Storm-generated Holocene and historical floods in the Manawatu River, New Zealand

Abstract

This paper reports the first reconstruction of storm-generated late Holocene and historical river floods in the North Island of New Zealand. The sedimentary infills of nine palaeochannels were studied in the lower alluvial reaches of the Manawatu River. Floods in these palaeochannels were recorded as a series of sand-rich units set within finer-grained fills. Flood chronologies were constrained using a combination of radiocarbon dating, documentary sources, geochemical markers, and palynological information. Flood units were sedimentologically and geochemically characterised using high resolution ITRAX TM X-Ray Fluorescence (XRF) core scanning and laser diffraction grain-size analysis. The longest palaeoflood record extends back ca.3000 years. The temporal resolution and length of the Manawatu record reflects accommodation space for fluvial deposits, channel dynamics and mobility, and high sediment supply. Floods that occurred in the Manawatu during the mid-1800s at the time of European land clearance and in the first decade of the twentieth century appear to be among the largest recorded in the last 3000 years.

Fluvial palaeohydrology in the 21st century and beyond

“…Similar results have been obtained in New Zealand (e.g. Clement & Fuller, 2007;Fuller et al, 2018Fuller et al, , 2019.…”

High‐resolution mapping of Manawatu palaeochannels

“…yr BP, based on 14 C ages of organic material recovered from palaeochannels cored at the northern margins of the confined and unconfined floodplain (Fuller et al . accepted). Since the river assumed its current course before 1859, the dynamic of the lower Manawatu River captured in this palaeochannel analysis reflects natural behaviour and response to unmodified catchment conditions.…”

“…Further work of this nature is under way in the Manawatu, using sediments recovered from several of the palaeochannels mapped in this study to reconstruct flood histories (Fuller et al . accepted). The methodology described in this paper furthermore allows an accurate identification of the feature to a large extent, avoiding time‐consuming and cost‐prohibitive field surveys.…”

“…Furthermore, rigorous palaeochannel identification also serves to identify potential pre‐historic flood archives, which are potentially useful for flood series extension (Fuller et al . accepted).…”

River flood reconstruction in the Lower Rhine valley and delta

“…The majority of palaeoflood studies have focused on geomorphologically stable confined fluvial settings, altough the alluvial reaches of most rivers are more densely populated and more at risk to flooding. In such lowland areas, sediment records in floodplain lakes have provided frequencies and relative magnitudes of past floods (Werritty et al, 2006;Cremer et al, 2010;2010a;Toonen et al, 2015;Minderhoud et al, 2016;Fuller et al, 2018;Munoz et al, 2018;Peng et al, 2019;Toonen et al, 2020). However, the recorded events have to be calibrated to more recent events in the same series to arrive at absolute discharge estimates, which introduces significant uncertainties.…”