In previous work an analysis of the primary flow and secondary circulation on a spiral was developed, based on the use of the Manning equation to describe the flow on the inner part of the spiral and the free vortex equation to characterise the flow on the outer section. More recent studies have advocated the use of the Laminar equation or a new relationship based on the Bagnold shear force. The theoretical basis is modified in the present contribution to permit the estimation of the primary flow on large diameter spiral troughs where the whole of the free surface is part of the vortex flow. Flow predictions for two different large diameter spirals are presented. (C) 2008 Published by Elsevier Ltd.