Each major landslide was scaled - at least as far as the top of its debris cone and typically to the top of its scar, which means that records of scar and outfall dimensions are quite precise Figure 4. Figure 3. Landslide debris on the Almora Lower Mall. Figure 4. Field survey of landslides : Almora Lower Mall, Table 2. Rock discontinuities per m 2 horizontal plane. These included local slope orientation, angle upslope and down-slope of the road, rock-type, geological dip, and the number of joints and fractures per metre, ground cover by ground vegetation and by tree canopy above the roadbed, road width, the angle and height of the road-cut and the depth of soil and any other loose debris there exposed.
Crozier, The original aim was to explore the environmental correlates of landslide activity. However, in the aftermath of September , both the media and local opinion sought explanations that linked landslide occurrence to human activities and development.
These included the construction of buildings, the effects of other roadways upslope of the roadcut, and the collapse of abandoned agricultural terraces Rautelaa and Paulb, The project also recorded engineering attempts to prevent landslides, mainly the construction of retaining walls, which, of course, also provides recognition of potential instability at the site.
Correlation is used to determine associations between variables, T-testing to establish differences between categories, and discriminant analysis to effect classification cf. Haigh, , Haigh et al. The test employed is one-tailed for the reason that theory predicts whether or not an association with landslide volume should be positive, as in the case of all human impacts, or negative as in the case of mitigating factors such as vegetation cover, rock strength, or the construction of retaining walls.
T-tests are used to contrast landslide attributes both at sites where a particular variable is present or absent and to contrast sites where the test variable score is above its upper third percentile and that where it is below its lower third.
Discriminant analysis is a technique that seeks to determine by a stepwise process of adding and removing variables if the differences between two or categories in a data are significant by calculating that weighted combination of variables that best differentiates between the two populations.
It then uses the resulting discriminant function to reclassify data back into the two originally defined sets and its efficiency at this task is recorded Klecka, Here, discriminant analyses are used for data screening purposes, in an attempt to link variables associated with key landslide conditions. In the correlation tests, these null hypotheses argue that there is no significant association between landslide volume and any particular variable.
In the T-tests, the null hypotheses argue that there is no significant difference between landslides at sites where the test-variable is present and where it is absent or where it is strongly or weakly developed. In the Discriminant Analyses, the argument is that it is not possible to effect a statistically significant differentiation between the two categories using any combination of the recorded variables.
A detailed description of the application of these formal statistical procedures is published in Haigh et al. Despite many significant differences between the two sites, the greatest difference is the fact that the Kilbury Road runs through relatively undisturbed reserved forest while the Almora Lower Mall is a busy and rapidly developing suburban highway.
Table 3. Human Influences at Each Landslide Site. At the time of the disaster, the main assumption in Almora was that the major causal factor was recent change in the urban landscape, human action outweighing any of the geological factors that are usually cited in the causal arguments of academic commentators Sah and Pande, ; Pande et al.
Figure 5 suggests, immediately, that there is little direct association between landslide incidence and suburban land use. The main landslide clusters are outside the suburban area. Table 4 charts the micro-geographies of landslide debris volumes along this roadway. An association is suggested between high volumes of debris, high roadcuts, steep hillsides and, to a lesser extent, more forest - even in this aggregated data.
Figure 5. Landslides and land use, Almora Lower Mall, This survey recorded landslides, compared to in , in , in and 88 in the original survey of Haigh et al.
Of course, the volume of debris generated was unusually large in , so the appropriate research question is did human activities help make the landslides larger than they might otherwise have been?
If they did, then there should be an association between landslide volumes and human development activities along the road. Formally restated, the multiple working null hypotheses for statistical correlation are that there is no significant association between landslide volume and either any aspect of human development work or any aspect of the geological and geomorphological environment.
The statistically significant results of these tests are displayed as Table 4. Table 4. Landslides per km : Almora Lower Mall from 0. These are from a spur-end site beneath a playing field. Almora Lower Mall Core 7 km. Tree cover upslope of landslide m 2. A negative correlation between the angle of the failure plane and landslide volume also indicates the importance of flow processes since gravitational rockfalls link to steeper failure planes Haigh et al.
A weak but negative association links debris cone volume and agricultural terrace collapse, which suggests that terrace collapse was associated with smaller outfalls. There are also a weak, positive, correlations between new building construction incidence, and also between reactivated landslides, and larger landslide scars but there is no mutual correlation between these two factors.
These four are the collapse of a road-cut retaining wall, the effects of vibrations and drainage from another road upslope, new building construction, and the collapse of agricultural terraces. T-testing is used to establish significant differences between landslides in places where a particular influence is detected and those where it is not. In the case of the two scalar variables, the test compares that third of sites where the score of the variable is greatest with that third where it is smallest.
Once again, the null hypotheses argue that recorded landslide volumes are not significantly different in either of the so-defined paired data sets. These variables divide into two types : those connected with human-induced environmental changes in the wider environment and those connected with road engineering, which are tackled first.
Key significant findings are listed in Table 5. Table 5. It is self-evident that the process of cutting a road into a steep hillside permanently undermines and weakens the slope above and, the greater this undermining, the more likely landslides will develop.
In Almora, this initial disturbance is now receding into the past but, possibly, it remains a key impact through accelerating rock weathering, the pressure-release of rock joints, and by making the hillside more vulnerable to other kinds of landslide trigger. T-test comparison between the largest and smallest third of the roadcuts shows that the highest roadcuts are associated with significantly larger landslide scars Of course, the highest roadcuts are found also on steeper slopes These results support the correlation analysis, which identifies the height of the roadcut as a key human impact on landslide debris production.
Of course, this activity is not always successful, especially if the retaining walls are not maintained. Their sites had lower slope angles 9. However, the landslides that resulted from retaining wall failures are very small compared to the others on the road debris volume : Table 6. Largest vs smallest third of roadcuts m 3. Landslides associated with failure of retaining walls m 3.
Landslide sites affected by roadways upslope m 3. Tree cover Upslope m 2 per m 2. However, T-tests show that this impact is associated with smaller landslide outfalls and smaller landslide scars The conclusion remains that impacts from other roadways on the hillside do not contribute significantly to the enhancement of landslide volumes Figure 6.
In sum, for the roadway, only roadcut height positively affects the size of landslides. Figure 6. Landslides on the north-eastern part of the Almora Lower Mall. Haigh et al. In Uttarakhand, most agricultural terraces are constructed on former landslide deposits, which, although inherently less stable, provide more gentle slopes, deeper substrates and usually more available soil moisture than undisturbed slopes Bartarya and Valdiya, As reported previously, these relationships suggest that trees survive, mainly, on slopes that are less stable and, consequently, less suited for development, but inherently more prone to landslide activity Haigh et al.
This provides a nice circle of causality but the fact remains that these results do not suggest that deforestation positively affects landslide size in this context. Here, T-testing compares those 21 sites where landslides are linked to new building with those 87 where they are not. This confirms the suggestion that these new structures were constructed on more vulnerable sites. However, ambiguously, landslide outfalls at sites associated with new construction were smaller than those where there was no new construction vs m 3 even though they emerged from larger scars vs m 3 , and while these differences are far from statistically significant, they suggest some kind of healing process.
In sum, it is impossible to argue that new building construction was a major factor in the enhancement of landslide activity. Heavy rainfall had caused large volumes of slope collapse material to be incorporated into the river waters, giving the mud flows and debris flows, which have overwhelmed villages within the mountain valleys.
Villages built a little further up from the floor of the valleys will be much less prone to the destructive debris flows, which occur in the base of the mountain valleys. Landslides tend to be most frequent and destructive in steep mountainous areas, as they are an expression of a natural process which reduces steep slopes to less steep slopes. Data collected over many years suggests that landslides are ranked 7th in the natural disaster table well after the major historical killers of droughts, floods and storms, but close on the heels of earthquakes and volcanic eruptions.
Landslides generally require a trigger, most commonly extreme rainfall or large earthquakes. Earthquakes initiate landslides by locally — and very briefly — changing the gravity experienced by a slope, which tips it beyond its stability point. Extreme rainfall temporarily drives the water pressure within a slope to a critical level; the stresses within the slope then exceed their stability point and the land begins to slip down the slope. Landslides are then driven simply by gravity, often assisted by a loss of strength in the region at the base of the slide.
So parts of the world with steep slopes, intense rainfall and large earthquakes tend to be most prone to this kind of geological disaster. If engineers or builders mess up and make a slope too steep, it may no longer be held up by the intrinsic strength of the rock. In the same way, apparently simple things like leaky water pipes or inadequate drainage on man-made slopes can also start landslides.
This is particularly so water is either retained in, or drains particularly slowly from, the rock or soil of the slope. On an average about earth-quakes of smaller magnitude occur every year in the hills of UP.
Going by the geological clock, the Himalaya is a "young" mountain range, and is prone to natural disasters. Experts also say that they are one of the most ero-sion-prone ranges. Intense rainstorms and earthquakes make these mountains prone to frequent landslides. But it is intense rainfall that usually triggers landslides see box: Sliding mountains. Prolonged downpour often reactivates old landslides.
Apart from bringing down large quantities of sedi-ments, landslides become a major cause for devastating floods because they block the narrow gorges. They are nor-mally marked by a sudden change in the gradient of a trib-utary stream, constriction at the point of confluence and weak geological conditions in the catchment of the tributary. The formation of landslide dams is a common sight in the Himalaya.
Large scale deforestation and faulty farming practices have also led to soil erosion, according to local people. Says V Sharma, department of geology, Delhi University; "It is not for the first time that these landslides arc occurring. This time it has attracted attention due to the large scale of deaths and destruction". The government must plan develop-ment activities to check landslides," he adds.
Ironically, the debris generated is thrown down the slopes, which in turn devours vegetation and even fields. Experts say that about 40, to 80, cubic metre of debris have to be removed to construct a road I km long. Moreover, cubic metre of debris have to be removed every year to main-tain the road. The increase in human activities along the slopes has changed the exist-ing land use pattern.
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