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Cfjjl01" Westerly-North-Easterly Direction Is Formed by Sand Accumulation on the West of the Peninsula with Transport by the Prevailing Winds

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Cfjjl01 Journal of Coastal Research 647-653 Fort Lauderdale, Florida Summer 1993 Recent Changes to a Climbing-Falling Dune System on the Robberg Peninsula Southern Cape Coast, South Africa G.B. Hellstrom and R.A. Lubke Schon land Botanical Laboratories Rhodes University Grahamstown, South Africa ABSTRACT ...... ...... _ HELLSTROM, G.B. and LUBKE, R.A., 1993. Recent changes to a climbing-falling dune system on the Robberg Peninsula Southern Cape Coast, South Africa, Journal of Coastal Research, 9(3), 647-653. Fort .tflllllllt. Lauderdale (Florida), ISSN 0749-0208. e •• • The Robberg peninsula climbing-falling coastal dune system is a possible source of sediment to the ~ ~ peninsula's associated bay (Plettenberg Bay) and beaches. The climbing-falling dune which lies in a south­ t!CFJJl01" westerly-north-easterly direction is formed by sand accumulation on the west of the peninsula with transport by the prevailing winds. Progressive dune stabilisation on the climbing-falling dune, by Acacia 4i 1b-- cyclops, an Australian introduced woody plant, has occurred over the last 54 years curtailing the movement of aeolian sands and thus reducing the potential to supply the bay with beach sediments. The rate at which stabilisation has occurred was determined by measuring and calculating the decrease in exposed surface sands of the dune system from orthophoto maps and aerial photographs. The mobile free sand surface of the dune has decreased its visible surface area by 55\~ over the last 54 years. Results suggest ~hat sed.iment input into the Plettenberg Bay has steadily decreased from the time of early Acacia cyclops ~nf~statlOns. Although these aliens, which invade the natural vegetation, are now being removed, there IS little change to the open surface of the dune system due to irreversible changes to the vegetation dynamics and soil properties following stabilisation. ADDITIONAL INDEX WORDS: Climbing-falling dune, dune stabilisation, tombolo, Witsand, dune vegetation, South Africa. INTRODUCTION ing of the marina would be required because of The Robberg Nature Reserve of the Cape Pro­ continual sand input (and budgets) to the area. vincial Administration is a rocky peninsula lying One possible sediment source to the marina/ in an east-west direction on the south-eastern Cape townhouse area is the climbing-falling dune sys­ coast of Southern Africa. It is a phytochorial re­ tem (TINLEY, 1980) which moves from south to fugium of different South Coast Dune vegetation north approximately half-way across the Robberg types, with a rocky coastline bordering mobile peninsula. This dune system originates on the sands, palaeo-dunes and stabilised phytogenic southern side of Robberg at a tombolo between dunes. In a detailed study of the vegetation of the the Island and the peninsula and rises to the sum­ reserve HELLSTROM (1990) identified the different mit of the peninsula, a height of 80 m, where it vegetation types of the reserve and put forward falls down the steep rocky face into Plettenberg management plans for the region. The greater Bay (Figures 1 and 2). It is commonly known as Plettenberg Bay area, in which the Robberg pen­ Witsand (white sand) by the locals and fishermen insula occurs, has shown rapid development in in the area. the last 10 years. Starting in the 1930's coastal It is thought that the dune system was once resorts have developed this coastline into one of larger than at present, and over approximately the most prestigious holiday regions on the Cape the last fifty years it has become reduced in sur­ South Coast of South Africa. Development is con­ face area (BRETT, M., personal communication, tinuing in the region and on the northern side of 1989) There is general infestation and encroach­ Robberg, at the point where the peninsula meets ment by the introduced Australian Acacia cyclops the mainland, a marina/townhouse complex was throughout the Cape coastline, and large areas of proposed. This would include part of Robberg the reserve's dunes are invaded, and thus, possibly Beach, one of the excellent sandy beaches adjoin­ responsible for the reduction in the mobile free ing the holiday homes of the town. Regular dredg- sand surface of the Witsand dune area. Invasion of indigenous shrubs onto the now stabilising dune 91088received 24 November 1988; accepted in revision 3 September 1992. appears to account for further reduction of the 1 , 648 Hellstrom and Lubke I PLETTENBERG BAY 34°06'5 ~ ~ r- ~ 'Z. """"("") J KILOMETRE 23°24'E Figure 1. The Robberg Peninsula, including Witsand and the proposed marina complex, in relation to the South Coast of South Africa. free sand of the dune. It is necessary to under­ and stabilization of the dune system by invader stand the dune system dynamics in order to elu­ species, document the recent vegetation history cidate the problem of invader species and for­ associated with the climbing-falling dune system, mulate possible management guidelines. and draw conclusions on the vegetation status of this dune system. Sand Dune Dynamics The energy regime responsible for the sand METHODS movement up the Witsand dune is dictated by Aerial and orthophotographs of this region are the Island. Sediment transported by long-shore available for the years 1936, 1942, 1959, 1975, 1980 drift from west to east is deposited directly behind and 1990. Photographs of the Robberg peninsula the Island resulting in a build-up of potentially were reproduced at the same scale for each of mobile sand (Figure 2a and b). The Island there­ these years. The Witsand region was mapped and fore acts as a sediment trap, where high potential surface aerial measurements of drift sand com­ wave energy is reduced by the barrier effect of the pared for each of the years. The area of drift sand Island. This accumulated sediment is then trans­ on the Witsand region and the tombolo was mea­ ported by the strong prevailing south-westerly sured for both the tombolo and the dune system winds (Figure 3), forming the climbing-falling dune onto tracing film and overlaying on graph paper. (Figure 2). The orientation of Witsand is parallel These graphic measurements were converted to with these winds (compare Figures 1 and 3) which square metres of mobile free sand surface re­ are funnelled by the dune topography. maining. It was not possible to distinguish the This dune system, like all coastal sand dunes, alien invasive (Acacia cyclops) from the dune fyn­ is formed and modified by wind, and the basic bos vegetation of the more recent photographs processes of aeolian sand transport (PETHICK, and hence ground truthing was necessary. A sur­ 1984). The requirements for dune growth are vey of the vegetation of the whole reserve was partly climatic, the occurrence of strong onshore undertaken at the same time (HELLSTROM, 1990). winds, an abundant sand supply and a vegetation cover, or lack thereof. The prevailing south-west­ RESULTS erly winds occur for most of the year (Figure 3) The wind-sand regime at Robberg is a complex and have enough potential energy to carry the interaction of wind direction and threshold ve­ sand up the steep gradient to a height of 80 m at locity, the effect of the Island and peninsula geo­ the summit of the climbing-falling dune on the morphology as shown diagrammatically in Figure peninsula. 4. The direction of prevailing winds (A) results in The general aim of this paper is to assess and the sediment laden ocean currents (B) approach­ highlight the problems associated with infestation ing the Witsand area from the south-west. The Journal of Coastal Research, Vol. 9, No.3, 1993 Stabilisat ion of Cape Coast, South Africa 649 Figure 2. Th e Robbe rg Peninsula showing the climbing- falling Dune system. (a) Th e Island and To mbolo where sand accumulates. (b) A nort hern aspect up th e peninsula showing the invasion of indigenous pioneer species into th e du ne. (c) South view of sand accumu lation at the summi t before falling into the bay. Acacia cyclops is still present on the top of th e dune. J ourn al of Coastal Research, Vol. 9, No.3. 1993 650 Hellstrom and Lubke 1 JANUARY APRil JULY OCTOBER KEY 0-6 14-21 Arcs represent 10% Intervals Percentage of calms within the circle ---.7-13 22 + Figure 3. Windroses for the months January, April, July and October for the Robberg Peninsula (data modified from Weather Bureau Annual Reports, Department of Transport, South Africa). low-energy environment (C), caused by the Is­ winds over the crest. Once the angle of repose is land, acts as a sediment trap resulting in the ac­ exceeded sand slides into Plettenberg Bay (E), to cumulation of beach sediments in this area. The be carried away by the east-to-west Bay currents mobile aeolian sands are transported up the (F) (Figure 4). "climbing" aspect of Witsand due to the prevail­ Changes in the Witsand dune system from 1936 ing winds (D) where the aeolian sands accumulate to 1990 are clearly evident from the maps (Figure at the lip of the dune until transported by the 5). The area of sand on the tombola has deceased I N Area of mobile sands Island vegetation Figure 4. Schematic representation of the sand dune dynamics of the Witsand climbing-falling dune system (for explanation see text). Journal of Coastal Research, Vol. 9, No.3, 1993 Stabilisation of Cape Coast, South Africa 651 --:._--~~,-- _.--r---- -. ~-._ 1942 Figure 5. Changes in the extent of the mobile sand in the Witsand dune system. 1936, Prior to massive invasion of Acacia cyclops. 1942-1980, During Acacia cyclops invasion. 1990, After removal of Acacia cyclops. only slightly, while there is a marked decline in in the higher lying areas.
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