GIS ANALYSIS OF DRIVERS OF EROSION AND GULLYING IN SOUTHEAST NIGERIA

Transcription

1 GIS ANALYSIS OF DRIVERS OF EROSION AND GULLYING IN SOUTHEAST NIGERIA Mayowa FASONA & Ademola OMOJOLA Department of Geography University of Lagos, Nigeria

2 Outline Background Methodology Results Conclusions

3 Background

4 The Southeast (SE) States of Nigeria is the area most affected by erosion in the country The erosion and gully problem has a long history - dated back to over 100years (Igbokwe et al. 2008), About 2500 Active gully sites and gullies in different stages of development in the SE States The Agulu-Nanka-Ekwulobia-Oko gully complex (in Anambra State) covers over 1000km 2 and expanding at about 20-50m per year (Ajaero and Mozie, 2011; Egboka & Okpoko, 1984)

5 Part of Agulu-Nanka-Ekwulobia-Oko gully complex, Anambra State

6 Erosion threatens the Natural and Built environment

7 Nigeria showing the erosion prone states

8 The study area in SE Nigeria

9 The interplay between physical and human factors - as the key drivers of erosion and gully Key variables identified Complex hydro-geological and geotechnical properties of underlying aquifer system Soil friable, unconsolidated Surface runoff from high intensity rainfall Vegetal removal High population density - Land-use pressure Poor design of roads, culverts and gutters

10 Objectives To explore an integrated data modeling approach using GIS to explain some of the drivers of erosion and gully formation To explore if canopy ecosystem management possesses any potential for erosion remediation and management

11 Methodology

12 Some of the spatial data used

13 Other data utilized include: Average rainfall data Population data Drainage from drainage maps Road network data

14 Procedure Conversion of data to GIS format include: Landuse/landcover mapping Second level derivatives e.g. slope, Parameterization and assignment of scores to candidate drivers based on susceptibility to erosion: Elevation Slope Geology Soil Population distribution Drainage network Road network Rainfall Canopy cover

15 Population density and gullies

16 Slope map, drainage network and gullies

17 Geology and gullies

18 Road Network and gullies

19 Scenario outlook for canopy cover under modelled under: Business as usual scenario - gullies and areas exposed to erosion assigned 0% Medium intervention - gullies and areas exposed to erosion restored from 0% to 20%, mosaic of grassland, farmland and bare surfaces from 20% to 40% Strong intervention - gullies and areas exposed to erosion restored up to 60%, and mosaic of grassland, farmland and bare surface to 80%.

20 Canopy Cover Scenarios

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22 Analysis Landuse/Landcover Change Analysis Transfer of the spatial driver values to the Polygon Attribute Table of the LULC map by collocation Transfer of the PAT to Statistical Package Logistic Regression Analysis to derive importance of the drivers and Simulation of Probability Surface Map of Erosion and Gullying Principal Component Analysis (PCA) to derive the relationship between Canopy Cover with other drivers under different scenarios

23 Results

24 LULC statistics (in Percentage)

25 LULC map for 1976

26 LULC map for 2006

27 Logistic regression of Erosion Drivers

28 The function for constructing probability surface maps for erosion and gully formation process in the SE was written as: P=1/[1+exp(-1*( (p- density)+0.09(slope)+0.003(elv) 0.003(riv_d)- 0.11(rd_dis)+0.149(geol) 0.132(sol) 0.003(rain90).

29 Simulated probability surface map of erosion and gullying

30 Principal Components Canopy cover vs Others

31 Conclusions

32 The results suggest the trend of eroded areas has been on the increase Geology, slope, elevation and population density emerged as the most important predictors of erosion. Results suggest that improved canopy cover has strong potentials to mitigate erosion in the SE Nigeria A GIS-based integrated and holistic basin-wide approach to erosion and gully management offers a promising strategy to provide data to support pointbased structural measures in the SE

33 ..Thanks