the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 31 Aug 2022
Elucidating the role of soil hydraulic properties on the aspect-dependent landslide initiation
Abstract. Aspect-dependent landslide initiation is an interesting finding and previous studies merely address the role of plant roots on this observed connection between landslide probability and slope aspect. In this work, the aspect-dependent landslide initiation in catchment with same plant species and high vegetation coverage was examined by pore water pressure and hillslope hydrology behavior. Remote sensing interpretation using the high-resolution GeoEye-1 image and digitalized topography found that the landslides on south-facing slope have higher probability, larger basal area and shallower depth than those on north-facing slope. The lower limit of upslope contributing area and slope gradient condition for south-facing landslides is no less than north-facing landslides. The higher basal area of south-facing landslides over north-facing landslides may attribute to the high peak values and slow dissipation of pore water pressure. The absorbed and drained water flow in given time interval, together with the calculated water storage and leakage during the measured rainy season, sufficiently prove that the soil mass above the failure zone for the south-facing slopes are more prone to form pore-water pressure and result in slope failures. In comparison, the two stability fluctuation results from finite and infinite models imply that landslides on south-facing slopes may fail on condition of prolonged antecedent precipitation and intensive rainfall, while those on north-facing slopes may fail merely in response to intensive rainfall. The results of this work provide an insightful view on the aspect-dependent landslide initiation from both classical mechanics and the state of stress.
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Notice on discussion status
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Preprint
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2022-798', Anonymous Referee #1, 09 Oct 2022
The paper investigates the question whether land-sliding initiation at slopes is aspect-dependent. The obvious reason for potential differences - the radiation budget - is not discussed explicitly, but resulting processes - differences in plant root strength, the pore water pressure (or larger evapotranspiration at the south-exposed slope) are.
The reviewer wants to know what exactly is your definition of north and south facing - which angle ranges (relative to North) is "allowed"? From the GoogleEarth images, there might very well also be west- and east-facing slopes.
Were they excluded from the analysis?The mechanisms leading to landslides are considered to some detail, including pore water dissipation, water storage and drainage, and stability fluctuations. The description of the latter is incomplete; there are equations shown for the
"finite slope model" and the "infinite slope stability model", involving many parameters (such as angles) usually not easily to obtain in the field. Were these modelled by Hydrus-1D? How reliable are these estimates?
Obviously (Fig. 11), the two Fs (eqs. 8 and 9) are dynamic quantities - which of the variables on the rhs are time-dependent? At least for the finite model, the time dependence seems to be marginal, but the difference in the mean values
seems to be huge in comparison (Fig. 11 lower left panel). What is precisely the origin of this discrepancy? The lower right panel shows that for the south-facing slope, the situation is rather similar (very stable values for F_s),
whereas for the north-facing, F_s varies a lot. Why is that the case, i.e. what property or variable of eq. 9 is responsible for that? The reviewer disagrees with the statement that (thus) the infinite slope model would better support
the observations, since the only rationale for that is by confirming the prejudice that south-facing slopes are more prone to landslides than north-facing ones. This is circular reasoning.However, the fundamental problem of the paper is that there is only a single site investigated, where landslides have occurred both at north-facing as well as south-facing slopes. The statistics of that particular location shows that more landslides for south-facing slopes have been recorded: 71 versus 20 - this is probably the result of a field survey in the area, but the time span over which these happend should be mentioned as well, if known. The whole area is densely tree covered, with a larch species dominating. In the GoogleEarth image, it seems that there are a lot of terraces surrounding the local peaks - is that due to management? If so, what was done there? This striking feature is not mentioned in the manuscript but could of course impact on landslide probability (either way).
However, how could an investigation at one particular area say something conclusive about aspect-dependent landslide probabilities in general?
In that regard, the paper seems to be way too ambitious. To do justice to the paper, systematic differences between north- and south-facing slopes are investigated to some detail. The slopes are rather steep but not different between S- and N-slopes (Fig. 3 left panel); grain diameter distributions are rather similar (Fig. 5); the physical properties reveal some differences, in particular for
the saturated hydraulic conductivity, which of course can imply different water routing during and after rainfall events. On the other hand, in the unsaturated domain, it is not obvious that there are any differences in the pF curves
(Fig. 8); they look strikingly similar for the two slope aspects. The shear tests (Fig. 6), on the other hand, seem to indicate that the two slope types have different pore water pressure behaviour (NB the reviewer wonders what the legend
of that figure ("Time (10-sec)") would mean? Do you intend to say that the time axis is in logarithmic units (to base 10)? It doesn't seem to make sense).However, cause and effect are totally unclear here: are these differences induced by the different aspects, or just geological properties of the area, or random variation due to small sample sizes?
A technical problem is that the language quality has to be improved. There are a non-negligible number of grammar errors and incomplete sentences which inhibits comprehensibility at times. Before resubmission, this issue should be
carefully addressed.Summarizing, the observational investigation for the selected sites is profound, and the processes and phenomena considered are numerous. However, the presentation is incomplete and in part difficult to follow, and most importantly, the conclusions
drawn from this small field study seem to be too far-fetched. The paper deserves a major revision.Citation: https://doi.org/10.5194/egusphere-2022-798-RC1 - AC1: 'Reply on RC1', Ma Chao, 16 Dec 2022
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RC2: 'Comment on egusphere-2022-798', Tammo Steenhuis, 19 Nov 2022
The authors present a complicated explanation for the greater number of bank failures on the south slope than on the north slope
COMMENT
I am not sure if the analysis is correct since hillslope stability is not my field. However, I know that when the soil becomes saturated, the hillslope could fail, given that roots do not keep it in place. Based on this simple principle, we can explain, based on the data given in this manuscript, the difference between the north and south-facing slopes in simple terms as follows:
The conductivity of the subsoil is greater on the north-facing slope than on the south-facing slope. Thus, the north-facing slope drained faster than the south-facing slope, and as shown in Figure 9, the soil on the north-facing slope does not saturate. In contrast, on the south-facing slope, the rainfall rate at some point is greater than the water that can be carried off laterally, and the soil saturates, as shown in figure 9. The saturation causes the soil strength to decrease, and failure occurs. Hence more failures on the south slope than on the north-facing slope.
COMMENT
Figure 9 is hardly discussed in the manuscript. It is likely the most significant finding as it shows that the oil becomes saturated on the south slope while not on the north slope.
COMMENT
On line 377, the authors write that
“the saturated hydraulic conductivities by variable-head permeameter and TRIM methods coincide with each other, which together prove that the soil mass on north-facing slope has a relatively larger water infiltration.
The amount of water infiltrated on a slope depends on the amount of rainfall and not the conductivity as long as it is greater than the rainfall rate. Moreover, laboratory-derived conductivity is a poor predictor for field hydraulic conductivity in the topsoil where plant roots and animal life provide vertical preferential flow paths
COMMENT
As I indicated before, I leave it up to the experts if the hillslope analysis is correct or not. It seems too complicated for the little information that is available on this site. The fact that the soil strength decreases greatly at the time the soil becomes saturated is important and is not well addressed. In addition, the fact that soil saturates should be stressed in the manuscript that claims to be a hydrologic analysis
Citation: https://doi.org/10.5194/egusphere-2022-798-RC2 - AC2: 'Reply on RC2', Ma Chao, 16 Dec 2022
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2022-798', Anonymous Referee #1, 09 Oct 2022
The paper investigates the question whether land-sliding initiation at slopes is aspect-dependent. The obvious reason for potential differences - the radiation budget - is not discussed explicitly, but resulting processes - differences in plant root strength, the pore water pressure (or larger evapotranspiration at the south-exposed slope) are.
The reviewer wants to know what exactly is your definition of north and south facing - which angle ranges (relative to North) is "allowed"? From the GoogleEarth images, there might very well also be west- and east-facing slopes.
Were they excluded from the analysis?The mechanisms leading to landslides are considered to some detail, including pore water dissipation, water storage and drainage, and stability fluctuations. The description of the latter is incomplete; there are equations shown for the
"finite slope model" and the "infinite slope stability model", involving many parameters (such as angles) usually not easily to obtain in the field. Were these modelled by Hydrus-1D? How reliable are these estimates?
Obviously (Fig. 11), the two Fs (eqs. 8 and 9) are dynamic quantities - which of the variables on the rhs are time-dependent? At least for the finite model, the time dependence seems to be marginal, but the difference in the mean values
seems to be huge in comparison (Fig. 11 lower left panel). What is precisely the origin of this discrepancy? The lower right panel shows that for the south-facing slope, the situation is rather similar (very stable values for F_s),
whereas for the north-facing, F_s varies a lot. Why is that the case, i.e. what property or variable of eq. 9 is responsible for that? The reviewer disagrees with the statement that (thus) the infinite slope model would better support
the observations, since the only rationale for that is by confirming the prejudice that south-facing slopes are more prone to landslides than north-facing ones. This is circular reasoning.However, the fundamental problem of the paper is that there is only a single site investigated, where landslides have occurred both at north-facing as well as south-facing slopes. The statistics of that particular location shows that more landslides for south-facing slopes have been recorded: 71 versus 20 - this is probably the result of a field survey in the area, but the time span over which these happend should be mentioned as well, if known. The whole area is densely tree covered, with a larch species dominating. In the GoogleEarth image, it seems that there are a lot of terraces surrounding the local peaks - is that due to management? If so, what was done there? This striking feature is not mentioned in the manuscript but could of course impact on landslide probability (either way).
However, how could an investigation at one particular area say something conclusive about aspect-dependent landslide probabilities in general?
In that regard, the paper seems to be way too ambitious. To do justice to the paper, systematic differences between north- and south-facing slopes are investigated to some detail. The slopes are rather steep but not different between S- and N-slopes (Fig. 3 left panel); grain diameter distributions are rather similar (Fig. 5); the physical properties reveal some differences, in particular for
the saturated hydraulic conductivity, which of course can imply different water routing during and after rainfall events. On the other hand, in the unsaturated domain, it is not obvious that there are any differences in the pF curves
(Fig. 8); they look strikingly similar for the two slope aspects. The shear tests (Fig. 6), on the other hand, seem to indicate that the two slope types have different pore water pressure behaviour (NB the reviewer wonders what the legend
of that figure ("Time (10-sec)") would mean? Do you intend to say that the time axis is in logarithmic units (to base 10)? It doesn't seem to make sense).However, cause and effect are totally unclear here: are these differences induced by the different aspects, or just geological properties of the area, or random variation due to small sample sizes?
A technical problem is that the language quality has to be improved. There are a non-negligible number of grammar errors and incomplete sentences which inhibits comprehensibility at times. Before resubmission, this issue should be
carefully addressed.Summarizing, the observational investigation for the selected sites is profound, and the processes and phenomena considered are numerous. However, the presentation is incomplete and in part difficult to follow, and most importantly, the conclusions
drawn from this small field study seem to be too far-fetched. The paper deserves a major revision.Citation: https://doi.org/10.5194/egusphere-2022-798-RC1 - AC1: 'Reply on RC1', Ma Chao, 16 Dec 2022
-
RC2: 'Comment on egusphere-2022-798', Tammo Steenhuis, 19 Nov 2022
The authors present a complicated explanation for the greater number of bank failures on the south slope than on the north slope
COMMENT
I am not sure if the analysis is correct since hillslope stability is not my field. However, I know that when the soil becomes saturated, the hillslope could fail, given that roots do not keep it in place. Based on this simple principle, we can explain, based on the data given in this manuscript, the difference between the north and south-facing slopes in simple terms as follows:
The conductivity of the subsoil is greater on the north-facing slope than on the south-facing slope. Thus, the north-facing slope drained faster than the south-facing slope, and as shown in Figure 9, the soil on the north-facing slope does not saturate. In contrast, on the south-facing slope, the rainfall rate at some point is greater than the water that can be carried off laterally, and the soil saturates, as shown in figure 9. The saturation causes the soil strength to decrease, and failure occurs. Hence more failures on the south slope than on the north-facing slope.
COMMENT
Figure 9 is hardly discussed in the manuscript. It is likely the most significant finding as it shows that the oil becomes saturated on the south slope while not on the north slope.
COMMENT
On line 377, the authors write that
“the saturated hydraulic conductivities by variable-head permeameter and TRIM methods coincide with each other, which together prove that the soil mass on north-facing slope has a relatively larger water infiltration.
The amount of water infiltrated on a slope depends on the amount of rainfall and not the conductivity as long as it is greater than the rainfall rate. Moreover, laboratory-derived conductivity is a poor predictor for field hydraulic conductivity in the topsoil where plant roots and animal life provide vertical preferential flow paths
COMMENT
As I indicated before, I leave it up to the experts if the hillslope analysis is correct or not. It seems too complicated for the little information that is available on this site. The fact that the soil strength decreases greatly at the time the soil becomes saturated is important and is not well addressed. In addition, the fact that soil saturates should be stressed in the manuscript that claims to be a hydrologic analysis
Citation: https://doi.org/10.5194/egusphere-2022-798-RC2 - AC2: 'Reply on RC2', Ma Chao, 16 Dec 2022
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Yanglin Guo
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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