An assessment of pedo-hydrological characteristics at medium sized landslide sites in Manafwa catchment, mountain Elgon.

Abstract

Landslide hazards are prevalent and continue to manifest seasonally on the slopes of Mountain Elgon, especially in the Manafwa catchment. While their occurrence has attracted several studies, limited attention has been given to pedo-hydrological characteristics underpinning these landslides. The thrust of this study was to assess pedo - hydrological characteristics at medium-sized landslide sites in the Manafwa catchment. Purposive sampling was done and an inventory of the medium-sized past landslides in the catchment was undertaken. This yielded a total of 27 scars with sizes ranging from 1,500 m2 to 37,850 m2. Four top-sized landslide scars were selected for the study. The selected scars were mapped using a Global Positioning System (GPS) receiver to capture the extent of each scar. Land use characteristics were observed or established through interviews were recorded at each section. Hydrological characteristics were conceived through the measurement of steady-state infiltration rate (IR) and saturated hydraulic conductivity (Ksat). At each site, 12 infiltration measurements representing three-segment positions i.e. head scarp position, main body position, and deposition position, and four replications were taken. In total, 48 infiltration measurements were conducted at the four sites using a double-ring infiltrometer. Composite soil samples were picked from the top (0-15 cm) and lower depth (15-30 cm) and taken for laboratory analysis to determine the content of soil texture and soil organic matter. Soil cores were also extracted to determine the Ksat and bulk density. The study revealed IR ranging from 0.03 cm min-1 to 1.22 cm min-1, averaging 0.5 cm min-1 across the studied sites. IR was slightly higher at the head scarp > the deposition > the main body section but not statistically significant. Overall, the magnitude of IR observed at the landslide scar sites is high. Ksat ranged between 230 mmh-1 and 856 mmh-1, averaging 566 mmh-1. The head scarp recorded the highest Ksat followed by deposition and main body section but not statistically significant. Overall, the magnitude of Ksat observed at the landslide scar sites is very rapid. The soil organic matter (SOM) content was below the critical value ranging from 1.5 to 3.6%, averaging 2.4% in the top (0-15 cm) and 1.0 to 3.0%, averaging 2% in the lower depth (15-30 cm). SOM variation was statistically significant across the soil depth. The clay content varied from 14.5 to 48%, averaging 35.5% in the upper section (0-15 cm) and 14.2 to 50.0%, averaging 39.8% in the lower section (15-30 cm) and the variation was statistically significant. Overall, the clay magnitude was high. Bulk density varied from 1.01 to 1.47 g/cm3, averaging to 1.26 ± 0.1 g/cm3 lower than the critical value (1.6) for clay loam soils. Although the IR was higher at sites, it was also found that the sites have a higher clay content which alters the pore water pressure contributing to slope failure. This challenge can be mitigated by strengthening the sites with indigenous trees to improve the cohesion of soil properties.