Abstract
Gully erosion is a major environmental problem in Gombe town, a large area of land is becoming unsuitable for human settlement, hence the need for a gully erosion susceptibility map of the study area. To generate a gully inventory map, a detailed field exercise was carried out, during this investigation one hundred gullies were identified and studied extensively within the study area of about 550 km2. In addition to the mapped gullies, Google EarthPro with high-resolution imagery was used to locate the spatial extents of fifty (50) more gullies. Ten gully erosion predisposing factors were carefully selected considering the information obtained from literature, and multiple field survey of the study area, the factors include elevation, slope angle, curvature, aspect, topographic wetness index (TWI), soil texture, geology, drainage buffer, road buffer and landuse. In this study, a GIS-based Frequency Ratio (FR) and Analytical Hierarchy Process (AHP) models were employed to predict areas prone to gully erosion in Gombe town and environs. The result obtained from FR shows that drainage, soil texture, and slope have the highest correlation with gully occurrence, while the AHP model revealed that drainage buffer, soil texture, geology have a high correlation with the formation of a gully. Gully erosion susceptibility maps (GESM) were produced and reclassified into very high, high, moderate, and low zones. The overall accuracies of both models were tested utilizing area under the curve (AUC) values and gully density distribution.FR and AHP model have AUC values of 0.73 and 0.72 respectively, the outcome indicates that both models have high prediction accuracy. The gully erosion density distribution values revealed that gullies are concentrated in the very high susceptibility class and it decreases towards the low class, therefore the GESM produced using these models in this study area is reliable and can be used for land management and future planning.
Highlights
One of the factors that endanger water and soil is soil erosion (Magliulo 2012)
Prediction rate The prediction rate result shows that drainage buffer, soil texture, slope degree, and landuse, have the major contribution to gully occurrence with a prediction rate of 4.21, 3.25, 2.49, 2.27, respectively as shown in (Table 3) Others include: elevation (2.03), topographic wetness index (TWI) (1.79), Geology (1.68), Road buffer (1.64), Aspect (1.61), curvature (1)
The produced susceptibility index maps for the frequency ratio and Analytical Hierarchy Process (AHP) models were reclassified into four zones according to the quantile classification scheme (Fig. 8a and b)
Summary
One of the factors that endanger water and soil is soil erosion (Magliulo 2012). The major cause of land degradation around the world is soil erosion by gully erosion (Nampak et al 2018; Rizeei et al 2016). The changes in the quantity of moisture content resulting from dry and wet seasons are a major factor contributing to cracks and grooves in clay formations forming rilled erosion and gullies (Torri et al 2012) Runoff accumulates within these cracks at the first sudden rainfall and gully develops. Several researchers have assessed control measures and the effect of gully erosion in Gombe town, but no work has been done to produce a reliable gully erosion susceptibility map (GESM) adapting Frequency Ratio (FR) and Analytical Hierarchy Process (AHP) models. The objective of this research includes: To develop a gully inventory map of the study area, secondly, to produce all relevant thematic maps for the conditioning factors and establish the correlation between predisposing factors and gully occurrence, to apply Frequency Ratio and Analytical Hierarchy Process (AHP) models to produce GESM and to validate the produced susceptibility maps using AUC and gully density distribution
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