Chasing the mud : analysing the geomorphic impact of the 19-21 June 2015 rainstorm and flood events in southern North Island, New Zealand : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Physical Geography at Massey University, Palmerston North, New Zealand
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Date
2021
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Massey University
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Abstract
Heavy and prolonged rainfall during the 19-21 June 2015 storm event, triggered the largest recorded flood in the Whanganui River and resulted in widespread flooding in adjacent Whangaehu and Turakina drainage basins. In addition, this high intensity rainstorm triggered many thousand landslides in the soft-rock hill country of these catchments, impacting productivity of vast areas of the landscape within the region. The principal objective of this research was to analyse and better understand the internal dynamics of the catchment sediment cascade and explain the manifestation of geomorphic change within the Whanganui, Whangaehu and Turakina catchments during the intense disturbance event of June 2015. To accomplish this, a geomorphic and sedimentological assessment of channel, floodplain and slope response to the 19-21 June rainstorm and flood events was undertaken to examine erosion, transportation and deposition zones that developed during this event. To facilitate a better understanding of the processes and mechanisms operating during this major storm event, this research provides an assessment of sediment sources and sinks that developed during the storm in the lower Whanganui, Whangaehu and Turakina catchments. It is surmised that the extreme nature of this rainstorm activated the landscape to a degree whereby slope-channel environments were highly connected. Using GeoEye satellite imagery, the distribution and extent of landsliding was mapped, and slope-channel connectivity quantified. Additionally, regions of overbank flood deposition within the flood corridors of these catchments were mapped and the areal extent quantified. Physical terrain attributes, such as slope aspect and angle, vegetation cover and rock type were analysed in relation to landsliding to assess the influence these factors had in determining slope instability. To evaluate the flow and sediment regime of these fluvial networks during the 19-21 June rainstorm event, field mapping and sampling of flood deposits was conducted, and an assessment of the thickness and character of these flood drapes was undertaken. Subsequent analysis of flood deposit samples incorporated grain size analysis so that a better understanding of the hydraulic processes that transported these eroded sediments could be formed. Additionally, sedimentary analysis facilitated the development of new and local knowledge of the processes that produced distinct vertical layering within the flood drape deposited during the 2015 flood event. A linkage is made between the developing flood hydrograph, event sediment flux hydrograph and the stratigraphic layering which was found at various sites within the June 2015 flood drape of the Whanganui River.
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Chapter 3 (Literature Review) is not available online owing to the copyrighted Figures it contains, but the print copy may be consulted in the Library.