Abstract
This thesis evaluates the temporal variability of the behaviour of mixed sand and gravel beaches (MSG) along the Waitaki coastline. MSG beaches are more morphologically and texturally complex than either pure sand or gravel shorelines. This is due to the hydrodynamic interactions with the mixed sediment populations being more complicated than those found in pure sands or gravels. Consequently the morphological response of MSG beaches over both short and long timescales remains poorly studied relative to sandy beaches despite recognition 20 years ago that they are an important defence against coastal erosion. The link between the short and long-term response has been difficult to establish. The behaviour of MSG beaches across these scales is critical for the understanding of the evolution of MSG beaches and how they respond to climate change.
Short-term behaviour (hours to days) of a MSG beach over different wave environments was examined using repeated profile surveys and Structure from Motion (SfM) derived Digital Elevation Models (DEMs) at three intervals between May and August 2021. Emphasis was on the short-term response to high energy events as this is when most geomorphic work is done to the beach. The analysis of short-term beach response contributes to the understanding of the long-term function of mixed sand and gravel coastal systems. Short-term morphological change of three profiles at Hook Beach a MSG beach located along the Waitaki coastline were analysed over three survey periods characterised by different wave conditions. Wave conditions with a maximum Hs of 1.1 m, 2.1 m and 3.1 m were captured. Response of the beach profile particularly to high energy events is influenced by the antecedent condition of the beach, rather than the event itself. The slope of the foreshore, the presence and dimensions of intermediate berms and volume of sediment in the foreshore are most important. Beach profiles with large intermediate berms, wide foreshores and high volumes are less likely to sustain erosion of the profile. Using repeated profile surveys reveals the general behaviour of the beach, however these are restricted in time and space. This study found that beach monitoring through SfM allows greater representation of the spatial variability of the beach compared to traditional techniques but is confined by the extent of wave run-up and the configuration of the ground control points used to construct accurate DEM’s.
Mobility of the active sediment layer is a short-term process that provides a link to long-term beach behaviour. The depth to which sediment is activated by waves drives profile change and helps determine the effectiveness and extent of sediment abrasion. Abrasion accounts for up to 90 % of all volume loss in annual sediment budgets which is a regulator of the long-term behaviour of MSG beaches. Active layer depth was measured using Depth of Disturbance rods deployed along the swash zone of each of the three profiles at the beginning of each survey period. The findings of this thesis revealed that the depth of the active layer on MSG beaches varies both alongshore and across shore on MSG beaches. Minimum active layer depth ranged between 1.7 cm and 116 cm. These results revealed that the depth of the active layer has been significantly under-estimated by up to half a metre. This is significant especially in terms of the contribution of the abrasion to sediment budget models. Active layer depth on MSG beaches is strongly influenced by the slope of the foreshore, rather than the wave height.
The morphological response of MSG beaches over long time scales (years to decades) is thought to be determined by sediment supply and sea level rise superimposed on changes in climate forcing. The balance between these two factors determines shoreline stability. However, this thesis reveals that processes operating over short timescales are superimposed on the longer term change and trends. The long-term response of a section of the Waitaki coastline was examined through the manipulation of long-term profile data between 1977 and 2020 and georeferenced historical aerial photographs. Studies have highlighted that the Waitaki coastline is in long-term retreat as a result of sea level rise and a negative sediment budget. However the findings in this study found that there is variation in the long-term response of the section of the Waitaki coastline examined in this study, where in general most profiles are either stable or prograding over time. This section of coastline is located north of the eroding alluvial cliffs that are major source of sediment to the sediment budget. Due to the strong northward component of longshore transport, a positive feedback loop between the MSG beaches backed by the eroding alluvial cliffs and the section of MSG coastline examined in this study has occurred. Results from this study provide insight into the dynamic and complex response of MSG beaches over both temporal and spatial scales. There is a clear linkage across scales where short-term abrasion rates, active layer data and insight into the profile response to a high energy event is needed to understand long-term sediment budgets and beach behaviour.