Three‐dimensional submerged wall jets and their transition to density flows: Morphodynamics and implications for the depositional record

Abstract

Jets that expand from an orifice into an ambient water body represent a basic flow model for depositional environments related to expanding flows. Momentum-dominated jets evolve into gravity-dominated density flows. To understand this transition and its sedimentological relevance, three-dimensional tank experiments with submerged wall jets were conducted, systematically varying parameters such as the initial density difference, bed slope, grain size and sediment supply. Bedform successions could be subdivided into those related to the jet and those related to the density flow. Jet deposits included early-stage bedforms, scours and mouth bars. Early-stage bedforms are asymmetrical dunes that spread concentrically from the orifice. Sediment entrainment by eddies from the expanding jets led to the formation of scours and mouth bars. Flows with lesser initial density difference produced more elongate scours. Conversely, scours became deeper for denser incoming flows. Coarser-grained sediment caused the formation of higher and steeper mouth bars and vice versa. The transition from momentum-dominated jets to gravity-dominated density flows occurred approximately at the mouth-bar crest. Hydraulic jumps were absent in the expanding jet regions and at the transitions to density flows. Instead, complex flow patterns and circulations were inferred from the velocity measurements within the scour and at the mouth-bar crests. Bedform trains related to the density flow were controlled by the grain size and sediment supply. Coarse-grained sediment and high supply rates caused strong mouth-bar aggradation and flow splitting, leading to the formation of bedform trains laterally adjacent to the mouth bar. Finegrained sediment and low supply rates caused the formation of bedform trains downflow of the mouth bar. The symmetrical bedforms deposited by the density flows always displayed an in-phase relationship with the flow, indicating that they were antidunes. The experimental jet deposits resemble successions known from subaqueous ice-contact fans and deep water channel-lobe transition zones.

Submarine Channel Mouth Settings: Processes, Geomorphology, and Deposits

“…The sediment built a channel extending beyond the fixed channel, and this widened over a distance of a metre from ~20 cm (the fixed channel width) to ~36 cm; this channel was filled with antidunes. Downstream of this widening constructional channel mouth is a lobe similarly covered in antidunes in proximal areas, and asymmetrical inphase bedforms in distal parts, interpreted as supercritical dunes (Fedele et al, 2016;Lang et al, 2021). Lang et al ( 2021) used a 3component velocity measurement technique, and observed no lowering of the velocity maximum, even though the flow expanded laterally by ~8.9 °in the initial metre, and presumably continued with a similar expansion rate.…”

“…Process studies on channel mouth settings have been predominantly undertaken using physical modelling (e.g., Pohl et al, 2019;Spychala et al, 2020;Lang et al, 2021). In addition, there is a broader body of experimental work on density flows crossing slope breaks (e.g., García and Parker, 1989;García, 1993;Gray et al, 2005;Gray et al, 2006;Pohl et al, 2020), which can provide insights into channel mouth processes.…”

“…The experiments, however, differ in their parameters relative to those of Pohl et al (2019). The Lang et al (2021) flows were supercritical throughout the measurement section, were depositional (several cm hr −1 ), and were mixed saline-particulate suspensions (excess density 50 kg m −3 1/3rd sediment, 2/3rd saline). The sediment built a channel extending beyond the fixed channel, and this widened over a distance of a metre from ~20 cm (the fixed channel width) to ~36 cm; this channel was filled with antidunes.…”

“…Many of these are dependent on the criticality of the flow, and the presence or absence of a pronounced break of slope. Supercritical flow on a relatively smooth slope produced channel mouth widening populated by a range of supercritical bedforms (i.e., a CMEZ) (Lang et al, 2021). Supercritical to subcritical transitions across a slope break may be associated with the production of a single hydraulic jump, as in plunge pools, if the incoming flow has a high Froude number (e.g., García, 1993) and the slope break angle is high (e.g., >4 °; Lee et al, 2002).…”

Deposit‐Derived Block‐and‐Ash Flows: The Hazard Posed by Perched Temporary Tephra Accumulations on Volcanoes; 2018 Fuego Disaster, Guatemala

“…However, as the Las Lajas block‐and‐ash flow waxed and the aggrading deposits gradually filled the alluvial channel, it increasingly spilled out, splaying laterally across the golf course and, 2 km downcurrent of this, overtopped interfluves into local drainages, most notably on the outside of a marked bend where, as flow conditions waxed, a significant stripped part of the current escaped southwards into a minor catchment (Figure 3) where it partially buried and then battered the village of San Miguel. Where this flow‐stripping and overflow event occurred, there is evidence for marked erosional scour, as in crevasse‐splay events of turbidity currents (Lang et al., 2021; Piper et al., 2007). Where the current overtopped the interfluve, standing trees were felled and soil was eroded, leaving longitudinal furrows and partly excavated large alluvial boulders with stoss‐side crescent‐scours and lee‐side gravel tails (Figure 3c; Allen, 1984).…”