Improved simulation of mixed flow conditions within building and local drainage systems

Building drainage systems, and the local systems that connect buildings and their curtilages to main sewer networks, are often characterised by a large number of small-medium diameter pipes, incorporating many junctions and a variety of different pipe lengths and slopes. Although normally designed to operate under free surface conditions, such systems will regularly experience full bore flow events, and may hence be defined as mixed flow systems. Whilst there are a number of fully dynamic modelling suites targeted at large scale urban drainage systems, there are no similar models to accurately simulate the full range of flow conditions that can occur within local drainage systems. Similarly, the numerical techniques employed in the existing DRAINET building drainage model mean that it is not particularly suited to the widespread simulation of mixed flow events within complex systems. This paper outlines the development of a new modelling technique to simulate mixed flow conditions within building and local drainage systems. The underlying principle behind the technique is to ensure that, where numerical stability permits, the appropriate set of governing equations are applied to the relevant flow regimes. To achieve this, a combination of shock capturing and shock fitting techniques is employed, and a new Time Varying Preissmann Slot (TVPS) is introduced.