A multi-method approach to analyze changes in gully characteristics between 2009 and 2018 in southeast Nigeria

Gully erosion is the dominant environmental problem in southeast Nigeria

and has led to loss of human and material resources. In this study, we

evaluated changes in gully characteristics in southeast Nigeria between

2009 and 2018 using a multi-method approach including geotechnical

investigations, analysis of high-resolution satellite imagery (2 – 5 m)

and focus group discussions. Gully numbers increased from 26 to 39 and

mean gully length increased from 0.39 to 0.43 km. We found that the

soils in the study area have low cohesion and high sand content which

make them susceptible to dispersal by erosive forces. Land adjacent to

rivers had the highest concentration of gullies, and there was a sharp

rise in slope from 10 – 58.2% within a distance less than 500 m from

the river. Regarding potential gully-drivers, land-use changes were

observed. Non-vegetated lands increased from 58.6 km2 to 144.7 km2

between 2009 and 2018, while reductions in fallowed lands from 281.2 km2

to 57.8 km2 were observed. Results from focus group meetings indicate

there was no gullying in the area before the Nigerian civil war.

Although war-time activities which led to initiation of oldest gullies

have ceased, land-use changes likely increase volume of surface runoff.

We infer that interactions between soil conditions which potentially

enhance seepage erosion and higher surface runoff resulting from

land-use changes have propagated gullying in the study area post-civil

war.