Preliminary investigations into recent changes of the Goukamma Nature Reserve frontal dune system, South Africa — With management implications

The Goukamma ‘delta’ dune field, pre-1930s, functioned more or less ‘naturally’ since the post-glacial nested-parabolic dune pulses. This dune field, approximately 204 ha in extent, acted as a sediment sink for wind-blown marine derived sands and a surface over which the Goukamma River meandered. Stabilisation of this dune system with Acacia cyclops and Ammophila arenaria in the early 1930s resulted, through the process of dune succession, in the formation of a large frontal dune system which is now acting as a sediment trap, and therefore preventing the supply of sediment to the associated headland-bypass dune system. This has resulted in the marine derived sediment supply to the Buffalo Bay beaches being cut off. Evidence of this phenomenon has been shown on many stabilised headland-bypass dune systems within the ‘half-heart’ bays of the south and south-east coasts of South Africa. The stabilisation of the delta dune field has resulted in an eastward river mouth migration and elongation of the sand spit at the mouth of the Goukamma River. Stabilisation of the spit by Ammophila arenaria, accompanied by natural colonisation of the river margins with Juncus spp. has narrowed the river course causing increased stream velocities and exacerbated river channel erosion. Recreational facilities and a Regional Services Council road is under threat of erosion in the not too distant future. A recent flood event on the 24 September 1993, with an average stream discharge of 380 m3 s−1, measured 1 km upstream of the mouth, resulted in the erosion of approximately 50 linear meters of the frontal dune. Lateral erosion of the stream bank in a northerly direction by 10 m also occurred. The high-intensity, short-duration storm event was not a sufficient flood pulse to break through the vegetated frontal dune spit, resulting in an eastward meander of the river mouth. This river meander has increased the potential for the recreational facilities and Regional Services Council road from being undermined by erosion and thereby creating major management decision-making by Conservation Departments and Local Authorities.