Options
Dahm and Kench undertook a study to assess and report on the feasibility for improving navigability at the mouth of the Opotiki Harbour. The study was carried out in two stages with the first stage reported in 2002 and the second in 2004.
The 2002 study contained a number of components, including a synthesis of existing technical reports on coastal and river processes and the application of a conceptual model to assess the likely effects of a range of options to improve navigation through the river mouth.
The following options for improving the river mouth navigability were evaluated with the main findings briefly listed:
- No intervention – no improvements of navigability
- No intervention and minor dredging – limited and short-term benefits. Insignificant impacts but moderate maintenance requirements.
- Mechanical dredging – larger improvements, short life expectancy with related high maintenance requirements to keep the channel open. Potentially significant coastal impacts.
- Fluidisation – not practical with the type of sediments and climate found at the site.
- Single mole (rock wall inserted perpendicular to the coast) – larger improvements with medium life expectancy. High maintenance requirements with significant coastal impacts. High capital costs and significant maintenance costs.
- Dual moles (parallel moles on either side of channel) – large improvements with long life expectancy. Significant coastal impacts with potential for adverse flooding impacts. Higher capital costs but lower maintenance costs than a single mole.
Overall, it was concluded that well-designed dual moles are likely to provide significant navigational improvements for several decades with a high level of operational availability. The structures will, however, have potentially significant adverse effects which must be taken into account in the design.
Physical Feasibility
Feasibility studies undertaken in 2004 by Eco Nomos Limited and Coastal Consultants Limited confirmed that the construction of entrance training walls would “promote scouring of the (Opotiki Harbour) entrance and the collapse of the existing ebb tide delta”. The studies also concluded that the improvements would be likely to persist for several decades because the establishment of a new bar seaward of the walls would probably be slow (80 – 100 years or more).
Having established that significant improvement of the harbour entrance was technically feasible, the Opotiki District Council engaged consultants DHI Water and Environment to undertake access modelling and preliminary design. DHI’s Stage 2 Preliminary Layout Assessment was completed in June 2007.
Based on detailed studies of the “wave climate” and sediment transport patterns in the area of the harbour mouth, DHI identified six possible training wall, or groyne designs and undertook further detailed testing to evaluate:
- How they perform with regard to river flow and flood release requirements;
- What impact river sediment discharges may have;
- Whether scouring of the entrance channel could undermine the groyne structures;
- What impacts they would have on the coastline in the vicinity of the river mouth.
Navigation
Duffill Watts has prepared a report to formally confirm the navigation requirements including channel depth for the design of the harbour entrance. This report forms part of the Modelling Report (Volume 2) and can be downloaded from this website from the Technical Data page.
Preferred Option | Options | Assessment of Effects
