Carefully selected data for the threshold of sediment movement under unidirectional flow conditions have been utilized to re‐examine the various empirical curves that are commonly employed to predict this threshold. After a review of the existing data, we employed only that data obtained from open channel flumes with parallel sidewalls where flows were uniform and steady over flattened beds of unigranular, rounded sediments. Without these restrictions, an unmanageable amount of scatter is introduced. This selected data is used to develop a modified Shields‐type threshold diagram that extends the limits of the original diagram by three orders of magnitude in the grain‐Reynolds number. The equally general but more easily employed Yalin diagram for sediment threshold is also examined. Although the Shields and Yalin diagrams are general in that they apply to a wide range of different liquids, in both cases somewhat different curves are obtained for threshold under air than for the liquids. The often used empirical curves of the friction velocity u*, the velocity 100 cm above the bed u100, the bottom stress θt, and Shields’ relative stress θt, all versus the grain diameter D, are limited in their ranges of application to certain combinations of grain density, fluid density, fluid viscosity and gravity. These conditions must be selected before the curves are generated from either the more general Shields or Yalin curves. For example, on the basis of the data selected for use in this paper, empirical threshold relationships for quartz density material in water are where the velocity u100 measured 100 cm above the sediment bed is given in cm/sec and the grain diameter D is in cm. The limitations on any of the threshold relationships are severe. These limitations should be properly understood so that the empirical curves and relationships are not improperly employed.

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Dredging of the Columbia River navigation channel has raised concerns about dredging-related impacts on Dungeness crabs (Cancer magister). The overall objectives of this effort are to synthesize what is known about disposal effects on Dungeness crabs (Phase 1) and to offer approaches to quantify the effects, including approaches to gain a population-level perspective on any effects found in subsequent studies (Phase 2). This report documents Phase 1, which included 1) development of a conceptual model to integrate knowledge about crab biology and the physical processes occurring during disposal, 2) application of physics-based numerical modeling of the disposal event to understand the physical forces and processes to which a crab might be exposed during disposal, 3) conduct of a vulnerability analysis to identify the potential mechanisms by which crabs may be injured, and 4) recommendations of topics and approaches for future studies to assess the potential population-level effects of disposal on Dungeness crabs. The conceptual model first recognizes that disposal of dredged materials is a physically dynamic process with three aspects: 1) convective descent and bottom encounter, 2) dynamic collapse and spreading, and 3) mounding. Numerical modeling was used to assess the magnitude of the potentially relevant forces and extent of mounding in single disposal events. The modeling outcomes show that predicted impact pressure, shear stress, and mound depth are greatly reduced by discharge in deep water and somewhat reduced at longer discharge duration. The analysis of numerical modeling results and vulnerabilities indicate that the vulnerability of crabs to compression forces under any of the disposal scenarios is low. Modeling results and other information suggest that crabs may be vulnerable to injury from surge currents under some but not all disposal scenarios. For the shallow-water, short-duration disposal scenarios, the shear stress and surge currents estimated from the modeling and observed in the field at Palos Verdes appear to be sufficiently high to mobilize and transport bottom sediment and at least juvenile crabs. The surge currents estimated from modeling the deep water scenarios are not sufficient to mobilize sediment greater than 1 mm or juvenile crabs. The effects of such movement, if any, are not known and can be reduced by behavioral responses by crabs. There is evidence that burial by dredged materials can affect crab respiration and survival, but confounding factors in previous experiments preclude conclusions about thresholds and extent of effects. We recommend that future studies focus on burial effects during shallow water, short duration disposal scenarios and take into account the potential for behavioral responses to mitigate any effects.iii During disposal, the characteristics of the dredged materials (e.g., size distribution, cohesiveness), the water column (e.g., depth, stratification, currents), and the bottom (e.g., slope, grain-size distribution) interact to determine the characteristic...

As the velocity of the water motion near the bottom under oscillatory waves is increased, there comes a stage when the water exerts a stress on the particles sufficient to cause them to move. This study reviews data on threshold of sediment motion under wave action and compares the results with the established curves for threshold under a unidirectional current.

Generalized curves for grain threshold under liquid flow and under gas flow (air), based on laboratory data summarized in a previous paper, are utilized to generate a series of threshold curves for unusual sedimentary environments and little‐studied sedimentary materials of interest to geologists. These are aeolian and water flow threshold on the planet Mars, and the threshold of foraminiferal sands in the deep‐sea. Such extrapolations from the generalized curves are necessary because of the difficulty or impossibility of obtaining satisfactory data for these sediments or environments, and the procedures demonstrate the importance of the general threshold curves.