TERRAIN ANALYSIS FOR EVALUATING POST-FIRE GEOMORPHIC PROCESSES

Topography, although little affected by fire, is a key determinant of accelerated geomorphic changes that occur following fires. Initiation sites for debris flows, flow-constriction areas that produce deep flow and scour, and flow expansion areas that produce deposition, are conditions that can be mapped automatically from digital elevation data. Maps of the spatial distribution of these conditions assist in assessments of post-fire flood and debris-flow hazards.

Terrain analysis tools in most geographic information systems are unsuited to these topographic analyses; they treat flow as moving in discrete directions, from each elevation cell to one of the eight nearest neighbors. In catchments or basins, discrete flow directions make little difference—but on hill slopes, discrete flow directions distort the nature of water movement and obscure features of critical interest. Methods presented here allow flow to move in any direction, following the line of steepest descent on a mathematical surface that passes through the elevation points of the digital elevation model (DEM).

One useful analysis product is a map of flow convergence. Areas where water accumulates on the landscape are areas of high flow convergence and are immediately identifiable. Numerically, flow convergence is identical to contour curvature; thus, the automated method yields results highly analogous to manual methods, such as analysis of contour crenulations, that geomorphologists have used for decades. Areas and lengths of convergent flow zones can be measured automatically and abstracted to statistics such as hill-slope drainage density.

Flow confinement is measured starting with a line of high flow convergence (a channel or swale axis). A two-sided “front” is expanded from this line at a rate inversely proportional to slope. This front is a line of constant height above the starting line, and the distance between the two sides of the front measures flow confinement. Derivatives of flow confinement in the downstream direction indicate flow constriction (or expansion), which is a determinant of scour (or deposition). The quality of such products is highly dependent on the quality of the digital elevation model, with high-resolution LIDAR (LIght Detection And Ranging) data being the preferred data source.