A waterfall highlights the locus of active fluvial
erosion of bedrock, and its mechanism has been the subject of several studies;
however, erosional processes remain to be clarified for specific rock
structures composing a waterfall. Herein, the detailed morphology of cliffs
around a waterfall is examined by a terrestrial laser scanning (TLS) approach
to analyze erosional processes occurring in the cliffs. The study site is Kegon
Falls in Japan, which has a vertical drop of surface water from the top of its
cliff and groundwater outflows from its lower portion. The entire cliff is
mostly overhanging and minor rockfalls are often observed. The latest major
rockfall occurred in 1986, causing an approximate 8-m upstream shift of the
waterfall lip. From the point cloud obtained by TLS measurement, a digital
elevation model on a vertical plane was generated, and cross-sectional profiles
were extracted. A distinct 5- to 10-m depression was found at the bottom of
the upper andesite layer of the waterfall cliff, which appears to have been
formed by freeze-thaw and wet-dry weathering following the upstream shift of
the surface water drop. Stability analysis of the waterfall cliff with an
undercutting notch indicates that the igneous rock composing the cliff is
sufficiently strong to maintain its current overhanging shape and that
catastrophic collapse of the entire waterfall face rarely occurs. Following the
formation of the depression, the upper cliff face appears to have been
gradually eroded by gravitational collapses of relatively small blocks bounded
by columnar and platy joints.
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