Effect of unsteady flows on the development and magnitude of bed forms

<p>Sediment transport and associated morphological changes in alluvial rivers occur primarily under unsteady flow conditions that are manifested as well-defined flood hydrograph events. At present, typical bed forms generated by such unsteady flows is far less studied and, thus, more poorly understood, than equivalent bed forms generated under steady flow conditions. In view of this, the objective of this work is to investigate the development of morphological bed features, and specifically variability in the length, height and steepness of bed forms that develop in a mobile coarse-sand bed layer under unsteady flow hydrographs under zero sediment feed conditions. A series of laboratory flume experiments is conducted within which different flow hydrograph events are simulated physically by controlling their shape, unsteadiness and magnitude. Experimental results indicate that different categories of bed forms such as dunes, alternate bars or transitional dune-bar structures develop within the erodible bed layer when subject to varying hydrograph flow conditions. Examination of relative importance of three parameters used to describe the hydrograph characteristics (i.e. asymmetry, unsteadiness and total water work) on bed form dimensional descriptors (i.e. wavelength, height and steepness) reveals that hydrograph unsteadiness and total water work are the primary and second-order controls on bed deformations or corresponding bed form dimensions. By contrast, hydrograph asymmetry appears to have minimal or negligible influence on bed form development in terms of their type and magnitude. Based on these findings, a physical model was developed and tested to describe the effect of unsteady flow hydrographs with varying unsteadiness and total water work on the nature and size of resulting bed forms that are generated in sand-bed layers. </p>

Form friction factor of armored riverbeds

Resistance to flow because of the presence of bed forms over armored riverbeds is of paramount importance, leading to the effective design of water-resources-related projects. Based on the findings over bed armored surfaces, it is shown that the controlling roughness (ks) can be taken as equal to the median diameter of the armor layer. Analytical methodologies for total and grain friction factors have been proposed here that take flow non-uniformity into account using the velocity distribution and friction slope. The percentage composition of form friction factor in the total friction factor was estimated to be 40%. The results were explained in light of the coupling of the sediment threshold problem with the friction factor and coarse-grain rearrangement in armor layer. The computed form friction factor by proposed method was compared with Keulegan’s method and is found to give satisfactory results, showing 80% agreement of all field data sets.

Applications in New River-meander Model

If the sediment transport behaves as bed-load, the sediment surface at meandering channel will deform into transverse waves. This investigation isa new model for prediction of river-meander models in nature. The aim ofthis research is to give a precise method whose bed forms can have a variety of scales ranging from ripples through small dunes to fully developeddunes or sandwaves. Its mathematical model will be investigated.

The role of surface cohesion is wind-driven snow transport

<p>The wind-driven saltation of sediments, such as snow and sand, is responsible for a wide range of geophysical processes. Blowing-snow, in particular, affects snow surface properties and drives snow redistribution in alpine terrain. As such, it is of fundamental importance for avalanche mechanics. One of the most important controls on initiation and development of snow saltation is the surface cohesion induced by ice particle sintering. Although inter-particle cohesion is known to limit the number of grains lifted from the surface through aerodynamic entrainment and granular splash, the role of cohesion in the development of saltation from onset to steady state is still poorly understood. Using a numerical model based on the discrete element method, we show that saltation over cohesive beds sustains itself at wind speeds one order of magnitude smaller than those necessary to initiate it, giving rise to hysteresis in which the occurrence of transport depends on the history of the wind. Our results further suggest that saltation over cohesive beds requires much longer distances to saturate, thereby increasing the size of the smallest stable bed forms.</p>

Active and fossil aeolian bedforms in Arabia Terra (Mars): climate and sedimentological implications

<p>Aeolian bed forms such as dark dunes and ripples are abundant and widespread on Mars and can be used to constrain present-day wind conditions at the surface. Fossils aeolian bed forms are usually fractured, cemented and useful to constrain paleo wind conditions. Here we describe active dark dunes and fossil megaripples from an area in Arabia Terra and we discuss theirs climate implications. This area shows dark-toned domes and barchans dunes 1.5 – 10.5 m in height. Dunes slip faces, dipping SW, suggest NE dominant winds. Dunes were targeted in 2006 and 2016 (ΔT = 9.37 Earth years) by the HiRISE camera onboard of the NASA Mars Reconnaissance Orbiter (MRO). By tracking the position of the dunes in the 2006 and 2016 images, we measured an average SW displacement of 1.1 m (0.12 m yr<sup>-1</sup>). This translates to an average flux of 0.82 m<sup>3 </sup>m<sup>-1 </sup>yr<sup>-1</sup> (median 0.78 m<sup>3 </sup>m<sup>-1 </sup>yr<sup>-1</sup>), which is almost three times the median dune flux in the MSL Curiosity landing site but ¼ of the flux measured in McLaughlin and Nili Fossae, areas where active megaripple migration were measured for the first time. Flux distribution (dune by dune) in the study area provides insights on the topographic effect, with the dunes located in depressed areas showing the lower fluxes. The dunes monitored over the 9.37 Earth years’ time-span migrated on the top of light toned layered deposit, which show a stair-stepped pattern of bright and dark layers showing different resistance to erosion. The different albedo and erosional pattern may represent different cementation/lithology, chemical composition and/or different grain sizes (bimodal). Eroded mounds 50 – 400 m-large, are the remnants of the widespread-layered unit in the studied area and are surrounded by a set of NW-SE trending linear ridges 10 – 20 m spaced. The morphology and regular spacing of the ridges suggest they are aeolian in origin. The ridges show a clear sinuous morphology that is typical of terrestrial megaripples. Megaripples are a particular type of ripples forming in bimodal sand which have coarse grains (> 1 mm) accumulating over the crest. In this scenario, the light-toned unit erosion could result in the production of bimodal sediment then re-organized in megaripples by the blowing winds and finally fossilized as suggested by the presence of fractures cutting through the megaripple crestlines. The capability of the winds to move coarse grains give hints on the transport capacity of the flows blowing in the past. The trend of the sinuous megaripples, matching the orientation of the dunes, suggests that the wind regime was consistent through time. The results reported here show how different aeolian features both active and fossils can be used to better constrain Martian climate and sedimentology.</p>

Lithostratigraphic classification of the Tsodilo Hills Group: a Palaeo- to Mesoproterozoic metasedimentary succession in NW Botswana

The Tsodilo Hills Group strata exposed in the Tsodilo Hills are an association of meta-arenites, meta-conglomerates, quartz-mica schists, sandstone, red siltstone and sedimentary breccia deposited on an open siliciclastic marine shelf between the Late Palaeoproterozic and Late Mesoproterozoic, and outcropping in NW Botswana. The succession is dominated by three micaceous quartzite units interlayered with subordinate lenses and wedges of other rock types. Facies gradients from S to N are expressed by decreasing content of muscovite at all levels of metasediment organisation from thin wedge-shaped units to thick quartzite complexes, as well as a decrease in pebble content and increase in the arenaceous matrix in some metal conglomerate beds, matching regional palaeotransport direction. Well-rounded pebbles of rocks are flat, suggesting redeposition from a beach environment. Lenticular conglomeratic bodies with erosional lower boundaries represent infills of local incisions in the sandy bottom sediments. The abundance of laterally discontinuous lithological units reflects shelf palaeotopography controlled and modified by deposition and migration of large bed forms, ranging from megaripple marks (or submarine dunes) to sand waves. Deposition was influenced by tides and two regressive events. The older regression resulted in a marker unit of tidal mudflat-related red-bed facies: mudstone, siltstone, channel-fill sandstone and sedimentary breccia. The second regression is indicated by a tabular conglomerate marker reflect-ing increased input of coarse terrigenous material.