Kingscliff seawall design

This seawall would extend along the foreshore between the existing rock seawall at the Kingscliff Beach Bowls Club and the existing secant pile seawall at the Cudgen Headland Surf Life Saving Club to protect built assets at immediate risk from coastal hazards. It would replace existing interim rock and sand-filled geotextile container structures along this stretch of coast.

Following consideration of a range of preliminary seawall design types (including an alternative protection scheme with a groyne field); a composite seawall design was adopted with:

• a rock seawall section at the northern end
• a stepped concrete section at the southern end.

As a result of two-dimensional physical modelling undertaken by WRL, the rock armour mass for the rock seawall section was reduced by more than 25%, the structure slope was steepened (reducing its footprint) and the expected wave overtopping rates were found to be lower compared with the preliminary design. The primary objectives of monitoring at Clybucca are to observe changes to the hydrology, water quality and ecology; to enable impacts (potential or actual) of rehabilitation works on the wider floodplain drainage to be assessed, and to provide ongoing data to inform adaptive management and optimise rehabilitation outcomes.

The Clybucca monitoring network has been collecting real-time data since November 2021 and includes:

• Three telemetered water quality stations (measuring water level, pH, conductivity and dissolved oxygen).
• One telemetered water level and conductivity station.
• Four telemetered water level stations (three surface water and one groundwater).
• One telemetered camera station.

The maximum horizontal wave forces measured on the stepped concrete section in the wave flume were found to be between preliminary design estimates using two different desktop techniques. The physical modelling also demonstrated that, under the same wave conditions, wave overtopping on the stepped concrete section was comparatively higher than the rock section with the same crest level and similar slope. As a result, the crest level of the stepped concrete seawall was increased by 0.5 m (an extra bleacher) from the preliminary design to maintain acceptable wave overtopping under design conditions.