Wainui Te Whara Flood Assessment

Client:  Whakatane District Council and Bay of Plenty Regional Council
Date:  2014 – 2015

The urban area of Whakatane downstream of Valley Road is prone to flooding from the Wainui te Whara stream. Two large flood events in May and June 2010 caused significant damage to a number of urban properties. In order to mitigate this flooding, Whakatane District Council (WDC) and Bay of Plenty Regional Council(BOPRC) investigated a number of options including the construction of a detention dam in the upper reaches of the stream.

PDP were commissioned to review and update the hydrology for the Wainui te Whara catchment and produce design inflow hydrographs for the sizing and preliminary design of a proposed detention dam.

Several methods were used to estimate peak flood flows and develop design inflow hydrographs for the dam.  A calibrated rainfall-runoff model was used to estimate flood flows for a range of return periods. This included probable maximum flood (PMF) estimates using probable maximum precipitation (PMP). The design inflow hydrographs were routed through the proposed detention dam to estimate the required storage volumes at the dam site and the resulting attenuated peak flows in the downstream channel.

The results of the routing indicated that:

  • A detention dam would provide a significant reduction in peak flows at Valley Road;
  • The difference in peak flows at Valley Road with the proposed dam in place is relatively large for the modelled scenarios due to the contribution of the Wainui te Whara catchment area below the dam;
  • The size of the dam outlet pipe size makes a large difference in terms of required storage volume.

Based on the results of the assessment PDP provided WDC with a number of recommendations to progress the project.

 

Key Aspects:

  • Rainfall analyses for a range of recorded extreme events.
  • Rainfall-runoff modelling to determine catchment peak flows
  • Probable Maximum Precipitation (PMP) and Probable Maximum Flood (PMF)
  • Catchment hydrological assessment
  • Flooding assessment and mitigation
  • Sizing of flood detention dam (hydraulic routing)

Client:  Whakatane District Council and Bay of Plenty Regional Council
Date:  2014 – 2015

 

 

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