Sediment transport in a gravel-bottomed stream

Graduation date: 1973Presentation date: 1973-05-03Sediment transport in a gravel-bottomed stream located in the Oregon Coast Range was studied to determine the effects of a single\ud layer of large particles (the armour layer) located at the surface of the bed material. The bed load transport system was studied jointly with suspended sediment transport to understand the total transport system. The bed load was sampled using a vortex trough in the stream bed which transported the bed load material into a sampling pit adjacent to the stream. It was found that the armour layer controls the bed load transport system by preventing sand and finer material from the bed from being entrained in the flow unless the armouring particles are first\ud moved. The bed load of an armoured stream can be calculated using the simplified Einstein bed load function with a representative diameter for the stability parameter equal to the particle size of armouring material at which 35% of the material is finer (D35 size) and a representative size for the transport parameter equal to the median site of\ud bed material below the armour layer. The critical discharge for disturbing the armour layer is related to a size equal to 69% of the D65 size. The critical shear stress of the armouring material is at a minimum for a particle equal to the 0.69 D65 size. Smaller particles are hidden in the armour layer and larger particles are heavier than the critical particle. From observation of the maximum size of particles transported for various stream discharges, the Shields parameter was found to be 0.025 for a rough bed and for a transport rate of very nearly zero. The suspended sediment transport system was found to be partially related to the past history of the stream because the past history f flows and sediment load controls the ability of the armour layer to remove sand and finer material from the water in the stream or to supply these smaller particles to the water. Both the bed load and suspended load of Oak Creek are quite variable when the discharge is below the critical discharge of the armour layer