Clevedon Plan Change 32 Flood Model Peer Review

Flood Model Peer Review and Expert Witness

Clevedon village in East Auckland is located on a ridgeline between the Wairoa River and the Taitaia Stream. Plan Change 32 for Clevedon provides for the future expansion of Clevedon village within an area bounded by the floodplain from these two watercourses, the extent of which was determined by flood hazard modelling.

Our client owned land on one side of the village, part of which was in the expanded village and part of which was within the floodplain. We were commissioned to review the flood hazard modelling study and present expert evidence on this matter at the PC32 public hearing.

Our review raised several issues and challenged the possibly over-conservative nature of the model, leading to the Council hearing being adjourned, allowing time for Council’s consultants and Fraser Thomas to meet, discuss issues raised and work out an action plan for addressing any unresolved issues. The review also investigated how a proposed landscape based development of our client’s land could be accommodated involving relatively minor cut-fill earthworks within the floodplain, essentially shifting soil from one location to another whilst maintaining the flood storage volume at the same level as existing. Expert caucusing resulted in Council deciding to undertake an extensive revision and extension of the Taitaia Stream component of the flood model, involving additional cross-section surveys, splitting up the Taitaia catchment into a larger number of sub-catchments, extension of the 1D model and update of the model to comply with Council’s recently released Flood Modelling Specifications. The revised model resulted in a modest reduction in flooding on the Hattaway land, consistent with our expectations.

We then undertook more detailed earthworks modelling and design work for the proposed modified landform to confirm that the floodplain storage volume within our client’s land is maintained, that the elevation-storage volume curve remains the same and that flowpaths are maintained. The revised landform, proposed landscaping and critical proposed culvert data were then input into the flood model, with remodelling confirming that the proposed development of our client’s land will have less than minor environmental effects, while further engineering design work will be undertaken as the proposed development advances through the preparation of resource consent applications that will further reduce and/or eliminate these minor effects, thereby improving the management of stormwater in the area.

In parallel with this process, we developed on-site wastewater management systems for the proposed development of our client’s land and reviewed Watercare’s own wastewater options analysis for the Clevedon village, presenting expert evidence at the PC32 hearing on this aspect as well.

Expertise provided:

  • Environmental
  • Civil

Project cycle:

  • Desktop study
  • Investigations
  • Earthworks and flood modelling
  • Preliminary design
  • Expert witness