Mapping earthflows and earthflow complexes using topographic indicators

Abstract

Practical methods of mapping earthflows and earthflow complexes using topographic recognition keys are briefly profiled in this article. These methods can be employed to tentatively identify earthflow features that may not be recognized on stereopair aerial photographs because of vegetation, mollification of the features with age, and/or unfavorable sun angle on the images. Conventional USGS 7.5-min topographic maps (1:24,000 scale) with contour intervals of >20ft/6.1m are not generally useful in identifying earthflows with much less than ~18 to 60m of vertical relief. Useful recognition keys for the topographic expression of earthflows are presented. These keys include divergent contours along a slope fall line with headward cutting upslope and depositional fans downslope. Fans tend to widen and deepen downslope. Earthflows in first- and second-order watersheds may exhibit more dissection of their toe lobes than those in zero-order basins. Earthflows commonly occur in large coalescing complexes, with one event, or lobe, superceding another. This forms a series of superposed lobes, with the most recent lobes being easiest to discern, while older lobes are increasingly mollified with time. The soft cohesive clay and silt debris deposited by earthflows is most easily eroded in the headscarp area, and compacted naturally in the depositional lobe. This makes the recognition of earthflows increasingly difficult with age. Earthflow debris is often interpreted to be colluvium when deposited on slopes without appreciable shearing to form slickensided contacts. Discernment of earthflows using topographic recognition techniques depends on the scale and quality of the topographic maps being evaluated.