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The change in the horizontal position of the dune toe and vegetation line on beaches within the Waikato Region

Beaches are highly variable landscapes that are constantly changing in response to a range of climatic variables. For example, local wind can cause the movement of sand across the beach and accretion, whilst storms that occur far from the beach create waves and surges which also changes the shape and morphology of the beach. The lower beach face is inundated for longer during a tidal cycle and therefore is subjected more often to wave action conversely, the upper part of the beach is inundated less often and therefore is less affected by wave energy. When storms occur, water levels are higher along the beach, waves are brought into contact with the upper beach face and erosion occurs. This erosion causes escarpment of the dune face and landward movement of the beach face, where sand is transported to offshore bars in the surf zone. Changes also occur during fair weather conditions where accretion occurs across the beach face and dune area and sand is transported back up onto the beach and sand dunes. Beach morphology has traditionally been monitored using beach profiles, however new methods are emerging that provide more information with the potential for greater spatial and temporal data collection. In New Zealand the vegetation line and the position of the dune toe is often used as a proxy for the shoreline and setback distances are measured relative to this feature. Beach morphodynamics have been studied extensively but typical monitoring and predictive methods are not optimised to track and predict changes in the dune toe position. Beach profile surveys are the most common and reliable monitoring method used for monitoring beaches and they are used extensively throughout the world. Although beach profiles give valuable information on several different aspects of beach processes, they may not be the optimal method for tracking dune toe movement and there are other methods that might be more effective. The aim of the research was to provide information to help optimise the monitoring and prediction of changes in the dune toe by determining: (i) the horizontal and vertical variation of the dune toe to establish how often the horizontal and vertical position of the sand dune toe changes, (ii) the alongshore variation of the dune toe along the beach length, (iii) what causes the changes in the dune toe, (iv) compare using the dune toe for beach monitoring with traditional beach monitoring methods, (v) testing different methods to measure the dune toe and vegetation line, (vi) predicting the dune toe in the future using a model. The main study sites for the research were Whangapoua Beach, Matarangi Beach, Buffalo Beach, Hot Water Beach and Tairua beach of eastern Coromandel Peninsula, and Ngarunui Beach, Raglan, where field surveys were conducted. An historical dataset of beach profiles from 20 Eastern Coromandel beaches were analysed for long term patterns. A new method for measuring dune toe using video analysis and a model for predicting the future dune toe changes was tested at Tairua Beach, Coromandel Peninsula. The results show that, from the field survey of beach profiles, there was low variability at the dune toe compared with further down the beach. The range of distribution of the vegetation line was larger than the range of distribution for the dune toe and the dune toe was generally more seaward than the vegetation line. The vegetation line was generally not at the same location as the dune toe which has strong implications for using vegetation as a measure of dune movement. The alongshore dune toe underwent small changes throughout the survey period and there was alongshore variation of the dune toe, along with changes in the dune toe horizontal position and height of the dune toe throughout the year survey period, which was expected. Video analysis was shown to be a potential new technique for measuring the vegetation line which could be used for beach monitoring. A model was used to predict the change in the dune toe, and it predicted that there was no change in the dune toe between 1998 and 2011; however, the analysis of the imagery showed there were numerous occasions where water reached the dune toe and therefore erosion of the dune toe occurred.
Type of thesis
Davies-Calway, C. J. (2020). The change in the horizontal position of the dune toe and vegetation line on beaches within the Waikato Region (Thesis, Master of Science (Research) (MSc(Research))). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/13592
The University of Waikato
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