Geotechnical controls on erodibility in fluvial impact erosion
Abstract. Bedrock incision by rivers is commonly driven by the impacts of moving bedload particles. The speed of incision is modulated by rock properties, which is quantified within a parameter known as erodibility that scales the erosion rate to the erosive action of the flow. Although basic models for the geotechnical controls on rock erodibility have been suggested, large scatter and trends in the remaining relationships indicate that they are incompletely understood. Here, we conducted dedicated laboratory experiments measuring erodibility using erosion mills. In parallel, we measured compressive strength, tensile strength, Young’s modulus, bulk density and the Poisson ratio for the tested lithologies. We find that under the same flow conditions, erosion rates of samples from the same lithology can vary by a factor of up to sixty. This indicates that rock properties that may vary over short distances within the same rock can exert a strong control on its erosional properties. The geotechnical properties of the tested lithologies are strongly cross-correlated, preventing a purely empirical determination of their controls on erodibility. The currently prevailing model predicts that erosion rates should scale linearly with Young’s modulus and inversely with the square of the tensile strength. We extend this model using first-principle physical arguments, taking into account the geotechnical properties of the impactor. The extended model provides a better description of the data than the existing model. Yet, the fit is far from satisfactory. We suggest that the ratio of mineral grain size to the impactor diameter present a strong control on erodibility that has not been quantified so far. We also discuss how our laboratory results upscale to real landscapes and long timescales. For both a revised stream power incision model and a sediment-flux-dependent incision model, we suggest that long-term erosion rates scale linear with erodibility and that, within this theoretical framework, relative laboratory measurements of erodibility can be applied at the landscape scale.
Jens Martin Turowski et al.
Status: open (until 21 Jun 2023)
- RC1: 'Comment on egusphere-2023-76', Anonymous Referee #1, 29 May 2023 reply
Jens Martin Turowski et al.
Jens Martin Turowski et al.
Viewed (geographical distribution)
The submitted manuscript presents a study designed to explore the controls on rock erodibility. The manuscript is interesting and well written. I recommend publication after the following minor comments have been considered.
Line 12: "uniaxial compressive strength"
Line 13: "Poisson's ratio"
Line 21: "presents"
Line 93: So, these discs were 191 to 193 mm in diameter? Perhaps this should be stated here?
Line 95: What lithology is the Passwang Formation? Perhaps this should be stated here?
Line 104: The samples were dried following preparation? If so, how were they dried?
Line 136: What does LDPE stand for?
Line 137: How was trapped air removed exactly? Typically, fully saturating samples in the laboratory requires a vacuum pump. How are the authors sure that their samples were completely saturated?
Line 141: What type of glass were the beads made from? What was the diameter of the glass beads? They are spherical?
Line 149: Did any material spall from the samples during rinsing? Was this material collected?
Line 161: This sentence suggests that bulk sample density was measured using an MTS load frame. Reword?
Line 164: I think it's important to clearly state, if true, that these measurements were performed on dry samples. The strength of rock is typically lower when saturated with water. Since this "water-weakening" varies from rock type to rock type, it could explain some of the scatter in the experimental data (since the erosion tests are performed on wet rock and the petrophysical parameters are measured on dry rock).
Line 165: I assume this is the dry bulk density? How did the authors dry their samples prior to the measurement of dry mass?
Line 172: By "conversion rate", do the authors mean "displacement rate"? Can the authors also quote the strain rate here too? How was the displacement measured?
Line 174: Surely this is the stress, not the pressure? Unless the authors are talking about the pressure inside the piston that is used to calculate the force, which is in turn used to calculate the stress using the sample radius?
Line 176: Why would you use a constant force? Do the authors mean a constant force rate?
Line 177: By "convergence speed", do the authors mean "displacement rate"?
Line 178: "before fracture" is ambiguous. The authors mean the maximum force obtained immediately prior to the formation of the tensile fracture?
Line 181: The authors are sure that the behaviour is elastic at 50% of the UCS in all the experiments?
Line 182: "Poisson's ratio"
Line 188: The authors are sure that the behaviour is elastic at 50% of the UCS in all the experiments?
Line 212: What about bedding orientation? Did this influence the erosion rate?
Line 244: Poisson's ratio and UCS do not look particularly correlated (Figure 5d).
Figure 5: "Poisson's ratio"
Figure 5: Why were these cross-plots chosen? I'm not suggesting that the authors should plot every combination, but why do the authors only show how tensile strength, Young's modulus, Poisson's ratio, and density vary as a function of UCS?
Line 245: Can the authors explain Kendall's rank coefficient? How is this calculated?
Line 259: How can the authors explain the large differences in erosion rates between samples cut from the same core (e.g., the data on Figure 4a)? This cannot be explained by differences in grain size etc.
Figure 7: These data were collected using the same (similar) impactor material and geometry (shape and diameter)? If not, is it not useful to state these parameters in the figure caption?
Line 265: Tensile strength can also be influenced by pore size and shape, as shown in Heap et al. (2021). Although this study focusses on volcanic rocks, the modelling results are relevant for other rock types, such as porous sedimentary rocks.
Heap, M. J., Wadsworth, F. B., Heng, Z., Xu, T., Griffiths, L., Velasco, A. A., ... & Deegan, F. M. (2021). The tensile strength of volcanic rocks: Experiments and models. Journal of Volcanology and Geothermal Research, 418, 107348.
Line 269: If the impactor type/size is important, I think this information should be provided in the figure caption for each of the datasets.
Line 276: Perhaps the authors should/could cite a few papers here that have previously demonstrated these relationships for rocks?
Line 302: The "elastic modulus" is the Young's modulus?
Line 308: This constant does not therefore depend on rock type or rock properties? This has been previously demonstrated?
Figure 9: Do these plots show the same experimental erosion rate data?
Line 355 and elsewhere: The authors often refer to a "linear fit", which is actually a power law. Is this not misleading?
Line 363: Also pore size and shape.
Line 383: Unless I'm mistaken, I don't think the size of the impactors (the glass beads) used in this study is stated in the manuscript. Surely, based on this paragraph, it's very important for future studies to clearly state the impactor size?