An integral model of the behaviour of a single horizontally discharged buoyant flow in a coflowing ambient fluid is presented. The usual integral equations for the conservation of mass, momentum and buoyancy flux are used. However, the model differs from those presented previously by using the spread assumption rather than the more usual entrainment assumption and by explicitly forcing the initially gaussian velocity distribution to become a thermal distribution. The position and the form of this transition is determined from laboratory data (Brown [1984], Knudsen [1988]) and field data (Lee and Neville-Jones [1987]). The performance of the model is compared with both laboratory and field data and it reproduces the data for a wide range of cases. A model of this type is essential in gaining an understanding of the range of behaviour of merging plumes (Cheng, Davidson and Wood) and is a preliminary to the understanding of the more general case where the buoyant discharge is ejected at an angle to the flow.
