Heritage Cases

Gansbaai Harbour is situated approximately 170 km south east of Cape Town and consist of the old and new harbours. The coastline is orientated South-West to North-East (facing North-West) and is exposed to south westerly swell. Harbour breakwaters generally trap longshore transport and induce accretion and erosion. However, from satellite images, no apparent accretion and/or erosion of the shoreline adjacent the breakwaters are evident. This is due to the rocky coastline at the site and potentially the absence of high longshore sediment transport rates. Gansbaai consists of two harbours: the old harbour and the new harbour. The old harbour is situated directly north from new harbour and is primarily used for fish processing and ship repair services. The marine infrastructure of the old harbour consists of a 370m long main breakwater, a 140m long secondary breakwater, two quays with fixed mooring facilities, 7.5 ton Derrick crane, two slipways and a small offloading jetty. The new harbour is primarily used for mooring, taking on supplies and minor maintenance for vessels. The harbour contains a 550m long main breakwater, a 210m long secondary breakwater, 4 berths, and a jetty. A sunken vessel is wrecked on the outside of the secondary breakwater. The proposed development entail the maintenance and repair of the above-mentioned existing infrastructure as well as maintenance dredging in the harbour. Gansbaai is on a rocky shoreline with little sediment transport across the entrance channel. The Dock Master mentioned that there is currently no dredging maintenance plan in place and there has been little build-up of sand during his tenure in the old harbour. He, however, noted that at the end of the mechanised slipway near to the off load quay, rocks have become a hazard to vessels at low tide. These rocks could not be observed at the time. Similarly, to the old harbour there has been no maintenance dredging to the new harbour; however, sediment build-up is occurring on the leeside of breakwater 3. The Dock Master mentioned that the build-up of sand is affecting the port operations. The Gansbaai facility requires dredging of the old harbour and new harbour in order to re-instate the water-depths to acceptable levels. The sediment present in the Gansbaai facility mostly comprises sandy alluvial deposits. From the information on the small number of grab samples taken at the facility during the Bathymetric Survey, it appears to be fine to medium well graded slightly silty sand with some shell and gravel fragments present. Large portions of the Gansbaai harbour area’s seabed floor exhibits typical characteristics of an exposed rocky seabed. The littoral drift process seems to be the main source of sediment beingtransported to and deposited in the harbour area, as there are negligible exposed beach areas in the facility supplying sediment load here. The areas that require dredging include the entrance to the old harbour where a sandbank seems to have formed, and the Eastern face of Breakwater 3 of the new harbour. Both the old harbour’s Eastern extent and the new harbour’s Southern extent will benefit from dredging.