- Title:
-
Surface Fine Fuel Hazard Rating Forest Fuels in East Gippsland
- Date:
- October 2004
- Organisations
- DSE
- Authors:
- Gregory J. McCarthy
- Location:
- Victoria, VIC, Australia
Overview
The method of assessing surface fine fuel, based on measurements of surface fine fuel load, was examined to investigate whether this could be replaced by measurements of structural fuel. A faster and more cost-efficient method of estimating surface fine fuel hazard was sought.
Surface fine fuel load, litter-bed height, stand height and near-surface fuel abundance were assessed on 12, one-hectare study plots dispersed through the foothill and coastal sclerophyll forests of East Gippsland, Victoria. Average litter-bed heights ranged from 20 to 48 mm, surface fine fuel loads from 6 to 20 t/ha and stand heights from 20 to 38 m.
Relationships were investigated between:
- surface fine fuel load and stand height
- litter-bed height and stand height
- surface fine fuel load and litter-bed height.
Surface fine fuel load and litter-bed height were both found to be correlated with stand height, and it is suggested that measurements of stand height over broad areas may be useful for predicting surface fine fuel hazard levels. Further research is required to confirm this, however. Surface fine fuel load was found to correlate with litter-bed height, the correlation improving when the plots with litter-only fuels (i.e. little or no near-surface fuels) were included in the analysis.
Nine of the study plots, located in current fuel reduction burning areas, were subjected to experimental burning. Forest Fire Danger Indices (FDIs) at the time of the experimental fires ranged from FDI 2 to FDI 6, forward rates of spread ranged from 29 to 100 m/hr and flame heights ranged from 0.4 to 1.3 m.
Models of fire behaviour were developed which related:
- forward rate of spread to surface fine fuel load
- forward rate of spread to litter-bed height
- flame height to surface fine fuel load
- flame height to litter-bed height.
Better correlations were found between the two fire-behaviour variables and litter-bed height than between these two variables and surface fine fuel load, indicating that, at low FDIs, fire behaviour was better related to litter-bed height than it was to surface fine fuel load. It is therefore proposed that measurement of litter-bed height could be used to replace measurement of surface fine fuel load for assessments of surface fine fuel hazard.
Five classes of litter-bed height were suggested to correspond with the five categories of surface fine fuel hazard—Low, Moderate, High, Very High and Extreme—described by Wilson (1993) but which, in that report, were based on measurement or estimation of surface fine fuel load.
This proposed method of assessing surface fine fuel hazard could then be combined with assessments of bark hazard and elevated fuel hazard to derive an Overall Fuel Hazard rating for a site. A series of reference photographs of the various categories of surface fine fuel hazard are presented to assist quick visual assessments.
Increases in forward rates of spread and flame height were observed to be associated with certain levels of near-surface fuels. It was proposed that the occurrence of near-surface fuels that interact with surface fine fuels should be incorporated into the assessment procedure. It is suggested that the surface fine fuel hazard rating should be increased when the cover of near-surface fuels exceeds 40 percent. This needs to be confirmed by further fire behaviour studies.
