Recent studies have modelled the flow of particle-driven gravity currents over horizontal boundaries using either shallow-water equations or simple 'box' (integral) models. The shallow-water equations are typically integrated numerically, whereas box models admit analytical solutions. However, the theoretical validity of the latter models has not been fully established. In this paper a novel mathematical technique is developed which permits the derivation of analytical solutions to the shallow-water model for gravity current motion. These solutions, confirmed by comparison with the results of numerical integration, are in good agreement with experimental observations. They also indicate why the simplified box models have been so successful. Moreover, they reveal how the internal dynamics of particle-driven flows are different from gravity currents arising solely due to compositional density differences. While compositionally driven gravity currents, which have a fixed density difference between the intruding and ambient fluids, may be modelled using similarity solutions to the governing equations, particle-driven gravity currents do not possess such solutions because their density is progressively reduced by particle sedimentation. Instead the new analysis determines how their behaviour progressively diverges from the similarity solution. By a change of independent variables, it is possible to develop convergent series expansions for each of the dependent variables which characterize the motion. It is suggested that this approach may find application to a number of other problems in which the dynamics are initially governed by a simple dynamical balance which is progressively lost as extra physical effects begin to influence the motion.

The intrusion of a polydisperse suspension of particles over a horizontal, rigid boundary is investigated theoretically using both an integral (‘box’) model and the shallow-water equations. The flow is driven by the horizontal pressure gradient associated with the density difference between the intrusion and the surrounding fluid, which is progressively diminished as suspended particles sediment from the flow to the underlying boundary. Each class of particles in a polydisperse suspension has a different settling velocity. The effects of both a discrete and continuous distribution of settling velocities on the propagation of the current are analysed and the results are compared in detail with results obtained by treating the suspension as monodisperse with an average settling velocity. For both models we demonstrate that in many regimes it is insufficient to deduce the behaviour of the suspension from this average, but rather one can characterize the flow using the variance of the settling velocity distribution as well. The shallow-water equations are studied analytically using a novel asymptotic technique, which obviates the need for numerical integration of the governing equations. For a bidisperse suspension we explicitly calculate the flow speed, runout length and the distribution of the deposit, to reveal how the flow naturally leads to a vertical and streamwise segregation of particles even from an initially well-mixed suspension. The asymptotic results are confirmed by comparison with numerical integration of the shallow-water equations and the predictions of this study are discussed in the light of recent experimental results and field observations.

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Quality attributes of packaged and unpackaged vegetables generally decreased nonlinearly during storage at 10°C or ZO"C, and most of the decrease was greater at 20°C than at 10°C. Packaging reduced weight loss of green beans and spinach kept at 20°C; and reduced chlorophyll loss of green beans at 10°C and of spinach at 20°C. Ascorbic acid (vitamin C) benefited by packaging of green beans and spinach kept at 10°C. Packaging had an effect on thiamin (vitamin Br) of only spinach held at 10°C and 20°C and had no effect on riboflavin (vitamin Bz). Loss of these quality attributes appears to be enhanced with water loss.