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 ul,,,,, the bottom stress T, and Shields’ relative stress Bt, 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 uloo = 122.6 Do.2B for D 0.2 crn where the velocity uloo measured 100 cm above the sediment bed is given in cmjsec 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.

Threshold of sediment motion under unidirectional currents

Data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size, t* c 50 . Calculated t* c 50 values were stratified by initial motion definition, median grain size type (surface, subsurface, or laboratory mixture), relative roughness, and flow regime. A traditional Shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition; t* c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel Shields curves, with the latter generally underlying the former; values derived from competence functions define a separate but poorly developed field, while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous, rather than time-averaged, critical shear stresses. The available data indicate that for high critical boundary Reynolds numbers and low relative roughnesses typical of gravel-bedded rivers, reference-based and visually based studies have t* c50 ranges of 0.052-0.086 and 0.030-0.073, respectively. The apparent lack of a universal t*50 for gravel-bedded rivers warrants great care in choosing defendable t* c50 values for particular applications.

/pdf/a-systematic-analysis-of-eight-decades-of-incipient-motion-4jq1z95rbv.pdf

A systematic analysis of eight decades of incipient motion studies, with special reference to gravel-bedded rivers

http://basin.earth.ncu.edu.tw/download/courses/seminar_MSc/2012/1018_1_1.pdf

Marine hyperpycnal flows: initiation, behavior and related deposits. A review

The composition of any environment or object is determined by a particular balance between material transport processes and chemical reactions within and around it. In the case of marine sedimentary deposits, the dominant agents of mass transport are often large bottom-dwelling animals that move particles and fluids during feeding, burrowing, tube construction, and irrigation. Such biogenic material transport has major direct and indirect effects on the composition of sediments and their overlying waters. In this chapter I review some of what is presently known about these effects, their implications for both chemical and biological properties of a deposit, and how they can be conceptualized in quantitative models.

The Effects of Macrobenthos on Chemical Properties of Marine Sediment and Overlying Water

CHANGES in delivery of river-borne nutrients such as dissolved phosphate, nitrate and silicate, owing to land-use changes and anthropogenic emissions, are known to result in eutrophication1— enhanced phytoplankton blooms—and more severe hypoxic events2–1 in many enclosed bays and seas. Although similar ecological effects might be expected on continental shelves, the occurrence of such eutrophication has remained unresolved5. Here we present evidence of eutrophication of the continental shelf near the outflow of the Mississippi river, obtained by quantifying biologically bound silica (BSi) in diatom remnants within dated sediment cores. BSi accumulation rates are greatest in water depths of 20 to 50 m within 100 km of the river mouth, and have increased by as much as 100% this century. The increases were substantial by 1980, by which time riverine nitrogen loading had doubled relative to the beginning of the century, even though the silica loading had declined by 50% over the same period. Thus changes in river-borne nutrient loadings can modify coastal food webs and affect the amount and distribution of oxygen in bottom waters on the scale of continental shelves.

/pdf/coastal-eutrophication-near-the-mississippi-river-delta-1po16la1cd.pdf

Coastal eutrophication near the Mississippi river delta

The use of Pb-210 geochronology as a sedimentological tool: Application to the Washington continental shelf

A sediment transport study conducted on the Amazon continental shelf as part of AmasSeds (A Multi-disciplinary Amazon Shelf SEDiment Study) revealed extensive regions of dense nearbed suspensions of sediment, known as fluid mud (suspended-sediment concentration > 10 g1−1 ). Fluid mud was found near the river mouth on the inner- and middle-shelf, in the region of the bottom salinity front, and was most extensive during periods of rising and high river discharge. Fluid mud, up to 7.25 m thick, but generally 1–2 m thick, appears to form by processes similar to those occurring at an estuarine turbidity maximum, i.e. enhanced settling and lateral convergence of near-bottom flows. A modeling study showed that vertical mixing was controlled by the suppression of turbulence, due to the stratification induced by suspended sediment, and established an upper bound for the total amount of suspended sediment that may be carried in suspension. Sediment leaving the Amazon River appears to go through cycles of trapping and resuspension at the river mouth, before being partially advected seaward and alongshelf, where it is largely incorporated into fluid mud along the bottom salinity front. The fluid muds have far-reaching effects on the Amazon shelf system by reducing boundary shear stresses, affecting water-column seabed exchange, and serving as the agent of outward growth of the subaqueous delta through episodic offshore transport.

Richard W. Sternberg

Papers

The use of Pb-210 geochronology as a sedimentological tool: Application to the Washington continental shelf

Fluid-mud processes on the Amazon continental shelf

Observations of storm and river flood-driven sediment transport on the northern California continental shelf

The formation of sedimentary strata in an allochthonous shelf environment: The Washington continental shelf

Shelf-to-canyon sediment-transport processes on the Eel continental margin (northern California)