Root Reinforcement and Bank Shear Strength Along a Tropical Deltaic Distributary: Field Evidence for a Limiting Vegetation Effect

Vegetation is widely promoted as a nature-based solution for streambank stabilisation, yet field measurements of the vegetation–strength–erosion linkage remain scarce for tropical deltaic channels, and the mechanism established in temperate systems may not transfer directly. This study quantifies bank shear strength, vegetation cover and measured bank erosion along a 5,000 m reach of the Erowha River, a distributary of the Forcados system in Delta State, Nigeria. At 101 chainage-referenced stations spaced at 50 m, vegetation cover was characterised by the quadrat method, land use was classified into urban, agricultural and natural classes, and cohesion and the angle of internal friction were determined by direct shear testing of undisturbed cores recovered from the upper bank and the water-level edge. Cover ranged from 45.00% to 65.00% (mean 55.00%) and was perfectly inversely associated with measured erosion (r = −1.000) and directly associated with deposition (r = 0.888). Cohesion ranged from 5.77 to 11.04 kN/m², with mean upper-bank strength (10.23 kN/m²) nearly twice mean edge-of-water strength (5.66 kN/m²), demonstrating saturation-driven softening at the bank toe. Undisturbed swamp forest (L15–L28) recorded the maximum profile cohesion of 11.01 kN/m² with stabilised erosion, whereas cultivated encroachment (L7–L14) coincided with declining cohesion from 8.29 to 7.01 kN/m². Measured cohesion lies within the range attributed to root reinforcement in the geotechnical literature. The results support vegetation-based stabilisation but indicate a limiting reinforcement beyond which further gains are marginal.