INTRODUCTION 



Slash or debris created by harvesting and thinning are a major fire management 

 problem because these residues can create unacceptable fire behavior hazards. Treating 

 slash to maintain an acceptable fire hazard is expensive and requires skillful decision- 

 making. An inexpensive, simple-to-use, yet objective means of appraising the potential 

 fire behavior of slash is needed to aid decisions in managing slash. Knowledge of 

 potential fire behavior can help determine treatment alternatives, the financing of 

 slash treatment activities, and even determine whether the slash should be created. 

 This report describes a method for predicting depth of slash fuels for analytical model- 

 ing of fire behavior. 



The capability to predict debris and to model fire behavior has made possible a 

 quantitative system for appraising fire behavior potential. Rate of fire spread and 

 flame front intensity can be mathematically modeled using Rothermel's (1972) fire model, 

 which is the basis for computing spread and energy release indexes in the National Fire- 

 Danger Rating System (NFDRS) . An area-growth-rate model by Anderson, 1 crown scorch 

 model by Van Wagner (1973), and flame length models by Byram (1959) and Thomas (1962) 

 are also available. A method for assessing total heat release and period of flaming 

 that incorporates large-diameter fuels has been theorized by Albini. 2 The fire models 

 require fuel loading by size class and fuel bed depth as inputs. 



Loading and depth of debris vary considerably by cutting prescriptions, therefore 

 must be estimated for each individual cutting situation. Other fuel inputs such as 

 particle density, heat content, silica-free ash content, and particle surface area-to- 

 volume ratio tend to be species dependent and can be approximated from known values 

 (Albini 1976; Brown 1970b, 1974). 



Office memo of August 10, 1973, on file at the Northern Forest Fire Laboratory, 

 Missoula, Montana. (Manuscript in preparation by Hal E. Anderson.) 



2 Albini (1976) outlines the total heat load computation; the computation of burning 

 time is based on empirical work reported by Harmathy (1972) describing structure fires 

 and pile burning, modified slightly to agree with single-particle burning times reported 

 by Anderson (1969) . 



1 



