Impatti, adattamento e vulnerabilita Le cause e le responsabilita dei cambiamenti climatici sono state trattate sul numero di ottobre della rivista Cda. Approfondiamo l’argomento presentando il documento: “Cambiamenti climatici 2007: impatti, adattamento e vulnerabilita” votato ad aprile 2007 dal secondo gruppo di lavoro del Comitato Intergovernativo sui Cambiamenti Climatici (Intergovernmental Panel on Climate Change). Si tratta del secondo di tre documenti che compongono il quarto rapporto sui cambiamenti climatici.

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Climate Change 2007: Impacts, Adaptation and Vulnerability.

Recent development in 2D materials beyond graphene

Foreword 1. Introduction 2. Runoff, erosion and delivery to the coastal ocean 3. Temporal variations 4. Human impacts Appendices. Global River Database: Appendix A: North and Central America Appendix B: South America Appendix C: Europe Appendix D: Africa Appendix E: Eurasia Appendix F: Asia Appendix G: Oceania References Index.

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River Discharge to the Coastal Ocean: A Global Synthesis

Morphodynamics of Holocene salt marshes: a review sketch from the Atlantic and Southern North Sea coasts of Europe

Submarine sediment density flows are one of the most important processes for moving sediment across our planet, yet they are extremely difficult to monitor directly. The speed of long run-out submarine density flows has been measured directly in just five locations worldwide and their sediment concentration has never been measured directly. The only record of most density flows is their sediment deposit. This article summarizes the processes by which density flows deposit sediment and proposes a new single classification for the resulting types of deposit. Colloidal properties of fine cohesive mud ensure that mud deposition is complex, and large volumes of mud can sometimes pond or drain-back for long distances into basinal lows. Deposition of ungraded mud (TE-3) most probably finally results from en masse consolidation in relatively thin and dense flows, although initial size sorting of mud indicates earlier stages of dilute and expanded flow. Graded mud (TE-2) and finely laminated mud (TE-1) most probably result from floc settling at lower mud concentrations. Grain-size breaks beneath mud intervals are commonplace, and record bypass of intermediate grain sizes due to colloidal mud behaviour. Planar-laminated (TD) and ripple cross-laminated (TC) non-cohesive silt or fine sand is deposited by dilute flow, and the external deposit shape is consistent with previous models of spatial decelerating (dissipative) dilute flow. A grain-size break beneath the ripple cross-laminated (TC) interval is common, and records a period of sediment reworking (sometimes into dunes) or bypass. Finely planar-laminated sand can be deposited by low-amplitude bed waves in dilute flow (TB-1), but it is most likely to be deposited mainly by high-concentration near-bed layers beneath high-density flows (TB-2). More widely spaced planar lamination (TB-3) occurs beneath massive clean sand (TA), and is also formed by high-density turbidity currents. High-density turbidite deposits (TA, TB-2 and TB-3) have a tabular shape consistent with hindered settling, and are typically overlain by a more extensive drape of low-density turbidite (TD and TC,). This core and drape shape suggests that events sometimes comprise two distinct flow components. Massive clean sand is less commonly deposited en masse by liquefied debris flow (DCS), in which case the clean sand is ungraded or has a patchy grain-size texture. Clean-sand debrites can extend for several tens of kilometres before pinching out abruptly. Up-current transitions suggest that clean-sand debris flows sometimes form via transformation from high-density turbidity currents. Cohesive debris flows can deposit three types of ungraded muddy sand that may contain clasts. Thick cohesive debrites tend to occur in more proximal settings and extend from an initial slope failure. Thinner and highly mobile low-strength cohesive debris flows produce extensive deposits restricted to distal areas. These low-strength debris flows may contain clasts and travel long distances (DM-2), or result from more local flow transformation due to turbulence damping by cohesive mud (DM-1). Mapping of individual flow deposits (beds) emphasizes how a single event can contain several flow types, with transformations between flow types. Flow transformation may be from dilute to dense flow, as well as from dense to dilute flow. Flow state, deposit type and flow transformation are strongly dependent on the volume fraction of cohesive fine mud within a flow. Recent field observations show significant deviations from previous widely cited models, and many hypotheses linking flow type to deposit type are poorly tested. There is much still to learn about these remarkable flows.

Subaqueous sediment density flows: Depositional processes and deposit types

The transport and fate of fine grained cohesive sediments in estuarine and coastal waters is influenced by the settling velocity of the sediment, which in turn is affected by flocculation effects. The flocculation processes depend on the physico-chemical properties of the sediment and the water, and on several physical mechanisms, of which turbulence is a major one. In the present paper the effects of turbulence are further analyzed. The floes are treated as self-similar fractal entities. As a result, the settling velocity is shown not to scale with the diameter D squared, as predicted by Stokes' law, but with Dnf-1 , where nf is the fractal dimension. Based on a collision frequency function and a floe breakup formulation, a simple flocculation model is proposed that can be solved analytically for uniform conditions. The flocculation and floe breakup coefficients were obtained from experiments described in literature. It is shown that the maximal attainable floe size and settling velocity are determined t...

A simple model for turbulence induced flocculation of cohesive sediment

On the flocculation and settling velocity of estuarine mud

The erosion behaviour of sand–mud mixtures is not well understood. As a first step, a conceptual framework for the erosion behaviour of sand–mud mixtures is proposed and validated with data from field and laboratory experiments based on appropriate parameters for cohesion and network structure. Six bed types with different characteristics are distinguished in relational diagrams. Validation with experimental data shows that observed transitions in erosion behaviour are clearly delimited in these diagrams. Mud content (%

A conceptual framework for the erosion behaviour of sand-mud mixtures

This study focuses on stratification effects induced by the interaction between suspensions of fine-grained cohesive and noncohesive sediment and the turbulent flow in estuarine and coastal environments. A concise literature review reveals that this interaction may cause an appreciable modification of the vertical profiles of velocity, vertical eddy viscosity/diffusivity, and Reynolds stresses. This interaction is further studied with a one-dimensional vertical (1DV) numerical model, which includes the standard k- turbulence model with buoyancy destruction terms. Application of this model to the experimental results by Coleman (1981) shows that the measured changes in the velocity profile can be explained entirely by sediment-induced buoyancy effects. It is argued that capacity for cohesive sediment is .....   ,cc,e,, a fluid mud layer is formed upon the deposition of the cohesive sediment flocs, contrary to the case with noncohesive sediment, where the depositing grains immediately form a rigid bed. Thus a two-layer fluid system develops causing significant damping of the vertical mixing processes, decreasing the carrying capacity further. This positive feed back results in a catastrophic collapse of the turbulence field and the concentration profile. A scaling law for this saturation behavior is derived from classical stratified flow theory. This law is sustained with a series of numerical experiments with the 1DV model. It also predicts the suspended sediment concentrations observed in the Yellow River to the right order of magnitude. It is concluded that sediment-induced buoyancy yields appreciable stratification effects at already moderate suspended sediment concentrations. In the case of cohesive sediment these stratification effects can result in a catastrophic collapse of the turbulent flow field and the vertical profile of suspended sediment concentration.

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Johan C. Winterwerp

Papers

A simple model for turbulence induced flocculation of cohesive sediment

On the flocculation and settling velocity of estuarine mud

A conceptual framework for the erosion behaviour of sand-mud mixtures

Stratification effects by cohesive and noncohesive sediment

The characterisation of cohesive sediment properties