ShorelineandCoastalPlannersGroupSteepSlopesPresentation
BradMurphy,SeniorPlannerThurstonCountyResourceStewardship
TheUrbanNews,AshevilleNC
Geologically Hazardous Areas: Two Main Relevant Types
Landslide hazard areas Erosion hazard areas
Ecology guidelines are not specific to other geologicalhazards addressed under GMA (seismic, mineandvolcanichazards)
WAC 173-26-221(2)(c)(ii)
HaveseparateChaptersinThurstonCountyCodefor:
• GeologicHazardAreas(Chapter24.15TCC)
• SeismicHazardAreas(Chapter24.16TCC)
• VolcanicHazardAreas(Chapter24.17TCC)
• MineHazardAreas(Chapter24.18TCC)
Specific Critical Areas
Geologically Hazardous Areas: keyprinciples
“Do not allow newdevelopment or the creationof new lots that would causeforeseeableriskfromgeological conditions topeople orimprovementsduring thelifeof thedevelopment.”
“Do not allow newdevelopment thatwouldrequire structural shorelinestabilization overthelife ofthe development.”
WAC 173-26-221(2)(c)(ii)(B) and (C)
24.15GeologicHazardsStandardsetback(landslideandmarinebluffhazard) isGREATERof
following:
• 50feetfromtoeandtopofslope,or:
• Distancefromtoeofslopeupwardatslopeof2:1untilintersectingwithtopographyofsite.
• Minimumdistancerecommendedingeotechnicalassessment
TCC24.15GeologicHazards
Atleast15feethighw/>40%slope
DeepSeatedLandslideSunsetBeachLandslide,Eld Inlet,Olympia,Washington
Thirddocumentedre-activationwasin2007Threehomesseverelydamaged
Typicalgeometrywitharc-shapedscarpsin“Stepformation”
RockCandyMountainSlide– ThurstonCounty
2014ThurstonCountyWaterResourcesStudyIdentified:
• +70Landslidesingeomorphicexercise• Preliminary‘risks’• 7PhysiographicAreasforHigherLandslidePotential• TwoLakeswith“Seiche”susceptibility(couldbemore)• Scopingoutlatestresearchonwhatisknown• IncludingWADNRViewer• Theydidnotidentifyan“Oso”TypeSlidepotentialinThurstonCounty
Geo-hazards: Typical CAO approaches can be adequate
Setbacks from edge of hazard areas
Requiring projects within 300’ of a hazard toinclude geotechnical report, to demonstratehow hazards are to be avoided
For erosion-proneareas, require SoilErosion and Sediment Control Plan
Commonpractice: Require Reportfrom licensed civil engineer orengineering geologist.Photo by Len Palmer
Geologically hazardous areas: exceptions allowed when…
“Exceptions may be madefor thelimited instances wherestabilization is necessary to protect allowed uses wherenoalternative locations areavailableand no net loss ofecological functions willresult.”
“Stabilization structures or measures to protect existingprimary residential structuresmay beallowed wherenoalternative exists and ifno net loss of ecological functionswillresult.”
Useof “NoNetLoss” = requiremitigationforunavoidableimpacts…
WAC 173-26-221(2)(c)(ii)(C) and (D)