[1] This paper describes a laboratory study of the dynamics of flow associated with three different stages of bed form amalgamation across the ripple-dune transition. Measurements of flow velocity were obtained over simplified fixed bed forms, designed to simulate conditions at the ripple:dune transition, using a 2D laser Doppler anemometer. This yielded information detailing the mean velocity field, turbulence statistics, local turbulence production, local deviations from the mean pressure caused by dynamic effects, turbulent kinetic energy and the contributions to the total Reynolds stresses from different coherent turbulent events. The results broadly confirm previous hypotheses that as bed form amalgamation proceeds across the ripple-dune transition, a superimposed bedstate can induce a series of critical changes to the flow structure, with higher Reynolds stresses being produced near and downstream of flow reattachment. In particular, quadrant 4 events (turbulent flow structures with a downstream velocity greater than average, and directed towards the bed) dominate the vertical turbulent diffusion of longitudinal momentum in near-bed regions close to the crest of the next downstream ripple, thus providing the potential for increased erosion and sediment transport. These experiments using simple fixed beds also provide support for recent measurements that document increased suspended sediment concentrations across the ripple-dune transition Robert, 2004, 2005), and that at the transition the largest bed forms are not necessarily those associated with the most intense shear layer activity (Schindler and Robert, 2005). Bed form superimposition and amalgamation may thus significantly alter the mean and turbulent flow field as compared to bed forms without superimposition.Citation: Fernandez, R., J. Best, and F. López (2006), Mean flow, turbulence structure, and bed form superimposition across the ripple-dune transition, Water Resour. Res., 42, W05406,

Ponds are often identified by their small size and shallow depths, but the lack of a universal evidence-based definition hampers science and weakens legal protection. Here, we compile existing pond definitions, compare ecosystem metrics (e.g., metabolism, nutrient concentrations, and gas fluxes) among ponds, wetlands, and lakes, and propose an evidence-based pond definition. Compiled definitions often mentioned surface area and depth, but were largely qualitative and variable. Government legislation rarely defined ponds, despite commonly using the term. Ponds, as defined in published studies, varied in origin and hydroperiod and were often distinct from lakes and wetlands in water chemistry. We also compared how ecosystem metrics related to three variables often seen in waterbody definitions: waterbody size, maximum depth, and emergent vegetation cover. Most ecosystem metrics (e.g., water chemistry, gas fluxes, and metabolism) exhibited nonlinear relationships with these variables, with average threshold changes at 3.7 ± 1.8 ha (median: 1.5 ha) in surface area, 5.8 ± 2.5 m (median: 5.2 m) in depth, and 13.4 ± 6.3% (median: 8.2%) emergent vegetation cover. We use this evidence and prior definitions to define ponds as waterbodies that are small (< 5 ha), shallow (< 5 m), with < 30% emergent vegetation and we highlight areas for further study near these boundaries. This definition will inform the science, policy, and management of globally abundant and ecologically significant pond ecosystems.

A series of large-scale experiments on nonchannelized, depositional turbidity currents show the evolution and complex stratigraphy of channel-lobe systems developed updip and downdip of a break in slope. Two different sets of experimental turbidity currents with different sediment concentrations were run. The results provided a comparative picture of the gross structure of the fans, with information on their surfaces, growth sequences, and times of activity of the incised channels and lobed features. In particular, data analysis focused on: (a) velocity and suspended-sediment concentration of the flows themselves; (b) time and spatial sequences of channel and lobe construction and modification, and (c) spatial trends in grain-size distribution along the deposit. Significantly, the floor geometry employed in this study allowed investigation of adjustments in deep-sea fan deposition associated with natural changes in bed slope. We show here that the break in slope played a very important role in governing channel aggradation and lobe architecture over the deposit. More specifically, the slope break tended to break up the formation of long channels and enhance the formation of lobate features. A comparison with field submarine lobe analogs demonstrates that the morphodynamics and stratigraphy associated with lobed fans can indeed be modeled, within limits, at laboratory scale.