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We are a heterogeneous group of writers, loosely linked by professional acquaintance and in some cases friendship, but recently brought together by concern about reports that the Royal Society of [...]

The Curiosity rover discovered fine-grained sedimentary rocks, which are inferred to represent an ancient lake and preserve evidence of an environment that would have been suited to support a martian biosphere founded on chemolithoautotrophy. This aqueous environment was characterized by neutral pH, low salinity, and variable redox states of both iron and sulfur species. Carbon, hydrogen, oxygen, sulfur, nitrogen, and phosphorus were measured directly as key biogenic elements; by inference, phosphorus is assumed to have been available. The environment probably had a minimum duration of hundreds to tens of thousands of years. These results highlight the biological viability of fluvial-lacustrine environments in the post-Noachian history of Mars.

In an alternative 1982, our narrator, Charlie, has just purchased a limited-edition robot, Adam, 'the first truly viable manufactured human with plausible intelligence and looks'. Upstairs, Charlie's neighbour, Miranda, is [...]

The goal of this paper is to discuss the spatial averaging concept in environmental hydraulics and develop it further by considering transport equations for fluid momentum, passive substances, and suspended sediments. The averaging theorems, the double-averaged (in time and in space) fluid momentum equation, and advection-diffusion equations for a passive substance and suspended sediments are introduced and their limitations and applications for modeling rough-bed flows, experimental design, and data interpretation are discussed. The suggested equations differ from those considered in terrestrial canopy aerodynamics and porous media hydrodynamics by accounting for roughness mobility, change in roughness density in space and time, and particle settling effects for the case of suspended sediments. We show that the form of the double-averaged equations may depend on the type of decomposition of flow variables and that this difference may have important implications for modeling. We also show that the suggested methodology offers better definitions for hydraulic characteristics, variables, and parameters such as flow uniformity, flow two dimensionality, and bed shear stress. DOI: 10.1061/(ASCE)0733-9429(2007)133:8(873) CE Database subject headings: Hydrodynamics; Fluvial hydraulics; Open channel flow; Boundary layer flow; Overland flow; Turbulence; Numerical models; Experimentation; Flow characteristics.

We investigate the turbulent structure of shallow open channel flows where the flow depth is too small (compared with the roughness height) to form a logarithmic layer but large enough to develop an outer layer where the flow is not directly influenced by the roughness elements. Since the log layer is not present, the displacement height d, which defines the position of the zero plane, and the shear velocity [u.sub.*], cannot be found by fitting the velocity data to the log law. However, these parameters are still very important because they are used for scaling flow statistics for the outer and roughness layers. In this paper we propose an alternative procedure for evaluating d in laboratory conditions, where d is found from additional experiments with the fully developed log layer. We also point out the appropriate procedure for evaluating the shear velocity u, for flows with low submergence. These procedures are applied to our own laboratory flume experiments with uniform sphere roughness, where velocities were measured using Particle Image Velocimetry. Results were interpreted within the framework of the double-averaged Navier-Stokes equations and include mean velocities, turbulence intensities, Reynolds stresses, and form-induced normal and shear stresses. The data collapse well and show that in flows without a developed log layer the structure of turbulence in the outer layer remains similar to that of flows with a log layer. This means that even though the roughness layer in the experiments reported herein was sufficiently high to prevent the development of the log layer, influence of the bed roughness did not spread further up into the outer layer. Furthermore, the results show that flow statistics do not depend on relative submergence except for the form-induced stresses which increase when relative submergence decreases. DOI: 10.1061/(ASCE)0733-9429(2007)133:8(896) CE Database subject headings: Open channel flow; Submerged flow; Roughness; Velocity.

The paper briefly outlines the double-averaging methodology for studying environmental rough-bed flows. It focuses on the applications of this methodology in environmental hydraulics by providing several examples illustrating advantages of this methodology over conventional approaches. Examples include: (1) identification of specific flow layers and flow types; (2) vertical distribution of the double-averaged velocity between the roughness tops and troughs; (3) vertical distribution of momentum fluxes and sinks for typical roughness types due to turbulence, mean flow heterogeneity, secondary currents, form drag, and viscous drag; (4) estimates of form-induced (dispersive) stresses and evaluation of their structure using quadrant analysis; and (5) closure development for mass-transfer-uptake processes for stream periphyton. These examples illustrate the advantages of the double-averaging methodology over conventional approaches as well as highlight its potential for studying flows over very rough beds, highly mobile beds, permeable beds, and surface--subsurface exchanges of mass, heat, and momentum. This methodology may also significantly improve research tools for studying a wide range of flow-biota interaction phenomena such as those related to aquatic plants, mussel communities, biofilms, and many others. DOI: 10.1061/(ASCE)0733-9429(2007)133:8(884) CE Database subject headings: Hydrodynamics; Fluvial hydraulics; Open channel flow; Boundary layer flow; Overland flow; Turbulence; Experimentation; Flow characteristics.

Standard measurement devices do not normally provide detailed information about flows in porous media because either they are too intrusive (e.g., hot/film wire anemometers) or because the solid phase impedes their physical access to the flow as in the case of laser Doppler anemometers or particle image velocimetry (PIV). In this study, a Metflow ultrasonic velocity profiler (UVP) is used to measure instantaneous velocity fields within porous media composed of plastic balls packed in a cubic pattern. Simple considerations of the ultrasound beam properties and propagation, plus a comparison with measurements performed by a PIV system, demonstrate that UVP devices are suitable for such a purpose. DOI: 10.1061/(ASCE)0733-9429(2006)132:9(983) CE Database subject headings: Porous media; Turbulence; Streams.

The extent to which turbulent structure is affected by bed-load transport is investigated experimentally using a nonporous fixed planar bed comprising mixed-sized granular sediment with a [d.sub.50] of 1.95 mm. Three different sizes of sediment ([d.sub.50]=0.77, 1.99, and 3.96 mm) were fed into the flow at two different rates (0.003 and 0.006 kg/m/s), and subsequently transported as bed load. Particle image velocimetry (PIV) was used to determine the turbulence characteristics over die fixed bed during clear water and sediment feed cases. Mean longitudinal flow velocities at any given depth were lower than their clear water counterparts for all but one of the mobile sediment cases. The exception was with the transport of fine grains at the higher feed rate. In this case, longitudinal mean flow velocities increased compared to the clear water condition. The coarse grains tended to augment bed roughness, but fine grains saturated the troughs and interstices in the bed topography, effectively causing the influence of bed irregularities to be smoothed. The PIV technique permitted examination of both temporal and spatial fluctuations in flow variables: therefore many' results are presented in terms of double-averaged quantities (in temporal and spatial domains). In particular, the form-induced stress, which arises from spatially averaging the Reynolds averaged Navier-Stokes equations and is analogous to the Reynolds turbulent stress, contributed between 15 and 35% of the total measured shear stress in the roughness layer. Flow around protrusive roughness elements produced a significant proportion of the turbulent kinetic energy shear production, suggesting that this process is highly intermittent near rough beds. CE Database subject headings: Sediment transport: Bed loads: Turbulence: Open channel flow: Hydrodynamics: Channel beds.

Several models for the vertical distribution of the double-averaged (in time and in the plane parallel to the mean bed) longitudinal velocity in the flow region between roughness troughs and roughness tops are suggested. We found that the same model for velocity distribution may be applicable to a range of flow conditions and roughness types, which share some common features. The suggested models for velocity distribution in the near-bed region are: (1) Constant velocity: (2) exponential velocity distribution; and (3) linear velocity distribution. The measured velocity distributions may be approximated by a single model or by a combination of models depending on roughness geometry and flow conditions. The validity of these models for velocity distribution is supported by laboratory data. DOI: 10.1061/(ASCE)(1733-9429(2004)130:10(1036) CE Database subject headings: Open channel flow; Turbulent flow; Velocity distribution; Roughness; Channel beds.

This paper proposes that the changes which occur in composition of the bed load during the transport of mixed-grain-size sediments are largely controlled by the distributions of critical entrainment shear stress for the various size fractions. This hypothesis is examined for a unimodal sediment mixture by calculating these distributions with a discrete particle model and using them in a probabilistic calculation of bed-load composition. The estimates of bed-load composition compare favorably with observations of fractional transport rates made in a laboratory flume for the same sediment, suggesting that the hypothesis is reasonable. The analysis provides additional insight, in terms of grain mechanics, into the processes that determine bed-load composition. These insights strongly suggest that better prediction methods will result from taking account of the variation of threshold within size fractions, something that most previous studies have neglected. CE Database subject headings: Sediment; Entrainment; Probabilistic models; Bed load; Grain size; Shear stress.

A new sampling technique called wax coring is presented in this paper that enables the determination of the composition of the surface and subsurface of a sediment bed formed during flume experiments. A wax-impregnated circular core is removed from the bed and melted in a series of well-defined layers that are then sieved separately to provide data describing the variation with depth of the particle-size distribution. The volumetric equivalence of the grain-size distribution obtained for an internal layer is demonstrated theoretically and empirical data are obtained concerning the active layer concept that is often used in the modeling of mixed-grain-size sediment transport.

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