CALIBRATING TURBIDITY AS A SUSPENDED-SEDIMENT SURROGATE IN A BACKWATER-AFFECTED TROPICAL DISTRIBUTARY

Turbidity is the most widely used surrogate for suspended-sediment concentration, yet its calibration is sensitive to particle size, sediment composition and hydraulic regime, and the assumption of a stable reach-wide relation is rarely tested in backwater-affected channels. This study evaluates the turbidity–suspended-solids relationship along a 5,000 m reach of the Erowha River, a distributary of the Forcados system in Delta State, Nigeria, during the wet season, when backwater from the receiving channel governs local hydraulics. Water samples were collected at 101 chainage-referenced stations spaced at 50 m and analyzed for turbidity, total suspended solids and total dissolved solids by filtration and evaporation. TSS ranged from 20.49 to 277.00 mg/L (mean 24.13 mg/L), turbidity from 84.30 to 180.00 NTU (mean 88.91 NTU) and TDS from 16.44 to 38.50 mg/L (mean 24.84 mg/L). Turbidity and TSS correlated at r = 0.948. A reach-wide linear calibration (TSS = 2.4605·T − 194.64) returned R² = 0.899 with a 95% confidence interval on the slope of 2.296 to 2.625, while a power-law form (TSS = 2.94 × 10⁻⁵ · T^3.018) returned R² = 0.903. Cross-application of the calibrations produced errors of −82.2% at the high-energy station, −64.0% and +56.6% across the depositional zone, and a reach-wide mean absolute percentage error of approximately 30%. A single reach-wide calibration therefore fails across the backwater transition, and zone-specific coefficients developed across the full hydrological regime are required.