2H:1V buffer equal to 12 ft (4 m).
Buffer for Factor of Safety Less Than 1.5:Th is buffer w as applied to all areas w ith acalculated FOS less th an 1.5. Th e bufferconsists of a 2:1 h orizontal to verticaldistance (2H:1V ). For example, if th emaximum depth for sh allow landslides is 15ft (4.5 m), th en th e 2H:1V buffer w ould equal30 ft (9 m).
Burns, W .J., and Madin, I.P., 2009, Protocol for inventory mapping of landslide deposits from ligh t detection and ranging (lidar)imagery: Oregon Department of Geology and Mineral Industries Special Paper 42, 30 p.
Burns, W .J., Madin, I.P., and Mickelson, K.A., 2012, Protocol for sh allow -landslide susceptibility mapping: Oregon Department ofGeology and Mineral Industries Special Paper 45, 32 p.
Cornforth , D.H., 2005, Landslides in practice: Investigation, analysis, and remedial/preventative options in soils: Hoboken, NewJersey, Joh n W iley and Sons, Inc., 596 p.
High land, L., compiler, 2004, Landslide types and processes: U .S. Geological Survey Fact Sh eet 2004-3072 (ver. 1.1), 4 p.
Turner, A.K., and Sch uster, R .L., eds., 1996, Landslides: Investigation and mitigation: Transportation R esearch Board, NationalR esearch Council, Special R eport 247, 673 p.
Several limitations are w orth noting and include th e follow ing.
1) Every effort h as been made to ensure th e accuracy of th e GIS and tabular database, but it is not feasible to completely verify all ofth e original input data.
2) Th e sh allow landslide susceptibility maps are based on th ree primary components: a) calculated factor of safety, b) landslideinventory, and c) buffers. Factors th at can affect th e level of detail and accuracy of th e final susceptibility map include th e follow ing:
a) Factor of safety calculations are strongly influenced by th e accuracy and resolution of th e input data for material properties,depth to failure surface, depth to groundw ater, and slope angle. Th e first th ree of th ese inputs are usually estimates (materialproperties) or conservative limiting cases (depth to failure surface and groundw ater), and local conditions may varysubstantially from th e estimated values used to make th ese maps.
b) Limitations of th e landslide inventory, w h ich w as discussed by Burns and Madin (2009).
c) Infinite slope factor of safety calculations are done on one grid cell at a time w ith out regard for th e adjacent grids. Th eresults may underestimate or overestimate th e level of stability for a certain area. W e developed buffers for areas w ith lowfactors of safety to try to counter th e tendency to underestimate susceptibility. W e developed th e focal relief meth od to try toreduce th e problem of overestimation of susceptibility due to steep slopes w ith low relief. How ever, th e overestimation andunderestimation of susceptible areas is still likely in some isolated areas.
3) Th e susceptibility maps are based on th e topograph ic and landslide inventory data available as of th e date of publication. Futurenew landslides may render th is map locally inaccurate.
4) Th e lidar-based digital elevation model does not distinguish elevation ch anges th at may be due to th e construction of structureslike retaining w alls. Because it w ould require extensive GIS and field w ork to locate all of th ese existing structures and remove th emor adjust th e material properties in th e model, such features h ave been included as a conservative approach and th erefore must beexamined on a site-specific basis.
5) Some landslides in th e inventory may h ave been mitigated, th ereby reducing th eir level of susceptibility. Because it is not feasibleto collect detailed site-specific information on every landslide, potential mitigation h as been ignored.
LOCATION MAP
S cholls
Linnton
Ca nby
P ortla ndHills boro
Dunde e
GladstoneBe a verton
Newb e rg
Va ncouver
S he rwood
Laure lwood
Mount Ta borFore s t Grove
Oregon City
La ke Oswego
S a uvie Is la nd
OREGON
U.S . Geologica l S urvey 7.5-m inute quadra ngle s a nd countie s arela b e le d. This m a p exte nt s hown a s gray recta ngle.
IMPORTANT NOTICE
Th is product is for informational purposes and may not h ave been prepared for or be suitable for legal,engineering, or surveying purposes. U sers of th is information sh ould review or consult th e primary dataand information sources to ascertain th e usability of th e information. Th is publication cannot substitutefor site-specific investigations by qualified practitioners. Site-specific data may give results th at differfrom th e results sh ow n in th e publication. See th e accompanying text report for more details on th elimitations of th e meth ods and data used to prepare th is publication.
*S e e expla na tion of corre s ponding contributing fa ctors b e low.
*High Moderate Low
le s s tha n 1.25 1.25 - 1.5 gre ater tha n 1.5included — —
2H:1V (he ad scarps ) 2H:1V (FOS le s s tha n1.5) —La nds lide De pos its & He ad S carps (S ha llow)Factor of S a fe ty (FOS )
Buffe rs
Final Hazard ZoneContributing Factors
1
1
2
2
3
3
STATE OF OREGONDEPARTMENT OF GEOLOGY AND MINERAL INDUSTRIES
w w w .OregonGeology.org
Ian P. Madin, Interim State Geologist
Th e project described in th is publication w as supported in part byW ash ington County, Oregon, Intergovernmental Agreement 14-0641.
Cartograph y by W illiam J. Burns, Oregon Department of Geology and Mineral Industries.
Th is map also benefited from internal review and comments by DOGAMI geologist Th omas J. W iley.
