Modelling mixed flow conditions within building and local drainage systems

Lead Research Organisation: Heriot-Watt University
Department Name: Sch of the Built Environment


Piped drainage systems form the backbone of urban drainage infrastructure, both in terms of foul and surface water drainage. The piped systems located in the upstream reaches of urban drainage networks include those installed within buildings and those local systems that connect buildings and their curtilages to the main sewer network; examples of local systems range from those serving a single residential property to those draining large retail parks. The purpose of this research is to improve the simulation of flow conditions within such systems, and hence facilitate the development of the integrated design methodologies required to meet the extra demands associated with the future impacts of climate change and water conservation measures.Flow conditions within building and local drainage systems are often complex, partly due to the highly unsteady nature of system inflows and partly due to their relatively complex and compact layouts; in particular, such systems commonly experience mixed flow conditions, characterised by both free surface and full bore flow regions separated by a hydraulic jump. In spite of this complexity, and the underlying importance of such systems to all sections of society, there are currently no numerical models available to accurately simulate the full range of mixed flow conditions that occur within building and local drainage systems. Without the ability to simulate such conditions, the challenges presented by system design to accommodate transitional flows can not be fully understood, and thus performance benefits remain unrealised. Whilst this situation is undesirable under current loading conditions, the consequences of these shortcomings is bound to increase in the future. It is now generally accepted that climate change will increase the frequency and severity of extreme rainfall events, and will hence result in increased surcharging of drainage systems conveying stormwater. Additional demands will also be placed on building and local drainage infrastructure due to changing demographics, increasing urbanisation and decreasing confidence in the long term viability of existing water supplies; these factors will lead to an increased emphasis on water conservation, as already highlighted by imminent changes to UK Building Regulations (which are likely to set minimum standards for water efficiency within buildings). There is clearly a very real need for enhanced tools to enable the wide range of stakeholders to develop the type of integrated designs necessary to meet both current and future performance requirements. The proposed research aims to meet this need by developing improved simulation models. The project will commence with a benchmarking exercise to assess the state of the art of mixed flow modelling. This will include the identification and experimental quantification of the key physical process, as well as a thorough assessment of existing techniques and their suitability to building and local drainage applications. These initial investigations will help drive model development activities, which will concentrate on formulating a novel numerical technique for the simulation of mixed flow conditions within small-medium diameter piped drainage systems (up to approximately 200mm). The developed technique will be incorporated into 1-D finite difference models for the simulation of conditions within building and local drainage systems. Dissemination of project findings will be critical in order to persuade relevant stakeholders of the benefits associated with the developed techniques and models, and to encourage uptake of the project recommendations and tools. In addition to traditional academic dissemination routes (journal and conference papers), project outcomes will also be publicised to a wider audience through presentations and seminars to professional bodies, industry organisations and wider research initiatives.


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Description 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 only existing fully dynamic building drainage model (DRAINET) mean that it is not particularly suited to the widespread simulation of mixed flow events within complex systems.

This project led to the development of a new modelling technique to simulate mixed flow conditions within building and local drainage systems. The underlying principle behind the developed technique is to ensure that, where numerical stability permits, the appropriate set of governing equations are applied to the relevant flow regimes. As such one of three different solution techniques is employed.

1. Purely free surface flow conditions are simulated using a TVD MacCormack solution of the governing equations of free surface flow.

2. Mixed flow conditions within the same pipe are simulated using the same TVD MacCormack solution of the governing equations of free surface flow, in conjunction with new Time Varying Preissmann Slot (TVPS).

3. Purely full bore flow conditions are simulated using the Method Of Characteristics solution of the governing equations of full bore flow.

The introduction of the novel Time Varying Preissmann Slot (TVPS), whereby the slot dimensions decrease with time as flow pressurisation occurs, helps to minimise the continuity errors and numerical instabilities that can occur during the transition between free surface and full bore flow regimes. In addition to existing building drainage and junction boundary conditions, the developed model also incorporates a new gully boundary condition, through which rainfall can enter local drainage systems.

Work continues on minimising the numerical errors associated with the TVPS, and on reducing runtimes. At the same time, work is also continuing on developing additional BCs and incorporating the new modelling technique into the existing DRAINET building drainage model.
Exploitation Route The research undertaken during this project has the potential to help improve the design of both building drainage and local drainage systems; the substantial costs associated with building and local drainage systems (capital, maintenance and damage costs), and the potential for these to rise significantly in the future, means that small performance improvements will yield considerable long term financial benefits. Initially, the research outputs could be used to help those groups involved in the design, installation and maintenance of building and local drainage systems, and whose simulation needs currently fall out-with the immediate scope of existing software. These groups include standards bodies as well as practitioners, manufacturers of system components and advocates of novel water conservation approaches. The proposed models will offer such groups the opportunity to accurately simulate system conditions, under all types of realistic loading scenarios, using the fully dynamic governing equations. This will assist in the development of the type of integrated design philosophies and mitigating strategies necessary to deal with the future impacts of climate change and water conservation measures. In the longer term, the potential improvements facilitated from the developed models could help to transform the perception of such systems from unsophisticated elements of the overall network to flexible, key components within the urban drainage cycle; such a transformation is a vital step to ensure that society can meet the additional demands presented by climate change and water conservation. If the techniques developed cross-over into related fields, and in particular large scale urban drainage modelling, the overall impacts could be multiplied many times over.
Numerical modelling of the transition from free surface to full bore flow regimes is an ongoing problem, and the approach developed in this project offers a novel solution. This could potentially help researchers develop new strategies to help improve the design of both building drainage and local drainage systems. This has clear implications for both water conservation and flood risk mitigation. In particular, the ability to more accurately assess flow conditions within building drainage systems should encourage the design and use of more innovative appliance designs, which use less water. In addition, it is anticipated that the developed models will assist current research initiatives in the modelling of building drainage vent systems, where accurate water flow data is required to drive the simulations.
Sectors Environment

Description The work undertaken as part of this project has led to the improved simulation of mixed flow conditions within building drainage systems.
First Year Of Impact 2011
Sector Environment
Impact Types Societal