Transient flows in sewers are multi-regime, multi-component and multi-phase and involve rapid spatial and temporal variations. The regimes include open channel, waterhammer and unsteady gas flows. Transitions from one flow regime to another are governed by flow instabilities that are generally not well understood. The main instabilities that could arise in transient sewer flows are reviewed and discussed and the relevance of these instabilities to model development are highlighted. In addition, existing transient flow models for sewerage lines are summarized and their strengths and weaknesses discussed. The inabilities of exiting models in handling some of the important flow instabilities are highlighted. Needs for future research are discussed.
