Landslide Morphology and Digital Terrain Analysis of the Roughs Landslide in Broadway - Research Paper Example

LANDSLIDE MORPHOLOGY AND DIGITAL TERRAIN ANALYSIS OF THE ROUGHS LANDSLIDE IN BROADWAY Introduction There are moments that wide range of ground movements take place leading to activities of landslip. Once such actions take place, they are known in geographic terms as landslides (Chandler, 2007). The ground movement that takes place may come in many different forms including rock falls, shallow debris flows and deep failure of slopes (Eyers, Moore, Hervas and Lui, 2005). The occurrence of landslide activities is regarded to take place as a result of the action of gravity as a major driving force. This notwithstanding, current studies have actually revealed that the state of slope stability can be a major contributing factor to the cause of landslides (Wilson and Gallant, 2000). The differences in causes of landslides give rise to the issue of morphology, which generally describes classification of landslides. In geological studies, the morphology of landslides at a particular site is very important in making a lot of predictions about landslide activities within a site. It is for this reason the studies about landslide morphology at various sites continues to be an important research area for geologists. Landslide morphology researches will remain very important in predicting cases of landslide and giving appropriate warnings where necessary. For example, there are current studies that have attributed the effect of climate change to landslide activities of various sites, depending on the morphology of those sites (Dikau, Brunsden, Schrott and Ibsen, 1997). Such outcomes of research are very important in avoiding the worse cases of landslide activities that could be devastating to people. To give meaningful outcomes of landslide morphology, there are six major areas that are focused on. These are type, sub-type, activity, depth, vegetation and body (Lacroix and Martz, 2006). In this research paper, Broadway in the United Kingdom is used as a research site where there is a critical study on the landslide morphology of the place. According to Eyers, Moore, Hervas and Lui (2005), geologists continue to be faced with major challenges in undertaking successful landslide morphology and coming out with accurate findings. The reason for this is that these geologists use ineffective and unfavourable tools and methods for engaging in landslide morphology. Meanwhile, Wilson and Gallant (2000) mentioned digital terrain analysis as an effective digital elevation model that gives 3D representation of a terrain’s surface. With the use of terrain elevation data and 3D representation, digital terrain analysis has been found to be an effective model for studying the landslide morphology of a given site. It is based on this that this research paper combines landslide morphology with digital terrain analysis of the Roughs Landslide in Broadway to have a vivid geological understanding of the landslide situation of the site. The research shall give key findings on the project site information, architecture and environment of Broadway, as well as a vivid description of the data collection tools used in the study. After the data collection, results shall be presented on the digital elevation data sources and structures of Broadway. Based on the outcome of the digital terrain analysis model used, results shall also be presented on three major categories of landslide activities in Broadway, which are active, suspended and relict. It is hoped that the findings from the research paper will be useful in making very important decisions regarding the geological behaviour of Broadway. Project Site Information Broadway is located in an area just beneath the southern extent of some of the last glacial advances that were experienced not too long ago (Whitworth, Murphy and Petley, 2000). Whitworth, Murphy and Petley (2000) therefore identify Broadway to be one of those areas that have been “strongly affected by successive periods of periglacial conditions” (p. 1). Up to date, there is sufficient evidence of relict periglacial features within the area of Broadway, making the site one of the major concern areas for geologists to look out for events and recent landslide activities. One of the most characteristic features of Broadway is the slope movements, which form a part of the larger movement activities in relict periglacial terrains (Dikau et al, 1997). Looking into the landslides recorded on the escarpment at Broadway, Whitworth, Giles and Anderson (2006) observed that landslides in Broadway are not different from what is found in other areas near the parts of Cotswolds. This means that there is no geographically distinct feature about the landslides of Broadway. Figure 1: Broadway and its Area of Study Source: Whitworth, Giles and Anderson (2006) Even though Broadway may have some characteristic similarities with other areas such as Cotswolds, there are key landslide sequence activities that are unique to Broadway alone. For example, Broadway takes its landslide sequence from the edge of the Cotswolds plateau and upper slopes of the escarpment (Chandler, 2007). Broadway has been found to be one of the areas within Worcestershire part of the Cotswolds in England where is there is extensive instability in terms of land movement activity. According to Whitworth, Giles and Anderson (2006), such extensive instability is the result of activities “including lobate mudslides, shallow irregular mudslides, translational and rotational landslides” (p. 2). Such characteristics make Broadway very distinct geological site for research interest. This is because apart from landslides, there are several other geological units, each of which has their own formation, description and surface morphology. Some of these geological units include birdlip limestone formation, bridport sand formation, whitby mudstone formation, marlstone rock formation, dryham formation, and charmouth mudstone formation (Westen, 2009). Architecture and Environment The architecture and environment of Broadway can best be discussed by looking at the distribution of landslides in the area. This is in line with the general purpose of the research work, which seeks to find the landslide morphology and digital terrain analysis of the Roughs landslide in Broadway. As far as the distribution of landslides is concerned, there are four major unique architectural developments in the environment of Broadway. The first of this is found in the upper part of the escarpment; a place known as Inferior Oolite where cambered strata can be found. As noted by Forster (1992), cambering occurs where is there is disturbance of the strata to a depth measuring up to 30-40 metres. In the case of Broadway, these cambering are known to be in place as a result of the presence of the more gentle slopes, which have not had any major defects resulting from activities of subsequent erosion (Bromhead, 1986). The second distribution of landslides found on the type of Jurassic strata in Broadway is located beneath the Inferior Oolite and comprise a zone of large scale rotational landslides (Whitworth, Giles and Anderson, 2006). As noted by Varnes (1996), rotational slides circulate along surface of rupture that is always curved and concave. By implication, the architecture of the land below the Inferior Oolite is curved and concave. The third classification of distribution of landslide that makes up the architecture of Broadway is a zone made up of not just a single case of rotational landslides but successive shallow rotational landslides (Flageollet, 1993). The successive nature of these rotational landslides notwithstanding, they possess the same features of any other dominant rotational landslide. That is to say that these successive shallow rotational landslides occur at speed that range from extremely slow to being extremely rapid (Green, 1992). These types of landslides also occur at angle 45-90°. The successive zone has their rotational landslides being caused by actions of vibration, undercutting, differential weathering, excavation or stream erosion (Varnes, 1996). The last classification of distribution of landslides has been noted by Forster (1992) to be made up of extensive shallow mudslides and translational landslides. This means that unlike other areas that possess a single type of landslide, Broadway also has translational landslides, even though these come only as a result of extensive shallow mudslide activities. Digital Terrain Analysis Tools Given the nature of landslide morphology of the Roughs Landslide in Broadway, a number of researchers have recommended the use of data collection tools that are directly digitised (Flageollet, 1993). Based on this, two major digital terrain analysis models are considered for Broadway. These digital terrain analysis models are TOPAZ and SLURPAZ. TOPAZ is a digital terrain analysis model that was first devised by Garbrecht and Martz (1993) with the full name of topographic parameterisation. The TOPAZ works as a digital terrain analysis tool by deriving a range of topographic and geometric variables from a raster digital elevation mode (DEM) situated at the site where the data collection takes place (Lacroix and Martz, 2006). For the TOPAZ however, its functionality is based on variables that are physically related to watershed runoff processes. Some of these watershed runoff processes that are found in the Broadway area have been noted to include subcatchment for each channel link, distances to nearest channel, distances to subcatchment outlet, relief corrected DEM elevations, change in elevation to nearest channel, and change in elevation to subcatchment outlet. As depicted below, TOPAZ has the advantage of generating raster files comprising of drainage network, subwatershed areas and other drainage-related topographic variables that account for the eventual landslide activity of the area. Figure 2: Possible Raster File Outputs from TOPAZ (a) Sub-basins (b) Changes to Channels (c) Distance to sub-basin outlets (d) Aspect Source: Lacroix and Martz (2006, p. 82) The SLURPAZ on the other hand functions as a data collection tool made up of the combination of two major digital landscape analysis models, which are the TOPAZ version 1.20 and SLURP version 11.0 hydrological models. Consequently, when used for digital terrain analysis in the Broadway area, there is going to be a combined functionality (Felicísimo, 1994). This combined functionality system has been displayed in the diagram below. Figure 3: SLURPAZ Combined Functionality Lacroix and Martz (2006, p. 84) As a computerized digital data collection system, the SLURPAZ combines model settings and parameter information that are contained in aggregated simulation areas (ASA) and all the land cover within each ASA (Lacroix and Martz, 2006). Consequently, there are files developed in weights and Morton, taken from the various areas of influence under the climate station available for each ASA. For very specific outcome of data, the area where data collection is taking place is further subjected to time series data for each ASA to ensure that geological activities including landslide activities taking place within each time frame is captured. Identification and Treatment of Error and Uncertainty While using various digital terrain analysis methods, there are two major forms of errors that may be identified. These are non-systematic and systematic errors in the DEMSs. According to Wilson and Gallant (2000), the effect that such errors have on the whole process of data collection from the site is that is leads to a confounding of the expected relationships that exist between the computed terrain attributes an terrain-controlled site conditions. By implication, if any of non-systematic or systematic errors in DEMs take place, it defeats the notion of there will be a direct reflection of the outcomes produced from the computed terrain attributes on the terrain-controlled site conditions available at the location. According to Garbrecht and Martz, 1993), there are specific conditions that increase the effects of errors. Some of these conditions can be noted to include first order derivatives and second order derivatives including slope and convexity. Between the effects that the first and second order derivatives however, Varnes (1996) observed that it was the secondary attributes that produced the worse consequences when encountered. Most forms of secondary attributes that are derived are associated with topographic wetness and sediment transport capacity indices (Wilson and Gallant, 2000). Meanwhile these two forms of indices are noted to be highly sensitive to any errors that take place within the elevation data in landslide areas. Because of the effect that identified errors produce, it is important that pragmatic treatment solutions and interventions be taken towards their occurrence. In an attempt to correcting DEMs errors, Green (1992) noted that there are always two ways out, each of which depends on whether the problem identified is with a linear DEM or a matrix DEM. Where a linear DEM prevails, the solution can be as simple as deleting any identified wrong data such as a point in a line (Whitworth, Giles and Murphy, 2001). In the linear DEMs, such corrections will not have any significant impact on information loss for the whole system. This is however not the case with matrix DEMs when such deletions are done. This is because such approaches may lead to the massive loss of data. As a solution therefore, it is important to manually detect the conflictive points. Once this is one, the right values must be introduced to make way for the erroneous ones. There could also be the option of using automatic corrections, which require the use of algorithms to estimate acceptable values that can be used to substitute all erroneous values (Felicísimo, 1994). In the first case the right value is introduced once the conflictive point has been detected. Automatic correction uses algorithms to estimate an acceptable value (though not necessarily real) that will substitute the erroneous value. Landslide Activity Categorisation at Broadway In line with the need to identify the very geo-morphological hazard that the Broadway site posses due to its landslide susceptibility, it is important to know the exact landslide frequency that exists in this site. To do this effectively in knowing what is present at Broadway, a landslide activity model has been devised by the digital terrain analytical records produced in various literatures. Most of these literatures sought to use the relationship that exists between slope movements and ridge and furrow cultivation remains at Broadway (Whitworth, Giles, Murphy and Petley, 2013). In all, three major categories of landslide activities are identified in Broadway namely active, suspended and relict landslides. Each of these is treated in details below. Figure 4: Landslide Categories at Broadway Source: Whitworth et al (2013, p. 3). From data collected through various methods and procedures, active landslides in Broadway are categorised as those that have shown movement during the last three years. The last three years is used as the basis for categorisation as it presents the outlook of what has been captured by various instrumental data and direct observations at the site (Whitworth et al, 2013). As a general description, Bromhead (1986) noted that active landslide is a landslide that is currently moving. This means that at any point in time, these landslides could cause any massive action associated with the normal activities of landslides. In terms of the classification of risk and geo-morphological hazard, active landslides have been noted to be the most prolific and threatening. Meanwhile, such displayed in the figure above, such active landslides currently exist in portions of Broadway, close to colliers Knapp. Comparatively though, not many of these active landslides exist in Broadway when compared to the other times. Of the type of active landslides that exists in Broadway, Whitworth, Giles and Murphy (2001) noted that they come in three major sub-classes. The sub-classes are lobate mudslide, shallow mudslide, complex landslide and landslide scarps. Of the four sub-classes of active landslides in Broadway, Whitworth et al (2013) observed that the complex landslides are the most predominate, having the description of complex translational landslide. By implication, it means that the type of movement of the active landslide at the Broadway location are slides, as translational landslides fall under this type of landslide movement. From the translational landslide, Cooper (2007) also pointed that there is the presence of many units, which go out to form rock slide. As with most other translational slides, those found as part of the active slides in Broadway in their mass displaces along a planar surface of rupture as noted in Varnes (1996). Consequently, there is the sliding out over of the original ground surface once this type of translational landslide movement takes place. For the second category of landslide, which Whitworth et al (2013) identified to be suspended landslides, they have been noted to have given sufficient evidence of movement over the past 228 years. For such suspended landslides, even though their previous movements are of relevance to geo-morphological decision making, it can be said in very general terms that their effect is not active in a present state. This notwithstanding, for those located in the Broadway site, there are a number of them with local cracking found in the crown of the topple. For most of the suspended landslides in Broadway, Whitworth et al (2013) argued that they have very similar spectral properties as active landslides. By implication, the suspended landslides can be justified to have maintained their morphological characteristics over the years, thereby giving off spectral signatures that are similar to active landslides. For this reason, the potential geo-morphological hazard that the suspended landslides can cause cannot be overemphasised. In the figure below, there is a comparison between the state of activity of active and suspended landslides in Broadway. By way of the mudslides also, their morphology can be seen to be composed of shallow concave niche that have flat lobate that are wider than normal transportational path (Westen, 2009) Figure 5: State of Activity of (1) Active and (2) Suspended Landslides Source: Cooper (2007, p. 348) Given the background that the morphological characteristics of the suspended landslides might have been retained from that of active landslides, Cooper (2007) argued that the morphology of the suspended are not very different from what exists for the active landslides. However, instead of having many units from the translational slides, it was noted that there were few units at the Broadway site (Hervas and Rosin, 2006). This subsequently produced a rock block slide material, leading up to earth block slide and debris block slide. The last categorisation that can be given at the Broadway site is the relict landslide, which for the past 228 years has had no evidence of movement at all. Regardless of the lack of activity, Cooper (2007) warned that such relict landslides could show active movements and characteristics when substantially disturbed. Apart from such substantial disturbances that may arise, very minute incidents such as the effect of changes in climate may not cause active movements for these landslides. Already, most of these relict land landslides have been said to come about as a result of periglacial conditions. Consequent to the current state of inactivity, further analysis on their morphology will not take place at this time. Meanwhile for geo-morphologists, the need to have significant observations in terms of long-term landslide hazard assessments has been recommended by Whitworth et al (2013). References Bromhead, E. (1986) The Stability of Slopes. Surrey University Press. Guilford. Chandler, R. J. (2007) The Degradation of Lias Clay Slopes in an area of the East Midlands. Quarterly Journal of Engineering Geology 2, 161 – 181. Cooper, R.G. 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No. 176, Nat. Acad. oi Sciences, pp. 11–33. Westen C. (2009). Introduction to landslides Part 2: Mapping landslides from airphotos. International Institute for Aerospace Survey and Earth Sciences: Netherlands Whitworth M, Giles D. & Murphy W. (2001). Identification of landslides in clay terrains using Airborne Thematic Mapper (ATM) multispectral imagery. SPIE Vol. 4545; pp. 42-56 Whitworth M.C.Z., Giles D.P., Murphy W. & Petley D.N. (2013). Spectral Properties of Active, Suspended and Relict. Landslides derived from Airborne Thematic Mapper Imagery. Proceedings of VIII International Symposium on Landslides June 2000, Cardiff Wales UK. Volume III, 1569 – 1674. Wilson J. P. and Gallant J. C. (2000). Terrain Analysis: Principles and Applications. John Wiley & Sons, Inc.: London Read More