Structure of a Horizontal Sediment-Laden Momentum Jet

Abstract

Partially treated wastewater discharge from marine outfall diffusers often contains inorganic and organic solids that may settle close to the source and give rise to sludge banks that affect benthic ecology. An experimental and theoretical investigation of the structure and settling mechanisms of a horizontal sediment momentum jet in stagnant ambient has been carried out. The cross-sectional particle concentration distribution of a horizontal sediment jet is measured for the first time. Bottom deposition measurements are made for a total of 41 experiments of momentum jets laden with sand particles or synthetic spherical glass particles. The sediment jet can be characterised by a sedimentation length scale $l_m$ defined by the jet momentum flux and particle settling velocity. The measured longitudinal sediment deposition rate can be described by a semiempirical log-normal distribution based on $l_m$. In the instantaneous particle distribution, the sediment fallout is concentrated in a central region and settles in a zig-zag trajectory. The time-mean transverse particle concentration shows that the upper half of sediment jet behaves like a pure jet with concentric contours for $x<0.5l_m$. For $0.5l_m<x<l_m$, the sediment cloud starts to depart from the water jet; for $x\gtrsim l_m$, the particle cloud separates significantly from the water jet. For $x<l_m$, the maximum particle concentration $C_m$ is located within the jet region and the decay of $C_m$ closely follows the classical jet theory. For $x\gtrsim l_m$, $C_m$ is significantly less than that given by the jet theory. The time-mean particle concentration distribution across the jet is horseshoe-shaped, with an open end at the bottom due to sedimentation. The concentration distribution is elongated in the vertical direction and shaped like an ellipse. A two-layer Lagrangian jet model is developed to predict the mixing of dilute horizontal sediment jets. The predictions are in excellent agreement with the layer-averaged particle concentrations and bottom deposition rate measurement.