Elsevier

Journal of Hydrology

Volume 380, Issues 3–4, 30 January 2010, Pages 338-355
Journal of Hydrology

Dam break – Outburst flood propagation and transient hydraulics: A geosciences perspective

https://doi.org/10.1016/j.jhydrol.2009.11.009Get rights and content

Summary

Dam break outburst floods are sudden, short-lived and high-magnitude discharges of water and sediment propagating over an initially ‘dry’ terrain. Both field observations by geoscientists and experimental measurements by engineers have produced understanding of outburst floods but neither approach has satisfactorily supplied data that is directly of use to hazard managers. This study draws together those two literature groups and provides new experimental data to test hypotheses concerning the effect of a series of external controls on resultant longitudinal flow propagation and transitory hydraulics. These external controls form experimental treatments and are; (i) the height of the lock gate raise as a surrogate for dam breach evolution and initial hydrograph, (ii) bed roughness, and (iii) suspended sediment concentration. These control both horizontal and vertical fluid motion, the latter of which is a key property for controlling aeration, bed shear stress and turbulence intensity. The experimental treatments show that outburst floods on horizontal beds can be identified to possess three longitudinal flow regimes. Firstly, there is a short acceleration due to the reservoir pressure level; i.e. the depth of impounded water. Secondly, channel flow quickly converges to an inertial regime. The third flow regime is viscous and dominated by channel bed friction. Flow depth increases in the inertial regime, then decreases in the viscous regime. Experiments herein show that the timing of the peaks at-a-point down channel of each hydraulic quantity varies; most notably peak bed shear stress precedes flow velocity, which precedes peak flow depth. A point of inflexion occurs with distance down channel that is a manifestation of the transition from accelerating to decelerating flows. These external controls, and the effects of them on longitudinal and vertical motion are analysed statistically and quantitative relationships are derived to underpin the resultant conceptual model. This conceptual model should serve as a platform for future probabilistic and process-based modelling of outburst floods.

Introduction

Dam break outburst floods are frequently generated by failures of natural dams (Carrivick and Rushmer, 2006). Particularly common sources of outburst floods include landslide-dammed lakes (e.g. Dai et al., 2005), glacial (e.g. Carrivick et al., 2009, Huggel et al., 2002, Tweed and Russell, 1999) and moraine lakes (e.g. Clague and Evans, 2000), artificial (e.g. Fread, 1996) and volcanic impoundments (e.g. Carrivick et al., 2009, Kataoka et al., 2008, Fenton et al., 2006, Macías et al., 2004, Waythomas, 2001), and the melting of ice by volcanic activity (e.g. Guðmundsson et al., 1997). They can be named ‘jökulhlaups’ if from a glacial source and as ‘lahars’ if incorporating volcaniclastic sediment. Generically such phenomena are referred to in the literature as ‘surging flow’, ‘flash/outburst flood’, or ‘dam break wave/flow/flood’. Across the different categorisations of these events by sources, trigger mechanisms and flow processes (e.g. Costa and Schuster, 1988, Tweed and Russell, 1999, Carrivick and Rushmer, 2006, Korup and Tweed, 2007) there remains what is essentially the propagation of a fluid wave over initially ‘dry’ terrain. Therefore the many types of dam break outburst floods will simply be referred to herein as ‘outburst floods’.

Outburst floods are the largest floods known to have occurred on Earth during the Quaternary (e.g. Baker, 2002, Korup and Tweed, 2007). Modern examples of these floods constitute a serious threat to life, property and infrastructure. Outburst floods can exert intense and widespread landscape change through erosion of both unconsolidated sediments (e.g. Gomez et al., 2002, Russell and Marren, 1999, Russell et al., 2006) and bedrock (e.g. Carrivick et al., 2004a, Tómasson, 1996, Baker, 1988). As climate change causes more ‘extreme’ weather outburst floods will likely increase in frequency, and perhaps also in magnitude (Bates et al., 2008).

Despite the pressing need to measure, model and to mitigate against outburst flood impacts, there are problems with each stage of this process. Such events are simply too sudden, powerful and short-lived to measure directly, and in situ field-deployed sensors and loggers are very likely to become destroyed. A few exceptional studies have succeeded in directly measured outburst flood properties; a most recent example being the March 18th lahar from Crater Lake on Mt. Ruapehu, New Zealand (Carrivick et al., 2009). In reaction to the problems of direct measurements, research aiming to better understand outburst floods has pursued two approaches. The first approach has sought to reconstruct characteristics of outburst floods from geological field evidence, using empirical relationships (e.g. Clague and Mathews, 1973, Walder and Costa, 1996), palaeocompetence (e.g. Carrivick, 2007) and various palaeohydraulic techniques including slope area methods. The same knowledge has also been incorporated into GIS-based hazard analyses (e.g. McKillop and Clague, 2007). The second approach has been adopted from the engineering community and is the application of fluid dynamics computational models, both with and without use of flume experiments. These studies are designed to develop simple physical-based equations that highlight the controls such as impounded water volume, reservoir geometry and dam height, for example, as well as to consider idealised dam breach evolution.

Nevertheless, individuals, authorities and organisations seeking to manage outburst flood hazards and to mitigate against outburst floods require knowledge of transient hydraulics. Specifically, the longitudinal evolution of a flood; i.e. flood propagation and time to inundation, time to peak discharge, persistence of peak discharge and rates of rising/falling stages, for example must be understood for effective outburst flood hazard analysis (e.g. McKillop and Clague, 2007). These properties, and the controls on them, have either not been directly measured or the literature has not reported them explicitly or coherently. Furthermore, although the fluid dynamics engineering provides some of the best approximations of the dam break processes, this literature is poorly known and understood by geomorphologists and sedimentologists who reconstruct outburst floods from field evidence.

Section snippets

Aim

The overall aims of this paper are to (1) To quantitatively analyse new experimental data on outburst flood propagation and transient hydraulics and (2) To summarise these analyses in a conceptual model of outburst flood controls and properties that will be of direct use for hazard management. These aims are achieved by explicitly testing a set of hypotheses derived from the literature concerning the key controls on longitudinal flow evolution. The framework for this work and the results are

Experimental insights to outburst floods

Outburst floods are unsteady non-uniform flow phenomena. In unsteady flows the key process parameter is friction velocity; which can be used to calculate bottom shear stress and must be measured within the (thin) viscous sublayer close to the channel bed. Friction velocity is frequently either not described or its dynamic behaviour is ignored (Schlütter, 1999). Additionally, turbulence, which is characterised by chaotic hydraulic changes; i.e. high momentum convection and rapid variation of

Methods

Flume experiments were conducted within the Sorby Environmental Fluid Dynamics Laboratory (SEFDL) at the University of Leeds. Experiments applied Froude scaling principles at 1:20 in order to best-match outburst flood conditions reported in the geological literature. Specifically, field observations and reconstructions of outburst floods commonly suggest Froude numbers of ∼1, high sediment concentrations approaching ∼25% by volume, and high channel roughness relative to flow depth (e.g.

Flow front character and propagation

The emphasis of the experiments in this study is not so much on absolute values; although these will be of use to modellers wishing to validate their results. Rather, the focus is on the controls on longitudinal flow propagation and hydraulics. For clarity and brevity, only results from a single experimental run of each combination of variables are graphically presented. However, the excellent repeatability of experiments can be demonstrated graphically as well as statistically. For example, an

Flow depth

Over fixed rough beds flow depths increase down channel (Fig. 6). Flow depths also increase with the height of the gate raise and with increasing bed roughness (Fig. 6). However, the duration of peak stages is rather more complicated. Whilst peak stage is long-lived with small gate raises, the stage hydrograph is more ‘flashy’ with large gate raises (Fig. 6). Increasing bed roughness does not appear to change the rate of rise to peak stage, nor the duration of peak stage and the recession from

Transient hydraulics and flow regime

So far the results described have emphasised effects on flow propagation and hydraulics of varying controls. However, varying flow conditions within a single outburst flood event are also important and best illustrated with recourse to high frequency velocity data. One example at 2 m channel distance from a 6 cm gate raise over a fine-grained fixed bed shows that such data is characteristically ‘noisy’ in its ‘raw’ form (Fig. 9A). Use of a running mean or standard deviation helps to compare these

Discussion

This section takes the results presented above and analyses them from a hazard management perspective. The embedded discussion emphasises external and internal controls on outburst flood propagation and transient hydraulics and seeks a process-based understanding of the observations. Both ‘at-a-point’ and ‘longitudinal’ variations are examined and the implications for managing outburst flood hazards are presented. Initially qualitative relationships are described, before expanding to consider

Conclusions

Previous experimental work has shown that outburst floods can be identified to possess three flow regimes. Firstly, there is a short acceleration due to the reservoir pressure level; i.e. the depth of impounded water. For the case of a narrow deep reservoir, such as an experimental lock box, this pressure rapidly falls and the channel flow quickly converges to an inertial regime (e.g. Stansby et al., 1998). The third phase is a viscous regime, dominated by channel bed friction and pronounced

Acknowledgements

This data presented in this paper is part of a project funded by a NERC New Investigator’s Research Grant NE/F000235/1 to J.L. Carrivick. Gareth Keevil is thanked for his tremendous technical assistance, and Jeff Peakall for his enthusiastic advice and support. Lee Brown and Lucy Rushmer gave advice on the quantitative analysis and flume modelling approaches, respectively. Insightful and helpful comments from two reviewers and an associate editor improved this manuscript.

References (92)

  • C.J. Legleiter et al.

    Geostatistical analysis of the effects of stage and roughness on reach-scale spatial patterns of velocity and turbulence intensity

    Geomorphology

    (2007)
  • J. Maizels

    Jökulhlaup deposits in proglacial areas

    Quaternary Science Reviews

    (1997)
  • V. Manville et al.

    Incipient granular mass flows at the base of sediment-laden floods, and the roles of flow competence and flow capacity in the deposition of stratified bouldery sands

    Sedimentary Geology

    (2003)
  • P.M. Marren

    Magnitude and frequency in proglacial rivers: a geomorphological and sedimentological perspective

    Earth Science Reviews

    (2005)
  • A.J. Russell et al.

    Morphology and sedimentology of a giant supraglacial, ice-walled, jökulhlaup channel, Skeiðarárjökull, Iceland: implications for esker genesis

    Global and Planetary Change

    (2001)
  • A.J. Russell et al.

    Icelandic jökulhlaup impacts: implications for ice-sheet hydrology, sediment transfer and geomorphology

    Geomorphology

    (2006)
  • F. Schlütter

    A conceptual model for sediment transport in combined sewer systems

    Water Science and Technology

    (1999)
  • C.F. Waythomas

    Formation and failure of volcanic debris dams in the Chakachatna River valley associated with eruptions of the Spurr volcanic complex, Alaska

    Geomorphology

    (2001)
  • S.-Q. Yang et al.

    Turbulence structures in non-uniform flows

    Advances in Water Resources

    (2008)
  • S.Q. Yang et al.

    Velocity distribution in a gradually accelerating flow

    Advances in Water Resources

    (2006)
  • F.-li. Yang et al.

    One- and two-dimensional coupled hydrodynamics model for dam break flow

    Journal of Hydrodynamics Series B

    (2007)
  • D.M. Admiraal et al.

    Entrainment response of bed sediment to time-varying flows

    Water Resources Research

    (2000)
  • V.R. Baker

    Flood erosion

  • V.R. Baker

    High-energy megafloods: planetary settings and sedimentary dynamics

  • J.E. Berlaymont et al.

    Shear stress distribution in partially filled pipes

    Journal of Hydraulic Engineering

    (2003)
  • F. Bigillon et al.

    Measurements of turbulence characteristics in an open-channel flow over a transitionally-rough bed using particle image velocimetry

    Experiments in Fluids

    (2006)
  • P. Brufau et al.

    Two-dimensional dam break flow simulation

    International Journal of Numerical Methods in Fluids

    (2000)
  • H. Capart et al.

    Formation of a jump by the dam-break wave over a granular bed

    Journal of Fluid Mechanics

    (1998)
  • F.G. Carollo et al.

    Analyzing turbulence intensity in gravel bed channels

    Journal of Hydraulic Engineering

    (2005)
  • J.L. Carrivick et al.

    Understanding high-magnitude outburst floods

    Geology Today

    (2006)
  • J.L. Carrivick et al.

    Inter- and intra-catchment variability in proglacial geomorphology: an example from Franz Josef Glacier and Fox Glacier, South Westland, New Zealand

    Arctic, Antarctic and Alpine Research

    (2009)
  • J.L. Carrivick et al.

    Geomorphological evidence for jökulhlaups from Kverkfjöll volcano, Iceland

    Geomorphology

    (2004)
  • J.L. Carrivick et al.

    A fluid dynamics approach to modelling the 18th March 2007 lahar at Mt. Ruapehu, New Zealand

    Bulletin of Volcanology

    (2009)
  • J.L. Carrivick

    Hydrodynamics and geomorphic work of jökulhlaups (glacial outburst floods) from Kverkfjöll volcano, Iceland

    Hydrological Processes

    (2007)
  • D.A. Cenderelli et al.

    Flow hydraulics and geomorphic effects of glacial-lake outburst floods in the Mount Everest region, Nepal

    Earth Surface Processes and Landforms

    (2003)
  • Chanson, H., 2004a. Unsteady two-phase flow measurements in surges and dam break waves. In: 5th International...
  • H. Chanson

    Free-surface aeration in dam break waves: an experimental study

  • H. Chanson

    Hydraulics of Open Channel Flow: An Introduction

    (2004)
  • Y.H. Chen et al.

    An experimental study of hydraulic and geomorphic changes in an alluvial channel induced by failure of a dam

    Water Resources Research

    (1979)
  • J.J. Clague et al.

    The magnitude of jökulhlaups

    Journal of Glaciology

    (1973)
  • J.E. Costa et al.

    The formation and failure of natural dams

    Geological Society of America Bulletin

    (1988)
  • J.E. Costa

    Rheologic, geomorphic and sedimentologic differentiation of water flows, hyperconcentrated flows and debris flows

  • S. Dey et al.

    Reynolds stress in open channel flow with upward seepage

    Journal of Engineering Mechanics

    (2005)
  • J. Eaket et al.

    Use of stereoscopy for dam break flow measurement

    Journal of Hydraulic Engineering

    (2005)
  • L. Fracarollo et al.

    Riemann wave description of erosional dam-break flows

    Journal of Fluid Mechanics

    (2002)
  • Cited by (0)

    View full text