Feasibility studies by Dahm and Kench in 2002 and 2004 identified the following options and likely outcomes for improving harbour navigability:
- 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.
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.
Feasibility studies by Eco Nomos Limited and Coastal Consultants Limited in 2004 confirmed that constructing entrance training walls would "promote scouring of the (Opotiki Harbour) entrance and the collapse of the existing ebb tide delta". The studies concluded that the improvements would be likely to last for several decades because 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, Opotiki District Council engaged consultants DHI Water and Environment to undertake access modelling and preliminary design. Based on detailed studies of the "wave climate" and sediment transport patterns in the harbour mouth, DHI identified six possible training wall, or groyne designs and undertook further detailed testing to evaluate:
- Performance against river flow and flood release requirements;
- Impact of river sediment discharges;
- Whether scouring of the entrance channel could undermine the groyne structures; and
- Impacts on the coastline near the river mouth.
Duffill Watts prepared a report to confirm navigation requirements, including channel depth, for design of the harbour entrance. This report forms part of DHI's Access Modelling and Preliminary Design Report - Volume 2 and can be downloaded from the Reports page.
Brian Perry Civil completed a refined design and costing report in 2014 following extensive geotechnical investigations. Brian Perry Civil Reference Design and Cost Report - April 2014 (PDF, 3.5MB)