A wide range of investigations was completed to identify the most suitable approach for enhancing beaches and creating protection for the new walkway. The deepwater and nearshore wave climate was defined through the application of numerical models of waves generated throughout the North Atlantic, hurricane wave and surge modelling and wave transformation modelling. A large number of numerical model simulations were completed to determine the erosion potential of sand beaches based on two key factors: wave height (and water level) and beach width (wider beaches lose sand faster).

 

1) An evaluation of the wave transformation, hydrodynamics and sand transport under the existing conditions;

2) Investigation of the performance of headlands and submerged breakwaters in enhancing and retaining expanded beaches; and

3) Evaluation of stone stability to provide information for final design of the coastal structures.

 

 

 

Sedimentological and sediment budget investigations were completed to better define the rate of sand supply to the shoreline for the southwest coast and the project area in particular. Sand is produced by coralline algae and foraminifera from the hardground (Pleistocene reef and beach rock) and coral rubble from live coral located further offshore in depths greater than 3 to 4m. Most of the sand supplied to the project reach is derived from the hardground areas. An estimate of the maximum possible rate of sand production from coral and hardground areas is 0.5 m3/m/year (i.e. per metre of shoreline). This assumes the hardground areas produce sand at the same rate as healthy coral. Therefore, the production rate is likely to be less. Aside from local production of sand, the other source of sand to the project area is the small band of sand moving around the toe of the Magic Isle Apartments revetment. Based on the sediment budget work, the total amount of sand coming into the project area is about 1,900 m3/year with approximately 50% from offshore and 50% from the small alongshore supply from Rockley Beach around Magic Isle's revetment.

 

A new technique was developed to determine the balance between natural supply of sand to the beaches (determined from the sediment budget and sedimentological investigations) and the losses during storm events (determined from the physical and numerical modelling work). This approach was tested with a long-term simulation of the existing conditions. Starting with no sand for a profile between Magic Isle and Hastings and assuming a long-term supply rate from offshore of 0.3 to 0.5 m3/m/year and modelling the period from 1980 to 2000, an equilibrium beach formed and was maintained (i.e. no further growth or loss). It was found that the equilibrium beach volume compared relatively well to existing situation (of approximately 5 to 10 m3/m) providing a direct confirmation of the validity of the approach.

 

With confidence in this approach, a range of different scenarios was tested including the headland-beach concept, beachfill only and offshore breakwaters. It was determined that a headland-beach concept would retain the beaches relatively effectively with a 30% loss projected over a 20-year period after construction (+/- 20% for accuracy and uncertainty of future hurricane frequency/intensity). This assumes the current supply rate of sand from alongshore and offshore is maintained at current levels. This approach was shown to have minimal impact to the environment and no net impact to downdrift beaches.