Mechanisms of riverbank failure and channel instability on the Nkisi River, Southeast Nigeria

Abstract

Sedimentation hazards pose several geoenvironmental problems in active alpine regions and fluvio-deltaic environments. Therefore, this research seeks quantitative knowledge of the mechanisms that trigger riverbank erosion which result in high volumes of sediment yield in the downstream reaches of the Nkisi River. Study of the mechanisms of mass wasting of the riverbanks was motivated by the high rate of siltation and accretion that are currently occurring at the mouth of the Nkisi River, occupying a total surface area of about 40,000 m2. Many riverbank stability and erosion assessment studies have been done using a wide range of geomorphometric and geotechnical analyses which consider the shear strength of the bank materials in relation to the geomorphometric characteristics of the riverbanks. With this insight, this research focuses on investigating the potential causes of riverbank retreat on the river, with particular interest on the effects of soil properties in triggering mass failures. Field surveys and laboratory investigations, coupled with Bank Erosion Hazard Index (BEHI) and Bank Stability and Toe Erosion Model (BSTEM) characterized the riverbanks as unstable and conditionally stable, revealing a potentially high rate of sediment loading from the composite banks, with a factor of safety that varies from 0.76 to 1.04.