Heritage Cases

The coastline at Arniston Harbour is orientated South to North (facing East) and is exposed to south westerly swell. The site is characterised by sandy beaches and rocky outcrops. The shoreline is exposed and subject to dynamic coastal conditions. From satellite images the breaker zone was measured > 400 m during a storm event. Furthermore, the sediment is clear on satellite images which indicate high concentrations of sediment in suspension. This is also an indication of high wave energy. A coastal setback study for the Overberg District was conducted by SSI during 2012. Apart from the coastal setback line, the study also estimated high risk areas with regards to coastal erosion based on wave run-up, topography, sea level rise, wave conditions, etc. The harbour area and slipway was identified as a high risk urban area during the study. The marine infrastructure for this harbour is limited to a 24m wide, 70m long slipway. All fishing vessels are launched and retrieved from this slipway by means of a retrieval winch located in a winch house at the top of the slipway. The harbour offers little protection from wave conditions. The overall dimensions of the concrete slipway are 70m long, 24m wide with a 1:16 slope. The leading walls (side walls) of the slipway divert outwards from the lower (24m) to top (34m) section of the slipway. The lower section of slipway in the intertidal zone is in poor condition. Plunging waves breaking onto the slipway have caused slab instability and breakup of concrete panels. Previous attempts of concrete repairs to these panels have been unsuccessful as a concrete repair section has failed and had to be removed. This has left a hole behind and become a hazard to both fisherman and their boats. Further cracks, patches “potholes” are observed throughout the structure. Loss of backfill material at the southern leading wall of slipway and subsidence in the slipway leading wall/pavement is evident. There is a misalignment of joints throughout. Ongoing localised remedial works have failed resulting in the slipway not being fit for purpose and creating a hazard to current harbour operations. It is evident that localised patching is a short-term solution, and a long term solution is required. To provide a long-term solution it is recommended that the lower portion of the slipway is to be replaced with new concrete precast panels. Construction methodology: • Lower half of the slipway deck to be removed and replaced • As far as possible the existing lead walls will be incorporated into new design • New insitu beams constructed to support precast concrete panels • New precast deck to be placed • New fencing to sides of slipway • Localised concrete repair and maintenance to rest of slipway as required A winch house is located at the top of the slipway where one winch is used to retrieve boats and stack them in parallel lines on the slipway deck above the high-water mark. There has been a recent fire in the winch room, (2015), but no structural concerns are immediately evident from the fire. The winch and other equipment required for retrieving the boats appears to be in good condition. This includes pulleys, wire ropes, hooks, chains, shackles and bollards that set the line for retrieving boats in different locations along the slipway. Construction Methodology • Refurbishment and maintenance as required to the entire operation