Synopsis: Integrated project involving the restoration of Awatapu Lagoon, a lagoon which historically constituted a meander of the Whakatāne River that was closed off in the 1970’s for the development of housing.  The restoration involves the formation of a wetland to improve flushing and water quality functions for the upstream urban catchment, and to address other drivers, including the recovery of historic, cultural and ecological values  and the creation of an attractive environment with improved amenity and opportunities for recreation and active participation by the local community.

 

The southernmost end of the Awatapu Lagoon (herein referred to as the “Awatapu Southern Lagoon” and “the lagoon”) experiences very minimal flushing (stagnant waters), due to the location of incoming flows and inlet / outlet configuration, as such has considerably poor water quality. In response, Whakatāne District Council (WDC) identified a long-term solution to improve water quality and the environment which is to convert the Awatapu Southern Lagoon into a wetland, hereafter referred to as ‘the Lagoon’ and ‘the Wetland’.

Two concept options were examined for the Awatapu Southern Lagoon improvement works, to address the key objective of improving water quality in the lagoon, while also achieving other drivers such as the enrichment of ecological values and the creation of an attractive environment with improved amenity and opportunities for active participation by the local community.

Concept 1 was based on the premise of allowing the existing inlets to drain through meandering channels within an engineered wetland system with wide sections of shallow aquatic vegetation interrupted by islands of seasonally dry land. This option however has a remnant risk of reduced or no flows (and consequently similarly diminished ability to flush) during dry periods, particularly given the likely intermittent nature of the stormwater outlets.

Concept 2 proposed the construction of a small earth dam at the outlet of the Wainui te Whara stream to drive baseflows from this catchment through a proposed new pipeline to the Wetland. The wetland is arranged such that inflow from the pipeline as well as existing inflows to the lagoon will be directed around a central infill peninsula, promoting flow around the perimeter of the existing lagoon with a more permanent flushing regime compared with Concept 1.

Both concepts are expected to result in water quality improvements and similar amenity benefits to the community. They also both have a relatively similar scale, timeframe and cost to complete. However, Concept 2 is considered to provide overall superior outcomes, due to the following reasons:

1. Concept 2 provides a more robust solution for the permanent flushing of flows, being a key objective of this project.

2. Concept 2 results in a more elongated flow path thereby improving opportunities for treatment through sedimentation and bioremediation by the plant media.

3. Concept 2 has the potential to offer a more comprehensive contaminant removal solution, due to the ability to provide treatment to Wainui te Whara stream flows.

4. The Concept 2 water quality benefits are also expected to be achievable within a slightly shorter timeframe and with less overall works compared to Concept 1.

AR & Associates recommended Concept 2 for the project, which is currently going through its consenting phase.