Attachment C - WUI Risk Index Methods

Study Session, October 22, 2026 · item 2: Enhancing Boulder’s Wildfire Resilience through (1) the Development of a Comprehensive Wildfire Home Hardening Implementation Hub and (2) a … · 26 pages

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ATTACHMENT C: WILDLAND URBAN INTERFACE (WUI)RISK INDEX METHODOLOGY CONTENTS I. Summary ....................................................................................................................................2 II. Assessment of East vs West WUI .............................................................................................2 Landscape Risk Pillars ..............................................................................................................2 Key Landscape Findings and Geographic Scope......................................................................2 III. Spatial Delineation of the Western WUI Sections ....................................................................4 IV. Scoring Overview .....................................................................................................................6 V. Quantitative Risk Analysis: 70% of Final Score ........................................................................7 1. Susceptibility of Values at Risk Pillar: 40% Weight ...............................................................7 2. Potential Fire Intensity Pillar: 30% Weight...........................................................................10 3. Exposure Pillar: 30% Weight ...............................................................................................15 VI. Qualitative Risk Analysis and Staff Evaluation: 30% of Final Score ......................................16 VII. Composite Prioritization and Risk Grouping .........................................................................16 Risk Categorization Groups.....................................................................................................17 Note on Risk Terminology .......................................................................................................18 VIII. Legal Disclaimer ..................................................................................................................19

I. SUMMARY The City of Boulder Wildland-Urban Interface (WUI) Risk Index is a decision-support framework designed to evaluate relative wildfire risk across residential areas adjacent to wildland fuels. Its primary purpose is to provide a consistent, data-informed foundation for prioritizing wildfire mitigation investments, establishing geographic priorities, and identifying areas where more ambitious risk-reduction actions may be warranted. Please see legal disclaimer at the end of this memo. The result of this work is the creation of 12 Western WUI sections, present in three functional risk groups: Highest Risk (ranks 1–4), Very High Risk (ranks 5–8), and High Risk (ranks 9–12). These groups provide a framework for concentrating work within the WUI and supporting decisions about where additional investment or more aggressive mitigation targets may provide the greatest benefit. The results are also shared on a web map, discoverable by searching “City of Boulder WUI Risk Index” or similar in any search engine.

II. ASSESSMENT OF EAST VS WEST WUI To establish the geographic scope for detailed analysis, City staff first conducted a broad, landscape-scale assessment of wildfire risk across the City-defined WUI.

LANDSCAPE RISK PILLARS The macro-level assessment evaluated three core parameters: • •

•

Likelihood of Occurrence: Historical ignition spatial density and wildfire burn probability modeling. Potential Fire Intensity: Integrated wildfire behavior modeling, including flame length/intensity, predicted fire type (surface versus passive/active crown fire), and rate of spread (ROS). Structural Exposure: Proximity of built structures to contiguous wildland fuel complexes.

For this initial macro-scale assessment, Susceptibility of Structures at Risk was held constant across the city. The purpose of the macro-scale assessment was to identify risk patterns at a city-scale, rather than to explore risk characteristics in depth.

KEY LANDSCAPE FINDINGS AND GEOGRAPHIC SCOPE The broad assessment identified a clear, expected West vs East geographic pattern in relative wildfire risk across the Boulder WUI. Areas east of Broadway generally exhibit lower potential wildfire intensity and structural exposure because of agricultural land uses, flatter slopes, natural areas that are less extensive 2

and more segmented by natural and constructed fuel breaks, including major road corridors and waterways. Conversely, areas west of Broadway contain combinations of steep topography, denser forest fuels, grass-dominated areas capable of supporting rapid fire spread, direct structural exposure to more extensive and continuous areas of wildland vegetation, and topographic and wind conditions that can contribute to significant wildfire behavior.

FIGURE 1 – MAP SHOWING BUILDINGS IN THE CITY OF BOULDER WILDLAND URBAN INTERFACE AND A FIRE INTENSITY RISK MODEL Based on these findings, staff focused the initial higher-resolution Risk Index on the Western WUI, where the combination of landscape conditions and structural exposure creates the greatest concentration of relative wildfire risk. Future phases may adapt this methodology for sectional analysis within the Eastern WUI.

LANDSCAPE COMPARISON AT A GLANCE

Western WUI — West of Broadway

Eastern WUI — East of Broadway

•

Dense and continuous wildland fuels in many areas Steeper topography Higher modeled potential fire intensity Greater direct exposure of structures to wildland vegetation Focus of the current 12-section deep Risk Index

• • • •

Greater influence of agricultural land uses More extensive road, waterway, and irrigation breaks Generally lower slope gradients Lower relative potential intensity and structural exposure in the broad landscape assessment Sectional analysis deferred to future phases

III. SPATIAL DELINEATION OF THE WESTERN WUI SECTIONS To transition from landscape-scale observations to actionable neighborhood-scale planning, staff subdivided the Western WUI into 12 distinct geographic sections. Each section was delineated using consistent spatial criteria: •

•

North and South Boundaries: Major east-west running roadway corridors, hydrological features such as lakes, ponds, creeks, and drainages, and prominent breaks between residential neighborhoods that make up the top (North) and bottom (South) boundary of each section. Western Boundary: WUI Natural Area: Extends approximately 1,500 feet westward from the edge of residential buildings into contiguous wildland and open-space fuels. This planning extent was selected to account for natural areas where wildfire behavior has a direct impact on residential values at risk, and areas where potential long-range ember transport poses an indirect risk to residential Boulder. Eastern Boundary: Captures City-defined Ignition Risk (IR) Zones 1 and 2, encompassing the highest-density structural edge immediately adjacent to the wildland interface.

These boundaries provide a consistent analytical geography for comparing relative risk and prioritizing mitigation across the Western WUI. See the interactive web map for higher resolution.

FIGURE 2 – MAP SHOWING THE 12 WESTERN WUI SECTIONS, WHICH EXTEND 1500’ INTO THE NATURAL ENVIRONMENT AND ENCOMPASS BUILDINGS IN CITY IGNITION RISK ZONES 1 AND 2

IV. SCORING OVERVIEW We sought a way to prioritize the Western WUI Sections by risk. We used a blended quantitative and qualitative approach. Box 2 provides an overview of the approach, which is then explained in more detail in the following parts of this memo.

THE SCORING FRAMEWORK Scoring Elements 1. Quantitative Rank (QuantRank) Quant = 0.40(VAR) + 0.30(INT) + 0.30(EXP) • VAR (Susceptibility of Values at Risk): Evaluated across 5 sub-metrics (structural separation distance, building coverage ratio, home construction age, canopy overhang percentage, building geometry). • INT (Potential Intensity): Evaluated across 3 sub-metrics (fire intensity layer, rate of spread, fire type). • EXP (Exposure of Values at Risk): Single metric derived from spatial analysis. 2. Qualitative Rank (QualRank) Assess the 12 sections against 4 qualitative criteria o Egress and Access Constraints o Operational Suppression Challenges o Localized Fire Behavior Interpretation o General Risk Interpretation 3. Final Rank (Final) Final = 0.70(QuantRank) + 0.30(QualRank) Steps Step 1: Calculate the baseline Quant score for all 12 sections. Step 2: Convert those scores into a QuantRank (1–12). Step 3: Evaluate the 4 criteria to assign a QualRank (1–12) to each section. Step 4: Enter both ranks into the Final formula to determine the definitive priority order.

V. QUANTITATIVE RISK ANALYSIS: 70% OF FINAL SCORE CORE QUANTITATIVE WEIGHTING FORMULA (0.40 × Susceptibility of Values at Risk) + (0.30 × Potential Intensity) + (0.30 × Exposure of Values at Risk) All three pillars are scored on a standardized 1.00–10.00 risk scale.

The quantitative component accounts for 70% of the final priority ranking. Across the 12 Western WUI sections, Likelihood of Occurrence was held constant, based on the assessment that all Western WUI sections share a high baseline probability of wildfire exposure, and that burn probability models are best interpreted at a larger scale than the WUI Sections. The quantitative model therefore differentiates sections using three risk pillars that vary spatially across the Western WUI: 1. Structural Susceptibility 2. Potential Fire Intensity 3. Structural Exposure Each pillar is standardized to a 1.00 to 10.00 scale, with 1.00 representing the lowest relative risk and 10.00 representing the highest relative risk. Examples of risk scoring for the three risk pillars and sub-metrics are shown throughout the following parts of this memo, these examples are taken from various WUI Sections and are noted in the Table and Figure captions.

1. SUSCEPTIBILITY OF VALUES AT RISK PILLAR: 40% WEIGHT The Susceptibility Pillar evaluates characteristics of the built environment associated with the potential for structural ignition and fire transmission. It consists of five sub-metrics, each weighted equally at 20% within the pillar:

SUSCEPTIBILITY SCORE 0.20(Structure Separation Distance) + 0.20(Building Coverage Ratio) + 0.20(Home Age) + 0.20(Canopy Overhang) + 0.20(Building Geometry) All five sub-metrics are scored on a standardized 1.00–10.00 risk scale.

A. STRUCTURAL SEPARATION DISTANCE (SSD): 20% SUB-WEIGHT Evaluates the median distance between adjacent structures as an indicator of structure-tostructure fire transmission potential. TABLE 1 – DAKOTA RIDGE: SSD SCORE Structure Separation Distances (20%)

MAX(1, MIN(10, 1 + (9 / 95) * (100 D10)))

Lowest 100'

1.00

Medium 52.5'

5.50

Highest Density 5'

10.00

Median SSD

8.97

SSD Score

9.62

B. BUILDING COVERAGE RATIO (BCR): 20% SUB-WEIGHT Evaluates structural footprint density relative to parcel area as an indicator of localized building density and potential structure-to-structure fire transmission. TABLE 2 - NORTHBRIAR: BCR SCORE Building Coverage Ratio (20%)

MIN(10, MAX(1, 1 + 22.5 * D20))

Lowest (0%) Low (10%) Moderate (20%) High (30%) Highest (40%)

1.00 3.25 5.50 7.75 10.00

BCR

16%

BCR Score

4.58

C. HOME CONSTRUCTION AGE: 20% SUB-WEIGHT Uses the age of structures as a proxy for the era in which they were constructed and the building standards, materials, and wildfire-related construction practices likely applicable at the time. Home age is not intended to directly measure current building condition or individual structure vulnerability; rather, it provides a consistent spatial indicator of the relative prevalence of older construction within each section.

TABLE 3 - NORTH WONDERLAND: HOME AGE SCORE Age of Home Construction (20%)

MIN(10, MAX(1, 1 + 0.09 * (2025 D30)))

Lowest (2025) Low (2015) Moderate (2005) High (1965) Highest (1925)

1.00 3.25 5.50 7.75 10.00

Median Age

1986

Home Age Score

4.51

D. CANOPY OVERHANG PERCENTAGE: 20% SUB-WEIGHT Measures the proportion of structural roof areas located directly beneath tree canopy. This represents potential exposure to leaf, litter, and branch accumulation, creating fire ignition sources on and around the home, as well as the potential for overstory vegetation to ignite and directly threaten structures. TABLE 4 - WONDERLAND HILLS: CANOPY OVERHANG SCORE Canopy Overhang Ratio (20%)

MIN(10, MAX(1, 1 + 45 * D40))

Lowest (0%) Low (5%) Moderate (10%) High (15%) Highest (20%)

1.00 3.25 5.50 7.75 10.00

Overhang Percentage

13%

Canopy Overhang Score

6.70

E. BUILDING GEOMETRY / COURNER COUNT: 20% SUB-WEIGHT Measures building footprint complexity by counting the number of corners for each building footprint, corners create areas at the base of buildings and on roofs which embers, flammable debris, and fire effects may accumulate around structures and roof geometry increasing ignition potential. High corner counts also correlate with the presence of attached decks, which likewise create void spaces where ember and debris accumulation can increase ignition potential.

TABLE 5 - SANITAS: BUILDING GEOMETRY SCORE Building Footprint Geometry (20%)

MIN(10, MAX(1, 1 + 0.75 * (D50 4)))

Low Complexity (4) Medium (7) High (10) Very High (13) Highest (16)

1.00 3.25 5.50 7.75 10.00

Median Corner Count

8.00

Building Geometry Score

4.00

2. POTENTIAL FIRE INTENSITY PILLAR: 30% WEIGHT The Fire Intensity Pillar is calculated from spatial wildfire behavior models derived from the 2022 Colorado Wildfire Risk Assessment (CO-WRA). It evaluates potential wildfire behavior using three sub-metrics, each weighted equally at 33.3% within the pillar.

POTENTIAL FIRE INTENSITY SCORE

0.33(Fire Intensity) + 0.33(Rate of Spread) + 0.33(Fire Type) All five sub-metrics are scored on a standardized 1.00–10.00 risk scale.

A. FIRE INTENSITY LAYER The Fire Intensity Scale estimates potential wildfire energy output, expressed as kW/m, and classifies potential intensity on a 1-4 scale, with each unit on the scale representing an order of magnitude increase in potential fire intensity and energy input. The Fire Intensity model incorporates surrounding fire behavior when assigning potential intensity to a location and provides an aggregate indicator of wildfire behavior across each WUI Section.

FIGURE 3 – MAP SHOWING THE FIRE INTENSITY LAYER CLIPPED AT 1500’ FROM BUILDINGS IN THE KNOLLWOOD SECTION

TABLE 6 - KNOLLWOOD: FIRE INTENSITY SCORE Fire Intensity (CO-WRA) (1/3)

Value

Raster Pixel Count 166 92 1082 885

Value * Count 166 184 3246 3540

Total Count

2225

7136

Average Value

3.21

1 2 3 4

MIN(10, MAX(1, 1 + ((C65 - 1) / 3) * 9))

Score

7.62

B. RATE OF SPREAD LAYER Rate of Spread is evaluated independently from Fire Intensity because rapid fire movement presents distinct challenges to residential protection. Faster-moving fires may reach residential areas before effective wildfire response can mobilize and before residents can safely evacuate.

FIGURE 4 - MAP SHOWING THE RATE OF SPREAD LAYER CLIPPED AT 1500’ FROM BUILDINGS IN THE FLAGSTAFF SECTION

TABLE 7 - FLAGSTAFF: RATE OF SPREAD SCORE Rate of Spread (CO-WRA Value 4-9) (1/3)

Value 4 5 6 7 8 9

MIN(10, MAX(1, 1 + ((C82 - 4) / 5) * 9))

Raster Pixel Count Value * Count 289 1156 91 455 80 480 259 1813 1115 8920 702 6318

Total Count

2536

Average Value

7.55 Score

19142

7.39

C. FIRE TYPE LAYER The Fire Type layer describes the potential for different types of wildfire behavior, classified as: 1. 2. 3. 4.

Surface Fire Individual Tree or Small Group of Tree Torching Potential Crown Fire Active Crown Fire

Fire Type provides a distinct indicator of potential ember production and transport toward residential areas and of the potential for more intense fire types that challenge suppression and pose greater risk.

FIGURE 5 - MAP SHOWING THE FIRE TYPE LAYER CLIPPED AT 1500’ FROM BUILDINGS IN THE CHAUTAUQUA SECTION

TABLE 8 - CHAUTAUQUA: FIRE TYPE SCORE Fire Type (CO-WRA) (1/3)

Value

Surface (2) Intermediate (4.5) Passive (7.5) Active (10)

Raster Pixel Count 1 2 3 4

Total Count IF((C91 / C93) >= 0.15, 10, MIN(10, MAX(1, SUMPRODUCT(C88:C91, {2;4.5;7.5;10}) / C93)))

Score

Value * Count

2243 299 216 149

2243 598 648 596

2907

4085

3.08

3. EXPOSURE PILLAR: 30% WEIGHT The Exposure Pillar evaluates the potential for wildfire to directly affect residential neighborhoods based on landscape configuration, barriers to fire spread, topography, and prevailing wind alignment. Staff evaluated available roadway, trail, and landscape data and applied defined wildfire exposure criteria. Sections receive higher relative exposure scores when neighborhoods are predominantly unprotected by barriers to wildfire spread and when their orientation aligns with slope or prevailing winds. Sections receive lower relative exposure scores where roads, trails, or other landscape features provide meaningful breaks in potential fire spread and where neighborhood orientation is less aligned with major fire-spread pathways. The Exposure Pillar is therefore intended to distinguish not simply whether wildland fuels are present, but how directly wildfire could affect a particular residential area. TABLE 9 - NCAR NIST: EXPOSURE SCORE Exposure

Analysis of Wildfire Barriers

Score Protected by multiple roads or significant barrier Protected by single road or significant barrier Protected only by a trail, minor barrier or less aligned with wildfire influences Mostly unprotected, and mostly aligned with wildfire influences; OR unprotected but not mostly aligned with wildfire influences Unprotected and aligned with wildfire influences

6.00

2.00

4.00

6.00

8.00

10.00

VI. QUALITATIVE RISK ANALYSIS AND STAFF EVALUATION: 30% OF FINAL SCORE GIS-based modeling cannot fully capture the operational and localized conditions that influence wildfire risk. To incorporate these factors, staff conducted a qualitative evaluation representing 30% of the final priority ranking. Each Western WUI section received a 1–12 qualitative ranking based on four operational criteria: • • •

•

Egress and Access Constraints: Single-point access to neighborhoods, narrow road widths, street-parking constraints, and potential evacuation bottlenecks. Operational Suppression Challenges: Tactical and geographic constraints that may limit effective wildfire response. Localized Fire Behavior Interpretation: Canyon wind-funneling effects, localized topographic features, and microclimate or wind patterns that may not be fully represented in regional modeling. General Risk Interpretation: Professional and experiential staff perspectives on wildfire risk, recognizing that spatial models cannot fully capture all conditions that influence actual wildfire behavior or community vulnerability.

The qualitative assessment supplements, rather than overrides, the quantitative analysis by incorporating operational knowledge and localized conditions that are difficult to represent consistently in GIS-based models.

VII. COMPOSITE PRIORITIZATION AND RISK GROUPING The quantitative analysis produces a risk score, and a relative quantitative rank from 1–12. The qualitative analysis produces a separate relative qualitative rank from 1–12. These rankings are then combined to produce a unified final risk score:

FINAL RISK RANK (0.70 × Quantitative Rank) + (0.30 × Qualitative Rank)

Final priority ranks are calculated in Excel using ascending-order logic. Because Rank 1 represents the highest priority, the smallest weighted composite value receives the highest priority.

The resulting ranking is a relative prioritization of the 12 Western WUI sections, rather than an absolute measure of wildfire risk. The methodology is intended to identify where relative risk is greatest and where additional mitigation investment may warrant prioritization.

RISK CATEGORIZATION GROUPS The 12 Western WUI sections are grouped into three functional risk groups: • • •

Highest Risk: Priority Group 1: Ranks 1–4 Very High Risk: Priority Group 2: Ranks 5–8 High Risk: Priority Group 3: Ranks 9–12

FIGURE 6 - MAP SHOWING THE 12 WESTERN WUI SECTIONS IN THEIR FINAL RISK GROUPINGS

These groups are intended to support differentiated wildfire resilience strategies and more ambitious mitigation targets in sections with the greatest relative risk. Where staff capacity, funding, or other resources constrain the ability to implement comprehensive wildfire resilience measures across the entire Western WUI simultaneously, the groups provide a consistent framework for sequencing investments and establishing geographic priorities. TABLE 10 - FINAL RISK RANKING AND GROUPING FOR 12 WESTERN WUI SECTIONS

Section Name Knollwood Red Rocks Wonderland Hills Flagstaff Devil’s Thumb Shanahan Ridge Chautauqua Sanitas East Chautauqua NCAR NIST Northbriar North Wonderland Shanahan Ranch Dakota Ridge

Risk Adjectiv e

Final Ranking (Quantitative 70%; Qualitative 30%)

Weighted Rank Score (Quantitative 70%; Qualitative 30%)

Qualitative Rank

Quantitativ e Rank

Quantitative Risk Score

Highest

1.30

7.46

Highest

2.40

6.94

Highest

2.90

7.20

Highest Very High Very High Very High Very High

3.70

6.40

5.60

6.32

6.70

6.25

6.90

6.31

7.50

5.77

High

8.60

5.87

High High

10 11

10.10 10.60

8 12

11 10

4.96 5.18

High

11.70

4.35

NOTE ON RISK TERMINOLOGY The grouping framework intentionally does not include Moderate or Low risk categories. All 12 sections are located within Boulder’s Western WUI and therefore share a high baseline level of wildfire risk. The purpose of the grouping system is not to distinguish between areas that

are “safe” and “unsafe,” but to identify relative differences in wildfire risk within an already highrisk landscape. Accordingly, a section categorized as “High Risk: Priority Group 3” should not be interpreted as having low or moderate wildfire risk. It indicates that, relative to the other Western WUI sections, it falls lower in the current prioritization sequence.

VIII. LEGAL DISCLAIMER The purpose of the WUI Risk Index is solely to provide a community and landscape-level overview of general wildfire risks within the assessment area as of the date hereof, and to provide a potential resource for community pre-fire planning. This risk assessment is premised on various assumptions and models which include and are based upon data, software tools, and other information provided by third parties (collectively, “Third-Party Information and Tools”). The City of Boulder relied upon various Third-Party Information and Tools in the preparation of this risk assessment and the City shall have no liability to any party in connection with this risk assessment including, without limitation, as a result of incomplete or inaccurate Third-Party Information and Tools used in the preparation hereof. The City hereby expressly disclaims any responsibility for the accuracy or reliability of the ThirdParty Information and Tools relied upon by The City in preparing this risk assessment. The City shall have no liability for any damage, loss (including loss of life), injury, property damage, or other damages whatsoever arising from or in connection with this risk assessment, including any person’s use or reliance on the information contained in this risk assessment. Any reproduction or dissemination of this risk assessment or any portion hereof shall include the entirety of this plan disclaimer.