Inflow and infiltration assessments - easy to get it wrong

Jan Heijs and Dean Watts (Morphum Environmental Ltd), Tim Lockie (Watershed Ltd), Nathan Shaw (Stategy), Bridget O’Brien (Christchurch City Council).


As wastewater networks age, managing stormwater inflow and groundwater infiltration is likely to become increasingly important in controlling wet weather overflows and in servicing growth. In theory there should no rainfall ingress or groundwater entering a separated wastewater network, but in practice significant volumes do occur.

An Inflow and Infiltration (I/I) assessment recently undertaken for Christchurch City Council was based on two methods: using flow gauge results and using a hydraulic model. This provided a rare opportunity to compare these assessment methods and results. If the I/I assessment undertaken for Christchurch City Council was based only on wastewater network modelling results, a number of catchments would have been prioritised without identifying confidence issues in some of the modelled results.

This paper will address a range of typical problems that are common when undertaking inflow and infiltration assessments and will demonstrate that undertaking these assessments is not easy and the consequential risks are significant.

Jennifer Howe

Digital Content Manager