Browsing by Author "Coulston, Ethan James"

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    Contrasting morphological responses to a singular flood event in neighbouring rivers and the implications for river management : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Geography at Massey University, Manawatū, New Zealand
    (Massey University, 2023) Coulston, Ethan James
    Geomorphic response to flood events is spatially and temporally variable and is influenced by many natural and anthropogenic processes. Research in recent times has shown the adverse geomorphic effects of rivers that have been managed by straightening, narrowing, and disconnecting them from their floodplain. This work attempts to evaluate the morphological response of small, rural gravel-bed rivers to discrete flood events and to put this response into the context of decadal-scale channel adjustments and river management practices. The Tauanui and Turanganui Rivers in South Wairarapa, New Zealand, were monitored and analysed to identify sediment dynamics within them and how they respond to discrete flood events and river management practices. This was achieved by analysing existing historic aerial and satellite imagery, cross-sectional survey data, and geomorphic change analyses using Structure from Motion (SFM) photogrammetry datasets collected in this project. Historical aerial imagery revealed that both rivers have significantly changed over time, with the area of active gravels reducing 38% in the Tauanui and 48% in the Turanganui River from the 1940s to 2013. A narrowing and straightening of both rivers and a proliferation of heavily vegetated banks was observed. It is suggested that these changes are linked to river management strategies, which have helped to develop floodplains for agriculture and occupation by people. Following a storm event on the 20th of June 2021, flooding caused significant geomorphic change. Geomorphic change analysis before and after suggested net aggradation of 1,564 m³ in the Tauanui River and 3,430 m³ in the Turanganui River. Although geomorphic change was significant in both study reaches, it contrasted. This contrast has been interpreted as a result of differences in river resilience and geomorphic thresholds. Similar to other studies, it is suggested that river management interventions have reduced resilience and brought both rivers closer to geomorphic thresholds. This has resulted in geomorphic change that is disproportionate to the flood magnitude. River management that homogenises river corridors is also detrimental to habitat diversity and increases the exposure of the surrounding land to flood risk.