The University of Southampton
University of Southampton Institutional Repository

Particle comminution defines megaflood and superflood energetics

Particle comminution defines megaflood and superflood energetics
Particle comminution defines megaflood and superflood energetics

The concept of ‘megafloods’ and ‘superfloods’ was introduced at the end of the last century to define exceptionally large-discharge floods, primarily those associated with the failure of Quaternary ice-dams. These floods exceeded, by one or two magnitudes, historically recorded floods, megafloods having peak discharges equalling, or exceeding, 1 M m 3 s −1. Herein, the sediment associations of exceptional floods are found to be distinctive, being dominated by comminuted (smashed) grain-size distributions, which contrast to the sediment deposits of more moderate floods. A consideration of the energetics of moderate floods, superfloods and megafloods with respect to the entrainment, dis-entrainment, suspension and comminution of coarse gravel allows thresholds of energy expenditure to be proposed that reflect general transformations in flood behaviour that relate to distinctive flood deposits. A threshold of energy expenditure - 20 kJ s −1 m −2 - is proposed to separate moderate floods from superfloods; the latter by this definition can be ~ 0.1 M m 3 s −1. This threshold separates floods competent to entrain cobbles and boulders (with limited comminution of the load) from superfloods that comminute the suspended cobbles and boulders to produce fine granules in suspension. The granules are subsequently deposited to form distinct depositional units that are characteristic of superfloods and megafloods. The energetic threshold is consistent with the lower range of power expenditures associated with the original definition of a megaflood in terms of peak fluid flux. Significance: Exceptional floods occurred in prehistoric times; the largest are referred to as ‘megafloods’ or ‘superfloods’. To distinguish such exceptional floods from other large floods, megafloods were defined as having a peak discharge exceeding 1 M m 3 s −1. However, many reports of ‘megafloods’ are flows with a peak discharge smaller than 1 M m 3 s −1. Recent research has shown that megafloods and superfloods deposit distinctive layers of smashed gravel, which is not the case for smaller floods. The gravel is smashed by exceptionally high-energy expenditures. Consideration of the energetics of floods to transport gravel allows a robust quantitative definition of ‘superflood’ based on the energy needed to smash a range of gravel sizes.

Megafloods, communition, flood energetics, streampower
0012-8252
Carling, Paul
8d252dd9-3c88-4803-81cc-c2ec4c6fa687
Fan, Xuanmei
04ff993f-9188-455a-8a18-5f2735901aaf
Carling, Paul
8d252dd9-3c88-4803-81cc-c2ec4c6fa687
Fan, Xuanmei
04ff993f-9188-455a-8a18-5f2735901aaf

Carling, Paul and Fan, Xuanmei (2020) Particle comminution defines megaflood and superflood energetics. Earth-Science Reviews, 204, [103087]. (doi:10.1016/j.earscirev.2020.103087).

Record type: Article

Abstract

The concept of ‘megafloods’ and ‘superfloods’ was introduced at the end of the last century to define exceptionally large-discharge floods, primarily those associated with the failure of Quaternary ice-dams. These floods exceeded, by one or two magnitudes, historically recorded floods, megafloods having peak discharges equalling, or exceeding, 1 M m 3 s −1. Herein, the sediment associations of exceptional floods are found to be distinctive, being dominated by comminuted (smashed) grain-size distributions, which contrast to the sediment deposits of more moderate floods. A consideration of the energetics of moderate floods, superfloods and megafloods with respect to the entrainment, dis-entrainment, suspension and comminution of coarse gravel allows thresholds of energy expenditure to be proposed that reflect general transformations in flood behaviour that relate to distinctive flood deposits. A threshold of energy expenditure - 20 kJ s −1 m −2 - is proposed to separate moderate floods from superfloods; the latter by this definition can be ~ 0.1 M m 3 s −1. This threshold separates floods competent to entrain cobbles and boulders (with limited comminution of the load) from superfloods that comminute the suspended cobbles and boulders to produce fine granules in suspension. The granules are subsequently deposited to form distinct depositional units that are characteristic of superfloods and megafloods. The energetic threshold is consistent with the lower range of power expenditures associated with the original definition of a megaflood in terms of peak fluid flux. Significance: Exceptional floods occurred in prehistoric times; the largest are referred to as ‘megafloods’ or ‘superfloods’. To distinguish such exceptional floods from other large floods, megafloods were defined as having a peak discharge exceeding 1 M m 3 s −1. However, many reports of ‘megafloods’ are flows with a peak discharge smaller than 1 M m 3 s −1. Recent research has shown that megafloods and superfloods deposit distinctive layers of smashed gravel, which is not the case for smaller floods. The gravel is smashed by exceptionally high-energy expenditures. Consideration of the energetics of floods to transport gravel allows a robust quantitative definition of ‘superflood’ based on the energy needed to smash a range of gravel sizes.

Text
Carling_main - Proof
Download (1MB)

More information

Accepted/In Press date: 13 January 2020
e-pub ahead of print date: 15 January 2020
Published date: May 2020
Additional Information: Funding Information: Part of this research has been funded by the National Science Fund for Outstanding Young Scholars of China (Grant No. 41622206 ), the Funds for Creative Research Groups of China (Grant No. 41521002 ), the State Key Laboratory of Geo-hazard Prevention and Geo-environment Protection Independent Research Project ( SKLGP2019Z002 ). Christoph Tuitz is thanked for the provision of the data plotted within Fig. 3 . Jonathan Carrivick and Jim O'Connor are thanked for their careful insightful reviews of the manuscript that led to improved presentation of the arguments. Publisher Copyright: © 2020 Elsevier B.V.
Keywords: Megafloods, communition, flood energetics, streampower

Identifiers

Local EPrints ID: 437614
URI: http://eprints.soton.ac.uk/id/eprint/437614
ISSN: 0012-8252
PURE UUID: 7f4533a4-894e-465a-a066-547306e10077

Catalogue record

Date deposited: 06 Feb 2020 17:32
Last modified: 02 Sep 2022 04:01

Export record

Altmetrics

Contributors

Author: Paul Carling
Author: Xuanmei Fan

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×