Cohesiveness of the Nigerian Mahin mud coast sediment: Implications for erodibility and morphodynamic modelling

Abstract

Cohesive sediments in coastal zones and estuarine mudflats have transport mechanisms that are not well understood due to the complex sediment properties that influence their erodibility. However, understanding coastal sediment composition and behaviour is important to determine its resistance to hydrodynamic forcing. This study investigates the properties of mudflats along the Mahin coast of southwestern Nigeria to identify the dominant factors controlling their resistive forces and erodibility potentials. Sediment samples obtained from 6 stations along this muddy coast were subjected to physical, mechanical, electrochemical and mineralogical analyses. Our results show that the sediments are inorganic silt of high plasticity, containing inactive clay minerals. With cohesion and frictional angles (φ) that ranged from 42.4 to 46.9 kN/m2 and 10.2–11.7°, respectively, the sediments can be described as highly cohesive and overconsolidated. Organic contents in the muds are <20% of the entire sediment mass, reflecting the impact of vegetation removal along the coast. Whereas fairly neutral to alkaline pH shows the role of seawater in maintaining the chemical balance in the mud. Sediments with proximity to the western and eastern estuaries in the study area show similar characteristics compared to those farther from the estuaries. In addition, sediments closer to the estuaries are dominated by an active illite-montmorillonite mixed clay layer, which makes up <20% of their bulk composition, while those farther are dominated by illite and kaolinite. The correlation analysis revealed that the physical and mechanical properties of the mudflat sediments along the Mahin coast, southwestern Nigeria are influenced by their chemistry and mineralogical composition. Estuarine dynamics also play a key role in recent sediment transport along the mudflat. This study thus provides baseline data on sediment properties for modelling morphological changes of the Nigerian Mahin mud coast. In-situ erodibility experiments are recommended to ascertain the erosion parameters of the area.