Large Scale Modelling of Melbourne Water Catchments - Lessons Learnt

This paper discusses the technical challenges of carrying out a rain on grid assessment covering Melbourne Water’s area of responsibility in Victoria Australia, a combined catchment of approximately 12,500km2, equivalent to approximately 10% of the North Islands land mass or 12 times the urban area of Auckland.

A key requirement of the project was to test and determine confidence in the modelling approaches compared with current flood modelling approaches and then utilise this to deliver for the first time catchment scale flood modelling assessments for areas that have been unmapped in the past.

Melbourne Water’s goal was to develop a better understanding of broad scale drainage and flood modelling and move towards developing integrated urban drainage models. This Model Pilot Project helped Melbourne Water to determine how best it could develop a whole of catchment modelling approach, deliver an integrated urban drainage model and work collaboratively within the company to deliver multiple objectives to best meet customer’s expectations.

The paper presents the organisational and technical challenges associated with delivering a project of this scale, concentrating on developing a robust technical approach, with a short project timescale.

Technical challenges included the long times of concentration (in the range of days) associated with large catchment modelling (which would be of interest to those involved in catchment modelling across New Zealand). The paper also discusses how the project team determined and overcame the limitations of the Australian Rainfall Runoff approach when applied on this scale. Additionally, the paper shares some key lessons associated with delivering large models (with long simulation times), as well as how the team worked through the validation stages to approach the sensitivity to depression storage / mesh resolution.

Finally, the paper will share an appropriate methodology for presenting the modelling results so that all stakeholders can best understand the uncertainties associated with such large scale coarse assessments, yet still understand the response required to help protect communities at risk of flooding. This approach would help overcome some of the reservations with releasing broad scale flood modelling to the communities involved.