Effects of Wrack Deposits on Sediment Characteristics

The amount of wrack deposited and retained on a beach may be affected by the beach’s morphology. Conversely, wrack deposits have the potential to modify sediment dynamics on the beach face through accumulation and erosion of sediments around wrack deposits, and the incorporation of organic matter into beach sands. The aims of this chapter were thus to determine whether beach morphology differed temporally and spatially and whether the cover of wrack differs between beaches of different morphologies. I also assessed the effects of wrack deposits on sediment characteristics (organic matter content, sand particle size and sand compaction). Seventeen beaches in three regions of South Australia were sampled on 2 occasions; April (‘summer profile’) and August (‘winter profile’) of 2006. Beach profiles were plotted and four measures of beach morphology were calculated (beach width, beach fall, beach-face slope and Beach Index, BI, which can be used to classify beaches into their morphological types). Sediment samples were taken above, below the driftline and within the driftline (DL), including in both wrack patches and bare sediments in the driftline. Beach profiles varied spatially and temporally but there was no systematic trend for wider, taller or steeper beaches, or towards beaches of a Reflective or Dissipative type, to occur on either visit or among the three study regions. There were no relationships between wrack cover and either beach width, beach fall or beach-face but as BI increased (i.e. on more Dissipative beaches) the cover of wrack also increased. At the scale of the whole beach, there was no relationship between wrack cover organic matter content but bulk density was negatively correlated with wrack cover, i.e. beaches with more wrack had less compacted sediments. Sediments from the South East region were consistently higher in organic matter content than those from the Fleurieu or Metro regions. This may be driven by both the higher cover and higher proportion of algal and kelp wrack in the South East region compared to the Chapter 3: Beach morphology and sediment characteristics 80 other two regions. There were no differences in sediment characteristics (mean particle size, organic matter content and sand compaction) between wrack-covered and bare sand areas within the driftline. This result was somewhat surprising given the large wrack accumulations and visible differences in sediments (e.g. sand colour and the presence of wrack fragments) within the driftline. A possible explanation is the transience and variability in the size (cover and depth) of wrack deposits. Alternatively, the effects of wrack deposits may only occur in the surface sediments, and the depth of the sediment cores (15cm) taken in this study may have been too deep. Beach morphology may thus affect the amount of wrack present on sandy beaches. In addition, wrack deposits have a limited but potentially important capacity to modify beach sediments through the input of organic matter and the reduction of sand compaction.