CLIENT BRIEF/PROJECT CHALLENGES
In terms of topography, the Christchurch wastewater network occupies generally flat land west and south of the Avon-Heathcote estuary with hills to the south. This is made possible by a complex array of pumping and trunk main infrastructure. Post-earthquake changes to landform and network condition has resulted in a wastewater network with limited surplus capacity and which is sensitive to rainfall (inflow and infiltration, I&I).
The modelled catchment is approximately 18,770 ha in size and the wastewater network serves an estimate population of 369,000.The network consists of approximately 1,235km of foul gravity sewers, with some areas served by pressure and vacuum sewers.
Due to the topography of the catchment, there are 121 pump stations and 41 lift stations represented in the model. The 119 constructed overflows are designed to spill from the foul network to the stormwater network or watercourses. These act as relief points for the network during storm conditions to prevent uncontrolled manhole overflows.
As a number of scheduled capital works projects relate directly to achieving the required outcomes of Council’s resource consent for wet weather overflows the quality of the model build and re-calibration was of paramount importance.
To provide programme certainty and a robust project outcome, we brought together a local team that not only included highly experiences specialists with unparalleled knowledge of the Christchurch wastewater system, who were supported by international wastewater modelling experts from WSP UK. The updated base model was in ICM, capturing the data from within Council’s latest InfoNet database, with ancillary structures from the existing ICM Model and other Council sources.
The consolidation of the previous asset data, existing incrementally improved model/s, the SCIRT (earthquake rebuild) handover information, the Infonet database, Operator information and recent flow survey data provided a sound basis for creating a ‘current day’ model that reflects the performance of the system. This model can confidently be utilised by Council to deliver a ‘Future Ready’ network in the face of an uncertain future and challenges being faced: •
- Demographic -including the shifting population across the city •
- Environmental -such as an increased focus on freshwater outcomes, rising sea levels and impact on groundwater levels •
- Cultural -such as an increased scrutiny on wastewater overflows to the water environment •
- Financial -the post-SCIRT legacy performance and impact on operational affordability •
- Societal -increasing use of food waste disposal units, wet wipes and other ‘foreign’ objects added to the system •
- Temporal -ageing assets, seasonal fluctuations in groundwater levels, the earthquake impacts of accelerating worsened conditions
The ‘Future Ready’ model can support decisions across the trunk network and infrastructure in terms of the capital improvements that can be made to help meet the challenges above.
The Client was provided with an updated model which was re-calibrated using flow survey data from 48 short-term flow monitors and 23 pump station data loggers, supplemented with 17 long-term flow gauge sites. The overall dry weather and wet weather flow calibration results were both good with high confidence in most areas. The calibration validation results highlighted some issues, with our report recommending some further investigation work and possible model improvements, to further improve confidence in predicted overflows. The model allows the Client to make informed decisions and have confidence in achieving their required resource consent outcomes.