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Design options (Reach 2 and 3)

These options apply to Reach 2 and 3, from Rangiora Railway Bridge to the ocean.

Primary stopbank upgrades:upgrading existing stopbanks where required.

This option proposes raising and strengthening existing stopbanks where required to increase the level of flood protection provided. A geotechnical engineering consultant will investigate how and where this is possible. 


Pros

     •  Minimises impacts on private land. 

Cons:

  • Doesn’t address flow restrictions at the SH1 bridge, so a spillway may still be required. 

  

  • May not be feasible to upgrade all sections of stopbank. 


Option D1: Stopbank retreat on Environment Canterbury-owned land.

This option proposes building a new stopbank on Environment Canterbury-owned land on the north side of the river and, once established, lowering or removing sections of the existing stopbank. 

For Options D1 and D2, new flood protection vegetation would be planted and allowed to establish. The existing stopbank would then be lowered or removed. 


Pros

  • Increases flood-carrying capacity in this section. 

  

  • Reduces constriction created by the existing stopbank alignment. 

 

  • Primarily uses Environment Canterbury owned land.  

Cons:

  • Will take time for new flood protection vegetation to establish. 

  

  • Addresses capacity issues over a relatively short section of the scheme  
    (1.2 km). 


Option D2: Stopbank retreat on Environment Canterbury-owned land 

This option proposes building a new stopbank on Environment Canterbury-owned land on the north side of the river and, once established, lowering or removing sections of the existing stopbank. 

For Option D1 and D2, new flood protection vegetation would be planted and allowed to establish. The existing stopbank would then be lowered or removed. 
Pros

  • Increases flood-carrying capacity in this section. 

  

  • Reduces constriction created by the existing stopbank alignment. 

 

  • Primarily uses Environment Canterbury owned land.  


 Cons

  • Crosses privately-owned land so would require property acquisition or compensation arrangements. 

  


Managed spillway: Engineered spillway on north-side of the river upstream of the SH1 bridge 

The State Highway 1 bridge limits how much water can safely pass through this section of the river during flood events.  

If flood capacity is increased, a spillway could be used to allow water to overflow from the river in a controlled manner to the north and into either Saltwater Creek or across the highway and into the estuary.  

Pros

  • Reduces flood pressure on the SH1 bridge. 

  

  • Improves flood protection for Waikuku. 

  

  • Allows overflows to occur in a controlled and predictable way. 

  

  • Guidebanks can help direct flood flows along a defined path. 

Cons:

  • Will result in flooding of properties within the floodway, when activated (likely in excess of 3,000 cumecs – almost twice the recorded peak flow in May 2021).  

 

  • Some properties in this area are at risk of flooding under existing conditions.  


Estuary options: Modifications to stopbank on north side of the Rakahuri at the estuary

This option considers modifying the existing stopbank on the north side of the river downstream of SH1. 

This might include lowering or removing sections of the existing north-side stopbank to relieve pressure on the south-side stopbank, and to lower flood levels near Waikuku Beach.  

Pros

  • May reduce flood levels near Waikuku Beach. 

  

  • Allow for the future effects of sea level rise by retreating flood protection from land that may be inundated. 

  

  • Creates opportunities for large-scale estuary restoration. 

Cons:

  • May affect private land and farming activities.