Modelling the hazards posed by sudden release of a quantity of contaminants

The consequence of an accidental release of toxic, flammable or noxious material into an environmental flow needs to be assessed in terms of the territory that is subjected to the contaminant at unacceptable concentration levels. Environmental flows are turbulent and generally unsteady and inhomogeneous such that the contaminant concentration values in question are non-stationary, inhomogeneous random variables. Thus the practically important, basic, problem of describing the evolution of the concentration field in a contaminant cloud presents some serious theoretical and experimental challenges. A new approach is developed to describe the diffusion of a contaminant cloud in terms of its location, size and state. The state of the cloud will characterize the dilution of the contaminant concentration values within the cloud. A new measure — the ‘expected mass fraction’ function — is introduced to describe the state of cloud dilution. The advantages of this approach are discussed in terms of the experimental difficulties associated with taking averages. The theoretical advantages that follow from this new approach are illustrated in terms of recent, simple, models of the evolution of the moments of the one-point probability density function of concentration.