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City of Portland

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24.51.030 Wildfire Hazard Zone Map Adoption.

A. Wildfire Hazard Zone Map Adoption.

1. A Wildfire Hazard Zone Map (WHZM) has been developed for the City of Portland through a review of topography, weather, type vegetation and fuel density. This map is dated October 11, 2002.

2. The WHZM dated October 11, 2002, is hereby adopted by reference and incorporated into this ordinance.

3. The Chief shall provide the Director with a copy of the official map adopted in Subsection one of this Section. Copies of the map shall be available for review in the Development Services Center, First Floor 1900 SW 4th Avenue, Portland Oregon.

B. Revisions to the Wildfire Hazard Zone Map.

1. The WHZM may be amended from time to time to either include or exclude properties as the facts may warrant.

2. The Chief shall have the authority to revise the Wildfire Hazard Zone Map.

3. All Wildfire Hazard Zone map revisions shall be determined using the criteria set forth below. Any site having a cumulative hazard value of five (5) or more shall be included in a wildfire hazard zone.

a. Topography Hazard Factor Value. The topography hazard value shall be calculated as follows:

(1) Determine site slope using the appropriate 7.5 minute quadrangle map published by the U.S. Geological Survey, USDI.

(2) Select appropriate hazard value using Table 1.

TABLE 1

APPROPRIATE TOPOGRAPHY
HAZARD FACTOR VALUE

Site Slope as determined by

the 7.5 minute quadrangle map

Hazard Value

Slopes 00 to < 03%

0

Slopes 03 to < 12%

1

Slopes 12 to < 20%

2

Slopes 20% or greater

3

b. Natural Vegetative Fuel Hazard Factor Value. The natural vegetative fuel hazard value shall be calculated as follows:

(1) Divide the jurisdiction into geographic areas which best describe the natural vegetation expected to occupy sites for the next 10 to 15 years.

(2) Select the appropriate hazard value from Table 2.

TABLE 2

NATURAL VEGETATIVE FUEL
HAZARD FACTOR VALUE
 

Natural Vegetative Fuel Description 1

Hazard Value 2

Limited

Little or no natural vegetative fuels are present.

0

Grass

Very little shrub or timber is present, generally less than one-third of the area. Main fuel is generally less than two feet in height. Fires are surface fires that move rapidly through cured grass and associated material. (Fuel model 1)

3

Grass

Open shrub lands and pine stands or scrub oak stands that cover one-third to two-thirds of the area. Main fuel is generally less that two feet in height. Fires are surface fires that spread primarily through the fine herbaceous fuels, either curing or dead. (Fuel model 2)

3

Grass

Beach grasses, prairie grasses, marshland grasses and wild or cultivated grains that have not been harvested. Main fuel is generally less than four feet in height, but considerable variation may occur. Fires are the most intense of the grass group and display high rates of spread under the influence of wind.(Fuel model 3)

3

Shrubs

Stands of mature shrubs have foliage known for its flammability, such as gorse, manzanita and snowberry. Main fuel is generally six feet or more tall. Fires burn with high intensity and spread very rapidly. (Fuel model 4)

3

Shrubs

Young shrubs with little dead material and having foliage not known for its flammability, such as laurel, vine maple and alders. Main fuel is generally three feet tall or less. Fires are generally carried in the surface fuels and are generally not very intense. (Fuel model 5)

1

Shrubs

Older shrubs with foliage having a flammability less than fuel model 4, but more than fuel model 5. Widely spaced juniper and sagebrush are represented by this group. Main fuel is generally less than six feet in height. Fires will drop to the ground at low wind speeds and in stand openings. (Fuel model 6)

2

Timber

Areas of timber with little undergrowth and small amounts of litter buildup. Healthy stands of lodgepole pine, spruce, fir and larch are represented by this group. Fires will burn only under severe weather conditions involving high temperatures, low humidity and high winds. (Fuel model 8)

1

Timber

Areas of timber with more surface litter than fuel model 8. Closed stands of healthy ponderosa pine and white oak are in this fuel model. Spread of fires will be aided by rolling or blowing leaves. (Fuel model 9)

2

Timber

Areas of timber with heavy buildups of ground litter caused by over-maturity or natural events of wind throw or insect infestations. Fires are difficult to control due to large extent of ground fuel. (Fuel model 10)

3

1. Some areas may contain vegetative fuels other than those listed in Table 2.  Additional natural fuel hazard factors may be found  in “Aids to Determining Fuel Models for Estimating Fire Behavior” published by the Forest Service, USDA Intermountain Forest and Ranger Experiment Station in 1982 as General Technical Report INT-122.  Vegetative fuel hazard factors determined using General Technical Report INT-122 shall be used  as alternative factors, for review under this chapter, as the facts warrant.

2. Due to various factors, such as variations in local vegetation species or vegetation conditions, the fuel models used in Table 2 may not accurately portray wildfire behavior.  The Chief may make modifications to the hazard values as necessary to accurately reflect the following characteristics:

(a) A hazard value of 1 shall describe vegetation that typically produces a flame length of up to 5 feet, a wildfire which exhibits very little spotting, torching, or crowning, and which results in a burned area that can normally be entered within 15 minutes.

(b) A hazard value of 2 shall describe vegetation that typically produces a flame length of 5 to 8 feet, a wildfire which exhibits sporadic spotting, torching, or crowning, and which results in a burned area that can normally be entered within one hour.

(c) A hazard value of 3 shall describe vegetation that typically produces a flame length of over 8 feet, a wildfire that exhibits frequent spotting, torching, or crowning, and which results in a burned area that normally cannot be entered for over one hour.

c. Natural Vegetative Fuel Distribution Hazard Factor Value . To determine the natural vegetative fuel distribution hazard factor value:

(1) Determine the percentage of each individual area that is covered by vegetation.

(2) Using the calculated percentage, assign a value using Table 3.

TABLE 3

NATURAL VEGETATIVE FUEL
DISTRIBUTION HAZARD FACTOR
 

Natural Vegetative Fuel Distribution

Hazard Value

0 to 10% of the area

0

10 to 25% of the area

1

25 to 40% of the area

2