Geological record of marine tsunami backwash: The role of the hydraulic jump

Abstract

Tsunamis are marked by distinct phases of uprush during coastal inundation and backwash when tsunami water recedes. Especially in the case of a steep coastal profile, the return flow may operate in a Froude-supercritical regime, eroding the flooded area and transporting large volumes of sediment seawards. Important sediment accumulation occurs when the supercritical flow goes through a hydraulic jump where it becomes subcritical upon deceleration. An inferred example in coarse-grained, mixed carbonates from the Lower Pleistocene on Rhodes (Greece) is described, with offshore bars up to 10 m long with scour-and-fill structures and steep antidune stratification. In finer-grained sandy depositional systems such structures may be much longer, up to hundreds of metres. It is suggested here that, analogous to some turbidite beds, the apparent lack of structures or the presence of faint stratification that is common for graded sand layers within marine tsunamiites may in fact consist of extremely low-angle, landward-dipping backset-strata that formed under a landward-migrating hydraulic jump during the basinward retreat of tsunami water. Numerical simulations that focus on the internal stratification of backwash-generated offshore bars support this hypothesis. The recognition of such deposits in the sedimentary record enlarges the toolbox for assessing the past frequency of tsunamis in coastal areas.

Emplacement of massive deposits by sheet flow

“…These sequences are typical of storm and tsunami deposits, i.e. tempestites and tsunamites (Brenchley & Newall, ; Duke, ; Myrow & Southard, ; Benson et al ., ; Dawson & Smith, ; Gelfenbaum & Jaffe, ; Naruse et al ., ; Nakamura et al ., ; Perez‐Lopez & Perez‐Valera, ; Sugawara et al ., ; La Selle et al ., ; Fujino et al ., ; Slootman et al ., ) and turbidity current deposits, i.e. turbidites (Bouma, ; Parkash & Middleton, ; Mutti & Ricci Lucchi, ; Hiscott & Middleton, ; Piper & Kontopulos, ; Pirmez, ; Mutti et al ., ; Stow & Johansson, ; Piper & Normark, ; Babonneau et al ., ; Bourget et al ., ; Mas et al ., ; Zhang et al ., ).…”

Topographical controls on small‐volume pyroclastic flows

“…Thirdly, when a pyroclastic flow moves in a turbulent regime, any significant change in the topography (for example, slope breaks or the presence of topographical obstacles) can lead to localized erosion and to a sudden halting of the flow, and/or to an important dissipation of energy resembling hydraulic jumps in water flows (Chow, ; Komar, ; Duller et al ., ; Chanson, ; Slootman et al ., ; see also Kieffer & Sturtevant, ; Sulpizio et al ., ). Similar processes in pyroclastic flows have been described by Freundt & Schmincke ().…”

“…The largest clasts tend to move mainly along this instantaneous equipotential surface of lithostatic load, and keep moving for some distance once the lower flow unit has already deposited its load (Postma & Roep, 1985;Postma, 1986;Postma et al, 1988;Legros & Mart ı, 2001). By analogy with turbidity currents, marine tsunamis and tidal bores, at Arico, the presence of abundant accidental lithic fragments entrained into the pyroclastic flow, their wide range of sizes and their irregular sharp contact at the base of the deposit, all suggest that the flow moved in a locally intense turbulent regime (Chanson, 2009;Sumner et al, 2013;Slootman et al, 2018; see also Duller et al, 2008), or that the flow was granular. Turbulent transport in ignimbrites originates once a pyroclastic flow starts to move and its velocity increases down steep slopes, which can lead to locally high transport velocities (Kieffer, 1981;Suzuki-Kamata, 1988).…”

“…In this situation, therefore, the amount of fine particles transported at the base of the flow would increase progressively and not be elutriated upward, thereby implying a significant further increase in the density and, occasionally, a local transformation from turbulent to laminar flow (sensu single phase flow; see Fisher, 1983), or from supercritical to subcritical flows through a hydraulic jump (Cartigny et al, 2014;Slootman et al, 2018). The relatively thin and discontinuous fine-grained layer at the base of the ignimbrite (Figs 4 and 5) observed in some localities may thus correspond to a particular case of the basal layer described by Sparks et al (1973), i.e.…”

Clast imbrication in coarse-grained mountain streams and stratigraphic archives as indicator of deposition in upper flow regime conditions

“…Significant sediment accumulation may occur underneath the hydraulic jump upon deceleration of the flow's velocity (Slootman et al, 2018). Contrariwise, a downstream change from a lower to an upper flow regime has no distinct surface expression, neither in terms of flow depth nor flow surface texture.…”