The morphology of beaches at the base of cliffs: a field-study of the Port Campbell Limestone cliffs

Cliffs are erosional landforms that migrate slowly inland over time. Sea level rise, as a result of climate change, is predicated to accelerate these rates of erosion. These predictions are based on numerical simulation of cliff systems rather than field observations. In such predictions beaches are seen as a key element in the rate of cliff retreat. The proposed influences on cliffs retreat rates include: beach width and length, sediment size, wave dynamics and lithology. The measurements of these variables, and the thresholds for erosion versus protection, are yet to be tested in the field. Beaches at the base of cliffs can act to protect cliffs, when beach width is extensive enough to buffer between incoming waves and the cliffs. Conversely, when beach width is narrow wave-induced basal erosion is increased by abrasive beach sediments. The aim of this thesis is to examine the variation in headland-embayment morphology along a coastline and develop an understanding of the conditions required for beaches to form within these embayments. Field surveying and sediment samples were taken at various cliffed beaches to help infer sediment source and beach morphology. Marine and terrestrial LiDAR data was used to assess the topographic and bathymetric plan and profile variation for this stretch of coast. From this analysis three broad geomorphic assemblages were categorised in relation to their dominance within an individual embayment, they are; cliffs with beaches, cliffs without beaches and cliffs with rock falls. The field area chosen for the investigation were 61 embayments backed by the Port Campbell Limestone cliffs, in Victoria, Australia. This stretch of coast is microtidal and exposed to high energy swells originating from the Southern Ocean. The Port Campbell Limestone is relatively consistent alongshore, with locally variable cliff top dunes. Embayment morphology varied from highly indented, sheltered bays with alongshore length of 51.6 m to straight, long bays with an alongshore length of 1.52 km. Sediment size was found to be medium to coarse sands, with little variation in sediment size alongshore or between embayments. Cliffed beaches were found to be oriented towards the dominant south-westerly waves, with bathymetric lows running perpendicular to the coast allowing transport and the storing of sediments in the nearshore zone. Platform width was found to be variable, but displayed no significant difference between sites with beaches, compared to those without. The combination of sediment transport pathways and coastline orientation was determined to be the morphological drivers for cliffed beach formation. Additionally, the study area is sediment starved, as beach sediments are not supplied from eroded cliff material. Beaches form in areas with consistent nearshore sediment deposits and cliff top sand dunes. The findings of this study challenge some assumptions within current coastline retreat models