The response of high density turbidity currents and their deposits to an abrupt channel termination at a slope break: Implications for channel–lobe transition zones

The transition between the slope and basin floor is typically marked by a slope break, in some cases causing channels to terminate and turbidity currents to undergo a loss of confinement. It is thus essential to understand how these slope breaks and losses of confinement influence the hydrodynamic evolution of turbidity currents and impact their depositional variability within natural scale channel mouth settings. Flume experiments, utilizing Shields scaling, are conducted to study how channel slope angle (3°, 6° and 9°) and initial suspended sediment concentrations (12 to 18% by volume) impact the hydrodynamics and deposit geometries of high density turbidity currents, subject to a simultaneous break of slope and loss of confinement. Measured velocity and concentration profiles indicate that turbidity currents are supercritical, with mean velocities between 0.80 m s−1 and 1.04 m s−1 and depth‐averaged basal concentrations between 9.2% and 23.9%, yielding bed shear velocities between 0.050 m s−1 and 0.064 m s−1. Upon encountering the slope break and loss of confinement, turbidity currents exhibit increases to their densimetric Froude numbers and shear velocities. This is due primarily to two factors: firstly, turbidity currents continue to accelerate during an initial period of velocity lag as their residual momentum gradually dissipates; and, secondly, expansion via flow relaxation collapses their structure towards the bed. The corresponding depositional geometries of these processes reveal that turbidity currents produce elongate channel–lobe transition zones that disconnect channel and basin deposits. The length to width ratios of channel–lobe transition zones decrease as the initial sediment concentrations of turbidity currents increase, while a reduction in the channel slope break angle reduces their length to width ratios. Corresponding, lobe elements are observed to increase in length, width and thickness with increasing initial sediment concentrations, while a reduction in channel slope break angle reduces their dimensions due to enhanced slope deposition.


INTRODUCTION
Turbidity currents (TCs) are particle-laden density flows that travel down slopes due to their excess density (i.e. due to suspended sediment particles combined with the interstitial fluid). These currents are capable of transporting large volumes of sediment (Stevenson et al., 2018;Talling et al., 2022), organic matter (Hage et al., 2020) and anthropogenically-derived particles, such as microplastics (Kane & Clare, 2019;Pohl et al., 2020a;Bell et al., 2021), into deep oceanic basins. When TCs occur on slopes that are sufficiently steep they begin to bypass their sediment load, and, if erosive, will incise channels into these slopes (Fildani et al., 2013;de Leeuw et al., 2016). These channels then serve as future sediment transport conduits enabling less erosive turbidity currents to also bypass slope regions (Stevenson et al., 2015) and deposit their suspended sediment in more distal, low-angle basin floor settings.
This switch to deposition takes place via a channel mouth that often coincides with areas in which TCs encounter topographic slope breaks (SB) , such as the mouth of the Rhone Canyon, where there is a slope reduction of 0.3° (Wynn et al., 2002). The recent study by Hodgson et al. (2022) identifies four key channel mouth settings: (i) channel mouth expansion zones dominated by supercritical turbidity currents; (ii) plunge pools that mark the base of steep slopes and are also created by supercritical turbidity currents; (iii) channel-lobe transition zones at shallower slope breaks characterized by hydraulic jump arrays within turbidity currents that have Froude numbers close to unity; and (iv) subcritical channel-lobe transition zones related to slope breaks and/or flow expansion.
Within channel mouth settings, TCs undergo a loss of confinement (LOC) and flow relaxation (Pohl et al., 2019), spreading out laterally as they are no longer supported within the confined channel. This lateral spreading has the effect of lowering the height of maximum velocity within TCs towards the bed, promoting sediment bypass and enhancing basal shear stresses at the flowbed interface. This results in the formation of scour regions that are prevalent features in both ancient (Macdonald et al., 2011a;Ito et al., 2014;Pemberton et al., 2016;Brooks et al., 2018bBrooks et al., , 2022Navarro & Arnott, 2020;Rohais et al., 2021) and modern (Wynn et al., 2002;Macdonald et al., 2011b;Carvajal et al., 2017;Maier et al., 2020) channel-lobe transition zones (CLTZs). The dimensions of these scour regions range between metre and kilometre scales (Hofstra et al., 2015) and are maintained by subsequent turbidity currents that can remain active for <0.2 Myr (Macdonald et al., 2011a).
On slopes with complex localized topography, severe SBs can occur more often as a result of the surface expression of complex subsurface processes , leading to the formation of: (i) ponded accommodation (Spychala et al., 2015); (ii) slopes with stepped profiles, such as mid-slope terraces (Brooks et al., 2018a); (iii) seafloor topography resulting from geological folds (Howlett et al., 2019) and exposed faults (Ge et al., 2017); (iv) active salt diapirism (Howlett et al., 2020; at the base of steep continental margins (Lee et al., 2002). Localized topographic variations are therefore important as they induce rapid shifts in the hydrodynamics of TCs that alter their depositional patterns on both local (Amy et al., 2007;Patel et al., 2021) and regional (Soutter et al., 2019) scales. It is clear from previous studies that channel slope breaks have a different overall impact on the hydrodynamics of TCs than a loss of confinement. As such, many past experimental studies have focused on these two key geometric characteristics separately. For example, slope breaks have been shown to induce internal hydraulic jumps as the two-dimensional TCs transfer from a sloping to horizontal channel bed (Garcia & Parker, 1989;Mulder & Alexander, 2001). Similarly, while slope breaks occurring between two differential sloping channel beds typically result in the thickening of the TC wall region, the corresponding densimetric Froude numbers for the TCs may remain within the supercritical regime downstream of the SB (Pohl et al., 2020a(Pohl et al., , 2022. By contrast, turbidity currents that undergo a LOC typically experience a two to three-dimensional transition in their hydrodynamic characteristics, primarily associated with a flow relaxation after the LOC that drives lateral expansion of the TCs into an unconfined basin (Pohl et al., 2019).
This current study utilizes the Shields scaling technique (e.g. de Leeuw et al., 2016Leeuw et al., , 2018Pohl et al., 2019Pohl et al., , 2020aFerguson et al., 2020;Miramontes et al., 2020;Spychala et al., 2020) to study the combined effect of SBs and LOCs on TCs that are capable of bypassing sediment. Previous studies that have combined SBs and LOCs have revealed that not all TCs decelerate immediately downstream of a simultaneous SB-LOC (Kostic & Parker, 2006). In some cases, the TCs remain within a supercritical flow regime (at least temporarily), meaning that they do not produce a stationary hydraulic jump immediately downstream of the SB-LOC (Alexander et al., 2008;Pohl et al., 2019Pohl et al., , 2020b. The current study investigates the roles that slope break angle at the simultaneous SB-LOC and the initial suspended sediment concentration have on the hydrodynamics of the TCs generated and their resulting deposition patterns. This will address some of the outstanding questions on the specific hydrodynamic conditions of TCs that form erosional and depositional features within abrupt simultaneous SB-LOC settings. Previous analogies have been drawn to the impact pools produced by snow avalanches (Lee et al., 2002) that are attributed to stationary internal hydraulic jumps (Dorrell et al., 2016;Guiastrennec-Faugas et al., 2021) or to flow relaxation due to loss of confinement.
Whether attached or detached from the slope, the deposits of channel mouth settings represent condensed stratigraphic intervals formed by the complex interplay of erosion, deposition and bypass (Gardner et al., 2003(Gardner et al., , 2008. When this stratigraphic complexity is coupled with the relatively poor preservation potential (Brooks et al., 2018b) of channel mouth settings, it can be difficult to establish the hydrodynamic processes that produced bypass and erosion when the zones were active. That said, the sediments found in both ancient and modern CLTZs do provide an opportunity to understand the depositional processes in zones of flow expansion. For example, Mutti (1974Mutti ( , 1977Mutti ( , 1985 and Mutti & Normark (1987) identified channel mouth bypass zones and bars within deep-water systems, deducing that the dynamics, magnitude and frequency of TCs are likely to alter the spatial extent and preserved sedimentology. More recently, the criticality of TCs (Postma et al., 2016), relative change in slope gradient (Van der Merwe et al., 2014;Pohl et al., 2020b), basin character (Garnder et al., 2008) and sediment granulometry and mineralogy (Navarro & Arnott, 2020) have all been shown to impact on the geometries of channel mouth deposits. In this regard, experimental studies are therefore highly beneficial to the fundamental understanding of these relationships because they permit both geometrical simplification of complex channelbasin systems and control over key parameters. This provides a means of assessing the relationships that exist between the hydrodynamics and deposits of TCs within channel mouth settings.
The current study presents results from carefully controlled laboratory experiments that study the influence of suspended sediment concentration and slope break angle on the hydrodynamics of TCs within a CLTZ that forms as the TCs encounter a simultaneous and abrupt SB-LOC. The study also assesses the geometrical properties of deposits produced prior to and after the TCs encounter the SB-LOC (i.e. within the sloping confined channel and on the horizontal unconfined basin floor, respectively).

Flume channel-basin setup
The experiments are conducted in a purposebuilt, Perspex-walled flume facility ( Fig. 1) consisting of a straight 3.20 m-long, 0.1 m-wide and 0.3 m-high confined and sloping channel section that exits into a larger 2 m-long, 2 m-wide and 1 m-deep unconfined basin. The basin incorporates a raised, horizontal bed with overall dimensions 1.70 m-long × 1.50 m-wide × 0.11 mhigh ( Fig. 1), providing a trough around its perimeter to minimize flow reflections. The confined sloping channel bathymetry is adjustable between an along-channel bed slope S 0 = 0 and 0.176 (i.e. slope angle = 0-10°) and transitions abruptly at the SB-LOC onto the horizontal bed within the basin. In the initial experimental configuration, the channel-basin flume facility is filled with freshwater (ρ 0 = 998 kg m −3 ) to a total water depth of 0.90 m corresponding to the fixed height of the syphon overflow in the basin that maintains the free surface elevation (Fig. 1). For the experiments reported in this study, each individual experimental run is conducted with initially sediment-free channel and basin bed conditions. The TCs are generated at the upstream end of the sloping channel section by a pumped water-sediment feed through an inlet manifold of dimensions 0.044 m-high and 0.089 m-wide (Fig. 1B). Three series of experiments (Series S1, S2 and S3) consider different combinations of confined channel slope angle (i.e. 3°, 6°and 9°) and the volumetric suspended sediment concentration c 0 [i.e. 0.12, 0.14, 0.16 and 0.18 (12-18%)] at the source (see Table 1 for details). These conditions are chosen to ensure that velocities within the TCs are sufficiently high for erosion and sedimentological bypass to occur downstream of the SB-LOC location within the basin (Fig. 1C) concentrations also mean that all experimental runs develop high-density turbidity currents (HDTCs) (Kuenen & Migliorini, 1950;Cartigny et al., 2013). This is in accord with measurements of natural HDTCs (Paull et al., 2018) that found basal volumetric concentrations to be >10%, meaning that sediment exchange between the HDTC and the bed within the scaled laboratory setting is likely to be governed by the same physical processes (i.e. hindered settling and grain-to-grain interactions) (Heerema et al., 2020).

Flow scaling and characterization
The current experimental study is designed to investigate the transition between net depletive, sediment bypassing and erosive HDTCs at an abrupt SB-LOC. Shields scaling (Shields, 1936) is used to ensure that both the Shields parameter τ Ã and the particle Reynolds number Re p of the laboratory turbidity currents reside within appropriate flow regimes as those reported in natural HDTC systems. These parameters represent the ratio of bed shear stress to gravitational forces acting on the sediment and the hydrodynamic condition at the bed boundary of the HDTC, respectively. When these scaling conditions are satisfied, the laboratory HDTCs are able to erode and maintain suspended sediment transport in the downslope direction, as found in natural bypassing slope systems (Fig. 2), for example within the Monterey Canyon, USA (Xu et al., 2014). The Shields parameter τ Ã and particle Reynolds number Re p are defined for the current study as: where ρ t ¼ ρ s Àρ w ð Þc b þ ρ w is the density of the HDTC, v is the kinematic viscosity of water, g is the gravitational acceleration, D 50 is the median grain size of the sediment within the flow and U Ã is the shear velocity. Concentration c b represents the time-averaged concentration in the near-bed region only (i.e. c b between z = 0 and z = H max , where H max is the layer thickness between the time average position of U max relative to the bed). In order to calculate shear velocity U Ã , this study assumes that a logarithmic velocity profile exists between the bed surface z = 0 and z = H max within each HDTC (e.g. van Rijn, 1993), such that: where κ is the von Kármán constant (= 0.41). It is noted that the value of v for the experimental flows is likely higher than that of the ambient water due to the addition of suspended sediment (Boyer et al., 2011). However, this effect occurs equally within HDTCs at both the laboratory (Pohl et al., 2020a,b) and field scales (Paull et al., 2018) and is therefore not taken into account. Within Shields scaling, the particle Reynolds number Re p can be categorized as either hydraulically smooth (Re p < 5), transitionally rough (5 < Re p < 70) or rough (Re p > 70), with most natural scale HDTCs residing in the smooth to transitionally rough regimes (Fig. 2). The corresponding Shields parameter τ* typically varies between 10 −1 to 10 0 , corresponding to flow conditions where a suspended sediment profile develops. Within the current experiments, the particle Reynolds numbers range between Re p = 6.66 to 12.38 (i.e. transitionally rough) and the corresponding Shields parameter ranges between τ* = 0.66 to 2.78. These experimental conditions thus reside within the same flow regimes as field measurements of natural TC systems (e.g. Xu et al., 2004;Azpiroz-Zabala et al., 2017;Zhang et al., 2018).
It is also important to define the densimetric Froude number Fr D of the HDTC at the source, and its spatial variation through the channelbasin system. This parameter defines the criticality of the HDTC flow regime, the evolution of which is crucial to understanding the hydrodynamic influence of the SB-LOC system. The densimetric Froude number Fr D is defined as follows: where U is the depth-averaged velocity in the HDTC, g 0 is the reduced gravitational acceleration defined as g 0 = g ρ t Àρ 0 ð Þ=ρ 0 ½ , with ρ t ¼ ρ s Àρ w ð Þc þ ρ w being the depth-averaged Turbidity currents and their deposits at CLTZs 1169 HDTC density at the concentration syphon location downstream of the SB-LOC, ρ 0 = ρ w the density of the ambient fluid, c the layer averaged volumetric concentration and ρ s the sediment grain density. In Eq. 4, h max is defined as the time-averaged thickness of the HDTC layer, determined by the elevation at which current velocity U ¼ 0:5U max (Launder & Rodi, 1983;Gray et al., 2006). At the inlet manifold, the source velocity u 0 = 0.89 to 1.0 m s −1 and the current thickness h = h 0 = 0.044 m, with corresponding densimetric Froude numbers Fr 0 = u 0 /(g 0 0 h 0 ) 1/2 = 2.52 to 3.43 (i.e. supercritical flow regime, see Table 1). Corresponding Reynolds numbers at the inlet (i.e. Re = 4u 0 h 0 /ν) range from 1.5 × 10 5 to 1.8 × 10 5 and therefore reside in the fully turbulent flow regime.
The Rouse number ζ, which defines the shape of the suspended sediment concentration profile within the evolving turbidity current, is also calculated for the D 10 , D 50 and D 90 percentiles of the sediment grain-size distribution to assess the expected mode by which the initial sediment load within the TCs will be transported. The Rouse number ζ is given by: where w s is the settling velocity of each of the individual grain-size percentiles, calculated using an empirical equation for natural sand particle settling (Cheng, 1997). In this way, the settling velocities for the D 10 (95 μm), D 50 (168 μm) and D 90 (282 μm) grain sizes are calculated as w s = 0.005 m s −1 , 0.0147 m s −1 and 0.0311 m s −1 , respectively. It is also informative to consider the energy balance within the HDTC between the confined, sloping channel and the unconfined, horizontal basin by calculating the potential energy E p (Al Ja'Aidi et al., 2004) and kinetic energy E k per unit volume through the channel-basin system, as follows: Experimental procedure Prior to each experimental run, the channel bed is set at the appropriate slope break angle and the channel-basin facility is filled in with freshwater. A sediment-water mixture volume (ca 350 l) with the appropriate volumetric suspended sediment concentration (Table 1) is stirred vigorously by a motorized impeller within an external circular mixing tank for 30 min to ensure homogeneity. The sediment used in the study is a moderately well-sorted fine quartz sand (Redhill 110, see Appendix S1) with a specific grain density S s = ρ s /ρ 0 = 2.65 (where ρ s = 2650 kg m −3 is the sand grain density). The grain-size distribution for the sediment, measured by a laser particle size analyser (Malvern Mastersizer 2000; Malvern Panalytical, Malvern, UK), provides D 10 , D 50 and D 90 percentiles of 90 μm, 163 μm and 284 μm, respectively. The source volume flux inflow for the HDTCs is set constant at Q = 3.66 l s −1 (AE0.15 l s −1 ) for each individual experimental run (see Appendix S1). This inflow is generated from the water-sediment mixing tank via a centrifugal pump, with flow rate controlled by a ball valve, prior to delivery via a short supply pipe to the inlet manifold. In all experimental runs, inflow conditions are maintained over a 50 s run duration for each HDTC, with the discharge being monitored in the supply pipe by a magnetic flow meter (Siemens Mag6000, accuracy AE0.4% of flow rate, see Appendix S1 for discharge plots; Siemens, Munich, Germany).
Four ultrasonic velocimeter doppler profiler (UVP) probes (Met-Flow, UVP DUO, 2 MHz; Met-Flow SA, Lausanne, Switzerland) are positioned along the channel and basin centreline ( Fig. 1) to record the evolving flow structure of the HDTCs throughout each experimental run (see Appendix S1 for UVP measurement parameters). To sample the main body region of the HDTCs, a data measurement window for the current is set such that the UVP probes are initiated 5 s after the passing of the HDTC head. The velocity measurements end 5 s before the termination of the sediment-water mixture at the inlet manifold and, thus, prior to the HDTC tail. The UVP probes are orientated at θ = 50°relative to the local bed slope and positioned at 2.90 m (UVP 1), 3.20 m (UVP 2), 3.50 m (UVP 3) and 3.80 m (UVP 4) downstream of the inlet manifold. As such, probes UVP 2 and UVP 3 are sited immediately upstream and downstream of the SB-LOC in the confined, sloping channel and unconfined, horizontal basin, respectively (see Fig. 1A). The UVP probes measure the velocity component U of the HDTCs along the probe measurement axis, with the along-channel, bed-parallel velocity u x then calculated from u x = U/cosθ. This assumes that 1D flow exists within the HDTCs along the channel-basin centreline, such that vertical and lateral flow velocity A number of repeat runs are conducted for specific experimental conditions (i.e. c 0 = 12-18% in Series S1, S2 and S3, see Table 1) to measure the vertical concentration structure of HDTCs within the bypass region, 0.4 m downstream of the SB-LOC (Fig. 1). Sediment concentration samples are syphoned at four elevations above the basin false bed (i.e. z = 1, 2, 4 and 8 cm in S1, and z = 2, 3, 5 and 9 cm in S2 and S3), with depth-averaged flow velocities from UVP3 used to adjust flow abstraction rates using peristaltic pumps to ensure that extraction velocities at the sampling pipe openings match the streamwise current flow velocities. Estimates of the vertical concentration profiles c z ð Þ for each HDTC are also made using the following exponential function (Pohl et al., 2020b): where z is the elevation above the bed surface and l 1 to l 3 are empirical fitting coefficients. Corresponding depth-averaged concentrations c are also determined over the whole turbidity current thickness (i.e. between z = 0 and z = h max ).

Quantitative analysis of sedimentary deposits
Upon completion of each experimental run, the channel-basin facility is slowly drained to facilitate quantitative measurements of the sedimentary deposits produced by each HDTC. Raw images of the basin deposits ( Fig. 3A) are reconstructed via a photogrammetry technique (Penna et al., 2019). This method uses a Pentax K-70 DSLR camera (Pentax, Tokyo, Japan) and an 18 to 50 mm (f4-5.6) lens, capturing over 200 images of each deposit from a 360°field of view. Images are then processed via Agisoft Photoscan and aligned to one another via 48 fixed targets on the basin walls ( Fig. 3A), with the central point for each target providing a geo-referenced position as Cartesian coordinates (x, y, z). The targets are calibrated using a Leica Disto (D110) laser distance measure (accuracy AE1.5 mm; Leica Microsystems, Wetzlar, Germany), with the origin (0, 0, 0) set in correspondence to the basin floor on the nearest right-hand corner (in the flow direction) of the facility (Fig. 1).  Table 1), (B) scaled mesh of deposit produced from a high-density point cloud, (C) contoured mesh produced within Rhinoceros 3D and converted to a grid of Cartesian coordinates (X, Y, Z) at a 2 mm resolution, (D) spatial grid plotted via MATLAB to produce colourmaps of depositional thickness.
Once aligned and scaled, dense point clouds are converted into triangular meshes (Fig. 3B) and subsequently into structured grids (Fig. 3C) using the Rhinoceros 3D software (McNeel et al., 2010). Each structured grid is then analysed using MATLAB to produce 3D contours and basin deposit maps (Fig. 3D). By contrast, the deposit thicknesses within the confined sloping channel are measured manually with a scale at along-channel increments of 0.10 m between the inlet manifold and the location of the SB-LOC.
Within the current study, each experimentally generated HDTC and its resulting channel and basin deposits are considered in isolation as individual events. However, they are likely to better represent a more protracted phase of sediment delivery into a deep-water depositional system, as has been considered in other experimental studies (e.g. De Leeuw et al., 2016Leeuw et al., , 2018Pohl et al., 2019;Ferguson et al., 2020). The purpose of physical experiments is therefore not to attempt to replicate natural system depositional architecture directly but, instead, to provide a reference to the underlying physical conditions which form individual elements within these depositional systems. On this basis, cored samples are taken from the basin deposit centreline at 0.10 m intervals from 0.90 to 1.60 m downstream of the SB-LOC location (i.e. 4.10-4.90 m downstream of the inlet), while another set of cores is taken at 0.10 m intervals in the lateral direction at the location of maximum deposit width [for Series S1, run R1.3 (c 0 = 16%, 9°slope), Table 1]. Samples are prepared for size analysis by removing the upper 5 to 10 mm of the sediment sample to ensure the analysis is representative of the depositional conditions from the body of each HDTC . Measurements of grain-size distributions from core samples are obtained using the laser particle size analyser and GRADISTAT (Blott & Pye, 2001) to assess the spatial variation of grading with reference to the initial sediment size distribution.

Turbidity current evolution between channel and basin
Initially, the controlled experimental inlet manifold and defined source conditions (Table 1) generate a well-developed, highly turbulent head region within the confined sloping channel. Immediately following this is a quasi-steady body region that remains stable and confined within the channel for the remaining duration of the test (Fig. 4A). On passing the SB-LOC location, the head (Fig. 4B) and body (Fig. 4C) of HDTCs spread radially and symmetrically into the horizontal basin, with lobe and cleft instabilities forming along the radially expanding and propagating front of all HDTCs (Fig. 4D). The head of the HDTCs also increases in height (i.e. due to billowing) during this radial expansion away from the SB-LOC ( Fig. 4D and E), while the subsequent body of each HDTC reduces significantly in thickness (i.e. thinning body) immediately downstream of the SB-LOC region ( Fig. 4C and F). The termination of the inflow conditions in each run causes the bodies of the HDTCs to diminish rapidly as any remaining residual momentum dissipates, resulting in a rapid reduction in current velocity and the mass deposition of the remaining suspended sediment from the current.
Representative time-averaged streamwise velocity profiles obtained from the body region of HDTCs ( Fig. 5A) demonstrate that increases in the volumetric sediment concentration introduced at the source (i.e. c 0 = 12% → 18% for runs R1.1 → R1.4, Table 1) results in an increase in the maximum velocities U max of the HDTCs at all UVP measurement locations. These velocity profiles show that the elevation H max at which the maximum velocity U max occurs within the HDTCs, relative to the bed, is at its highest when HDTCs are confined within the channel, prior to encountering the SB-LOC. Once downstream of the SB-LOC, this height H max initially reduces, as observed at UVP 3, before increasing again upon reaching UVP 4 (Fig. 5A). The HDTCs are also observed to achieve their largest velocities (i.e. highest U max values) downstream of the SB-LOC between UVP 2 and UVP 3 (Fig. 5A), after which they begin to decelerate, as observed at UVP 4. The overall height or thickness h max of the individual HDTCs (see Flow scaling and characterization section) are measured directly from the time-averaged UVP velocity profiles (Fig. 5A). These show that the height of each individual HDTC remains approximately constant in the confined channel between UVP1 and UVP2 (i.e. varying between h max = 0.074-0.113 m and 0.074-0.092 m at UVP1 and UVP2, respectively, Fig. 5A). By contrast, measurements of h max at UVP 3 and UVP 4 show that, once downstream of the SB-LOC, the overall thickness of the HDTCs is significantly reduced (i.e. h max = 0.056-0.068 m and 0.041-0.050 m, respectively). Figure 5B compares the representative timeaveraged velocity profiles for experimental runs with a fixed input volumetric sediment concentration of 18% with varying channel slope angles from 9°, to 6°and 3°(i.e. R1.4, R2.4 and R3.2, see Table 1). The systematic reduction in slope angle between the three runs results in a reduction of the maximum velocity U max achieved by HDTCs at UVP 3, from U max = 1.23 m s −1 (for a 9°slope break) to U max = 0.96 m s −1 (for a 3°s lope break). A similar trend is also observed at the other UVP locations. Sediment samples were syphoned at different elevations within the individual HDTCs to measure the suspended sediment concentration profile immediately downstream of the SB-LOC (see Fig. 6 and Appendix S1). These measurements indicate a general exponential decrease in concentrations with increasing elevation within the HDTCs, as expected (Eq. 8). The suspended sediment profiles also indicate a systematic reduction in measured concentrations at all elevations as the initial volumetric concentration of sediment introduced at the source is reduced (i.e. c 0 = 18% → 12%, Fig. 6A). Furthermore, there appears to be some evidence of a marginal showing the effect of increasing the initial volumetric concentration of suspended sediment c 0 (i.e. 12% → 18%) for runs with a fixed 9°channel slope angle (Series S1, Table 1). (B) Time-averaged velocity profiles of HDTCs at UVP 3 (i.e. 0.3 m downstream of SB-LOC), showing the effect of a channel slope reduction (i.e. 9°→ 3°) for runs with a fixed initial volumetric sediment concentration of 18%. reduction in the suspended sediment concentrations (for a given source volumetric concentration) as the channel slope angle is reduced from 9°to 3° (Fig. 6B), potentially due to increased sediment bypass on steeper channel slopes. Depth-averaged volumetric concentrations c measured over the full thickness of individual HDTC runs range between 4.0 to 9.4%, 4.0 to 9.5% and 3.9 to 8.3% for S1 (i.e. channel slope = 9°, Fig. 6A), S2 (6°) and S3 (3°) runs, respectively. Corresponding depth-averaged basal layer concentrations c b , calculated by the exponential fitting function (Eq. 8) between the bed surface and elevation of the velocity maximum U max within each TC, range between 9.2% to 23.6%, 8.4% to 21.7% and 9.2% to 19.6% for S1 (9°), S2 (6°) and S3 (3°) runs, respectively.
The prevailing flow characteristics of the HDTCs are illustrated by the downstream variations of the shear velocity U Ã (Eq. 3) ( Fig. 7A and B). These plots indicate that, for the HDTC run out from the confined channel into the unconfined basin for a fixed slope break angle at the SB-LOC (for example, 9°, Fig. 7A), the shear velocities at all UVP locations increase with increasing initial volumetric concentrations of suspended sediment (i.e. c 0 = 12% → 18%). Shear velocities are also observed to increase consistently between UVP 1 (i.e. within the confined channel) and UVP 3 (i.e. in the unconfined basin, downstream of the SB-LOC), where it reaches a maximum value (U Ã = 0.059-0.074 m s −1 , Fig. 7A) before decreasing again at UVP 4 (U Ã = 0.055-0.062 m s −1 , Fig. 7A). In addition, for HDTCs that have the same initial volumetric concentration of suspended sediment (for example, c 0 = 18%, Fig. 7B), a reduction in the channel slope break angle generally leads to a reduction in the shear velocity U Ã , particularly in the unconfined basin. This plot also shows the same general increasing trend in U Ã between UVP 1 and UVP 3, followed by a reduction at UVP 4, as before. Overall, the range of shear velocity values measured at each slope break condition range between U Ã = 0.050 to 0.074 m s −1 (i.e. channel slope = 9°); 0.049 to 0.066 m s −1 (i.e. slope = 6°); and 0.057 to 0.0629 m s −1 (i.e. slope = 3°) (see Appendix S1 for additional U Ã plots).
Variations in the densimetric Froude numbers Fr D from the same representative HDTCs as considered in Fig. 7A (i.e. c 0 = 12-18%, channel slope = 9°) indicate that their flow regimes remain supercritical (i.e. Fr D > 1) throughout the transition between the confined channel and basin floor (Fig. 7C). Indeed, Fr D values are shown to increase with downstream distance between UVP 1 and UVP 3 (i.e. Fr D = 2.15-2.55 and Fr D = 3.3-3.8, respectively), before levelling off or even reducing slightly at UVP 4 (i.e. Fr D = Fr D = 3.2-4.1). In general, the magnitude of Fr D values also have an inverse relationship with the initial concentration of suspended sediment c 0 , with lower initial concentrations (i.e. c 0 = 12%) typically resulting in HDTCs with the highest densimetric Froude numbers (see Fig. 7C). By contrast, reducing the showing the effect of increasing initial volumetric concentration of suspended sediment c 0 (i.e. 12% → 18%) for runs with a fixed 9°channel slope angle (Series S1, Table 1). (B) Suspended sediment concentration profiles, showing effect of a channel slope reduction (i.e. 9°→ 3°) for runs with a fixed initial volumetric sediment concentration of 18%. Solid lines show the best fit of concentration measurements to exponential fitting function (Eq. 8). Calculation of the Rouse number ζ for the D 10 , D 50 and D 90 percentiles of the input sediment grain-size distribution (Fig. 8) indicate that all ζ values for the D 50 and D 10 percentiles lie in the washload regime (i.e. ζ < 0.8) throughout the channel basin transition. By contrast, the equivalent ζ values for the coarse D 90 percentile lie either in the 100% or 50% suspended load regime (i.e. ζ = 0.8-1.2 or ζ = 1.2-2.5, respectively) (Fig. 8). Furthermore, Fig. 8A indicates that, at a fixed channel slope break (i.e. 9°), the HDTCs generated under the lowest initial volumetric sediment concentration (i.e. c 0 = 12%) have the highest Rouse numbers ζ, and vice versa. This plot also shows that, as the HDTCs transition between the channel and basin (i.e. UVP 1 → UVP 3), the ζ values decrease with increasing distance from the inlet, suggesting that HDTCs become more efficient in transporting their sediment loads in suspension. Similarly, the increase in ζ values at UVP 4 indicates a reduction in the efficiency for suspended load transport further into the basin, particularly for larger grain-size percentiles. Reducing the slope break angle (i.e. for fixed c 0 = 12%, Fig. 8B) has an inconsistent effect on ζ values within the confined channel (i.e. UVP 1 → UVP 2), but leads to an overall increase in ζ values within the unconfined basin. In this case, the ζ values for the coarse D 90 percentile largely reside in the 50% suspended load transport regime (i.e. ζ = 1.2-2.5, Fig. 8B).
The kinetic energy E k (Eq. 6) and potential energy E p (Eq. 7) for the same representative HDTCs (as considered previously) are both shown to increase at all UVP locations as the initial volumetric concentration of suspended sediment c 0 increases (i.e. c 0 = 12% → 18% for channel slope = 9°, Fig. 9A and 9C). In each of these runs, there is a consistent downstream increase in E k values from UVP 1 to the maximum E k value at UVP 3 (i.e. 0.30 m downstream of the SB-LOC). Further downstream at UVP 4 (i.e. 0.6 m from the SB-LOC), the corresponding E k values indicate a significant reduction, but remain higher than E k at UVP 2 (i.e. at the SB-LOC). In contrast, the potential energy E p decreases consistently in the downstream direction from UVP 1 and UVP 4, with a more pronounced reduction occurring as the HDTCs pass downstream of the SB-LOC (i.e. between UVP 2 and UVP 3, Fig. 9C). Reducing the channel slope angle from 9°to 3°results in a reduction in E k at all UVP positions (Fig. 9B). This also diminishes the downstream variability in E k values between UVP1 and UVP 4, which remain largely unchanged through the channel-basin transition at this lower slope break angle (i.e. slope = 3°, c 0 = 12%, Fig. 9B). In contrast, the reduction in the slope break angle is shown to have minimal effect on the potential energy E p at each UVP position, or the general reduction in E p values in the downstream direction (Fig. 9D).

Depositional trends
Deposition in sloping confined channel Observations and direct measurements of the deposit thickness from the channelized HDTCs along the length of the sloping, confined channel are shown in Fig. 10 for the three bed slope angles (i.e. 9°, 6°and 3°) and the four initial suspended sediment concentrations c 0 (i.e. c 0 = 12%, 14%, 16% and 18%) tested. All HDTCs deposit sediments within the channel, and results indicate that the deposit thickness increases as the channel slope decreases from 9° (  Fig. 10A) to 3° (Fig. 10C). There is no observable bedform development on the surficial deposits along the channel length and the bed profiles remain smooth in all runs. In close proximity to the inlet manifold, the generated HDTCs produce a zone of scour due to the high inlet velocities (i.e. u 0 , Table 1) and Reynolds numbers in this region. This initial scour length reduces as the channel slope angle reduces (i.e. slope = 9°-3°, Fig. 10A to C), as the high inflow velocities are sustained over a shorter downstream distances at lower slopes. At the largest channel slope angle of 9° (Fig. 10A), the majority of channel deposition from the HDTCs is observed to occur towards the end of the discharge period, and the overall deposit thicknesses increase marginally as the initial suspended sediment concentrations of HDTCs decreases (i.e. c 0 = 18% → 12%, Fig. 10A). These channel deposits increase in thickness with increasing distance from the end of the scour region, with the maximum thickness between 1.5 cm (i.e. c 0 = 18%) and 2.7 cm (i.e. c 0 = 14%) occurring at the SB-LOC. The channel deposit for the HDTC generated with c 0 = 12% deviates from this trend, in that the maximum deposit thickness of 3.30 cm occurs 1.60 m downstream of the inlet (i.e. halfway along the channel between the inlet and the SB-LOC).
Reducing the channel slope angle to 6°and 3°r esults in the channel deposits (Figs. 10B and C) produced by HDTCs aggrading steadily over the 50 s duration of each test. Along-channel deposit profiles recorded at the slope angle of 6° (Fig. 11B) indicate maximum thicknesses between 6.8 cm (for c 0 = 18%) and 7.7 cm (for c 0 = 14%) at a distance of 1.3 m downstream of the inlet manifold. Lowering the slope to 3°further increases the overall channel deposit (Fig. 10C), with maximum thicknesses between 9.9 cm (for c 0 = 12%) and 10.1 cm (for c 0 = 18%), also recorded 1.30 m downstream of the inlet manifold.
The measured depositional profiles along the confined channel can also be used to estimate the overall volume of sediment deposited by the HDTCs within the channel. This can then be compared with the total sediment volume introduced into the channel-basin system via the inlet manifold, obtained from the initial volumetric sediment concentration c 0 and the flow rate data (see Fig. 9. Downstream variations in the calculated kinetic energy E k (plots A and B) and potential energy E p (plots C and D) between UVP 1-UVP 4 locations showing: (i) the effect of varying initial volumetric sediment concentration c 0 for runs with a fixed 9°channel slope angle (plots A and C), and (ii) the effect of varying channel slope angle for runs with a fixed initial volumetric sediment concentration of 12% (plots B and D). Appendix S1). This provides a means of estimating how much of the initial sediment volume discharged during each HDTC bypasses the confined channel slope and is therefore deposited in the unconfined basin. For the channel slope angle of 9°, the overall volumes of sediment deposited in the channel decrease (i.e. 20% → 13.8% → 7.5% → 4.2%) as the initial volumetric concentration of suspended sediment increases (i.e. c 0 = 12% → 14% → 16% → 18%). Lowering the channel slope angle from 9°to 6°also results in an increase in the proportion of the discharged Fig. 10. Bed deposit profiles within the sloping, confined channel for (A) series S1 (i.e. channel slope = 9°; c 0 = 12-18%, sediment input that is deposited within the channel ranging between 44.5% → 31.2% (for c 0 = 12% → 18%). A further reduction in the channel slope from 6°to 3°again increases the volume of channel deposited sediment, with the proportion of the total sediment input ranging between 52.5% → 43% (for c 0 = 12% → 18%).

Deposition in horizontal unconfined basin
Upon exiting the channel at the SB-LOC, all HDTCs form two flanking levées that have a convex geometry and extend out into the basin from the channel edges (Fig. 3D). These levées flank a central low deposition region formed by the body region of each HDTC, representing the sediment bypass zone immediately downstream of the SB-LOC. Further into the basin, sedimentation from all HDTCs forms lobate geometries. These depositional features increase abruptly in thickness along the basin centreline and downstream of the sediment bypass zone. Flanking all lobate bodies is a thin sediment fringe dominated by current ripples (Fig. 3A).
The geometries of the basin sediment bypass zones and lobate deposits produced by HDTCs are analysed in terms of their geometrical lengths, widths, and the maximum bypass zone depths and lobe deposit thicknesses (Fig. 11). The length L L and width W L of a lobate deposit are measured with reference to the z = 0.015 m contour, whilst maximum thickness D L is based upon subtracting the deposit profile generated by the photogrammetry technique (see Quantitative analysis of sedimentary deposits section) from the initial basin bed condition. Similarly, the length L B , width W B and depth D B of the sediment bypass zone are measured with reference to the flanking crest elevation of the levée   Table 1 for details). Deposit thicknesses are shown to highlight the sediment bypass region, flanked by two convex depositional features, and the downstream depositional lobe element.  (Fig. 11).
Colour-coded 3D basin deposit maps (Fig. 12) indicate that the plan-form spatial extent and thickness or depth of each lobate body (L L , W L , D L ) and bypass zone (L B , W B , D B ) are dependent on the slope angle and initial suspended sediment concentration c 0 . The geometrical dimensions of these lobes and bypass regions, along with the lobe centroid positions (i.e. the downstream distance of the maximum deposit thickness from the SB-LOC), are detailed for each experimental run in Table 2. Basin lobe deposits for the same representative runs as considered previously (i.e. slope = 9°, c 0 = 12-18%, Fig. 12A to D) show that when the initial volumetric sediment concentration c 0 is increased, the resulting HDTCs produce lobate deposits that are longer, wider and thicker. The downstream location of the lobate deposit centroid also increases from 1.07 m → 1.26 m (for c 0 = 12% → 18%). Non-dimensional aspect ratios L L /W L and L L /D L for these lobate deposits are found to decrease as c 0 is increased between runs (for example, L L /W L = 2.92 → 2.39 and L L / D L = 37.17 → 27.96 for c 0 = 12% → 12%, Table 2). Reducing the channel slope break angle (i.e. 9°→ 3°) between runs with a fixed c 0 value (for example, c 0 = 18%) has a less consistent influence of the lobate deposit aspect ratios L L /W L : L L /D L , which vary from 2.39 : 27.96 (at 9°slope); 2.70 : 25.75 (at 6°slope, see Fig. 12E) and 2.91 : 28.16 (at 3°slope, Fig. 12F). Corresponding distances downstream of the SB-LOC at which the lobate deposit centroid occurs also typically reduce (i.e. shift towards the SB-LOC) as the channel slope break angle is reduced (see Table 2).
The geometric dimensions of the sediment bypass zones (i.e. L B , W B and D B , Fig. 11) produced by HDTCs immediately downstream of the SB-LOC are shown to decrease in length L B , while increasing in both width W B and depth D B (Table 2), as the initial sediment concentration c 0 is increased between runs. Values of the bypass aspect ratio L B /W B therefore decrease as c 0 is increased, such that L B /W B = 2.80 → 2.36 (for c 0 = 12% → 18% runs at 9°slope); 3.37 → 2.33 (c 0 = 12% → 18% at 6°slope) and 2.51 → 2.06 (c 0 = 12% → 18% at 3°slope) (see Table 2). Ratios of L B /D B also generally decrease as c 0 is increased, although this reduction is most apparent between c 0 = 12% → 14% under each channel slope break condition, with L B / D B = 42.0 → 30.8 (for 9°slope) and 54.0 → 31.3 (for 6°slope). At the lowest slope break angle of 3°, L B /D B = 52.0 → 25.4 (for c 0 = 12% → 18%). Indeed, the reduction in slope break angle has the overall effect of decreasing L B , W B and D B values in the bypass zones (Table 2). However, the corresponding aspect ratios L B /W B and L B /D B show no general trend with these changing slope break conditions (for runs with fixed c 0 values).

Deposit grain-size trends
It is also informative to consider the nature of any spatial variability in the grain-size distributions within the basin lobate deposits. As such, cores taken along the centreline axis of lobate deposits (A-A 1 , Fig. 13A) and across a perpendicular transect at the point of maximum width W L (B-B 1 , Fig. 13A). Figure 13B shows the measured grain-size distributions along A-A 1 , for run R1.3 (i.e. slope = 9°, c 0 = 16%, Table 1), indicating that the lobe deposit has a general fining in the downstream direction (A → A 1 ). The corresponding percentile grain sizes (D 10 : D 50 : D 90 ) along the A-A 1 axis decrease from 133 : 229 : 400 μm at 0.90 m to 121: 198: 326 μm at 1.6 m (i.e. downstream distances from the SB-LOC). Figure 13C shows the corresponding grainsize distributions across the lateral deposit axis B-B 1 , with symmetrical fining from the centreline to the margins of the lobe deposit. Corresponding D 10 : D 50 : D 90 values across this B-B 1 transect decrease from 138 : 226 : 365 μm at the lobate centreline (i.e. intersect with A-A 1 ) to an average of 109 : 188 : 311 μm at the lobe margins. Similar grain-size distributions measured in other experimental runs with different channel slope break angles and/or initial volumetric sediment concentrations display similar along-lobe and cross-lobe fining trends to those described above (see Appendix S1).

Flow properties of confined and unconfined high density turbidity currents
Combining the measurements of HDTC velocity and concentration profiles from all runs demonstrates that an increase in the initial volumetric sediment concentration c 0 increases both the maximum velocities throughout the channelbasin system (Fig. 5) and the measured volumetric concentrations (Fig. 6). Furthermore, increasing the c 0 value also reduces the overall flow thickness of the HDTCs and, specifically, the vertical elevation h max within the current at which velocity U ¼ 0:5U max (Fig. 5A). Similarly, a reduction in the channel slope break angle from 9°to 3°results in reduced flow velocities for HDTCs generated under equivalent initial volumetric sediment concentrations c 0 (Fig. 5B), as well as marginally reduced suspended sediment concentrations (Fig. 6B) downstream of the SB-LOC. As such, the flow dynamics of the confined and unconfined HDTCs appear to be largely driven by two physical mechanisms. Firstly, for the range of initial volumetric sediment concentrations tested in the study (i.e. c 0 = 12-18%), the currents generated from higher c 0 values generally achieve higher flow velocities at all UVP positions in the channel and basin (Fig. 5A) for any channel slope break condition. This is primarily due to increased suspended sediment concentrations within the turbidity currents (for example, Fig. 6A) and, hence, the larger excess densities driving the flows. It is acknowledged here, however, that further increase in the initial sediment concentration c 0 may reach a point where the turbidity currents can no longer maintain this additional sediment load in suspension (i.e. when the transport carrying capacity of the current is reached), and the excess sediment load will be deposited within the confined channel. Secondly, higher channel slope break angles also increase the volume of the released sediment that bypasses the channel into the basin, as demonstrated by the relative sediment volumes deposited along the confined channel at different slopes (see Deposition in sloping confined channel section, Fig. 10).
On entering the unconfined basin, the velocity profiles of the HDTCs at UVP 3 (Fig. 5A) indicate that currents initially continue to accelerate downstream of the SB-LOC, before decelerating further into the basin (as identified at UVP 4, Fig. 5A). In particular, the increase in velocity between UVP 2 and UVP 3 is attributed to a 'velocity lag' effect (Mulder & Alexander, 2001;Spychala et al., 2020;Bell et al., 2021), as the residual momentum of the turbidity currents gradually dissipates upon experiencing the reduction in slope angle occurring at the SB-LOC. This velocity lag coincides with a significant reduction in the elevation H max of the maximum velocity U ¼ U max and the overall HDTC layer height h max relative to the bed (Fig. 5A, UVP 2 → UVP 3). Flow relaxation downstream Table 2. Geometrical dimensions and aspect ratios of the sediment bypass regions (or channel-lobe transition zones -CLTZs) and basin lobe elements produced downstream of the SB-LOC within Series S1-S3 runs (see Table 1).

Bypass zone dimensions
Lobate deposit dimensions Run Initial conc.  of the SB-LOC is the driver of these height reductions as the HDTCs are no longer confined by the channel walls, which prevent the formation of lateral density gradients (Pohl et al., 2019). After the SB-LOC, these density gradients drive lateral expansion of the HDTCs in the downstream unconfined basin, resulting in reductions of both H max and h max . When combined with the velocity lag effect, this flow relaxation also explains the significant increase in the shear velocity U Ã of the HDTCs at UVP 3 ( Fig. 8), that corresponds with a switch from a depositional to a bypassing state in all runs, and hence the formation of a sediment bypass zone immediately downstream of the SB-LOC (Fig. 3D). Flow relaxation may also contribute to the velocity lag as the rapid reduction in h max (i.e. UVP 2 → UVP 3, Fig. 5A) also corresponds with the conversion of potential energy E p to kinetic energy E k downstream of the LOC (Fig. 9). This energy conversion within other gravity-driven flows (for example, saline density currents) undergoing a LOC has previously been termed the 'slumping phase' (Huppert & Simpson, 1980), from the initial stage of density current expansion in lock-exchange experiments (Inghilesi et al., 2018;Lombardi et al., 2018).
In the current study, the combination of flow relaxation and velocity lag results in supercritical densimetric Froude numbers Fr D for the HDTCs that increase significantly in response to the SB-LOC at the channel-basin transition, and remain strongly supercritical at UVP 4 (i.e. 0.6 m downstream of the SB-LOC) ( Fig. 7C and D). This is true for all HDTCs in this study, irrespective of the slope break severity (i.e. channel slope angle) or the initial volumetric sediment concentration c 0 that generates the HDTCs. This poses the question as to where the downstream transition from supercritical (Fr D > 1) to subcritical (Fr D < 1) flow regimes may occur within the horizontal basin. This is important as previous experimental studies (e.g. Garcia & Parker, 1989;Baas et al., 2004;Islam & Imran, 2010) have shown that sudden reductions in slope angle can induce internal hydraulic jumps, where turbidity showing the location of bed cores taken for grain-size distribution analysis along transects (B) A-A 1 (i.e. centreline axis of deposit) and (C) B-B 1 (i.e. lateral axis at maximum deposit width) for run R1.3 (i.e. channel slope = 9°, c 0 = 16%, Table 1). currents transition rapidly from a supercritical (Fr D > 1) to subcritical (Fr D < 1) regime. In such cases, hydraulic jumps have been observed to form immediately downstream of a slope break, with the turbidity currents often depletive in nature (i.e. non-erosive and continually depositing) and decelerating on approach to the slope break. The resulting internal hydraulic jumps cause the currents to rapidly lose kinetic energy due to energy dissipation, as they decelerate rapidly and thicken (Komar, 1971;Baas et al., 2004). As these studies have been for laterally constrained (i.e. 2D) channels, they have not considered the effects of lateral spreading of the current (Kostic & Parker, 2006) or the flow obstruction caused by sediment deposition. The latter effect may also induce a hydraulic jump through the interaction of the current with an adversely orientated topographic feature, resulting in rapid flow deceleration. As such, hydraulic jumps are not always synonymous with breaks in slope, particularly in 3D experiments and simulations. For example, Kostic & Parker (2006) found that the rapid deposition of sediment can prevent turbidity currents from going through a rapid supercritical to subcritical flow regime transition via a strong hydraulic jump. Alexander et al. (2008) also showed that dilute TCs (i.e. volumetric concentration of 1.21% and velocity of ca 0.10 m s −1 ) can temporarily accelerate downstream of a SB-LOC if sufficient momentum has been generated on a high angle slope (20°).
The potential presence of larger hydraulic jumps immediately downstream of channel mouths, such as those observed by Garcia & Parker (1989), Garcia (1993), Baas et al. (2004) and Islam & Imran (2010) is not being questioned here. However, it is proposed that not all turbidity currents will necessarily produce internal hydraulic jumps immediately downstream of a SB-LOC. In the current study, the most likely location of the internal hydraulic jump appears to be further into the horizontal basin, downstream of UVP 4, close to the location of maximum deposition depth within the basin lobe element. Unfortunately, this can only be inferred as no such features have been observed or measured directly within the UVP data. However, the velocity profiles at UVP 4 generally show the HDTCs to be decelerating and, together with a corresponding increase in the height H max of the maximum velocity U max , indicate a reduction in the bed shear velocity U Ã . Consequently, supercritical HDTCs will only undergo flow regime transitions (i.e. via an internal hydraulic jump) when sufficient change occurs in the HDTC hydrodynamics, for example, when the current encounters an adverse slope or when it is ponded by severe downstream topography, resulting in rapid flow deceleration (e.g. Pohl et al., 2020a;Soutter et al., 2021). This indicates that the distance downstream of a SB-LOC at which the hydraulic jump occurs will not only depend on an adjustment length scale to the new slope angle (SB) and/or any change to lateral confinement (LOC), but it is also affected by other topographic bed features (for example, lobes) that the current interacts with during this adjustment.

Deposits of channelized turbidity currents
The thin sediment layers deposited in the confined channel at the highest bed slope angle (i.e. 9°, Fig. 10A) indicate that, when sufficiently steep, the channel itself acts as a sediment bypass zone. Under this condition, the majority of the deposition along the channel from the HDTCs is observed to occur after the cessation of the water-sediment flow at the inlet manifold (i.e. during the diminishing turbidity current tail). The bed profile measurement along the confined channel also indicates that an inverse relationship exists between the channel deposit thickness and the initial suspended sediment concentration c 0 . This is expected as the HDTCs generated under the largest c 0 values at the channel inlet have sufficient residual momentum to transport the vast majority of the sediment load into the basin under this high channel slope condition. By contrast, a reduction in the channel slope break angle from 9°→ 6° (Fig. 10B) → 3° (Fig. 10C) results in an increasing proportion of the sediment load being deposited along the confined channel, with this deposition occurring continually throughout the full duration of each HDTC. One surprising aspect of these channel deposition profiles that is particularly evident at these lower slope breaks (i.e. 6°and 3°, Fig. 10B and C) is that they appear to be largely unaffected by initial sediment concentration c 0 . This suggests implicitly that a larger overall proportion of the sediment load contained in HDTCs formed from lower c 0 values (i.e. c 0 = 12%) is deposited in the channel compared with HDTCs formed on the same channel slope condition from higher c 0 values (i.e. c 0 = 18%). The along-channel deposition profiles indicate the formation of an adverse slope 0.5 to 1.0 m downstream of the inlet, with the largest deposit thicknesses, in all cases, occurring ca 1.30 m downstream of the inlet. This finding suggests that, on expansion away from the channel inlet, the HDTCs quickly become over capacity [with capacity defined as the maximum sediment concentration that the HDTC can transport in suspension (Kuenen & Sengupta, 1970)]. This is evident in the alongchannel sediment deposition profiles at the lower slope conditions where, after an initial adjustment length on leaving the inlet, there is a rapid increase in deposition thickness (between ca 0.6 m → ca 1.30 m from the inlet), followed by a more gradual reduction in deposit thickness towards the SB-LOC. This downstream tapering of the deposit profile towards the SB-LOC indicates that the HDTCs are no longer over capacity, with deposition instead dominated by competency driven processes [i.e. where competency refers to the maximum supportable grain size that the HDTC can transport in suspension (Kuenen & Sengupta, 1970)]. The gradual loss of competency therefore leads to deposition of the coarser grain sizes from the initial suspended sediment size distribution, supported by high Rouse numbers calculated in the confined channel for the D 90 percentile (Fig. 8) and evidenced by the lack of coarse sediment in core samples taken from the basin deposits (see Deposit grainsize trends section and Fig. 13).

Depositional features downstream of the SB-LOC
To aid the comparison of the deposits downstream of the SB-LOC to those of natural systems, the lobe hierarchy of Prélat et al. (2009) is considered. This scheme breaks down a lobe complex into four fundamental components ( Fig. 14): (i) beds, which are the depositional product of individual TC events; (ii) lobe elements, formed by the stacking of multiple beds and generally no more than a few kilometres in length and width, and a few metres in thickness (Prélat et al., 2009); (iii) lobes, produced by the stacking of one or more lobe elements fed from a single channel; and (iv) lobe complexes, produced as lobes switch via an upstream avulsion or channel migration, and stack or onlap onto older lobes (Ferguson et al., 2020). By definition, individual HDTCs within this study produce beds as the deposits are formed by singular events. However, these basin deposits and those of similar experiments have a strong resemblance to lobe elements (Prélat et al., 2009;Ferguson et al., 2020;Spychala et al., 2020). Such lobate deposits are therefore considered to be lobe elements (Fig. 14), while the corresponding sediment bypass zones are considered to be equivalent to lobe element scale channel-lobe transition zones (CLTZs).
Effect of experimental parameters on the geometry of lobe elements At a given slope break condition (i.e. 9°, Fig. 10A to D), the resulting basin lobe element geometries become longer, wider and thicker (i.e. increasing L L , W L and D L , Table 2) as the initial sediment concentration c 0 increases from 12 to 18%. Increases in these lobe dimensions are driven primarily by the increased volume of suspended sediment within the HDTCs for higher c 0 values that bypasses the confined channel and is deposited in the basin. For example, at the slope break angle of 9°, HDTCs generated with c 0 = 12% and 18% transported 80% and 95.82%, respectively, of the total sediment volume introduced into the channel-basin system (i.e. 0.0217 m 3 for c 0 = 12% and 0.032 m 3 for c 0 = 18%) into the basin. In this context, HDTCs generated under higher c 0 values clearly have a much larger volume of sediment that bypasses the confined channel and is thus available to develop the lobe elements downstream of the SB-LOC. The effect of reducing the slope break angle from 9°→ 6°→ 3°is that the resulting basin lobe element geometries in general become shorter, narrower and thinner (i.e. reducing L L , W L and D L , Table 2). This overall decrease in the size of basin lobe elements is attributed primarily to the increased volume of deposition that takes place within the confined channel as HDTCs become less able to bypass the initial volumetric sediment concentration c 0 beyond the SB-LOC. This is clearly demonstrated by the significant increase in channel deposition by HDTCs at lower bed slopes (for the same initial sediment concentration c 0 ). For example, for runs with c 0 = 18%, in-channel deposition from the HDTCs generated increases from 4.2% (at 9°slope) → 31.2% (at 6°slope) → 43.0% (at 3°slope), indicating that the bypassed sediment volume beyond the SB-LOC reduces from 95.8% → 68.8% → 57.0%, respectively.
The relatively low proportions of coarse sand within the grain-size analysis also alludes to the HDTCs depositing the coarsest sediment fractions on the confined channel slope. This is supported by the estimated Rouse numbers for the D 90 percentile of the initial suspended sediment mixture, which lie within the 50% suspended sediment regime at UVP 1 and UVP 2 locations (Fig. 8). It is interesting to note that the overall grading of all sediment samples obtained from the lobe deposits are considerably coarser than the grain-size distribution of the initial sediment mixture (i.e. D 10 : D 50 : D 90 = 90 : 163 : 284 μm). This indicates that a proportion of the finer sediment grain sizes completely bypass the whole channel-basin system and are deposited within the trough at the end of the basin section.
Comparison of lobe geometries produced in the current study with those developed in previous analogous experimental studies are shown in Table 3. These previous studies have generally indicated that an increase in the initial concentration c 0 (for a given channel-basin slope combination) tends to result in lobe elements that have a higher aspect ratio L L =W L , on account that denser HDTCs have greater runout distances. This runout distance (and hence L L =W L value) is also increased by increasing: (i) the channel slope angle (e.g. Baas et al., 2004; Table 3); and (ii) the basin floor angle on which the lobe deposits are formed (e.g. Spychala et al., 2020; Table 3). These previous findings are somewhat contradictory to the current study, where: (i) an inverse relationship exists between c 0 and the lobe deposit aspect ratio L L =W L (i.e. L L =W L generally reduces with increasing c 0 , at a fixed channel slope condition); and (ii) a reduction in the channel slope increases L L =W L (for a given c 0 value). Overall, the range of aspect ratios L L =W L = 2.39-3.16 obtained in the current study are found to be significantly higher than in Baas et al. (2004), where L L =W L = 1.10-2.09 for depletive turbidity currents that formed internal hydraulic jumps immediately downstream of the SB-LOC. These depletive currents are thus dynamically different to the supercritical HDTCs in the current study that have much longer run-out distances into the basin (and, hence, higher L L =W L values). By contrast, the lobe aspect ratios measured by Spychala et al. (2020) (i.e. L L =W L = 1.66-3.66, Table 3) are in more general agreement with the current study. Their study also indicates that a similar relationship exists between sediment bypass into the basin, the slope angle and the sediment concentration in the turbidity current (as discussed above), with higher discharge conditions also resulting in lobe deposits that are fully detached from the simulated channel-levée system. The experimental results in the current study also broadly agree with a range of natural scale lobe elements identified by Pettinga et al. (2018) and Saller et al. (2008) to have aspect ratios L L =W L between 0.61-10.73 (average = 2.59, n = 38) and 1.0-7.2 (average = 2.18, n = 18), respectively. Effect of experimental parameters on the geometry of CLTZs In all experimental runs, the sediment deposits within the sloping confined channel are fully detached from the lobe deposits in the horizontal basin by a sediment bypass region immediately downstream of the SB-LOC (Fig. 11), as representative of the channel-lobe transition zone (CLTZ). Analysis of the geometry of these CLTZs (Fig. 12) shows that an increase in the initial sediment concentration c 0 typically reduces the nondimensional aspect ratios L B /W B and L B /D B of the CLTZs (Table 2). For example, for runs conducted at the 9°channel slope condition, L B /W B and L B / D B decrease from 2.80 → 2.36 and 42.0 → 28.66, respectively, as c 0 = 12% → 18%. By contrast, the effect of reducing the channel slope angle from 9°→ 6°→ 3°corresponds to a gradual reduction in the CLTZ length L B produced by HDTCs with comparable initial sediment concentrations c 0 , while changes to W B and D B are less consistent (Table 2). It is also informative to consider how the aspect ratios L B =W B of the bypass zones produced in the current study (Table 2) compare with others from similar channel mouth settings . Based upon the results obtained, it is perceived that the channel mouth setting modelled in the current study has hydrodynamic characteristics analogous to both supercritical channellobe transition zones and plunge pools. Hodgson et al. (2022) note that both of these features are generated by supercritical flows and the bypass zone produced by the HDTCs of this study shares similar geometrical properties with the latter (Table 4). In general, the L B =W B values in the current study (i.e. 2.06-3.37, Table 2) coincide with the upper end of the equivalent L B =W B value range (i.e. 0.4-2.5, Table 4) observed for natural CLTZ systems, thus indicating some degree of accord across scales.
Overall, the CLTZ and basin lobe geometries are indicative of the complex relationship that exists between the HDTC dynamics (for example, flow velocities, shear velocities, Froude numbers and energy transitions) under different experimental conditions (for example, slope break angle and initial concentration c 0 ) and the relative proportions of the sediment load that: (i) deposit along the confined channel; and (ii) bypass the SB-LOC to be deposited in the basin lobe. Indeed, it can be concluded that the scaling complexities that exist between the HDTC flow dynamics and the resulting nondimensional aspect ratios for both the CLTZs (i.e. L B /W B ; L B /D B ) and the basin lobes (i.e. L L / W L ; L L /D L ) most likely arise from the counteracting effects of the channel slope break and loss of confinement. Specifically, the slope break (SB) clearly results in deceleration of the HDTC, flow thickening and a reduction in the shear velocity, while the loss of confinement (LOC) results in acceleration of the HDTC, a reduction in the flow thickness, and an increase in shear velocity. A combination of these effects at the SB-LOC may well at least partly account for the lack of clear scaling patterns within the resulting CLTZs and lobe deposits.

CONCLUSIONS
Scaled laboratory experiments are conducted to investigate how supercritical high density turbidity currents (HDTCs) and their deposits are influenced by an abrupt transition between a confined, sloping channel and an unconfined, horizontal, basin. The key focus of the study is to determine the response of turbidity current flow dynamics and deposition geometries within the channel-basin system to changes in the initial volumetric suspended sediment concentration and the slope break angle at a simultaneous loss of confinement (i.e. SB-LOC). This is relevant to understanding the hydrodynamic and depositional variability within deep water sedimentary systems, particularly those associated with channel mouth settings such as supercritical channellobe transition zones and plunge pools. Velocity and concentration profile measurements within the HDTCs demonstrate that an increase in the initial volumetric sediment concentration (for a particular slope break condition) results in higher along-channel flow velocities both in the laterally confined (2D) sloping channel and the unconfined (3D) horizontal basin. A reduction in the slope break angle at the simultaneous SB-LOC (for the same initial sediment concentration condition), by contrast, results in an overall decrease in the current velocity. Concentration measurements within the HDTCs immediately downstream of the SB-LOC also show a systematic increase with increasing initial sediment concentration, but only a marginal reduction as the slope break angle is reduced.
The hydrodynamics of the HDTCs in the channel-basin system are also characterized by downstream variations in the shear velocity, densimetric Froude number, and the potential and kinetic energy. Importantly, the HDTCs are shown to remain supercritical throughout the system, and exhibit an increase to their densimetric Froude number and shear velocity due to both flow relaxation and residual momentum immediately downstream of the SB-LOC. These flow relaxation and momentum effects are believed to be analogous to the slumping phase observed as 3D gravity-driven flows expand laterally following lock release into an unconfined basin, and coincide with the conversion of potential energy to kinetic energy. In the current study, the HDTCs experience a short period of velocity lag on encountering the SB-LOC, during which they continue to accelerate into the basin. The overall thickness of the HDTCs, along with the height within the current at which maximum velocities occur, also reduce significantly immediately downstream of the SB-LOC. The resulting increase in the bed shear velocity produces a zone of sediment bypass that is analogous to a channel-lobe transition zone (CLTZ) that detaches the channel and basin deposits. This study therefore has important implications for the hydrodynamic processes that govern the formation and preservation of stratigraphy within CLTZs, and affirms the need of further research to help link the hydrodynamic behaviour of HDTCs and the resulting channel and basin deposit geometries within these settings. The dimensions of the CLTZs produced by HDTCs are shown to be impacted by the channel slope break angle and the initial sediment concentration, with a reduction in the slope break resulting in CLTZs with reduced dimensions (i.e. length, width and depth). By contrast, an increase in the initial sediment concentration results in CLTZs that have a more circular geometry, as demonstrated by a reduction in the measured aspect ratio L B /W B for the sediment bypass regions.
Basin lobe elements that are fully detached from the channel slope deposits by the CLTZs, typically increase in size (i.e. length, width and thickness), with their centroid located further downstream of the SB-LOC, as the initial sediment concentration is increased (for a given slope break condition). Conversely, a reduction in the slope break angle (for a given initial sediment concentration) results in basin lobe elements that are typically shorter, narrower and thinner. This occurs as the sediment bypass efficiency of HDTCs is reduced at lower slope angles, meaning that an increased proportion of the sediment load is deposited within the confined channel, upstream of the SB-LOC. Grain-size analysis of cores obtained along the axial centreline of the basin lobe elements and across the lobe deposits at their maximum thicknesses, shows that general fining occurs both in the downstream direction and laterally (in both directions) away from the centreline deposit axis. The measured grain-size distributions also indicate that the coarsest sediment fraction (i.e. coarse sand) is largely absent from the lobe deposits, suggesting that it must be deposited in the confined channel, prior to the SB-LOC, as predicted by the Rouse numbers for the D 90 percentile of the sediment mixture.
In summary, the scaled experimental study provides a plausible explanation for the sediment bypass zones and basin lobe deposits produced by supercritical high density turbidity currents as they undergo a simultaneous loss of confinement and break of slope, highlighting the geometrical dependencies. Results indicate that these sedimentary features need not always be formed by internal hydraulic jumps, but may instead result from flow relaxation of the turbidity currents during the transition from confined sloping channels to unconfined basin floors. This transition is characterized by the continued acceleration of turbidity currents during a period of velocity lag, during which flow relaxation takes place, causing a reduction in current thickness and elevated shear velocities. Results therefore have important implications for the stratigraphy associated with rapid channel mouth transitions, such as supercritical channel-lobe transition zones and plunge pools, as this study indicates that these zones may be characterized by increased densimetric Froude numbers. Future work should focus on the longevity of these bypass features, and the relationship between the feeder channel and CLTZ geometry.