Simulating of Groundwater Infiltration in Urban Drainage Networks

This paper discusses the representation of long term, highly attenuated runoff in collection systems, focusing on rainfall dependent groundwater infiltration. Traditionally, the contribution to surface water systems, or the surface water element of a combined sewer system, has been represented by a combination of overland runoff and a constant baseflow. It is recognised that there are types of contribution to urban drainage systems, such as stream ingress or the contribution from large permeable areas, which do not fall easily into either the runoff or baseflow categories. In addition, the increasing prevalence of Sustainable Urban Development (SUD) structures is promoting a need to represent highly attenuated groundwater contribution. Until now, representation of this type of contribution has been attempted with varying degrees of success, by attenuating the direct overland runoff. It has been identified that new mechanisms are required. A feature of model verification and application to design, at least in the UK, is the use of long duration rainfall data. In the past a model would be verified against three storms recorded during a short term flow survey and then applied to design using standard ‘normal distribution' storms. It has now been identified that there is a need to verify against one continuous flow survey data set and then to apply to design using several years of recorded data or standard annual time series rainfall events. An important element of the groundwater infiltration module is in the application to long term data. This paper presents existing overland runoff models and discusses their limitations in the representation of groundwater contribution. The groundwater infiltration model as implemented within InfoWorks CS is then detailed. The potential for application of the groundwater model to sustainable urban development structures is then briefly described. It is recognised that the inclusion of the groundwater infiltration model in InfoWorks CS is a recent innovation, the paper therefore concludes with a recommendations of further work to enhance the model and improve its application in engineering design studies.