A Comparison of Alluvial and Bedrock River Sedimentary Archives for Reconstructing Holocene Palaeofloods in New Zealand

Toonen, W, Fuller, I, Macklin, Mark and Holt, K (2016) A Comparison of Alluvial and Bedrock River Sedimentary Archives for Reconstructing Holocene Palaeofloods in New Zealand. In: Ex-Aqua 2016.

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Padova_abstractNZ.pdf - Abstract

Item Type:Conference or Workshop contribution (Presentation)
Item Status:Live Archive


Current assessment of flood risk in New Zealand is compromised by short instrumental flow records (less than 50 yr) in most large river catchments. These typically do not include the largest floods that have occurred in the past and which would cause the most damage to life, property and infrastructure.
This paper reports the first Late Holocene palaeoflood reconstruction in the North Island of New Zealand within the Manawatu river (c.5950 km2), based on a coring campaign to examine the sedimentary infill of oxbow lakes ('lagoons') in the lower part of the catchment. Study reaches are fully alluvial, with an
extensive suite of infilled palaeochannels developed on a series of low elevation cut-terraces and on very low-gradient fluvial plains. The geochemical proxies for the grain-size of individual flood units, recovered by percussion and piston coring, were compared with modelled overtopping discharges for each study site, and the monitored discharge record in order to correlate flood units with historicallyknown events. The sedimentary record extends beyond the historical period, and can be used to identify major flood events that occurred in the past. Flood chronologies are constrained using a combination of radiocarbon dating, documentary sources, palynology (pin-pointing the European colonisation), and
geochemistry (associated with heavy-metal pollution). The setting of the Manawatu river was compared with other flood-prone river systems in the same
hydroclimatic region (south-west North Island); the Whanganui (7380 km2) and Hutt Rivers (650 km2). Largest floods are for all these catchments generated by westerly storms throughout the year, but the specific local geological setting and potential for palaeoflood reconstructions varies greatly. In the Whanganui catchment two sedimentary archives have been investigated: 1. Slackwater deposits on an alluvial bench c.20 m above the present river level (on the so-called Taupo terrace, commonly used for settlement) within the bedrock reach of the Whanganui Gorge; 2. Palaeochannel fills on terraces 6-8 m above the river level within a mixed alluvial-bedrock reach located in the piedmont zone downstream. For the Hutt River, the shallow channel fills of the high-gradient wandering, semi-braided gravel-bed reaches at the margin of the greywacke Tararua Range were explored for potential palaeoflood records. Holocene palaeoflood records differ significantly in both length and nature between the three catchments, conditioned by tectonic and land use histories, which strongly influence local flood sedimentation styles. Terrains with elevated post-European settlement rates of sediment supply, particularly in the Manawatu and Whanganui, generated high-resolution palaeoflood records. Results of
this study demonstrate that fluvial sedimentary archives from a range of alluvial as well as bedrock river environments are required for reconstructing regional Holocene flood histories in New Zealand, but that palaeoflood studies and their expected results need to be tailored to fit the highly diverse local setting.

Keywords:Manawatu, Whanganui, Hutt River, New Zealand, oxbow, lagoon, palaeofloods, slackwater
Subjects:F Physical Sciences > F810 Environmental Geography
F Physical Sciences > F840 Physical Geography
Divisions:College of Science > School of Geography
ID Code:38184
Deposited On:06 Nov 2019 09:23

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