The analysis of rainfall-induced slope failures at Iva Valley area of Enugu State,Nigeria

This paper analyses the dominant mechanisms of slope failures and identifies potential obstacles to landslide-hazard reduction at the Iva Valley area, Enugu, Nigeria. The landscape is replete with landslide scars and gullies of varied sizes and the slope deposits comprise unconsolidated, friable sands inter-bedded with thin units of montmorillonitic claystone. Forty-three landslide events were identified in the study area with most being shallow, short run-out movements with slip-surface depth <2 m. The study found the landslides mainly occur in the beginning of rainy season characterized by short duration, high intensity rainfall. An integrated approach comprising field mapping, laboratory tests and numerical analyses reveals that the barren nature of the slopes prior to the outset of rainy season, high rainfall intensity, erosion, overgrazing, soil characteristics and the site’s unique lithologic sequence are the main causes of instability. Shearing tests under several conditions showed that the soils strongly strain-soften until low steady-state strength is achieved. A computer code, based on this strength reduction technique, used input parameters obtained from the field and laboratory studies to simulate a landslide with similar structure, travel distance and distribution area. It is noted that urbanization has gradually increased the vulnerability of the society’s poor to landslide hazards as they now erect unplanned residence (tents and blocks) on the slopes. This work is part of a regional study aimed at finding ways of protecting the vulnerable by generating data that could be used to build future landslide susceptibility map.     

WCoE/IPL projects in West Africa: application of Landsat ETM+ and ASTER GDEM data in evaluating factors associated with long runout landslides in Benue hills,North-central Nigeria

Under the WCoE/IPL project, Landsat Enhanced Thematic Mapper plus (ETM+) and ASTER GDEM data were used to analyze factors associated with frequent debris flow and rock-debris avalanche on the Nigeria-Cameroon mountain range. Detailed processing of the ETM+ imageries of the vulnerable portions on the western part of Adamawa plateau was carried out to identify structural trends relatable to the slope failures. The processing identified the structural characteristics of the study area and clearly distinguished the lineaments associated with the debris flow and rock-debris avalanches. The strongly weathered and fragmented gneissic rocks, shallow groundwater table, as well as the distribution of faults and joint sets were significant factors predisposing the slopes to rapid movements. Recent landslides occurred at the locations with high lineament density and in areas close to major faults and long lineaments. The interaction among the moving mass, slope geometry, local geology, topography, and drainage then resulted in a long runout and a high degree of spreading. Analysis of the ASTER–GDEM delineated landforms, slope morphology, and drainage patterns, which was not possible with conventional mapping techniques. Integration of these results yielded reasonable interpretation of the predisposing factors for the long runout failures and provided logical basis for future landslide susceptibility analysis in the area. Accurate investigation of the predisposing factors and characteristics of landslides in environments that have experienced past failures is therefore important in understanding areas that may be susceptible to landsliding in future.     

Interpretation of aeromagnetic data of Idah area in north central Nigeria using combined methods

Aeromagnetic data of Idah area (sheet 267), north-central Nigeria, has been interpreted by applying source parameter imaging (SPI), Euler deconvolution and forward and inverse modeling methods. Quantitatively, depth estimates obtained by employing SPI have shown minimum to maximum depth to anomalous source at 57.591m to 664.841m. Applying Euler deconvolution for various structural indices (SI), the depth obtained for SI=1, ranges from 5.6m to 197.6m, 22.0m to 204.7m for SI=2 and for SI=3, the depth to magnetic source obtained ranges from 38.0m to 205.5m. The results from forward and inverse modeling for profiles 1, 2, 3, 4 and 5 indicate depths of 95.6m, 103.9m, 477.5m, 1239.8m and 76.8m respectively. From the results of susceptibility values; the result obtained from profile 1 with susceptibility value of -0.0175 suggests that the body is associated with non-magnetic sedimentary deposits. Profile 2 with a susceptibility value of 0.07 is typical of basic igneous rock; gabbro and basalts. The results for profiles 3, 4 and 5 having susceptibility values of 0.01 to 0.013 are typical of intermediate igneous rocks; diorite and andesite.     

Interpretation of airborne geophysical data of Nsukka area,southeastern Nigeria

Airborne geophysical (aeromagnetic and gravity) data of Nsukka area was interpreted qualitatively and quantitatively with the aim of determining the susceptibilities of rock types, depth/mass of the anomalous bodies, possible cause of the anomalies and type of mineralization prevalent in the area. The estimated depths from forward and inverse modeling of aeromagnetic data for profiles 1, 2, 3, 4 and 5 were 1200m, 1644m, 1972m, 2193m and 2285m respectively. The respective susceptibility values were 0.0031, 0.0073, 1.4493, 0.0069 and 0.0016. These indicate dominance of iron rich minerals like limonite, hematite, pyrrhotite, and pyrite and forms lateritic caps on sandstones. SPI depth result ranges from 151.6m minimum (shallow magnetic bodies) to 3082.7 m maximum (deep lying magnetic bodies). Euler depths for the four different structural index (SI = 0.5, 1, 2, 3) ranges from 7.99 to 128.93m which are depths of shallow magnetic sources resulting from lateritic bodies in the outcrops in the study area. From the gravity data interpretation, Euler depth estimation reveals that depth to anomalous bodies ranges from 89.13 to 2296.92m. Density of the causative body obtained from modeling results for profile 1 was 1498kg/m³, which is in the range of clay material and the depth was about 923m. From models 2, 4 and 5, the densities of the causative bodies were 3523, 4127 and 3707kg/m³, while depths to the surface were about 604, 815 and 1893m respectively. These density ranges correspond to that of ironstone. From model three, the density of causative body obtained was 2508kg/m³, located at a depth of about 268m below the surface. This work has shown that Nsukka area is underlain by thick strata of shales, sandstones and ironstones, which together are suitable for ceramic production, and sufficiently thick sediments suitable for hydrocarbon accumulation.     

Remittances economy,remittances landscape: an analysis of the economic and socioecological implications of remittances to households in South Eastern Nigeria

This study evaluated the level of household dependence on remittances, its uses and linkages with landscape/environmental conditions in South Eastern Nigeria. The study area is a landscape unit defined by the Idemili River Basin of Anambra State. The concept of socio-ecological systems provided the basis for an integrated assessment of remittances environment linkages using primary data from a survey of 840 households in 25 localities in the basin and landscape data from a Nigerias at 32 m resolution image dataset. Based on this, linear and non linear regression modelling of locality level relationships between four indicators of household remittances (HR) and a key landscape variable (vegetation cover density-VCD) was implemented. About 45 % of the households had access to remittances; 10 % of receiving households depended on it as their main income source; and over 87 % used the receipts mainly to meet basic household needs. Only 4 % of the households used it in activities that directly affected the vegetation cover. The linear and non linear relationships between the indicators of HR and VCD were generally weak (R² < 0.250). However, the pattern of relationship was different for various indicators of HR. It was inverse for locality aggregate (total) and average HR and positive for level of household access to remittances/locality. This illustrates the complex relationship between remittances and landscape variables in the basin. The study shows that the impact of remittances on households and the environment in the region is both positive (through the alleviation of poverty and land pressure) and negative (through reduction of vegetation cover density).