The Raumanga and Lower Waiarohia Stream Assessment report, prepared by Zealandia Consulting, provides a comprehensive review of the environmental health and management needs of the Raumanga Stream, situated in Whangārei, Te Tai Tokerau/Northland.
Significant changes have been made to the stream, especially in the middle and lower parts, to enhance its stormwater management and flood control capabilities. As a consequence, natural characteristics like meanders have been lost, and historical floodplains have been transformed into urban zones. The impact of these alterations is evident in the sharp difference between the original, native vegetation in the upper sections and the heavily modified mid-to-lower sections now overrun by invasive plant species.
In August 2022, a stream assessment was carried out on-site by walking along 14km of the Raumanga stream. The tools utilized for this assessment included a high-resolution GoPro 360 Hero 8 camera and a data capture application designed for smartphone use. Lower sections of the stream were explored using a well-equipped canoe, while the upper sections were surveyed on foot. Post-processing and specially developed tools allowed for open access to the findings and visual documentation through
in 3D imagery format, facilitating easy viewing for the wider community. This method has brought about various advantages, such as opportunities for integration and increased engagement with stakeholders and the public.
Key Environmental Management Issues:
1. **Stream Modification and Habitat Degradation**:
- Extensive straightening and channelization have led to altered flow dynamics and loss of natural habitat.
- Grade control weirs constructed to mitigate peak flow have inadvertently created barriers to fish passage, impacting native fish populations.
- Lack of channel complexity and bed material in lower sections has resulted in dominance by fine sediments, impacting water quality and aquatic habitats.
2. **Erosion and Sedimentation**:
- 21 erosion hotspots were identified throughout the catchment, with the majority posing low risk to buildings but moderate safety concerns.
- Erosion is exacerbated by a lack of stabilizing vegetation and previous reclamation activities, resulting in soft, unconsolidated banks.
3. **Stormwater and Water Quality Concerns**:
- Untreated stormwater discharges contribute to contamination and ecological stress, with high levels of E. coli detected in some areas.
- Stormwater infrastructure (culverts and pipes) was found in varying conditions, with some structures missing in the GIS records, complicating management efforts.
4. **Fish Passage Barriers**:
- Several natural and artificial barriers hinder fish migration and connectivity.
- The report recommends addressing these barriers to restore access to upstream habitats.
Management Zones and Actions:
Six distinct management zones have been identified based on geomorphological and ecological conditions. Each zone has specific recommendations:
1. **Zone 1: Te Matau A Pohe Bridge to Maunu Road Bridge**:
- Focus on enhancing riparian vegetation and improving floodplain engagement.
- Community engagement and amenity restoration.
2. **Zone 2: Maunu Road Bridge to State Highway 1**:
- Implementation of erosion control measures, riparian restoration, and sediment management.
- Prioritize fish passage improvements and weed control.
3. **Zone 3: Maunu Road Bridge to Lower Tarewa Park**:
- Address significant erosion hotspots and enhance habitat complexity.
- Improve fish passage and establish native riparian buffers.
4. **Zone 4: Lower Tarewa Park to Raumanga Valley Road**:
- Emphasize weed management and restoration of riparian areas.
- Improve stormwater outfall conditions to reduce sedimentation.
5. **Zone 5: Raumanga Valley Road to 158 Raumanga Valley Road**:
- Conduct bank stabilization and prevent further erosion through targeted replanting and structural reinforcements.
6. **Zone 6: Hopua te Nihotetea Dam to Pompallier Estate Drive**:
- Maintain high natural character in the upper reaches and address minor erosion.
- Protect remnant stands of native vegetation and enhance in-stream habitat complexity.
Recommendations for Future Actions:
- Implement systematic weed control, replant native species, and engage local communities in restoration efforts.
- Conduct fish passage assessments and remove or mitigate barriers, particularly in the mid to lower reaches.
- Utilize a prioritization approach to target erosion control and sediment management interventions.
- Expand water quality monitoring to address contamination sources, especially at high-risk sites like Tarewa Park.
- Pursue re-naturalization opportunities to restore channel meanders, improve floodplain connectivity, and enhance habitat complexity in degraded sections.
Cultural and Heritage Considerations:
The Raumanga Stream is of high historical significance to local iwi and hapū, serving as a portage route and an important food source. Any restoration efforts should incorporate cultural values, seek guidance from local kaitiaki, and ensure the protection of significant cultural sites.
Overall, the report emphasizes the need for a coordinated management strategy that balances flood control, water quality improvement, habitat restoration, and community engagement. Such an approach would not only enhance the ecological health of the Raumanga Stream but also improve the overall resilience of the Whangārei catchment and its receiving environments.
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