Assessment of land use impact and seepage erosion contributions to seasonal variations in riverbank stability: The Iju River, SW Nigeria

Abstract

The Iju (Atuwara) River is an important component of the Owo River catchment (ORC), which supports the livelihood of millions of people in Lagos and Ogun States of southwestern Nigeria. However, rapid urbanisation and numerous anthropogenic activities have caused significant changes in the natural flow regime of the river, resulting in serious bank erosion. In many riparian areas, accelerated bank erosion has been attributed to the combined effects of seepage erosion and fluvial undercutting of the bank toe due to the hydraulic stress of the river on the bank. This research, therefore, investigated the combined effects of seepage erosion, riparian vegetation (root cohesion), tension crack, and land use/land cover (LULC) change on the stability of riverbanks on the Iju River, Nigeria. Multi-temporal LULC datasets acquired from remotely sensed Landsat images of 1999, 2009, and 2019 were used to map LULC changes in the ORC. Laboratory tests were conducted to determine the geotechnical properties of the riverbanks followed by bank stability analysis and evaluation of the erosion potential of the riverbanks using the Bank Stability and Toe Erosion Model (BSTEM) and Bank Erosion Hazard Index (BEHI), respectively. The BSTEM results indicate that the factor of safety (Fs) decreased with an increase in seepage undercutting distance (du), irrespective of the bank angle. The correlation of Fs with root cohesion (cr) and depth of tension crack (zc) shows that Fs was positively correlated with cr but decreased with an increase in zc. LULC change results show that the natural flow regimes of rivers within the ORC have been adversely affected by the combined decrease in vegetation and wetlands, with a corresponding increase in the spatial extents of the built-up area and cultivated land/open spaces. These research findings are essential for the development of a watershed-scale natural disaster management framework for the southwest ecoregion of Nigeria.