Smart Flood Modelling Protects Coastal Infrastructure in Araparera Catchment Area

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To address flood risks for a proposed broiler farm near Araparera, eCoast carried out detailed flood modelling. Using HEC-HMS and HEC-RAS, we tested how the site’s earth bund would perform during an extreme (1% AEP) flood, including future rainfall and sea-level rise. The results showed the bund effectively contains floodwaters on-site while having minimal impact on surrounding land—even under climate change scenarios.

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Information:

• Location: Araparera Catchment, New Zealand
• Client: Private landholder / Local Council
• Project Date: 2024

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Scope of Work:

• Site-specific flood risk modelling and analysis-Infrastructure exposure and drainage assessment
• Climate change (SSP5-8.5) and sea level rise impact projections
• Design review of the proposed bund and resilience recommendations

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Project Description:

eCoast performed a detailed flood modelling assessment for a proposed farm upgrade. The site is fully enclosed by an earth bund around the proposed shed, and the Auckland Council required an evaluation of how these earth works would alter flood flows to adjacent land and the highway. eCoast specifically modelled a 1% annual exceedance probability (100-year) event to represent extreme flood conditions.

HEC-HMS was used to develop rainfall–runoff hydrographs, which were input into the HEC-RAS model to simulate two-dimensional unsteady flows for the 1% AEP event under both current climate and future (SSP5-8.5) conditions. A spring high-tide cycle was applied at the downstream boundary, and a combined sea-level rise (SLR) plus vertical land movement allowance of 0.79 m (by 2080) was included via a stage hydrograph.

The modelling showed that the bund effectively contains floodwaters within the site and channels flows through its drainage outlets, protecting the proposed sheds while only causing minor, localised differences in inundation. In both present-day and future scenarios, flood extents and depths outside the bunded area remain very similar.

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Key Findings:

• The bund design effectively prevents on-site flooding under both present and future (SSP5-8.5) conditions.
• Climate change increases runoff (~33% higher peak flows), but flood extents and velocities outside the bunded area remain largely unchanged.
• The design already meets climate-adjusted standards, with minimal off-site impacts; regular monitoring is recommended to ensure long-term resilience.

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