Journal of Hydrology 408 (2011) 153–163

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Journal of Hydrology

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Experimental insights on geomorphological processes within dam breakoutburst floods

Jonathan L. Carrivick a,⇑, Richard Jones b, Gareth Keevil c

a School of Geography, University of Leeds, Leeds, West Yorkshire LS2 9JT, UKb Department of Earth Sciences, University of Durham, Durham DH1 3LE, UKc School of Earth and Environment, University of Leeds, Leeds, West Yorkshire LS2 9JT, UK

a r t i c l e i n f o

Article history:Received 15 April 2011Received in revised form 8 July 2011Accepted 23 July 2011Available online 2 August 2011This manuscript was handled byKonstantine P. Georgakakos, Editor-in-Chief,with the assistance of Ellen Wohl, AssociateEditor

Keywords:Dam breakOutburst floodFlash floodFlumeClast sizeSediment transport

0022-1694/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.jhydrol.2011.07.037

⇑ Corresponding author. Tel.: +44 (0)113 34 33324;E-mail address: j.l.carrivick@leeds.ac.uk (J.L. Carriv

s u m m a r y

Hydraulic behavior and unsteady sediment transport within dam break outburst floods are inherentlyinter-dependent, yet poorly constrained. This study experimentally determined how the mobility andclast size of channel bed sediments affected the peak flow depth and flow velocities, wave propagationand attenuation, sediment entrainment and duration of transport, flood power, energy transfer and geo-morphological impact of outburst floods. Due to our choice of clast size and of a unimodal size distribu-tion the predominant sediment transport mechanism was bedload. We found that frontal wavepropagation was slower and attenuated faster for flows over a mobile bed versus those over a fixedbed. Peak flow depth was greater and was attained earlier for flows over a mobile bed and with increasedclast size. Geomorphological power declined logarithmically with time after an initial period where grav-itational forces exceeded frictional forces. Both peak and time-to-peak geomorphological power variedinversely with clast size. Peak deposition rates were greatest with coarser bed sediment but time to peakdeposition rate was inversely proportional to clast size. Volume changes down channel were described byan exponential curve, which we suggest indicates that sediment transport proceeded in waves via bothtranslation and dispersion mechanisms. Peak suspended load occurred coincident with peak flow veloc-ity, peak bedload occurred coincident with peak flow depth, and peak sediment discharge lagged behindpeak water discharge. These descriptions of kinematic transformation, multi-phase flow and of geomor-phological work due to outburst floods help to provide data for numerical modelling, for mitigating out-burst flood hazards and for informing on the immediate and longer-term geological legacy of outburstfloods.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Dam break outburst floods from man-made and natural damsare rapidly-released high-magnitude water and sediment flowmixes that propagate over initially non-flooded terrain due to bothinertia and gravity. The largest floods known to have occurred onEarth during the Quaternary (e.g. Baker, 2002) were dam breakoutburst floods. Modern outburst floods constitute a serious haz-ard to life, property and infrastructure and can induce intense geo-morphological change through erosion of bedrock andunconsolidated sediments (e.g. Procter et al., 2010; Carrivicket al., 2010).

Dam break outburst floods can be identified to possess threehydraulic regimes in longitudinal space with kinematic passageof a wave form (Janosi et al., 2004; Carrivick, 2010). Firstly, thereis a short acceleration due to the reservoir pressure level; i.e. the

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depth of impounded water. Secondly, channel flow quickly con-verges to an inertial regime. The third flow regime is viscous anddominated by channel bed friction. All three hydraulic regimescan be observed at many points along the flood path and overthe entire (though often short-lived) wave-form passage and it isthis co-existence of flow regimes (Carrivick, 2010) that makes mostnumerical models of outburst floods necessarily highly idealizedand rarely inclusive of sediment transport. This is the first reasonwhy there is a general inability to directly link outburst flood pro-cesses to products; i.e. to resultant landforms and sediments.Numerical models of outburst floods that do include sedimenttransport generally do not change fluid flow behavior to accountfor changes in sediment concentration and particularly for interac-tions between clasts (e.g. Cao et al., 2004; Capart and Young, 1998;Fracarollo and Capart, 2002; Shieh et al., 1996). The few numericalmodels that do modify flow rheology in accordance with the con-stituents of the fluid phase are largely built on the theory ex-pounded by Takahashi (1991) and are compared in Rickenmannet al. (2006). However, like the earlier work mentioned, they donot iteratively update sediment concentration.

154 J.L. Carrivick et al. / Journal of Hydrology 408 (2011) 153–163

The second reason why it has generally not been possible to di-rectly link outburst flood processes to products is because fieldmeasurements necessarily concentrate on analyzing sedimentarydeposits to infer or reconstruct high-magnitude flow conditions.These field measurements show that both fine- and coarse-grainedclasts are commonly transported within outburst floods (e.g. Car-rivick et al., 2007; Russell et al., 2006; Manville and White,2003), and that sediment load within outburst floods is highlytransient. Outburst floods have been noted to become progres-sively more fluidal as initially voluminous sediment supplies areexhausted (e.g. Old and Lawler, 2005; Russell and Marren, 1999).Conversely, initially fluid flows can become bulked by rapid sedi-ment entrainment (e.g. Lavigne and Thouret, 2003; Manville,2004).

With the limitations of both numerical models and of field mea-surements it is thus laboratory flume experiments that perhapshave the most promise for understanding the effects of sedimenttransport on outburst flood behavior and for developing process–product knowledge. Therefore the overall aim of this study is thusto examine sediment transport effects on outburst flood hydraulicsand geomorphological work.

2. Previous experiments with mobile sediment

Previous experiments of outburst floods with mobile sediment;‘the geomorphic dam-break problem’, have been motivated bypractical, theoretical (numerical) and experimental considerations.These experiments (Table 1) have successfully isolated specificgeometric controls of reservoir length (Nsom, 2002), channel sinu-osity (e.g. Chen and Simons, 1979), channel slope (e.g. Lauber andHager, 1998), and channel roughness (e.g. Chanson, 2004c), and thehydraulic controls of fluid viscosity (e.g. Nsom, 2002), hydrographshape (e.g. Rushmer, 2007), and water depth ratios behind/in frontof the dam (Stansby et al., 1998), for example. Very few experimen-tal studies have explicitly examined flow-bed interactions; i.e. ero-sion and deposition. Those studies that have examined erosion anddeposition have been limited to one-dimensional observations and

Table 1Summary of some major previous dam break outburst flood experiments with mobile sed

Experimental channel geometry(length, width, height, all inmetres)

Channelplanform andgradient

Channel substrate

18.5 � 5.0 � 1.6 Horizontal,straight

Non-uniform coal ash

9.0 � 0.8 � 0.45 1�; straight Poorly sorted, rounded,fine to medium pebbles

3.0 � 0.25 � 0.7 Horizontal,straight

Saturated mobile granules

2.92 � 0.495 � 0.15 Horizontal,straight

Saturated mobile coarsesand

19.2 � 0.5 � 0.7 Horizontal,straight

Fixed bed, sand bed, andpumice bed

2.5 � 0.1 � 0.35 Horizontal;straight

Coarse cohesionlesssediments saturated withwater

12.0 � 0.2 � 0.7 Horizontal;straight

Mobile ‘wet’ granules ontop of a mesh

10.0 � 0.3 � 0.3 Horizontal,straight

Plastic, acrylic and sandgranules 0.13–3 mm

12.7 � 1.4 � ? 0.002; Straight Initially dry withManning’s ‘n’ of 0.012

2.4 � 0.12 � 0.12 0.0025; Straight Mobile sand-clay andgravel

measurements that are either side-looking (e.g. Capart and Young,1998; Fracarollo and Capart, 2002), longitudinal (e.g. Spinewineand Zech, 2007) or at single cross-sections (e.g. Soares Frazãoet al., 2007). Thus full consideration of spatial and temporal varia-tions in erosion and deposition due to outburst floods is lacking inexperiments, as is a consideration of the geomorphic (mobile sub-strate) controls on outburst floods. A notable exception is the re-cent work by Xia et al. (2010) that has demonstrated that‘sediment sorting’, or grain size distribution, influences scour holegeometry via selective transport capacity.

The previous findings outlined above lead us to pose the follow-ing research question; how does sediment mobility and hencechannel bed clast size control outburst flood flow hydraulics andgeomorphological work? We hypothesize that channel bed clastsize will determine (i) entrainment threshold, and (ii) duration oftransport, and (iii) mode of transport. This control of clast size iswell-known for steady flows and overbank floods but has yet tobe examined in outburst floods where both flow hydraulics andsediment transport are unsteady.

3. Experimental methods

Flume experiments were conducted in the Sorby Environmen-tal Fluid Dynamics Laboratory (SEFDL) at the University of Leeds.Flume channel geometry was 4.0 m long � 0.2 m wide � 0.5 mdeep with a gated lock-box at one end of 150 l volume; 0.56 mlong � 0.47 m wide and 0.69 m deep (Fig. 1A). We present resultsfrom a horizontal channel bed with uniform cross-section only,and justify this with two reasons. Firstly, many numerical modelsare validated against the simple geometry of a uniform cross-sec-tion and horizontal channel bed (e.g. Cao et al., 2006; Emmett andMoodie, 2009; Xia et al., 2010). Secondly, because in reality it isvery common for outburst floods either to be channelized withinriver banks or between valley sides. We acknowledge that in real-ity some outburst flood channels tend to have quite a high gradi-ent and that where some outburst floods run-out onto veryshallow gradient surfaces they are usually unconfined by

iment.

Other notes Reference

40 cm water depth in reservoir, 0.12 m water depth inchannel. D50 of sediment = 0.135 mm

Xia et al. (2010)

Sediment also input to recirculating system Rushmer (2007)

Sand (>2.4 mm) and Polyvinyl Chloride (PVC) pellets (=3.9 mm),

Spinewine andZech (2007)

Initial bed layer thickness = 0.08 m. Erodable banks includedas well as bed

Soares Frazãoet al. (2007)

Sand diameter = 0.8 mm As reported inLeal et al.(2009)

Pumice diameter = 1.2 mmInitial channel sediment infill = 0.07 m. Initial water depth inchannel = 0.21 mInitial water depth upstream = 0.40 m‘Sediment’ was cylindrical 3.2 mm diameter PVC pellets,initially 5–6 cm thick

Fracarollo andCapart (2002)

Granules; uniform 6.1 mm diameter, initially 0.06 m thick Capart andYoung (1998)

Recirculating water and sediment Sumer et al.(1996)

Flume has a curved constriction 0.6 m wide Bellos et al.(1992)

Initial channel sediment thickness was 0.03 m. The sand claybed layer was cut to form a rectangular sinuous channel

Chen andSimons (1979)

Fig. 1. Experimental set-up and design comprising; flume geometry and location of sensors (A), transcritical flow regime as determined by the Froude number achieved formobile bed experiments (B), and grain size distributions of gravel (C). Gravel was either fixed; i.e. glued to boards on the channel floor, or mobile; infilled to a depth of�0.08 m within the channel.

J.L. Carrivick et al. / Journal of Hydrology 408 (2011) 153–163 155

topography or channel banks. Further (future) experiments willbe required to elucidate the control of channel gradient and chan-nel expansions/contractions on flow transformation and erosion/deposition dynamics. To initiate the floods in our experimentswe raised a lock gate by 0.06 m only. Our experiments are thusmost analogous to flow under a sluice gate. We consider this toprovide an appropriate surrogate for outburst floods generatedby tunnelling, such as through unconsolidated sediment dams,for example. Our experimental outburst floods were initiatedfrom a lock box reservoir containing 100% water. This experimen-

tal set-up is analogous to floods initiated from a freshwater lakeor reservoir, for example, albeit one with idealized geometry.However, our primary motivation for 100% water in the lockbox (i.e. no suspended sediment in the reservoir) was that thismost clearly permits isolation of our primary control; that ofthe effects of channel bed sediment entrainment, and of subse-quent transport and deposition. Additionally, initiating dam-breakexperiments with pure water meant that resulting flows weresufficiently translucent to perform high-resolution image andvelocity analysis along the entire flume channel.

156 J.L. Carrivick et al. / Journal of Hydrology 408 (2011) 153–163

Our experiments aimed to best-match outburst flood conditionsreported in the geological literature, specifically: (i) transcriticalflows; i.e. Froude numbers of �1, (ii) sediment concentrationsapproaching �20% by volume, and (iii) high channel roughness rel-ative to flow depth (e.g. Carrivick et al., 2009; Carrivick, 2007a,b;Cenderelli and Wohl, 2003; Russell et al. 2001; Walder and Costa,1996). To this end, we necessarily sacrificed some degree of geo-metric similitude in favor of dynamic similarity. Geometricallywe had a fixed channel width that produced relatively low aspect(width-depth) ratios of 2.0–6.7, with typical flow depths of 0.03–0.1 m (Fig. 2A). The aspect ratio of our experimental flows is clearlytransient and is sometimes less than the desirable range of 4–6,which is recommended to avoid side wall effects (Chanson,2004a). However, as can be seen from Table 1, this experimentalchannel width is similar to that used by other researchers examin-ing sediment transport within outburst floods (e.g. Spinewine andZech, 2007 0.25 m; Fracarollo and Capart, 2002 0.1 m; Nsom, 20020.2 m; Capart and Young, 1998 0.2 m).

Dynamic similarity of process between our experiments andreality was necessary because the hydraulic scale of primary inter-est is at the bed sediment particle diameter (Peakall et al., 1996).Hence for our sediment transport analysis our experiments werescaled by the ratio of the roughness of the bed relative to the thick-ness of the viscous sub-layer, i.e. the grain Reynolds number Re�

(Peakall et al., 1996):

Re� ¼ qU�Dl

where q is fluid density, D is grain size, l is the dynamic viscosity ofthe fluid and U� is the shear velocity:

U� ¼ffiffiffiffiffiffiffiffigRS

p

Fig. 2. Influence of transport mode as dictated channel bed clast grain size, on temporal ecamera (A) and flow velocity as measured by a UVP sensor mounted 0.01 m above the chwidth channel. For clarity experimental runs ‘fixed fine’ and ‘mobile fine’ are not depict

where g is the gravity constant, R is hydraulic radius and S is chan-nel slope. Values of these experimental parameters are given in Ta-ble 2 with an indication of the corresponding real-world situation.The dynamic similarity of process scaling, which is �1:100, also ap-plies to the ratio of bed shear stress to the submerged particleweight; i.e. the Shields parameter (Table 2). The scaling is approxi-mate because we do not have a specific prototype (real-worldevent) in mind and because hydraulic parameters such as flowdepth and flow velocity vary considerably within a single experi-ment. The (transient) Froude number, Fr, was calculated from tem-porally-varying flow depth and instantaneous flow velocity using:

Fr ¼ uffiffiffiffiffiffigh

p

where u is the instantaneous flow velocity measured by a UVP sen-sor, h is flow depth as measured by a side-looking high-speed videocamera, and g is the acceleration due to gravity. Calculations of Fro-ude number demonstrate that the experimental outburst flows withmobile bed sediment were transcritical (Fig. 1B). For note, virtuallyno other experimental literature on outburst flood flows reportsRe�, or the scaled Reynolds or Shields numbers. In our experiments,our dynamic scaling meant that we were able to use the same fluidin the experiments as in reality and we were able to use real gravel,albeit appropriately scaled for size and density (Table 2). A conse-quence of focussing scaling towards bulk flow kinematics is thatthe non-dimensional products of kinematic forces in the experi-mental outburst flood are not correctly scaled to real-world flows.

The experimental parameter systematically varied for thisstudy was gravel clast size. We specifically examined gravel trans-port because unsteady coarse (gravel) sediment transport is under-represented in the experimental literature (Cao et al., 2006;

volution of outburst flood flow depth as measured by side-looking high-speed videoannel bed (B) at 1 m, 2 m and 3 m distance down an experimental horizontal fixed-ed.

Table 2Summary of parameters used to determine flume experiment geometric properties and scaling of hydraulic conditions. Note that Re� and the Shields parameter are variablewithin a single experiment given the highly transient hydraulics, such as flow depth and flow velocity, for example. �Real world clast density is typical for most igneous ormetamorphic rocks.

Typical flume experiment value Parameter Associated typical real-world (field) value

0.001 Channel slope (m m�1) 0.0010.2 Channel width (m) 750.08 Typical flow depth (m) 5.00.016 Flow cross-sectional area (m2) 3750.36 Flow wetted perimeter (m) 850.04 Flow hydraulic radius (m) 4.41000 Fluid density (kg m�3) 11001200 Clast density (kg m�3) 2550�

0.01 Typical particle diameter D50 (m) 0.11.3 Typical flow velocity (m s�1) 70.00152 Fluid dynamic viscosity (Pa s) 0.001520.78 Bed shear stress (N m�2) 54140 Grain Reynolds number 15,0000.00038 Shields parameter 0.037

J.L. Carrivick et al. / Journal of Hydrology 408 (2011) 153–163 157

Emmett and Moodie, 2009; Xia et al., 2010). Furthermore, the geo-logical literature suggests coarse-grained clasts are commonlytransported within outburst floods and are a key feature of resul-tant deposits (e.g. Carrivick et al., 2004a,b, 2010; Carrivick,2007a,b; Russell et al., 2006; Manville and White, 2003).

Quartz gravel clasts, with a mean density of 1200 kg m�3 (Table2) were laid to in-fill the experimental channel (Fig. 1A) to a uni-form depth of 0.07–0.08 m. The gravel for an individual experimenteither comprised ‘very fine’ (median grain size, D50 = 0.005 m),‘fine’ (D50 = 0.01 m) or ‘medium’ (D50 = 0.015 m) mixes, each witha narrow uni-modal clast size distribution (Fig. 1C). At the scaleof our experiments these clast sizes represent 0.1, 0.2 and 0.3 mclast sizes, or ‘cobbles’ in reality (Table 2). For each experiment,the flume channel floor comprised of fixed gravel clasts of the samegrain size as that in-filling the channel. Channel floor gravel wasglued using epoxy resin to create an immobile fixed surface ofthe same roughness as the mobile gravel surface. Bed roughnessfor the very fine, fine and medium gravel mixes was calculatedto be Manning’s n = 0.034, 0.049 and 0.057 respectively. Thisroughness measurement was obtained via a separate set of exper-iments where water discharge was iteratively varied until flowdepth and flow velocity were constant at a point through time;i.e. roughness was measured in steady ‘equilibrium’ flows overthe same fixed bed. Gravel within the channel was initially wet,but not flooded.

Our experimental flows lasted for ca. 30 s but only the first 10 sare reported in Fig. 2 for this is when the most rapid and significantdynamical changes occur. Reservoir (lock-box) water level andhence water volume was measured through time using a side-looking video camera. Reservoir discharge was both calculatedand numerically modelled from this volume data. Flow front ad-vance was measured with an up-channel looking high-speed50 fps video camera and by comparing the position of the frontalflow lobe to markings every 10 cm on the channel sides. Flowdepth (Fig. 2A) was measured via three side-looking 50 fps videocameras placed at 1 m, 2 m and 3 m channel distance. These samethree cameras recorded the onset, duration and character of bederosion and/or deposition. These three cameras also permittedsemi-quantification of sediment transport duration; i.e. the exper-imental time during which channel bed clasts were observed to bemobile, semi-quantitative data on the height of entrainment ofclasts into the flow, and visual qualitative observations of flowfront shape, flow aeration and mode of sediment transport. Instan-taneous flow velocity (Fig. 2B) was measured at a single point0.01 m above the bed in the center of the channel cross-sectionand at the same channel distances used for flow depth observa-tions, using 2000 Hz ultrasonic velocimeter profiling (UVP) sen-sors. Due to the location of these UVP sensors in the flow we

consider them to be measure bed shear velocity but we cautionthat the bed surface elevation beneath the UVP sensor changedslightly through an experiment, typically by 0.005 m. UVPs re-corded instantaneous (bed shear) velocity at 0.05 s intervals forthe duration of the flood. UVP data were smoothed using a movingaverage to derive a mean velocity and to derive turbulence inten-sity, I:

I ¼ u0

U

where I is the ratio of (root mean square) velocity fluctuations u0, tothe mean (Reynolds-averaged) velocity. Bed shear velocity mea-surements and turbulence intensity calculations are thus only fora single point near the center of a channel cross section; they arenot a cross-section average.

Pre-experiment and post-experiment bed elevation was mea-sured at high spatial resolution (3–4 mm or better) using a RieglLMS Z420i terrestrial laser scanner (TLS). Multiple scan sequenceswere taken from each end of the flume producing �15 million sur-face elevation point measurements for each experiment. These‘point clouds’ were merged and smoothed to average elevation val-ues for the same point and filtered using a standard octree filter. Inan octree filter the scan volume is divided into cubes; in this casewith a resolution of 0.0015 � 0.0015 � 0.0015 m. For each cube, allpoints within the cube were averaged and replaced with a singlepoint corresponding to the mean of the original points. This proce-dure drastically reduced the amount of data without losing signif-icant resolution at scales above the cube size. It also served toreduce noise and to eliminate very short wavelength irregularitiesin the topography caused by the shape of individual clasts. Finally,these points were interpolated using inverse distance weightingonto a regular grid with cell size 0.005 m � 0.005 m. The grid sur-face resolution is thus equal to the mean grain size of our ‘very fine’gravel. Vertical accuracy of the grid surfaces was ±0.001 m. Pre andpost-experiment bed elevation and bed elevation change mapswere created and analyzed quantitatively using grid-basedanalyses.

4. Results

As hypothesized, gravel grain size and hence bed mobility af-fected flow hydraulics. Flow depth within the main body of theflow; i.e. excluding the flow front, was�50% larger if medium grav-el was mobile versus being fixed (Fig. 2A). Flow velocity within themain body of the flow was �50% smaller for the same scenario(Fig. 2B). Mobile sediment caused flow depths to decrease with dis-tance downstream, whereas with a fixed bed flow depths increasewith distance downstream (Fig. 2A). Mobile sediment caused flow

158 J.L. Carrivick et al. / Journal of Hydrology 408 (2011) 153–163

velocity to attenuate faster with time than for a fixed bed (Fig. 2B).Slower frontal advances were measured with mobile beds versusfixed beds and slower frontal advances were measured with in-creased clast size (Fig. 3A). Thus whether a bed was mobile or fixedwas found to be a stronger control than grain size on outburst floodhydraulics.

The flow front at time relative to inundation at that point tr = 0was a highly aerated lobe (Fig. 3B). Bed erosion was recognizedwithin the side-looking cameras and initiated immediately behindthe aerated lobe. Clasts were entrained into the flow column insmall clusters or bursts, and were transported either in suspensionor by saltation (Fig. 3B). The resultant bed morphology after 0.6 swas hummocky (Fig. 3B). Isolated clasts continued to move as bed-load and thus gaps between hummocks became filled in. Totaltransport duration increased down channel with very fine gravel,remained approximately constant with fine gravel and decreasedwith distance down channel with medium gravel (Table 3). Themaximum height to which sediment was entrained within the flowcolumn decreased with distance down channel, and with increas-ing clast size (Table 3).

In terms of post-experiment morphology, each experiment pro-duced a scour hole of 0.04–0.07 m depth that extended to 0.7 mdown-channel (Fig. 4). This scour hole was nearly identical in planshape (Fig. 4A–C) and longitudinal profile shape (Fig. 4D) betweeneach experiment although the depth of scour varied with grainsize. Fig. 4D shows that volume change in the scour hole doesnot vary monotonically with grain size; it would be expected thatmost scour would occur with the smallest grain size. Indeed thereis therefore a clear pattern of the expected relationship betweenvolume of erosion/deposition and grain size being realized forchannel reaches with net aggradation; 0.7–2.2 m (Fig. 4D), andnot being realized for channel reaches with net erosion.

Beyond 1 m channel distance, the very fine gravel experimentproduced alternating erosion and deposition in zones of �0.4 mlength (Fig. 4A). In contrast, the medium gravel experiment pro-duced two clearly defined zones; one of proximal net bed aggrada-tion from 1.0 m to 2.5 m, and one of distal net bed erosion from2.5 m to 4.0 m (Fig. 4B). Overall, aggradation occurred up to0.04 m above the initial elevation (Fig. 4C) and deposition was vol-umetrically greatest with larger clast sizes (Fig. 4D). The longitudi-nal pattern of volumetric erosion and deposition was characterizedby peaks and troughs (Fig. 4D). If the peaks of an individual exper-iment are taken as a series of points, they decline in magnitudewith distance down-channel with a near-constant wavelength of�0.5 m, and an exponential decline in magnitude; r2 = 0.93 forthe curve illustrated in Fig. 4D. The point of maximum depositionshifted down-channel with increased clast size (Fig. 4D). All exper-iments produced a net loss of gravel (out of the end of the flume)and the magnitude of loss was inversely proportional to gravelclast size.

It is very important to examine transient geomorphologicalchanges because (i) these reflect energy transfers due to flow-bedinteractions within an outburst flood and (ii) because net geomor-phological change (e.g. Fig. 4) is a poor descriptor of total geomor-phological work; whilst considerable erosion and deposition canoccur at a given distance down channel the final bed elevationcould appear very similar to that of the pre-flood. Therefore we uti-lized our knowledge of frontal wave position (Fig. 3A) and sedimenttransport duration (Fig. 3B; Table 3) to calculate total sedimenttransport with time. Using a linear regression, we interpolatedthe duration of erosion observed at our at-a-point measurements(Fig. 3B; Table 3) along the channel (Fig. 5). We thus computed ero-sion rate at each 0.005 m increment of channel distance (Fig. 6A)and used this as an index of geomorphological power. The deposi-tional component is simply a bed aggradation rate (Fig. 6B). Bedaggradation rates for each experiment had very similar rising limb

gradients but these rates increased with increasing size of mobileclasts. Deposition rates with medium gravel continued to increaseup to 1.85 m channel distance, for fine gravel up to 1.3 m, and forvery fine gravel up to 0.8 m (Fig. 6B). After peak, deposition ratesdeclined for medium and fine gravel, but for very fine gravel depo-sition they remained approximately constant (Fig. 6B). A compari-son of water discharge; as determined from the volume of waterremaining in the lock box at a given time, versus sediment transportrate (Fig. 6A), clearly shows that all experiments increased in flowvolume; a phenomenon known as bulking, due to rapid sedimententrainment (Fig. 7). As expected, greatest flow bulking was evidentwith a mobile very fine gravel substrate, and bulking declined downchannel most rapidly with medium gravel (Fig. 7).

To consider transient forces and energy transfers and to under-stand the conditions at the lock gate immediately after release, weconduct a brief numerical analysis following the method of Xie(2010). This analysis solves the Navier–Stokes equations by theSIMPLE (Patankar, 1980) type algorithm in staggered grids usingthe PISO (Issa, 1985) algorithm to couple the pressure and velocityand a high resolution volume of fluid method (Ubbink, 1997) totrack the water surface. These analyses demonstrate that the pres-sure gradient immediately next to the lock gate declines exponen-tially from 420 J to 90 J over 4 s (Fig. 8). Momentum transfer due tomass exchange through the lock gate (Fig. 8) does not vary greatlyand thus has only a marginal influence on peak discharge. Rather,mobile sediment increases basal drag (e.g. Recking et al., 2008) andcauses kinetic energy to decline (Fig. 8) as geomorphological workprogresses, thereby retarding flow front propagation and flowvelocities and increasing water surface elevations and flow depths.Consequently we find that geomorphological power in our experi-mental outburst floods declines logarithmically (Fig. 6A) as fric-tional forces quickly overcame gravitational forces (Fig. 8). Usingthe rate of erosion as calculated at 1 m, 2 m and 3 m channel dis-tance (Fig. 6A) and reservoir discharge as determined from thewater level in the lock-box (Fig. 8), we estimate that sediment con-centration in our experiments reached 15–19% by volume for veryfine sediment, 10–15% for fine sediment and 4–8% for medium sed-iment. Sediment was entrained at the base of the flow where thewhole bed surface became instantly mobile (Fig. 3B).

5. Discussion

Experimental analogues of the (small) scale herein can neverreproduce real-world flows. We focus on dynamic similarity usingnatural fluids and particles to allow correct scaling of viscous andgravitational forces on a particle scale. This is permits an examina-tion of flow-bed interactions; i.e. of erosion and deposition pro-cesses in unsteady flow conditions. However, as a consequence ofthis focus bulk flow kinematics; the non-dimensional products offorces describing the kinematics of the outburst flood are not cor-rectly scaled to natural flows. Hence flow time-scales and pro-cesses of particle transport, the transport regimes themselves,time-scales of bedload transport and suspension are not scaled cor-rectly. Those results from this study therefore need to be inter-preted alongside other (field-based) data.

Experimental outburst floods over a mobile substrate have afrontal wave that is strongly aerated (Fig. 3B). This aeration isimportant because the rate of mass transfer in an outburst floodfront is proportional to the air–water interfacial area (Chanson,2004b). Aeration could thus be considered as a qualitative proxyfor erosive power. Our experiments indicate that the height of theaerated zone within the frontal wave is proportional to the flowdepth and that it reduces in depth gradually as the flood propagates(Fig. 3). This characteristic has also been observed for outburstfloods over fixed beds and was numerically proposed by Emmettand Moodie (2009). Wave depression; i.e. reduced wave heights

Fig. 3. Control on flow front propagation due to changing bed mobility as a function of clast size (A), and typical temporal evolution of flow front morphology (B). Exampledepicted in B is for fine gravel at 2 m channel distance. The solid black line denotes the initial bed elevation. The dashed black line delineates the free water surface and thewhite dashed line delineates the boundary between static (hence good image clarity) gravel clasts and actively mobile (hence blurred) gravel clasts. The upper boundary ofmobile bed sheets is indicated by the white dotted line, and single saltating (hence blurred) clasts are indicated by the white dotted circles.

Table 3Summary of sediment transport as measured from side-looking cameras. vf_1 m denoted results from an experiment with ‘very fine’ mobile gravel at 1 m channel distance.m_2 m denoted results from an experiment with ‘medium’ gravel at 2 m channel distance. Note that net bed elevation change does not reflect true geomorphic work achievedbecause both scour and infill can occur at-a-point.

Experiment vf_1 m vf_2 m vf_3 m f_1 m f_2 m f_3 m m_1 m m_2 m m_3 m

Transport duration (s) 3.68 3.24 2.64 3.40 2.64 2.20 3.24 2.56 1.56Maximum entrainment height into flow (cm) 5.2 3.5 3.1 4.7 2.2 2.1 4.3 2.2 1.5Net bed elevation change (cm) �1 0 0 �2 �2 �2 2 0 �1

J.L. Carrivick et al. / Journal of Hydrology 408 (2011) 153–163 159

(Fig. 2A), reflects increasing basal drag (e.g. Recking et al., 2008) andexpenditure of flow energy through geomorphological work.

Our experiments with fine sediment produced initial erosion(Fig. 4D) without significantly affecting flow hydraulics (Fig. 2Aand B). Whilst the fact that initially outburst floods have an iner-tia-dominated flow regime could partly explain this, it is nonethe-less surprising because our experiments were not supply-limitedvolumetrically and probably had potential to reach a hyperconcen-trated state. It appears that without sufficient time to reach capacity,outburst floods with predominant suspended sediment transportrapidly produce alternate zones of erosion and deposition

(Fig. 4D). A similar alternating pattern of erosion and depositionhas been documented by Procter et al. (2010) for the outburst floodthat occurred in 2007 at Mt. Ruapehu. We interpret the pattern ofalternate erosion and deposition in our experiments to indicate a‘dynamic equilibrium’ flow regime. We use this term to describe thatflow competence is sufficient to entrain voluminous sediment butthat rapidly exceeds the transport capacity of the flow and sedimentis deposited. The threshold at which capacity is reached declines asflow energy is absorbed by channel roughness and by geomorpho-logical work; i.e. erosion (Fig. 4D). Sediment concentration withinan outburst flood is thus highly variable in space and time.

Fig. 4. Control of clast size on patterns of erosion and/or deposition for clast size ‘very fine’ (A), ‘fine’ (B), and ‘medium’ (C). The pattern of erosion and deposition downchannel has peaks and troughs reflecting a near-constant wavelength and an exponential decline in magnitude (D).

Fig. 5. Sediment transport duration measured semi-quantitatively using side-looking high-speed video cameras at 1 m, 2 m and 3 m channel distance, andinterpolated linearly with distance down channel.

Fig. 6. Control of clast size on geomorphological power, which is defined ascumulative erosional work done/cumulative experimental time (A), and bedaggradation rate, which is defined as cumulative deposition/cumulative experimenttime (B).

160 J.L. Carrivick et al. / Journal of Hydrology 408 (2011) 153–163

We find that the exact influence of channel substrate clast sizeon outburst flood hydraulics is not absolute but rather is relative tothe magnitude of the initial hydrograph; it is stage dependent andcould be considered as a bed roughness parameter. Correspond-ingly, our experiments do not produce an initial scour hole orchannel bed erosion and deposition that are proportional to thegrain size of mobile clasts (Fig. 4) and we interpret this fact asindicative of a control of substrate micro-morphology on sedimenttransport in outburst floods. It can be considered that very finegravel has a relatively smooth bed surface because very fine gravelclasts pack closely together. In contrast, coarse gravel clasts do notpack closely, have many void spaces and clasts protruding into theflow; they present a rough surface (Hodge et al., 2009). A similarmicro-morphological control of pre-existing joints and voids hasbeen identified in the field by Lamb and Fonstad (2010) in the effi-cacy of erosion of bedrock by outburst floods.

Where bedload transport was predominant in our experimentsfree surface profiles (water depth), flow velocity and therefore theflood hydrograph were apparently modified by the grain size of

mobile clasts (Fig. 2A and B). These results suggest that it is the rel-ative mobility of the channel substrate that modified the flood hyd-rograph. These results are in direct agreement with the numericalwork by Cao et al. (2004) for a ‘rough’ or ‘complex’ mobile bedand also with the work by Fagents and Baloga (2005) for flows over‘complex topography’. Cao et al. (2006) also consider bed erodibilityto play a paramount role in dictating longitudinal evolution of peakdischarge.

The time-scales of the experiments reported herein do not scalewell with natural flows; such scaling is hard to achieve using real

Fig. 7. Flow bulking evident at 1 m, 2 m, and 3 m channel distance for outburst floods over mobile substrate of varying gravel clast size. Derivation of water discharge andsediment transport rate is given in the text.

Fig. 8. Numerical model results of the variation in time of energy and transfer ofwater mass at the dam, i.e. at the lock gate at zero channel distance.

J.L. Carrivick et al. / Journal of Hydrology 408 (2011) 153–163 161

gravel and water. Time scales are important for longitudinal andvertical accretion of sediments. Never-the-less we feel it is impor-tant to outline the pattern and style of deposition observed andmeasured in our experiments. Our (semi-quantitative) videoobservations (Fig. 3B) indicate that deposition ensued as flow

Fig. 9. Summary of transient dam break outburst flood hydraulics and sediment transporunimodal grain size distribution and at 2 m channel distance from the lock gate. The marespectively, is indicative only, as gained from side-looking high-speed video cameras.

depth and discharge were still increasing. Our video observationsalso indicate that suspended load diminished immediately follow-ing peak flow depth (Figs. 3B and 9). Bedform development wasminimal due to rapid loss of flow competence and negligible timefor longitudinal or vertical accretion (Rushmer, 2007). As flowcapacity diminished, erosion and bedload transport became re-stricted to discrete longitudinal zones (Fig. 4A). The fact that peaksand troughs in longitudinal volume changes are described by anexponential curve (Fig. 4D), which we suggest indicates that sedi-ment ‘waves’ at the base of outburst floods proceed via both trans-lation and dispersion mechanisms. This suggestion of sedimentwaves requires more thorough investigation to confirm its pres-ence and formative mechanisms. The fact that bedload transportpersisted for a longer time than suspended sediment transport(Figs. 3B and 9) was unexpected, but we attribute this to an ab-sence of fine-grained material; the gravel was essentially of auni-modal grain size distribution, and thus the flows subsequentlybecame incapable of suspending individual clasts.

Sediment transport within our experimental outburst floodswas dominated by bedload, and vertically within the flow columnthis bedload was restricted to a region overlain by a saltating zoneand covered by a turbulent flow suspending finer sediments(Fig. 3B). This flow stratification has been theorized from field databy Pierson (2005) and numerically considered by Cao et al. (2006).Cao et al. (2006) suggested that large-scale bed scour occursmainly during the rising phase of the flood via dynamic interplay

t. This is example is for experimental floods over ‘fine’ mobile sediment with a near-gnitude of suspended load and bedload transport; light and dark gray shaded areas

162 J.L. Carrivick et al. / Journal of Hydrology 408 (2011) 153–163

of granular and fluid motion. In our experiments we envisage thatthis interplay developed from intense transport capacity due torapidly-rising stage and due to an initially abundant supply of sed-iment. Hence both suspended and bedload transport rapidly devel-oped (Fig. 9) immediately behind the aerated wave front (Fig. 3B).Overall and in summation, our observations and measurements ofexperimental outburst floods (with unimodal grain size mobilechannel sediment) can be summarized as in Fig. 9 to emphasizethe transient nature of major hydraulic and sediment transportproperties (Fig. 9). Of primary interest is that peak suspended loadoccurs coincident with peak flow velocity, peak bedload occurscoincident with peak flow depth, and peak sediment discharge lagsbehind peak water discharge (Fig. 9).

Finally, we emphasize that for outburst floods; (i) simple stage-discharge relationships are invalid due to intense bed scour and in-fill, (ii) velocity varies non-linearly with discharge, and (iii) water–sediment discharge relationships are dynamic due to rapid bulkingand dilution (e.g. Fagents and Baloga, 2006). Feedbacks exist be-tween substrate clast size and clast size distribution (Xia et al.,2010), mode of transport, flow density, flow energy, and thus tran-sient hydraulics and geomorphological work. The low predictionaccuracy of final bed elevations that Xia et al. (2010) and Abderrez-zak et al. (2008) found in their models illustrates that more work isneeded to understand unsteady sediment transport processes, (i)in super-critical flow and transcritical flow, and (ii) where boththe magnitude and rate of change of bed elevation changes is sim-ilar to that of flow depths.

6. Conclusions

This study measured longitudinal and temporal variations inhydraulics and sediment transport within experimental outburstfloods over a mobile channel substrate. It quantified the verticaland horizontal patterns of bed elevation changes resultant fromthese flows. It thereby examined flow-bed interactions, which arefrequently neglected or grossly simplified within geologically-based reconstructions of outburst floods and within numericalmodels of outburst floods. Specifically, we tested the control ofchannel clast size on both flood character and on geomorphologicalwork. Our key findings are that: (i) frontal wave propagation wasslower and attenuated faster for flows over a mobile bed and forincreasing size of mobile clasts, (ii) peak flow depth was greaterand was attained earlier for flows over a mobile bed and withincreasing clast size if the predominant transport mechanismwas bedload, and (iii) the quantity and pattern of erosion anddeposition was controlled by the mode of sediment transport.The mode of sediment transport, which was predominantly bed-load, was determined by flow hydraulics and sediment grain size,and probably also by clast shape and packing. Furthermore, our re-sults identified magnitudes and rates of energy transfers and hencegeomorphological work. Flows with a predominantly suspendedload reached a dynamic equilibrium of sediment transport as re-flected by longitudinal zones of erosion and deposition alternatingat near-constant frequency. The associated volume change in bedsediment with distance down channel was described by an expo-nential curve. In contrast, flows with predominant bedload trans-port produced re-worked surfaces with net aggradation inproximal zones and net erosion in distal zones. The results of thisstudy should be useful for understanding channel adjustments inresponse to rapid-onset water and sediment flows, for understand-ing impacts and hazards of outburst floods, for provision of exper-imental data to numerical modelling studies, and for informing onthe immediate and longer-term geological legacy of outburstfloods. Future work should systematically examine super-criticalto sub-critical flow transitions over a mobile bed, the influence ofmobile bed clast size distribution and pre-existing suspended sed-

iment concentration, and the controls of channel gradient andchannel bank expansion/contractions on dam break outburst floodhydraulics.

Acknowledgements

This paper is a product of a project funded by a NERC NewInvestigator’s Research Grant [NE/F000235/1] to JLC. Zhihua Xiecreated the VOF numerical model for calculating the energy andmass fluxes reported in Fig. 7. Paul Carling and Gert Lube are boththanked for their extremely insightful and helpful reviews. VernManville, Lucy Rushmer, Kate Staines and Fiona Tweed are thankedfor their comments on an earlier version of this manuscript.

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