Spatially distributed detection of bedrock erosion is a long-standing challenge. Here we show how the spatial distribution of surface erosion can be visualized and analysed by observing the erosion of paint from natural bedrock surfaces. If the paint is evenly applied, it creates a surface with relatively uniform erodibility, such that spatial variability in the erosion of the paint reflects variations in the erosivity of the flow and its entrained sediment. In a proof-of-concept study, this approach provided direct visual verification that sediment impacts were focused on upstream-facing surfaces in a natural bedrock gorge. Further, erosion painting demonstrated strong cross-stream variations in bedrock erosion, even in the relatively narrow (5 m wide) gorge that we studied. The left side of the gorge experienced high sediment throughput with abundant lateral erosion on the painted wall up to 80 cm above the bed, but the right side of the gorge only showed a narrow erosion band 15–40 cm above the bed, likely due to deposited sediment shielding the lower part of the wall. This erosion pattern therefore reveals spatial stream bed aggradation that occurs during flood events in this channel. The erosion painting method provides a simple technique for mapping sediment impact intensities and qualitatively observing spatially distributed erosion in bedrock stream reaches. It can potentially find wide application in both laboratory and field studies.
Fluvial bedrock erosion is an important control on stream channel development
(and thus on whole landscape evolution) in steep mountainous terrain and
tectonically active regions. Bedrock erosion in stream channels is driven by
several interacting processes, of which the most efficient are hydraulic
shear detachment of weak bedrock, plucking of bedrock blocks, and abrasion of
small bedrock grains due to sediment impacts. Dissolution and cavitation can
also be important contributors to bedrock erosion under specific conditions
Spatially distributed measurements of natural bedrock erosion rates are
valuable for understanding the underlying process physics, as well as for
modelling landscape evolution and designing engineered structures. Repeated
measurements of local or reach-scale rates of vertical erosion (i.e. channel
incision), lateral erosion (channel widening), and downstream-directed erosion
of protruding bedrock surfaces are needed to better understand bedrock
channel evolution. However, quantifying spatially distributed bedrock erosion
rates in natural settings is challenging and few such measurements exist
Documenting subtle topographic changes in bedrock surfaces has typically
required sophisticated instruments and techniques, including photogrammetry,
total stations, laser scanners, and erosion meters
Here, we explore an easy, inexpensive method for monitoring spatial patterns of bedrock erosion, which we term erosion painting. We evaluate its applicability using a 3-year series of photographs of painted bedrock surfaces in a natural bedrock gorge in the Swiss Alps and illustrate how this simple method gives insight into sediment transport and erosion processes during high-flow events.
The demonstration field site for bedrock erosion painting:
We present a proof-of-concept field study demonstrating the scientific potential of the following general approach. We used environmentally safe and water-insoluble latex-based dispersion paint to cover natural bedrock surfaces that were expected to show varying patterns of erosion (see below for a description of the field site) and regularly photographed these surfaces from defined vantage points during visits to the sites. Comparisons of sequential photographs from the same vantage points were then used to document the removal of paint by erosive events. To compare specific details of interest over time, it was helpful to include retrievable features (benchmarks) in the pictures. The observed pattern of eroded and remaining paint indicates the spatial distribution of erosion. More precisely, to the extent that the paint provides a uniformly erodible surface, we suggest that the spatial pattern of paint erosion reflects the spatial pattern in the erosivity of the flow and the sediment that it carries (i.e. their erosive strength or potential to erode the bedrock). For useful results to be obtained, this erosivity must be high enough to remove some of the paint, but also low enough that some paint remains.
The field site for this study was a 30 m long and 5 m wide semi-alluvial
bedrock gorge of the Gornera glacial meltwater stream above Zermatt,
Switzerland (Fig.
The erosion pattern on the painted staff gauge on the left gorge wall (cf. Fig.
We repeatedly painted several patches of the gorge's bedrock surface over
a period of 3 years and photo-documented the resulting spatial patterns of
eroded paint, renewing the paint as needed. To visualize variations of
erosion with height above the stream bed, we painted several vertical stripes
of 0.15 m width and 2.0 m height on two opposing straight and smooth
bedrock walls, starting at the sediment bed surface
(Fig.
Even over short periods (i.e. a few flushing events), paint erosion was
visible over most of the studied bedrock gorge section. The painted stripes
on the opposing smooth bedrock walls revealed different erosion patterns: on
the left gorge wall, the painted staff gauge
(cf. Fig.
Painted stripe R2 on the right gorge wall (cf. Fig.
On the right gorge wall, both painted vertical stripes R1 and R2
(cf. Fig.
Characteristic spatial patterns of eroded paint were observed at the
laterally protruding wall section and at the boulder and slab protruding
from the stream bed (Fig.
In the following, we first assess the erosion painting method based on our proof-of-concept study. We then use this technique to draw inferences about spatial erosion processes at our study site and discuss potential future applications in the geosciences.
This study illustrates erosion painting as a straightforward technique for
visualizing the spatial distribution of the erosivity of sediment-laden
flows. The paint remained on bedrock surfaces that were frequently submerged,
showing that it could resist fluvial shear detachment and water dissolution
(see the inset in Fig.
Erosion painting is inexpensive, requires no fixed installations (apart from the paint itself), is straightforward to implement even in challenging locations, permits quick high-resolution field surveys (requiring only visual inspection of the surfaces and reference photographs), and can detect even low levels of streamflow erosivity. However, drawing quantitative inferences on erosion rates would require calibration against independent measurements because the erodibility of the paint and the underlying bedrock will typically differ by large factors (see further discussion below). Environmentally friendly paint should be used, and only small surface patches should be painted to limit paint consumption and the visual impact of the technique. Any necessary permission should be requested, particularly for sensitive field areas. The paint should be applied carefully (e.g. avoiding wet and dusty rock, and leaving sufficient time for drying), since incorporated air bubbles or insufficient drying could lead to shear detachment of the paint by flowing water alone, without abrasion of the surface.
The paint erosion pattern at the staff gauge (Fig.
Patterns of eroded paint at several sites in the gorge (cf. Fig.
Erosion patterns on the painted stripes in the gorge (left column) reflect likely cross-stream variations
in the sediment tools and cover effects during flushings (as indicated by interpretive diagrams in the right column):
Quantitative TLS-based spatial bedrock erosion measurements (over 2 years
with more than 200 flushing events of various discharges, lengths, and volumes;
see Fig.
At the right gorge wall, both stripes R1 and R2 were eroded in only
a restricted band situated more than 15 cm above the bed
(Fig.
Notably, the erosion pattern on the right gorge wall could be detected
repeatedly (cf. the three time periods in Fig.
The erosion patterns of the painted surfaces in Fig.
A comparative view on the erosion patterns of all the painted stripes on the
opposite bedrock walls (Fig.
Together, these interpretations indicate a strong difference in sediment
transport concentration across the gorge (Fig.
Our results demonstrate that erosion painting is a straightforward method for
(i) visualizing the spatial distribution of bedrock erosion (i.e. variations
with position and orientation), (ii) inferring the spatial distribution
of sediment transport (i.e. the sediment tools and cover effects), and (iii) localizing the transient elevation of the sedimentary stream bed under
some circumstances. Qualitative erosion patterns observable in the eroded
paint generally coincided with the quantitative bedrock erosion analysis of
Local erosion rates depend on both the erodibility of the surface and the
erosivity of the sediment-laden flow that abrades it. A general challenge in
surface erosion studies is that it is difficult to know whether spatial
variations in erosion rates are driven by variations in erodibility of the
surface or erosivity of the flow. Erosion painting provides an artificial
surface (the paint) that has a relatively uniform erodibility, and thus
patterns of paint erosion should mostly reflect variations in the erosivity
of the streamflow and its entrained sediment. A further step would be to
standardize the painting technique to a specified paint volume per unit area,
thus better constraining the thickness (and therefore erodibility) of the
paint layer. Laboratory tests
The simplicity of the erosion painting technique could lead to wide-ranging applications in geomorphology. Examples of advanced applications for field sites like the studied gorge would be (i) to more frequently check eroded paint patterns (e.g. after every erosive event) to find thresholds of paint erosion for constraining streamflow erosivity, (ii) to repeatedly paint entire walls, beds, or cross sections to study the spatial variations in streamflow erosivity due to varying sediment concentrations, or (iii) to paint below the sediment bed or below the on-site water surface to determine how the sediment bed varies during flushings and whether erosion also occurs below the level of the dry bed.
Erosion painting should be applicable to topics and settings well beyond the
framework of our study. The relative erodibility of paint by suspended
sediment and bedload could be tested in the laboratory, e.g. in experiments
similar to those of
Besides application in fluvial environments, erosion painting could also be
used to visualize spatial distributions of erosion by ice
Picture data of Figs. 1–5 are given in the text. For discharge and surface change data of Fig. 2, please contact the main author.
The authors want to thank Rafael Bienz, Jean-Pierre Bloem, Lorenzo Campana, Daniela Cervenka, Simon Etter, Kristen Cook, Mattia Sieber, Alexander Stahel, and Carlos Wyss for helping with painting of bedrock surfaces over the years. We are very thankful to Grande Dixence SA for providing logistic support and discharge data for the Gornera study site. Comments by Joel Johnson, Theodore Fuller, and an anonymous reviewer greatly improved this paper. This study was supported by SNF grant 200021 132163/1. We thank the handling associate editor Jane Willenbrink. Edited by: J. Willenbring Reviewed by: J. Johnson, T. Fuller, and one anonymous referee