Gully-Landslide interactions: an ecogeomorphic investigation

Abstract

Gully erosion and landsliding are important geomorphic processes that shape the Earth’s surface, yet they pose significant hazards. Gully-induced landslides occur due to extreme gullying creating favourable conditions (e.g. bare and irregular surfaces) for expansion of already existing gullies (landslide-induced gully expansion). These gully-landslide feedbacks are facilitated by interactions among ecogeomorphic factors, yet little is known about the mechanism of these interactions. The aim of this study is to improve understanding of ecogeomorphic processes of gully-landslide interactions using examples from Southeast Nigeria. To achieve this aim, multi-method research methods were adopted: analysis of remotely sensed data, geotechnical investigations, quantitative and qualitative techniques and hydrological modelling. Gullies were mapped using high resolution data (0.61 – 5 m) acquired between November 2009 and December 2018 while supervised land-use classification was undertaken for both years. Geomorphic variables were acquired from the 30 m SRTM-DEM. Geotechnical investigations were conducted by Loraj Consortium, a partner of the Nigeria Environment and Watershed Management Project (NEWMAP) and results were made available for this research. Multiple regression analyses were used to establish associations between gully drivers and changes in gully sizes. Two focus group meetings were held and 192 copies of a questionnaire were distributed. The Soil and Water Assessment Tool model was used to understand effects of land-use changes on catchment hydrology and relate these changes with changes in gully sizes. Results showed that major land-use changes, especially, reduction in fallow-cover, were recorded during the study period and interactions among ecogeomorphic factors were found to be significantly associated with changes in gully sizes. The soils in the study area are mainly sandy with low cohesion values which predispose them to dispersion by surface runoff and high seepage erosion. Modelling results showed there have been increased volumes of surface runoff between 2009 and 2018 due to increased non-vegetated surfaces, a view supported by focus group meeting attendees. Twenty six gullies covering an area of 0.36 km2 were mapped in 2009 but in 2018, 39 gullies occupying 0.62 km2 were mapped. Also, modelling results indicated that despite similarities in soils and geomorphology, hydrological responses of gully catchments varied, thus pointing to the uniqueness of catchments and possible variations in driving processes of individual gullies. Results from focus group meetings indicated there were no gullies in the area before the Nigerian civil war that lasted between 1967 and 1970. Military activities including digging trenches were said to have led to the initiation of the oldest gully in 1968. Participants at focus group meeting said a lag existed between rainfall events and occurrence of gully-induced landslides, suggestive of effects of cumulative rainfall and groundwater as drivers of gullying. At visited gullies, presence of springs was observed suggestive of groundwater-driven gullying. Modelling results suggested high sub-surface flow in the study area, thus, fieldwork, focus group meetings and hydrological modelling all suggest that sub-surface flow is a potential driver of gullying in the study area. It has been suggested that in the design of gully-control, different gully catchments should be treated individually as no two catchments are the same. The multi-method research approach adopted in this research was helpful to understand gully-landslide interactions considering potential effects of ecogeomorphic processes. Adopting this research approach in future studies, especially in data-scarce regions, will improve understanding of geomorphic process in those regions and thus enhance design of management measures.