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)