A large storm event occurred in Tauranga in April 2013. Consequently, Tauranga City Council (TCC) came under significant pressure to remedy the flood hazards apparent to the public. The first step in the flood mitigation efforts that were part of TCC’s Integrated Stormwater Project (ISP) was to develop computer-based models of all catchments across the city for flood-hazard assessment and mitigation-option testing. DHI was engaged by TCC to assist in the field of stormwater modelling.
At the beginning of the ISP, initiated in 2013, TCC had seven flood models already constructed by a number of different consultants based on a variety of approaches. As part of the ISP, four engineering consultants were selected to construct the twelve remaining stormwater catchment models. A significant effort was made to standardise the model-build approach, to ensure some degree of consistency between models.
DHI provided simulation technology, produced technical components of the tender briefs, undertook the technical peer review, and provided software and flood-modelling assistance to both TCC and the consultants.
Technical aspects that set the models constructed for TCC apart from previous studies include: explicit representation of all council-owned sumps, rain-on-grid hydrological approach, and the combination of raised footprints and high roughness to represent building blockage. Each of these approaches are justified by practical model-build experience and the intended use of the results.
A wide range of documents have been produced to support the ISP, including flood-modelling guidelines, largely-standardised technical tender briefs, a tested peer-review schedule, peer-review checklists and peer review reporting.
As a result of the ISP, Tauranga City Council has moved from providing an indicative level of service (too costly) to focusing on flooding that directly threatens life. Many of the flood models, developed as part of the ISP, have already been used successfully in options assessments.