Bedforms and sedimentary structures related to supercritical flows in glacigenic settings

This paper describes steady two-dimensional disturbances forced on the interface of a two-layer fluid by flow over an isolated obstacle. The oncoming flow speed is close to the linear longwave speed and the layer densities, layer depths and obstacle height are chosen so that the equations of motion reduce to the forced two-dimensional Korteweg–de Vries equation with cubic nonlinearity, i.e. the forced extended Kadomtsev–Petviashvili equation. The distinctive feature noted here is the appearance in the far lee-wave wake behind obstacles in subcritical flow of a ‘table-top’ wave extending almost one-dimensionally for many obstacles widths across the flow. Numerical integrations show that the most important parameter determining whether this wave appears is the departure from criticality, with the wave appearing in slightly subcritical flows but being destroyed by two-dimensional effects behind even quite long ridges in even moderately subcritical flow. The wave appears after the flow has passed through a transition from subcritical to supercritical over the obstacle and its leading and trailing edges resemble dissipationless leaps standing in supercritical flow. Two-dimensional steady supercritical flows are related to one-dimensional unsteady flows with time in the unsteady flow associated with a slow cross-stream variable in the two-dimensional flows. Thus the wide cross-stream extent of the table-top wave appears to derive from the combination of its occurrence in a supercritical region embedded in the subcritical flow and the propagation without change of form of table-top waves in one-dimensional unsteady flow. The table-top wave here is associated with a resonant steepening of the transition above the obstacle and a consequent twelve-fold increase in drag. Remarkably, the table-top wave is generated equally strongly and extends laterally equally as far behind an axisymmetric obstacle as behind a ridge and so leads to subcritical flows differing significantly from linear predictions.

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Comparison principles for free-surface flows with gravity

Journal of Fluid Mechanics ◽

10.1017/s0022112091000770 ◽

1991 ◽

Vol 230 ◽

pp. 231-243 ◽

Cited By ~ 3

Author(s):

Walter Craig ◽

Peter Sternberg

Keyword(s):

Solitary Waves ◽

Finite Depth ◽

Free Surface Flows ◽

Immiscible Fluids ◽

Steady Flows ◽

Two Dimensional ◽

Surface Flows ◽

Ship Hulls ◽

Geometrical Information ◽

Supercritical Flows

This article considers certain two-dimensional, irrotational, steady flows in fluid regions of finite depth and infinite horizontal extent. Geometrical information about these flows and their singularities is obtained, using a variant of a classical comparison principle. The results are applied to three types of problems: (i) supercritical solitary waves carrying planing surfaces or surfboards, (ii) supercritical flows past ship hulls and (iii) supercritical interfacial solitary waves in systems consisting of two immiscible fluids.

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An Examination of Froude-Supercritical Flows and Cyclic Steps On A Subaqueous Lacustrine Delta, Lake Chelan, Washington, U.S.A

Journal of Sedimentary Research ◽

10.2110/jsr.2015.48 ◽

2015 ◽

Vol 85 (7) ◽

pp. 754-767 ◽

Cited By ~ 22

Author(s):

Aaron T. Fricke ◽

Benjamin A. Sheets ◽

Charles A. Nittrouer ◽

Mead A. Allison ◽

Andrea S. Ogston

Keyword(s):

Lacustrine Delta ◽

Supercritical Flows

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Modern and Holocene microbial mats and associated microbially induced sedimentary structures (MISS) on the southeastern coast of Tunisia (Mediterranean Sea)

Quaternary Research ◽

10.1017/qua.2020.91 ◽

2020 ◽

pp. 1-21

Author(s):

Rached Lakhdar ◽

Mohamed Soussi ◽

Rachida Talbi

Keyword(s):

Spatial Distribution ◽

High Frequency ◽

Depositional Environment ◽

Microbial Mats ◽

Coastal Environment ◽

Tidal Dynamics ◽

Very High Frequency ◽

Intertidal Zones ◽

Sedimentary Structures ◽

Very High

Abstract On the southeastern Tunisian coastline, very diverse living microbial mats colonize the lower supratidal and intertidal zones, and locally may extend into the upper infratidal zone. The interaction between the benthic cyanobacteria and their siliciclastic substratum leads to the development of several types of microbially induced sedimentary structures (MISS). The mapping of the microbial mats has allowed the identification of the types of MISS that characterize the different segments of the coastal environment. The modern microbial mats have been compared with those recorded at the top of the Holocene deposits, which are composed of biodegraded microbial black mats alternating with white laminae made of clastic and evaporitic sediments, indicative of very high frequency cycles of flood and drought. A hypothetic profile showing their occurrences along the different areas bordering the coastline is proposed as a guide for the reconstruction of the ancient depositional environment. The roles of tidal dynamics, storms, and climate in controlling their genesis and spatial distribution, are discussed and highlighted. The modern MISS of southeastern Tunisia are compared with their equivalents that are well documented through the different geological eras.

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