Recent research has shown that conventional methods for the hydraulic design of two-stage channels are incorrect as they fail to allow for the loss in conveyance arising from the interaction between the main channel and flood plains. Recent research at large scale has been reviewed and interpretted in the context of the need for practical design procedures. Flow may lie in one of several regions of performance depending largely on the relative depths of flow on the flood plains and in the main channel, and a suite of functions has been derived for assessing overall conveyance, allowing also for a wide range of geometries and different roughness between flood plain and main channel. The new information has wide implications for openchannel hydraulics, river engineering and flood management.