hydrologic engineering community, this type of water flow is referred to as open channel flow. Examples of open channel flows include rivers, tidal currents, irrigation canals, and sheets of water running across the ground surface after a rain. The equations commonly used to model such flows are anchored in experimental measurements and decades of validation in many diverse applications. It is the turbulence of the flowing water in such flows that keeps the sediment particles in suspension. The Journal of Hydraulic Engineering is but one of several journals that has published a wealth of papers on turbulent open channel flow and sediment transport over the past many decades. Appendix A in Baumgardner (2018a) summarizes the observations, experiments, and efforts to formulate a mathematical description of fluid turbulence over the past two centuries. A description of turbulent fluid flow provided almost a century ago by the British scientist L. F. Richardson (1920) is still valid today. His description is a flow whose motions are characterized by a hierarchy of vortices, or eddies, from large to tiny. These eddies, including the large ones, are unstable. The shear that their rotation exerts on the surrounding fluid generates smaller new eddies. The kinetic energy of the large eddies is thereby passed to the smaller eddies that arise from them. These smaller eddies in turn undergo the same process, giving rise to even smaller eddies that inherit the energy of their predecessors, and so on. In this way, the energy is passed down from the large scales of motion to smaller and smaller scales until reaching a length scale sufficiently small that the molecular viscosity of the fluid transforms the kinetic energy of these tiniest eddies into heat. When a fluid is moving relative to a fixed surface, the speed of the fluid, beginning from zero at the boundary, increases—first rapidly, and then less rapidly—as distance from the surface increases. The region adjacent to the surface in which the average speed of the flow parallel to the surface is still changing, at least modestly, as one moves away from the surface is known as the boundary layer. When the speed of the fluid over the surface is sufficiently high, the boundary layer becomes turbulent and becomes filled with eddies that can span a broad range of spatial scales. Appendix B in Baumgardner (2018a) summarizes some of the prominent features of turbulent boundary layers, including the discovery that the mean velocity profile within the turbulent boundary layer is very close to a logarithmic function of distance from the boundary. Remarkably, the parameters specifying the profile can be determined merely from the thickness of the layer and its mean flow speed. The theory of open channel flow applies this mathematical representation of a turbulent boundary layer to describe sediment suspension, transport, and deposition by turbulent water flow for cases where the width of the flow is much greater than the water depth. Appendix C in Baumgardner (2018a) provides the derivation of a mathematical expression for the sediment carrying capacity of a layer of turbulent water as a function of sediment particle size. This expression is utilized in the numerical treatment to quantify the sediment suspension of the water flow. The expression requires the particle settling speed for each of the particle sizes that is assumed in the model. Appendix D in Baumgardner (2018a) describes how these settling speeds may be obtained via empirical fits to experimental data. Obviously, a prominent issue in the formation of the earth’s sediment record is the origin of the sediment. From the rock record it is clear that there were pre-Flood continental sediments. However, for sake of simplicity, these sediments are ignored in the current version of the model. Instead, it is assumed that all the sediment deposited is derived from erosion of continental bedrock. In terms of erosional processes, we restrict the scope to the mechanism of cavitation, again for simplicity. We suspect, however, that contributions from other processes by comparison were small. We further assume that the cavitation erosion of crystalline continental bedrock results in a distribution of particle sizes corresponding to 70% fine sand, 20% medium sand, and 10% coarse sand. Here the fine sand fraction also includes the clay and silt, which are assumed to flocculate to form particles that display settling behavior identical to that of fine sand. Mean particle diameters for these three size classes are 0.063 mm, 0.25 mm, and 1 mm, respectively. In this model we neglect carbonates which in the actual rock record represent on the order of 20-30% of the total sediment volume. We recognize that it is difficult to imagine how feldspar in the continental crustal bedrock, even when reduced by cavitation to 0.063 mm particle sizes and smaller, might be transformed to clay minerals in the brief time span available during the Flood. We acknowledge that a significant portion of the clay in the shales and mudstones in the Phanerozoic sediment record may well have been derived from shales and mudstones of the pre-Flood earth. For example, the Precambrian tilted strata exposed in the inner gorge of the Grand Canyon, rocks that include the Unkar Group, the Nankoweap Formation, and the Chuar Group, display total thicknesses of about two miles, mostly of shale and limestone (Austin 1994). Even more impressive, the Mesoproterozoic (Precambrian) Belt Supergroup, exposed in western Montana, Idaho, Wyoming, Washington, and British Columbia, is mostly mudstone (shale, fine sand, and carbonate) and up to 8 miles in thickness (Winston and Link 1993). These examples hint that there may have been a vast quantity of mudrocks on the pre-Flood earth, possibly enough to account for most of the clay and carbonate rocks in the Flood sediment record. Exploring the consequences of initial conditions that include a substantial layer of pre-Flood mudstone sediments is an attractive task for future application of this model. Appendix E of Baumgardner (2018a) provides a description of the cavitation submodel. It is implemented in the numerical code by means of a single equation involving three adjustable parameters. One of these parameters is the cavitation threshold velocity. For the calculation described in this paper, that threshold velocity is set to 15 m/s, below which no cavitation, and hence no erosion, occurs. Appendix E also describes the criteria for deposition and for erosion of already deposited sediment. Given that the average thickness of Flood sediments on the continents today is about 2,000 m, it is not surprising that a numerical model capable of eroding, transporting, and depositing that much sediment will yield sediment thicknesses in some locations that significantly exceed that average value. In early tests it was found that the calculations become unstable unless some degree of isostatic compensation is allowed in locations where the sediment thicknesses become large. Appendix F in Baumgardner (2018a) describes how isostatic compensation is included in the two studies published in BAUMGARDNER AND NAVARRO Large tsunamis and Flood sediment record 2023 ICC 367
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