Community Survivability in the WUI

HMM goes beyond the traditional approach of protecting individual homes and addresses the unified problem of protecting entire WUI communities to give them the best chance of survival.

Properties throughout a community need protection from fire and embers. The spacing between homes and fuels within a community affect its survivability. In a low-density community the biggest threat will likely be embers, whereas in a high-density community the biggest threat will likely be fire exposures from an adjacent home.

Low Structure Density (Low SSD): One Structure Ignition ⇨LEADS TO⇨ Limited Additional Losses

Moderate Structure Density (Moderate SSD): One Structure Ignition ⇨LEADS TO⇨ Variable Losses

High Structure Density (Low SSD): One Structure Ignition ⇨LEADS TO⇨ Large Losses

For WUI communities to survive, it is essential that high-density communities implement HMM and enforce complete structure and parcel hardening. It is desirable for moderate and low-density communities to implement the HMM.

High-Density Communities
Community participation is necessary in high-density communities. This is due to the disproportionate impact of a single structure ignition on the community. There are many ways to accomplish this via voluntary or mandatory community hazard mitigation programs. Different programs can be implemented using HOA, county, or state rules and regulations. Hardening structures and parcels for ember exposures is important in these communities, as these exposures can originate from outside the community, and also ensure that there are no fire spread pathways that can generate fire exposures to residences. In certain scenarios, additional protection may be provided by structure cladding (siding and roofing) hardening requirements (such as included in CA Chapter 7A). While these requirements add significant value for protecting against low and moderate exposures, they will frequently not be able to cope with the full exposure assault of a residence burning 6 ft to 10 ft away because of limitations of one of the components in the wall or roof assemblies. Parcel-level hardening is a critical requirement in these high-density communities, and auxiliary fuel removal (instead of displacement) will likely be necessary on very small lots. A key issue that will need to be addressed is the presence of vehicles in the community, as they can generate locally high fire exposures resulting in structure ignitions. Ongoing research by NIST, CAL FIRE, and IBHS aims to provide guidance on minimum spacing between vehicles and residences in the future.

Moderate-Density Communities
Moderate-density communities offer more options to residents for community participation in hazard mitigation. In moderate-density construction, the ignition of a single residence may directly impact one or more adjacent residences. However, direct fire (via radiation and convection) progression across the community will depend on local construction, spacing, and fuels more than in high-density. Here there is an opportunity to slow down that fire spread and reduce the structure ignition pathways by selectively hardening structure sides that may see increased exposures. This selective structure hardening reduces overall community hardening program costs while increasing community resilience. Hardening of all structures for embers is still necessary in these communities. Fuel displacement, reduction, and removal may also need to be implemented to reduce fire exposures.

Low-Density Communities
These community types, by the nature of their structure density, do not present a direct structure to structure fire spread pathway as can be expected in high-density and certain moderate-density configurations. These larger parcels also offer a significantly large number of fuel displacement options to residents, as auxiliary parcel level fuels can be placed where they will not impact other fuels, the primary residence, or neighboring structures. The notion of sacrificial auxiliary structures/features can be introduced in this setting; a resident with a large lot may be willing to accept the loss of an auxiliary structure that is located far from their residence or other features to be protected. In a low-density community this may be a readily acceptable hazard if the residence is hardened for ember exposures and all other potential fire propagation pathways near the residence have been disrupted (using the provided methodology).