Worksheet for creating the RORGEN Fire Assumptions for Your Community

Updated 01/16/2008

Table 1: The existing fire assumptions in RORGEN.

The existing RORGEN fire assumptions were constructed for the Meadow Vista community. The existing assumptions are likely to be inappropriate for your community. Thus, the firefighters in your community need to work with you and me to design a set of assumptions which will indicate (not predict exactly) to homeowners the likely consequences to them for implementing fuel load reduction.

Table 1 showed two types of assumptions: 1) Background (non fuel load) Assumptions, and 2) Fuel Load Reduction Assumptions. The background assumptions are given in the top half of Table 1 and the fuel load reduction assumptions are given in the bottom half.

1) Background (non fuel load) Assumptions: These are assumptions about the non fuel load aspects of wildfire survivability. As you can see in Table 1, currently there are three background assumptions:

1.1) Each structure has a fire-proof roof. The firefighters in Meadow Vista assume that fuel load reduction without fire-proof roofs is not effective. If the home does not have a fire-proof roof, then fuel load reduction is considered irrelevant. In some communities, firefighters will not try to save a home which lacks a fire-proof roof. This indicates the importance of roof to survivability in Meadow Vista.

If, at the beginning of an interview, the homeowner states that the structures do not have fire-proof roofs, the interviewer explains that RORGEN cannot be used to help the homeowner assess the benefits and costs of fuel load reduction. Either the interview ends at that point, or the homeowner decides to proceed with the interview so as to understand more about living in a fire ecology.

If a fire-proof roof is not important in your community, then remove this assumption. This means that the interviewer will not ask about roofs during the pre-interview.

1.2) Property is at least 2 acres but no more than 15 acres large:

The 2 acre lower bound comes from the estimate about what is the smallest acreage that can be protected by the homeowner acting by himself, acting without cooperation from adjacent neighbors. This assumption means that if a homeowner’s parcel is at least 2 acres, then that homeowner’s fuel load reduction can significantly improve the likelihood of survivability during a catastrophic wildfire even if adjacent neighbors do no fuel load reduction.

This assumption enable RORGEN to relate the expenses of a single homeowner to the benefits of that homeowner regardless of what the neighbors do. For the Meadow Vista area, parcels which are greater than 2 acres are considered defensible regardless of the actions of the adjacent neighbors. Thus, when RORGEN is used in Meadow Vista, homeowners must have parcels which are at least 2 acres large in order for RORGEN to give ballpark estimates of benefits and costs to any one homeowner.

In some communities, this 2 acre assumption may not be appropriate. Perhaps some communities are so steep and dense with vegetation that the minimum defensible parcel might be 5 acres. Conversely, other communities may be so flat and sparse with vegetation that the minimum defensible space might be 1 acre.

Note: If, in your community, the minimum defensible space is GREATER than the average parcel size, then most people will not be served well by the existing version of RORGEN. For example, suppose the average parcel size is 1/2 acre and the minimum defensible space is 2 acres. Then, many of the people interviewed would have parcels smaller than the smallest defensible parcel. In this case, RORGEN would have to be significantly rewritten to reflect the communal nature of fuel load reduction.

The 15 acre upper bound was created because of the cost considerations for most of the residential parcels in Meadow Vista. Most residential parcels are less than 15 acres. The average parcel size is between 2 and 3 acres. Thus, the generalized fuel load reduction practices (from which cost estimates came) come from the parcel size statistics. Firefighters in Meadow Vista expect that most parcels will have two zones of fuel load practices: Zone 1 is the area closest to the structures and Zone 2 is the rest of the property. If a residential parcel is larger than 15 acres, the existing cost parameters will overestimate the cost of fuel load reduction. Currently, if a homeowner’s property is greater than 15 acres, the homeowner is told that the costs from the RORGEN will be too large. This has happened once in 15 interviews.

If in your community the average size is very large you may have to consider three zones of protection, each with its own cost parameters.

1.3) Homeowner implements an approved plan of practices: This assumes that homeowners would implement fuel load reduction practices that are approved by fire fighting experts. This tells the homeowner that just removing some of the material he or she identifies may not be adequate. Fuel load reduction means specific practices much in the same way home construction requires specific, approved practices. There are professional fuel load reduction requirements and these must be met in order for RORGEN’s estimates to be useful to the homeowner.

2) Fuel Load Reduction Assumptions: The bottom half of Table 1 gives the fuel load reduction assumptions. These assumptions directly relate fuel load reduction activities to the likelihood of survival for mature trees, and structures with their contents. The numbers given in these assumptions require that the background assumptions are met.



2.1) No Fuel Load Reduction: This says that when a catastrophic wildfire comes to the boundary of a residential property of at least 2 acres whose structures have fire-proof roofs but which has no fuel load reduction, the fire will most likely totally destroy the structures and mature trees. Thus, even if adjacent residential properties have done fuel load reduction, the homeowner’s property size is large enough to isolate his unprotected property from the incidental protection due to a neighbor’s fuel load reduction activities.

This assumption is used to estimate the losses a homeowner is most likely to suffer without doing fuel load reduction. This assumption creates the base case upon which fuel load reduction benefits are estimated.

In your community, the base case may be different. Suppose that in your community the typical residential structure without the benefit of fuel load reduction would, in the face of a catastrophic wildfire, be 80% destroyed. Then, place the number 80 in the field named “Percent of Structures Destroyed”.

2.2) Zone 1 Only Fuel Load Reduction: “Zone 1” is a phrase used to describe the fuel load reduction in areas closest to the structures. Zone 1 is also known as “Defensible Space”. If Zone 1 is the only fuel load reduction treatment which has been installed, then only the areas closest to the structures have been cleared. The remaining property has received no fuel load reduction.

For the Meadow Vista area, the assumption is that fuel load reduction in Zone 1 only reduces the percent destruction. With Zone 1 only, about 30% of the structure and 50% of the mature trees are still destroyed during a catastrophic wildfire. Thus, Zone 1 fuel load reduction has reduced the level of structural destruction from 100% to 30%.

2.3) Zone 1 & Zone 2 Fuel Load Reduction: “Zone 2” is the rest of the residential property. In some communities this is also known as “Healthy Forest”. The vegetative management in Zone 2 is less intense than in Zone 1. For the Meadow Vista area, Zone 2 fuel load reduction costs in Zone 2 required placing an upper limit on the parcel size. The upper limit is about 15 acres. Parcels larger than 15 acres might have a “Zone 3” area in which the vegetative management is even less intense than that in Zone 2.

This part of the fire assumptions provides a general fraction for destruction during a catastrophic wildfire after both Zone 1 and Zone 2 fuel load reduction practices have been implemented. Notice, that in the Meadow Vista area, there is still some loss even after both Zone 1 and Zone 2 fuel load reduction is implemented: 10% of the structure is assumed to be destroyed, and 30 % of the mature trees are assumed to be destroyed. Even with fuel load reduction practices installed, the property could still be destroyed if the fire is hot enough and the flame lengths are long enough.

Worksheet: Table 2 on the next page is the worksheet for modifying the fire assumptions in RORGEN. If you use Table 2 as the framework for your discussions, you will allow RORGEN to be easily modified. This will hasten the implementation of RORGEN in your community.

Final Thought: Remember, the goal of RORGEN is to provide basic estimates to homeowners. There is no goal of giving precise estimates. Basically, RORGEN is designed to help people see the costs of fuel load reduction and get a fundamental idea about the benefits. Thus, it is not necessary to require precision in the fire assumptions. I know every fire is different, and fires (their ignition points, their extents, and the suppression resources available) are very difficult to predict. However, there are probably basic fire statements which could still help homeowners see the benefits and make a more informed decision than they would otherwise. The goal is to move decision-making in the right direction. If people can make higher quality decisions with your fire assumptions than they can without your fire assumptions, then the assumptions will serve them well in making decisions they will understand and agree with one year later.

Table 2: The worksheet for modifying the fire assumptions in RORGEN.
 

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