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https://www.nist.gov/el/fire-research-division-73300/wildland-urban-interface-fire-73305/hazard-mitigation-methodology-9

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Engineering LaboratoryFire Research Division

## Wildland-Urban Interface Fire Group

-  HAZARD MITIGATION METHODOLOGY (HMM)  Expand or Collapse    -  Why Implement HMM in your Community?  Expand or Collapse      -  Exposures During WUI Fires 
    -  HMM vs Traditional Hazard Mitigation 
    -  WUI Definitions 
    -  Structure Survivability in the WUI 
    -  Community Survivability in the WUI 
    -  Paths forward: Implementing HMM 
    -  Communities already implementing HMM 
  -  How Fire Spreads in the WUI  Expand or Collapse      -  How do Structures Survive WUI Fires? 
    -  Fire Spread Pathways 
    -  Why Exposure Matters, not Parcel Boundary 
    -  Hazard from Fuel Agglomeration 
    -  Community-scale WUI Fire Hazards 
  -  Community-Wide Structure- and Parcel-Level Hardening Actions  Expand or Collapse      -  How Density Impacts Community 
    -  Workflow for HMM Hardening Actions 
    -  Structure Hardening for Embers 
    -  Parcel Hardening for Fire: the Three R’s 
    -  Surrounding Parcel Hazard Mitigation 
    -  Primary Parcel Hazard Mitigation 
    -  Structure Hardening for Fire 
    -  Communities Hardening Actions 
  -  Hardening Actions: Construction Guidance 
  -  Contacts 

# WUI Definitions

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### **Traditional WUI Definitions**

The common conceptual definition of wildland-urban interface (WUI) is the geographical area where human development, including structures and other infrastructure, meets or intermixes with undeveloped wildlands. 

### **The WUI is often further distinguished into two WUI types based on housing densities:**

                  - Interface*:* High-density development adjacent to undeveloped wildland vegetation.
                  - Intermix*:* Lower-density housing mingled with undeveloped wildland vegetation.

While these definitions can be useful to identify general areas, they do not focus on the parcel-level details that characterize parcel-to-parcel and structure-to-structure fire spread that is characteristic of many WUI fire disasters.

----

### WUI Definition by SSD and Parcel Size

More specific WUI definitions are established in the HMM by explicitly adding the concept of structure separation distance (SSD). These definitions provide perspective and can help develop effective parcel-level mitigation strategies based on housing density and potential fire exposures.

### ***WUI Type 1 and Type 2:***

High-density interface communities with an SSD as small as 6 ft and parcel size of less than 0.5 ac. Type 1 is characterized by Perimeter and Type 2 as Interior WUI community. Significant portions of communities can get consumed in fire even with small number of ignitions in such settings.

### *WUI Type 3, Type 4, and Type 5:*

Medium-density construction with SSD ranging from 30 ft to 100 ft with parcel size in excess of 1 ac. Type 3 and Type 4 are characterized as interface and Type 5 as intermix communities.

### *WUI Type 6 and Type 7:*

Low structure density at the interface (Type 6) and intermix (Type 7).

**WUI Type 2**

**WUI Type 4**

***WUI Type 6.** Low Density Intermix, The Trails, Rancho Bernardo, CA. Note irrigated lawns, long paved driveways, pools and tennis courts. Some structure-to-structure distances are representative of Moderate Density Intermix (lower part of image). Also note limited high density non-irrigated vegetative loading.*

#### WUI Types classified by structure separation distance (SSD) and typical parcel size.

-  **‹** HMM vs Traditional Hazard Mitigation 
-  Structure Survivability in the WUI **›** 

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