Dr Amah's presentation on Landslides

Slide 1: 

APPLICABILITY OF HYDROMECHANICAL MODELS WITH GIS SUPPORT IN THE ANALYSIS AND PREDICTION OF LANDSLIDE HAZARDS WITHIN THE SCARPLANDS OF SOUTH-EASTERN NIGERIA Speaker: Amah,J.I.(JP) PhD MNMGS,COMEG Centre for Environmental Management and Control, University of Nigeria Enugu Campus International Workshop on landslides and other Environmental Disasters, University of Nigeria Nsukka.21st -26th March 2010.

Outline : 

Outline Statement of the Problems & Previous Efforts. The Protocol Physiography,Climate and Geology Hydrogeology/Hydrology Hydromechanical Models & their applicability. Model Simulations with JGRASS support. General Discussions and Conclusions.

Statement of the Problems & Previous Efforts : 

Statement of the Problems & Previous Efforts Landslides is a general term for the results of rapid mass movements. The five states of eastern Nigeria comprising Abia,Anambra,Ebonyi,Enugu and Imo have continued to be ravaged by landslides, reaching disaster levels. Landslide causal factors in the area include regional groundwater flow, Storm water regimes,topography,soil nature,landuse practices.

Statement of the Problem Cont’d : 

Statement of the Problem Cont’d Previous studies in the analysis and mitigation of landslide disasters in the area are not event-specific and are of little use in the prediction of landslide risk zones in that they are geospatially constrained and not qualitatively linked to events that trigger the landslide. The landslides are most intensive within the escarpment ridge that runs N-S of the Benue trough and cutting across most of eastern Nigeria .

The Protocols : 

The Protocols The geospatial distribution of hydraulic heads in the area was examined using a 2-D finite-difference groundwater software (ASMWIN) because of its suitability in modeling an aquifer with a horizontal extent much larger than its thickness. Input parameters for the model include hydraulic heads from over forty boreholes, elevation of aquifer top and bottom from borehole records, hydraulic parameters from analysis of pumping test, meteorological data i.e. monthly temperatures,precipitation,evapotranspiration,rainy days, and sunshine hours etc. For GEOtop,Consider the initial conditions (antecedent soil moisture conditions), soil types, rainfall intensity and duration.

The Protocol Cont’d : 

The Protocol Cont’d Determine physical ,mechanical and hydraulic properties of the sand,assuming reasonable value of bulk density b(g/cm3),effective angle of shearing resistance ‘ and saturated hydraulic conductivity Ksat. Import the DEM of interest and generate input files for the GEOtop program using the GIS JGRASS. Run the GEOtop program

Physiography,Climate & Geology : 

Physiography,Climate & Geology characterized by a Cuesta topography, an asymmetrical ridge stretching in a sigmoid curve for over 500km from Idah on the Niger River to Arochukwu on the Cross River (Reijers et al 1996). The area lies within the tropical rainforest belt of Nigeria with savannah-type vegetation. The escarpment is sparsely vegetated with thin shrubs that tolerate moisture deficiency like cashew trees and grasses. Two remarkable seasons; a dry season characterized by extreme aridity and bush burning from end of October to early April, and a rainy season characterized by heavy thunder storms and low to intense rainfalls that begins in early April and ends in October.

Physiography,Climate & Geology Cont’d : 

Physiography,Climate & Geology Cont’d The dominant formation is the middle Maastrichtian Ajali Sandstone. This comprises a sequence of friable poorly consolidated fine to medium and occasionally coarse sandstone. It is typically cross-bedded and contains minor clay lenses. It is highly porous and permeable with porosity estimated at 37%(%( Morris &Johnson, 1967; pettijohn et al 1973).Further south in Anambra,the dominant geological formation is the Eocene Nanka Sands. It is a sequence of unconsolidated or poorly consolidated sand, 305 m thick, underlain by the thick Imo Shale formation of Palaeocene age, and overlain by the lignite-clay seams of the Oligocène Ogwashi-Asaba formation. The Nanka Sands are predominantly sandy with thin claystone and siltstone bands, lenses and laminations. The sand is poorly-sorted, cross-bedded and medium to coarse grained.

DTM adapted from Igbokwe et al(2008) : 

DTM adapted from Igbokwe et al(2008)

Hydrogeology/Hydrology : 

Hydrogeology/Hydrology The Ajalli is highly permeable. The dominant lithology is fine-medium sandstone and hydraulic conductivity range of 3.6x10-5m/sec to1.13x10-4m/sec.the Ajalli sandstone is homogenous and anisotropic, exhibiting some form of layered heterogeneity.Waterlevel drawdown of up to 17m has been simulated at the escarpment while flows into the Cross river basin from the various leakage nodes have been estimated at 15.45m3/day per unit aquifer width normal to the flow direction. The hydraulic gradient is about 1/200 at the recharge area and increases to about a value of 1/150 at the eastern spring line where the hydraulic flux increases appreciably, indicating proximity to discharge areas. The Darcy velocity (Specific discharge) varies from 0.02m/day at the east to 0.06m/day at the west while linear velocity follows the same pattern from 0.05m/day to 0.16m/day(Amah,J.I. 2006)

Hydrogeology/Hydrology Cont’d : 

Hydrogeology/Hydrology Cont’d . Transmissivity (T) is highly variable from 35m2/day around the south-eastern boundary at Diogbe to 2219.17m2/day at Ibagwa-aka.The hydraulic conductivity varies from 0.56m/day to 30.52m/day following the same trend. Two factors account for this variation-the progressive increase in aquifer saturated thickness westwards and the varying permeability of the aquifer material. The hydraulic conductivity field shows a highly heterogeneous layering pattern with an equivalent horizontal conductivity Kx 0f 9.28m/day and vertical hydraulic conductivity Ky of 5.86m/day, giving an anisotropy factor of 1.58. Seasonal water level changes of 17m for the eastern boundary and 68m for the western boundary have been simulated. The western areas are prone to high evapotranspiration that consumes the excess moisture (Amah, J.I. 2006)

H/H Cont’d : 

H/H Cont’d The escarpment is characterised by high hydraulic head values and peak at 328m at Ogbozalla Opi and 329m in around Awhum . Water is critical to the process of mass wasting. Too little water in a mass of material such as sand prevents the grains from sticking together - a lack of cohesion that decreases shear strength. Enough water to coat the grains will cause a great increase in internal cohesion, increasing shear strength. If water is added to completely saturate the pore spaces, the weight of the mass will be greatly increased, increasing the shear force. Also, the pore water now acts as a lubricant between grains and the pore pressure helps force the grains apart. This decreases the shear strength. Because of the effect of water on slope stability, many mass wasting events are triggered or hastened by heavy or extended rainfall. The implication of high heads is the incidence of moisture deficiency in the escarpment areas, scanty vegetation cover, and enhancement of weathering conditions

Slide 14: 

Map of the hydraulic heads implemented with ASMWIN.Note the recharge(Opi,Ohodo,Edeobala,Awhum and entire Ngwo area) and the discharge towards Obimo, Nachi towards Oji)

Hydromechanical Models & their applicability : 

Hydromechanical Models & their applicability