Fire Spread in Chaparral: Comparison of Data with Flame-Mass Loss Relationships

The relationships between flame length, mass loss rate, and the Froude number have become well-established for many different fuels over the past 60 years. Chaparral, a mixture of shrub plants from the Mediterranean climate zone of southwestern North America, represents a fuel type—living plants—that has seldom been included in the development of these relationships. Flame length and mass loss data from single leaves to high bulk density fuel beds similar to wooden cribs were compared to existing correlations for circular and line burner configurations. Fuel moisture content of the fuels ranged up to 100 percent of dry mass, a value outside the range of data used to develop the correlations. Results confirmed the relationship of the Froude number to flame length for these fuels; however, many existing correlations fit the data marginally well. Data from single leaves and circular pans agreed with circular burner correlations better than fuel bed data with line burner correlations. The first evaluation of Albini’s thermochemical, wildland fire flame model in chaparral fuels revealed the need for improved data and perhaps changes to the model. Collectively, this data set provided strong support for Byram’s correlation of flame length and the square root of mass loss rate. Nelson’s effective wind speed related to Byram’s convection number significantly improved correlation between the Froude number and flame angle. This Froude number may provide a potential scaling parameter for experiments examining the combined effects of wind and slope.