Critical shear stress of trout feed and implications for particulate transport in flow through raceways

Aquaculture has been identified with many different types of environmental impacts. Of these, aquacultural sediments are noteworthy because of the associated nutrient and particulate transport from aquaculture facilities to the receiving waters. We investigated the critical shear stress needed to initiate sediment transport as a function of, bed roughness (smooth, 0.4 and 0.7 mm) and particle size (2.5, 1.4 and 0.71 mm). We estimated bed shear stress using logarithmic velocity profiles and near-bed velocity gradients and obtained a critical shear stress estimate with a relative error of 18.9% or 0.007 N/m2. Critical shear stress estimates were impacted by particle size and bed roughness with smaller particles and greater roughness associated with greater critical shear stress. The maximum (plus uncertainty) critical shear stress for all test conditions was 0.083 N/m2. The estimate is in agreement with other estimates of critical shear stress for aquacultural particles, and is greater than estimated available shear stress in typical Idaho raceways especially considering the algae growth in many raceways. Lack of adequate sediment transport increases the time that particles reside in the raceway which, in turn, increases mechanical and biological degradation, resulting in reduced effectiveness of sediment and nutrient management. Means to increase the critical shear stress offers opportunities for future reductions in nutrient and particle export from aquaculture operations.