Improving Fire Hazard Assessment at the Urban-Wildland Interface ...

United StatesDepartment ofAgricultureForest ServicePacific SouthwestResearch StationImproving Fire Hazard Assessment atthe Urban-Wildland Interface: CaseStudy in South Lake Tahoe, CACenter for UrbanForest ResearchInternal ReportFire-1Lisa de Jong, Ph. D.AbstractA fire hazard assessment was conductedon private, developed lots in South LakeTahoe, a high fire hazard urban-wildlandinterface community in Northern California.Fire hazard was assessed in terms of theminimum standards set forth in the NationalFire Protection Association’s (NFPA)Standard 299 and homeowner practicessuch as compliance with the fire safety lawPRC 4291, construction materials of thehome, and irrigation. In addition, theinfluence on small parcel fire hazard byneighbors was assessed.Results indicated that the overall firehazard rating for the city was relatively lowbecause of its good roads, water, signage,and firefighting resources. However, thecitywide non-compliance rate for propertymaintenance was 66%, the citywide noncompliancerate for defensible space was86% when adjusted for small parcel size,and 57% of the parcels were non-compliantfor both defensible space and maintenance.Clearly, homeowners in South LakeTahoe rarely choose for fire safety eventhough the city’s infrastructure is good.Furthermore, results demonstrate thatassessing the city’s fire hazard using NFPA299 alone will underestimate a parcel’s firehazard. Including an analysis of compliancerates and homeowner practices will providea more accurate estimate of individual firehazard.IntroductionThe matrix of homes and landscaping thatmake up the urban forest of urban-wildlandinterface (UWI) communities is defined as afuel matrix by the National Fire ProtectionLISA DE JONG is a research forester at thePacific Southwest Research Station, Center forUrban Forest Research, One Shields Ave.,Rm. 1103, Davis, CA 95616

Association (NFPA 299). In contrast towildland areas, this fuel matrix requiresactive homeowner involvement in firehazard mitigation. This necessity hasresulted in regulations governing thelandscaping practices by private landownersin the high fire hazard areas of California(CA PRC 4291). Designed with theimportance of fuels reduction and structuresurvivability in mind (Brown 1994, Cohen1995, Tran 1992, Foote 1991), PRC 4291requires defensible space for at least 10meters around the home. It limits ahomeowner’s choice of plant species,density, and placement and regulatesmaintenance practices such as pruningtrees and removing ladder fuels.Though compliance with PRC 4291 mayincrease fire safety, the law does notrecognize (1) the variability in the individuallandscaping preferences that people have,or (2) the impact of neighboring parcels on ahomeowner’s fire hazard. Residents at theUWI have unique, individual sets of valuesand preferences that are reflected in thelandscapes they create and maintainaround their homes. These values oftenconflict with the principles of fire safety butproperty owners resist fire-safe regulationsif they detract from what is valued in thelandscape (Manfredo 1990, Abt 1991,Bailey 1991, Cortner 1991, Foote 1991,Smith 2001, Hodgson 1993, Winter 2000).In some communities, the combination ofthese two problems – small lot size andlandscape preferences – poses a significantbarrier to effective individual and communityfire hazard mitigation.This report describes a fire hazard analysisconducted on private, developed lots inSouth Lake Tahoe, California. South LakeTahoe is a high fire hazard UWI communityin Northern California where many of thedeveloped lots are noncompliant with PRC4291 even though there is active agencyoutreach and public support of fuelsreduction on undeveloped lots (Garrett,pers. comm., Harcourt, pers. comm.). Firehazard was assessed according to thestandards in NFPA 299, compliance withPRC 4291, construction materials of thehome, irrigation practices, and the influenceon a parcel’s fire hazard by its immediateneighbors.Results indicated that the overall fire hazardscore for the city was relatively low becauseof infrastructural components such as goodroads, water availability, signage, andfirefighting resources. Analysis of thecomponents that involve homeowner choiceonly (e.g. defensible space, maintenance,irrigation, and construction materials)indicated limited individual, parcel-scale firehazard mitigation efforts.Study siteSouth Lake Tahoe has approximately24,000 inhabitants and is located on thesouth shore of Lake Tahoe, adjacent to theNevada border. The greater Lake TahoeBasin extends across 88,000 hectares intwo states (California and Nevada) and fourcounties (El Dorado, Placer, Douglas andNevada). Its elevation ranges fromapproximately 1900 m at the surface of thelake to about 3300 m at Freel Peak.Historically, fires in the Tahoe Basin wereprobably low and medium intensity surfacefires, occurring every 15 – 25 years andconsuming mostly light surface fuels, rarelybecoming stand-replacing events (Skinnerand Chang 1996). Eighty-five years of firesuppression in the basin (Murphy andKnopp 2001) combined with recent,prolonged drought conditions and extensivebug-kill have led to a build up of highlyflammable, hazardous fuel conditionsthroughout the area.The dominant conifer species in the lowermontane zone is ponderosa pine (Pinusponderosa), with some white fir (Abiesconcolor), incense cedar (Calocedrusdecurrens), and sugar pine (P. lambertiana).The upper montane zone (approx. 2000 –3000 m) is dominated by Jeffrey pine (P.jeffreyi) and also contains red fir (A.2

magnifica), white fir, western white pine (P.monticola) and some pure stands oflodgepole pine (P. contorta). In thesubalpine zone (above 3000 m) speciesinclude whitebark pine (P. albicaulis) andmountain hemlock (Tsuga mertensiana)(Manley and Schlesinger 2001).The average January temperature for thebasin is slightly below 0° C, the averageJuly temperature is approximately 16° C,and the average annual precipitation is 74cm. Average annual snowfall ranges from2.5 m at the elevation of the lake to almost 9m in the mountains. At lake level, there areon average only 70 – 100 frost-free daysper year (U. S. Environmental Data Service2000).Archaeological evidence suggests thathuman habitation of the Tahoe Basin beganwith the ancestors of the Washoe NativeAmericans, who entered the Basin after theSierran glaciers retreated 8,000 to 9,000years ago. The first non-Native settlersarrived in the basin after the discovery ofgold and silver in nearby Virginia City,Nevada. Early industry included logging,ranching, grazing and fishing. By the earlytwentieth century, less than half of the presettlementforest in the Tahoe Basinremained. While timber harvestingdecreased, grazing and ranching continuedbecause of the need for farm products forthe basin’s expanding population.Management agencies and consortiumswere developed in the Lake Tahoe Basin tomitigate the negative ecological impacts ofthe basin’s growing population. Among themost visible are the Tahoe RegionalPlanning Agency (TRPA), the USDA ForestService’s Lake Tahoe Basin ManagementUnit (LTBMU), and Tahoe Re-Green. TheTRPA is a powerful regulatory organizationwhose primary objective is to develop landuse and management standards thatmaximize environmental health and mitigatenegative environmental impacts fromdevelopment (Murphy and Knopp, eds.2000). Since the early 1970s, TRPA hasprohibited development on environmentallysensitive parcels and has regulated privatelandowners’ management of their ownparcels. To compensate landowners, theLTBMU and the California State TahoeConservancy have purchased many ofthese lots. The LTBMU also plays an activerole in fuel management on theundeveloped urban lots owned by theForest Service. Tahoe Re-Green is aninteragency consortium that aims to educateresidents and help them reduce fire hazardby removing fuels on privately owned land.MethodsSampling sites were chosen from apopulation of approximately 6,500 singlefamilyresidential parcels. These parcelswere stratified into low, medium, and highcanopy cover and into low, medium, or highdensity residential. A proportional allocationmethod was used to determine the numberof parcels to be sampled within eachstratum. In total, 102 parcels across the citywere sampled. The vegetation andstructural characteristics of each parcelwere measured and mapped.The city was divided into six neighborhoodsbased on observed differences invegetation, lot, and building characteristics(Fig. 1) (de Jong, in review). The initialocular classification was refined throughstatistical analysis for homogeneity in eachof the defined neighborhoods. Theboundaries of each neighborhoodencompass built areas within city limits andexclude unbuilt areas such as parks andgolf courses. Major roads defined theboundaries between adjacentneighborhoods.Neighborhood 1 was the Tahoe Keys,characterized by wide streets, canals, large,new homes and planted exotic vegetationand turf grass. There was no significantslope on any of the parcels found in thisneighborhood. Neighborhood 2 – 5 weresimilar to one another in terms of smallparcel and home size. Their vegetation was3

dominated by native conifer species with asparse assortment of exotic shrubs andother plants, though the speciescomposition and structure differed betweenneighborhoods. Some parcels were slightlysloped. Neighborhood 2 encompassed alarge area surrounding the “Y,” or thejunction of Highways 50 and 89, includingthe tracts of Gardner Mountain, TahoeVista, Tahoe Island, State Name streets,and the area around the hospital.Neighborhood 3 was the tract consisting ofthe north-central part of the city.Neighborhood 4 was the Sierra Tract.Neighborhood 5 contained the Bijou and AlTahoe tracts. Neighborhood 6 was theaffluent area of Heavenly Ski Resort andwas characterized by large, new homes,large lots, and dominance of native coniferspecies. Slopes in this neighborhood weresignificant.A fire hazard analysis was conducted oneach parcel and then compared qualitativelyto the fire hazard of neighboring parcels.The assessment was based on NFPA 299,which assigns a number score for riskfactors, compliance with PRC 4291,construction materials, and irrigation. Higherscores reflect higher fire hazard.Figure 1. -- Neighborhoods and characteristic lots as defined for this study. 1- Tahoe Keys; 2 – The Y; 3-North Central; 4- Sierra; 5- Bijou; 6- Heavenly.Lake Tahoe35 61424

The law (PRC 4291) requires homeownersto prune dead branches, clear needles andother litter from roofs and gutters, covervents with wire mesh, and clear treebranches for 3 m around chimney outlets.Characteristics known to contribute tostructural ignition potential, such as a woodroof and single-paned windows (Foote1991, White 2000, Quarles 2001, Quarles2002), were also rated.Compliance with PRC 4291 was analyzed interms of the creation of defensible spacealone, maintenance alone, and thecombination of defensible space andmaintenance. Parcels demonstrating little orno defensible space were rated noncompliant.Parcels that were non-compliantwith one or more of PRC 4291’smaintenance requirements were alsoconsidered non-compliant.Parcels were further assessed for presenceof irrigation, construction materials, parcelsize, and the presence of hazardous decks.Defensible space ratings were adjusted forsmall parcels to account for neighboringvegetation that would influence the parcel’sfire hazard. Decks were consideredhazardous if they were made of wood andgreater than 0.5 m high and were openunderneath or had flammable materialstored underneath them.Parcels were classified as small and underthe direct influence of the fire hazard ofimmediate neighbors if the distancebetween the house and the side boundariesof the parcel was less than 7 m on eitherside, if the difference between the totalwidth of the parcel and the total width of thehouse was less than 14 m, or if thedifference between the total length of theparcel and the total length of the house wasless than 14 m. Larger parcels wereconsidered independent of neighboringparcels.The fire hazard ratings of the individualsmall parcels were adjusted to include thefire hazard of neighboring parcels. Smallparcels with good defensible space and“relatively better” maintenance were ratedthe same for defensible space as a mediumor large parcel with moderate defensiblespace. Small parcels with good defensiblespace and “same” relative maintenancewere rated the same for defensible spaceas a medium or large parcel with gooddefensible space.Neighborhoods were assigned a mean firehazard rating based on the fire hazards ofthe parcels that were sampled within theneighborhoods. The point scoring system isfound in Table 1. The range of possiblescores is 9 – 80 or more, depending on thenumber of decks present.ResultsOverall fire hazard ratingThe mean citywide fire hazard rating was 30(s.d. 6), due in large part to the city’sinfrastructure, including good access (wide,paved roads), the availability of water, andthe presence of city and agency fire-fightingresources (Table 2). As expected, theTahoe Keys exhibited the lowest firehazard, with a mean fire hazard rating of 24(s.d. 5), and the Heavenly Ski Area had thehighest (38, s.d. 7). The rating for theremaining neighborhoods ranged from 28 to30.Lot sizeThe sampled lots in South Lake Tahoe weresmall. Mean lot size varied from 585 m 2 inthe Sierra tract to 1211 m 2 in Heavenly. Thedifference in size is explained by thevariation in the depth of the lots rather thantheir width. The mean lot width citywide was22 m (s.d. 8.5 m). The lot size in the Sierratract was significantly smaller than any otherneighborhood in the city.5

Table 1. Point scoring system for risk factors.RISK FACTORIngress/egressPrimary road widthAccessibilitySCORING1- two or more primary road3- one road, primary route5- one way in/out1- >6.1m3- 15m3- outside radius 40%Wall materials+1 – wood sidingWall, eave, roof vents+2 – some present without _ in. mesh coverPredominant number of window panes+1 – predominantly single-panedDeck height +1 – each deck with height > 0.5mOpen space below deck+1 – each deck with open space beneathStorage of flammable materials under deck +1 – each deck with storage of flammables beneathDeck materials+1 – each wooden deckParcel sizeAdjustments made for small parcelsRelative maintenance1- parcel is worse than neighbors3- about the same5- neighbors are worse than parcel6

Table 2. Fire hazard rating, non-compliance rates, and risk factors in South Lake Tahoe neighborhoods.Risk FactorCityNeighborhoodTotal(n=102)Keys(n=15)The Y(n=22)N. Central(n=13)Sierra(n=22)Bijou(n=21)Heavenly(n=9)Mean fire hazardrating (s.d.)30 (6) 24 (5) 30 (4) 30 (6) 30 (6) 28 (5) 38 (7)Maintenance noncompliancerateIndividual def.space noncompliancerateIndividual totalnon-compliancerateIndividual def.space noncompliancerate(adjusted forsmall parcels)Individual totalnon-compliancerate (adjusted forsmall parcels)Irrigation (% ofparcels with lessthan half irrigated)Mean slope %(s.d.)Wood exterior (%of homes)Wood roof (% ofhomes)Window hazard(% of homes)Deck hazard (%of homes)66 20 68 69 73 76 8975 47 86 85 77 62 10053 7 59 62 64 48 8986 80 91 92 82 81 10057 20 59 62 64 58 8952 13 45 69 59 58 782 (6) 0 (0) 0 (0) 0 (0) 1 (3) 0 (0) 15 (16)96 87 95 100 100 95 10031 27 18 54 27 29 5629 27 41 23 32 24 2267 60 68 77 73 48 89Compliance with PRC 4291Citywide, the majority of parcels hadincreased fire hazard ratings because theywere partially or wholly non-compliant withPRC 4291. Sixty-six percent of the sampledparcels were non-compliant with the law’srequirements for maintenance and 75%exhibited little or no defensible space. Intotal, 53% of the parcels were noncompliantfor both maintenance anddefensible space.When taking small parcel size intoconsideration, i.e. including the vegetationof neighboring parcels in the defensiblespace analysis, 86% of the parcels werenon-compliant for defensible space and57% were non-compliant for bothmaintenance and defensible space.Adjusting the defensible space rating toaccount for neighboring lots had thegreatest effect on the defensible spacecompliance rates for the Keys and for Bijou.Smaller changes were observed for the Y,

North Central, and Sierra. It had no effecton the 0% defensible space compliance ratefor Heavenly.IrrigationOver half the parcels citywide had irrigationon less than half of the vegetation found onthe parcel. The vegetation in the Keys,which was dominated by turf grass andplanted exotics, was well irrigated, whileover 75% of the parcels in Heavenly, whichwere dominated by native conifer stands,showed little evidence of irrigation. Lessthan one third of the parcels in the NorthCentral neighborhood were irrigated. From41% - 55% of the parcels in the otherneighborhoods were irrigated.SlopeMost of the parcels that were sampledexisted on little or no slope, with theexception of parcels in Heavenly, where themean was 15% and the range was from 0%to 53%.Wall materialThe preferred building material for homes inSouth Lake Tahoe was wood. 96% of thehomes in the sample had exterior walls thatwere shakes, logs, or wood siding. Thirteenpercent of the homes in the Keys werepredominantly brick, stucco, or stone, butfrom 95 – 100% of the homes in theremaining neighborhoods had woodexteriors.Roof materialCitywide, 31% of the sampled homes had asignificant increase in susceptibility toignition because of wood roofs.Neighborhoods where more than half thehomes had wood roofs were North Central(54%) and Heavenly (56%). The fewestnumber were found in the neighborhood ofthe Y (18%).Window panesCitywide, 29% of the sampled homes hadincreased fire hazard due to the presence ofsingle-paned glass in over half the windowsof the home. The highest percentage ofhomes that had predominantly single-panedwindows was in the neighborhood of the Y(41%), while the lowest was in Heavenly(22%).Hazardous decksHazardous decks were found on 67% of thehomes citywide. Deck construction andplacement was particularly problematic inHeavenly, where slopes were greatest. Inthat neighborhood, eight of the nine parcelssampled had hazardous decks. In Bijou only48% of the homes had hazardous decks,while 60 – 73% of the homes in theremaining neighborhoods had hazardousdecks.DiscussionThe results of this study clearly indicate thatstandard city-scale fire hazard rating inSouth Lake Tahoe will not providemanagers and planners with sufficientlydetailed information to implement aneffective fire hazard mitigation program.While the city’s infrastructure is good,individual homeowners in the communityrarely choose for fire safety in terms ofconstruction materials, propertymaintenance, and landscaping or defensiblespace. The problem of non-compliance isworsened by the fact that many of the city’slots are so small they are influenced by thefire hazard of neighboring lots.Furthermore, results indicate that firehazard rating should be improved toaccount for the fire hazard created byneighboring vegetation and houses in areasdominated by small lots. Also, eachcomponent of a fire hazard rating systemshould produce results than can be used asdecision support for fire management,including identifying priority areas fortreatment, identifying problematic areas in8

terms of non-compliance, and identifyingreasons for non-compliance. Each of the sixneighborhoods in South Lake Tahoe has aunique profile in terms of the suite of factorsthat contribute most significantly toneighborhood-scale fire hazard. Theneighborhood profiles can be used to directand focus management and homeownereducation efforts. The obvious differencebetween the Keys and Heavenly, forexample, provides managers with a clearset of management objectives, but there arealso important, less obvious differencesbetween the other neighborhoods.For example, results indicated that the Yhad an average fire hazard for the city, butcompared to the rest of the city thisneighborhood was characterized by a lowpercentage of wood roofs, better irrigation,average compliance with PRC 4291, and anaverage hazard created by decks.However, the fire hazard rating was thesame as the citywide average becausethese positive factors were offset by its 85%non-compliance rate for defensible space.This figure increased to 91% when adjustedfor small lot size. Compared to Sierra, the Yhad a lower compliance rate but rated betterin terms of construction materials anddecks. Compared to North Central, the Yexhibited comparable compliance rates andwas slightly better in terms of deck hazards,but the Y rated far worse in terms of thepercentage of wood roofs and single-panedwindows. These data can guidemanagement decisions, including both fuelsreduction programs and outreach andeducation that focus on the particular needsin each neighborhood.In addition to the education efforts thatfocus on defensible space andmaintenance, there is clearly a need toeducate residents about other practicesrelated to fire hazard. Education is neededin Heavenly and elsewhere about thebenefits of irrigation in terms of fuelmoisture content and the relationshipbetween drought stress, bug kill, and firehazard. Most homes already have doublepanedwindows for better insulation againstwinter weather, but many residents do notrealize that double-paned windows alsodecrease the risk of structural ignition.Hazardous decks are a chronic problem inHeavenly, where most decks hang oversteep slopes covered with continuoussurface fuels. In this community, which ischaracterized by small lots and manyseasonal residents, education on theimportance of neighborhood-scalecooperation is critical.In sum, standard fire hazard analysis inSouth Lake Tahoe and similar communitiesis likely to underestimate individual firehazard. First, the city’s fire-safeinfrastructure is included in the parcel-scaleanalysis, which offsets the increasedindividual fire hazard associated with a lackof defensible space and maintenancearound the home. Compliance with PRC4291 is perhaps the most important factor instructure survivability, but the influence ofthe city’s good infrastructure on its overallfire hazard rating obscures the fact thatthree-quarters of the parcels citywide arenon-compliant with defensible space codesand two-thirds are non-compliant withmaintenance codes. Second, the hazard onsmall lots will be further underestimatedbecause of the influence of the fire hazardof neighboring parcels.A more appropriate approach to fire hazardassessment in South Lake Tahoe is toassess parcels for compliance, lot size, andthe individual choices homeowners make interms of construction materials andirrigation. Analysis of compliance rates andhomeowner choices will provide a moreaccurate estimate of individual fire hazard.The analysis can also serve as decisionsupport for focusing outreach and educationefforts and for prioritizing areas that aremost in need of homeowner compliance andcooperation.9

AcknowledgementsThe author would like to thank SteveLennartz, Tommy Mouton, Nancy Strahan,Torry Ingram, Sabrina Mathis, and StephanStreiling for their field work. Also, the LakeTahoe Basin Management Unit (USDAForest Service) and the CaliforniaDepartment of Forestry and Fire Protectionwere very helpful with their insights andsupport. Much appreciation also goes to Dr.E. Gregory McPherson (Project Leader,USDA Forest Service, Center for UrbanForest Research), to Dr. Madalene Ransom(State Economist, USDA Natural ResourceConservation Service), and to Scott Maco(Urban Forester, USDA Forest Service,Center for Urban Forest Research) forcomments on the early drafts of this report.This study was part of a project fundedthrough the National Fire Plan.Literature CitedAbt, R. C., M. K. Kupyers and J. B. Whitson.1991. Perception of fire danger andwildland/urban policies after wildfire.USDA Forest Service. GTR SE-69, pp.257-259.Bailey, D. W. 1991. The wildland-urbaninterface: social and politicalimplications in the 1990’s. FireManagement Notes 52(1): 11-18.Cohen, J. 1995. Structure IgnitionAssessment Model (SIAM).USDAForest Service. PSW-GTR-158.Cortner, H. J. 1991. Interface policy offersopportunities and challenges: USDAForest Service strategies andconstraints. Journal of Forestry 89(6):31-34.de Jong, L. 2002 (in review). UrbanForestry at the Urban WildlandInterface: Case Study in South LakeTahoe.Foote, E., R. Martin, J. K. Gilless. 1991. Thedefensible space factor study: asurvey instrument for post-firestructure loss analysis. Proceedings,11 th Conference on Fire and ForestMeteorology. Society of AmericanForesters. p. 66 – 73.Garrett, Brian. Urban Lot Manager, USDAForest Service Lake Tahoe BasinManagement Unit (LTBMU). May 7,2002.Harcourt, Steve. Area Forester, CaliforniaDepartment of Forestry and FireProtection. Telephone conversation,February 11, 2002.Hodgson, R. W. 1993. Perceptions ofDefensible Space: PerceivedCharacteristics that InfluenceWildland-Urban Intermix Residents toAccept or Reject Fire SafeLandscaping. Prepared for theCalifornia Department of Forestry andFire Protection.Manfredo, M. J., M. Fishbein, G.E. Haasand A.E. Watson. 1990. Attitudestoward prescribed fire policies.Journal of Forestry 88(7): 19–23.Manley, P. and M. Schlesinger. 2001. FinalReport for the California TahoeConservancy and US Forest Service.April, 2001.Murphy, D.D., C. M. Knopp, eds. 2000.Lake Tahoe watershed assessment:Volume I. Gen.Tech.Rep. PSW-GTR-175. Albany, CA: Pacific SouthwestResearch Station, Forest Service, U.S.Department of Agriculture; 736 pp.NFPA. 1997. NFPA-299: Standard forProtection of Life and Property from10

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