Abstract. Landslides triggered by strong earthquakes often caused most of the global damage and most of all casualties related to the events, such as shown by the M = 7.7 Peru earthquake in 1970, by the M = 7.6 El Salvador earthquake in 2001 or by the M = 7.4 Khait (Tajikistan) earthquake in 1949. The obvious impact of a landslide on the population is directly related to its movement. Yet, prediction of future failure potential and hence future risk to population is necessary in order to avoid further catastrophes and involves the analyses of the origin of seismic instability. The seismic landslide potential is mainly determined by the interaction between the regional seismic hazard and local geological conditions. At a local scale, seismic factors interfering with geological conditions can produce site-specific ground motions. The influence of such Site Effects on instability is the principal topic of this paper, which is divided into two parts, A and B. The present Part A is concerned with the correlation of field data with observed instability phenomena. Field data were obtained on mainly three landslide sites in the Northern Tien Shan Mountains in Kyrgyzstan, Central Asia. Geophysical prospecting, earthquake recordings, geological observation, trenching and geotechnical tests were the main investigation tools. The collected information gives an insight in the geological background of the slope failure and allows us to roughly infer failure mechanisms from field evidence. A detailed analysis of the susceptibility of a mechanism to specific geological conditions will be shown in Part B.
Limbe town and surrounding areas, on the SE foot slopes of the active Mt Cameroon Volcano, have experienced numerous small-scale shallow landslides within the last 20 years. These resulted in the loss of *30 lives and significant damage to farmland and properties. Landslides and their scars are identified in the field, and their geometry systematically measured to construct a landslide inventory map for the study area. Specific landslides are investigated in detail to identify site-specific controlling and triggering factors. This is to constrain key input parameters and their variability for subsequent susceptibility and risk modeling, for immediate local and regional applications in land-use planning. It will also enable a rapid exploration of remediation strategies that are currently lacking in the SW and NW regions of Cameroon. Typical slides within the study area are small-scale, shallow, translational earth, and debris slides though some rotational earth slides were also documented. The depletion zones have mean widths of 22 m ± 16.7 m and lengths of 25 ± 23 standard deviation. Estimated aerial extents of landslide scars and volume of generated debris range from 10 1 to 10 4 m 2 and 2 to 5 9 10 4 m 3 , respectively. A key finding is that most slope instabilities within the study area are associated with and appear to be exacerbated by man-made factors such as excavation, anarchical construction, and deforestation of steep slopes. High intensity rainfall notably during localized storms is the principal triggering factor identified so far. The findings from this case study have relevance to understanding some key aspects of locally devastating slope instabilities that commonly occur on intensely weathered steep terrains across subtropical Africa and in the subtropics worldwide and affecting an ever denser and most vulnerable population.
This study aims to investigate the relation between mega-gully (N5 m width) distribution and urbanization in Kinshasa (D.R. Congo), to establish what governs mega-gully location and plan form and to illustrate the concepts behind mega-gully treatment. For this purpose, the diachronic distribution of mega-gullies has been mapped in Kinshasa. All mega-gullies have been reported in ArcGis 9.3 on the orthorectified SPOT 2007 image. A newly elaborated DEM enables the mega-gullies to be placed in their natural topographical context. The GIS inventory on the SPOT 2006/2007 anaglyph indicates the mega-gully situation in the high town of Kinshasa 5 years ago: 308 mega-gullies with a cumulated length of 94.7 km, a mean drainage density of 0.4 km km −2 and an average width and depth of 17 m and 6 m respectively. On the WorldView 1 (WV1) coverage, the number of mega-gullies has more than doubled between 2007 and 2010 from 160 to 334. The study shows that mega-gullies only develop within the urbanized perimeter of the high town of Kinshasa and only 5 to 10 years after incipient urbanization. The study also indicates that neither the location, the plan form or the downslope course of mega-gullies in Kinshasa are controlled by the natural topography. Forty-three point eight percent of the mega-gullies in Kinshasa are 'axial', occupying urban structures which function as artificial runoff drainage lines: roads, tarred or not, with or without side-road trenches, gutters in all forms and materials from concrete to sand, also foot paths and further all artificial runoff drainage lines. The study reveals that every mega-gully is directly or indirectly induced by human activities, but that every gully also finally ends to grow after an initial phase of sudden development. Mega-gully treatment follows two principles, often combined. The first is to stop the alimentation of the mega-gully head with water. The second includes a complete stabilization of the channel and walls inside the mega-gully. This study emphasizes that gully prevention can basically be achieved by control of the runoff discharges in the artificial stream network, as well as beside the roads.
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