Mechanisms of riverbank failure along the Arno River,central Italy

Riverbanks along the Arno River have been investigated with the aims of de?ning the main mechanisms of failure and retreat, their spatial distribution, and their causes. Geomorphological aspects were investigated by a reconnaissance of riverbank processes, for a number (26) of representative sites. Laboratory and in situ tests were then performed on a selected number of riverbanks (15). Based on the material characteristics, six main typologies of riverbanks have been de?ned, with homogeneous ?ne‐grained and composite banks representing the most frequent types. Slab‐type failures are the most frequent mechanism observed on ?ne‐grained banks, while cantilever failures prevail on composite banks. The role of river stage and related pore water pressure distributions in triggering the main observed mechanisms of failure has been investigated using two different types of stability analysis. The ?rst was conducted for 15 riverbanks, using the limit equilibrium method and considering simpli?ed hypotheses for pore water pressure distribution (annulment of negative pore pressures in the portion of the bank between low water stage and peak stage). Stability conditions and predicted mechanisms of failure are shown to be in reasonably good agreement with ?eld observations. Three riverbanks, representative of the main alluvial reaches of the river, were then selected for a more detailed bank stability analysis, consisting of: (a) de?nition of characteristic hydrographs of the reach with different return periods; (b) modelling of saturated and unsaturated ?ow using ?nite element seepage analysis; and (c) stability analysis with the limit equilibrium method, by adopting pore water pressure values derived from the seepage analysis. The results are compared to those obtained from the previous simpli?ed analysis, and are used to investigate the different responses, in terms of stability, to different hydrological and riverbank conditions. Copyright © 2003 John Wiley & Sons, Ltd.     

6: Classification of VMS deposits: Lessons from the South Uralides

VMS deposits of the South Urals developed within the evolving Urals palaeo-ocean between Silurian and Late Devonian times. Arc-continent collision between Baltica and the Magnitogorsk Zone (arc) in the south-western Urals effectively terminated submarine volcanism in the Magnitogorsk Zone with which the bulk of the VMS deposits are associated. The majority of the Urals VMS deposits formed within volcanic-dominated sequences in deep seawater settings. Preservation of macro and micro vent fauna in the sulphide bodies is both testament to the seafloor setting for much of the sulphides but also the exceptional degree of preservation and lack of metamorphic overprint of the deposits and host rocks. The deposits in the Urals have previously been classified in terms of tectonic setting, host rock associations and metal ratios in line with recent tectono-stratigraphic classifications. In addition to these broad classes, it is clear that in a number of the Urals settings, an evolution of the host volcanic stratigraphy is accompanied by an associated change in the metal ratios of the VMS deposits, a situation previously discussed, for example, in the Noranda district of Canada.Two key structural settings are implicated in the South Urals. The first is seen in a preserved marginal allochthon west of the Main Urals Fault where early arc tholeiites host Cu–Zn mineralization in deposits including Yaman Kasy, which is host to the oldest macro vent fauna assembly known to science. The second tectonic setting for the South Urals VMS is the Magnitogorsk arc where study has highlighted the presence of a preserved early forearc assemblage, arc tholeiite to calc-alkaline sequences and rifted arc bimodal tholeiite sequences. The boninitc rocks of the forearc host Cu–(Zn) and Cu–Co VMS deposits, the latter hosted in fragments within the Main Urals Fault Zone (MUFZ) which marks the line of arc-continent collision in Late Devonian times. The arc tholeiites host Cu–Zn deposits with an evolution to more calc-alkaline felsic volcanic sequences matched with a change to Zn–Pb–Cu polymetallic deposits, often gold-rich. Large rifts in the arc sequence are filled by thick bimodal tholeiite sequences, themselves often showing an evolution to a more calc-alkaline nature. These thick bimodal sequences are host to the largest of the Cu–Zn VMS deposits.The exceptional degree of preservation in the Urals has permitted the identification of early seafloor clastic and hydrolytic modification (here termed halmyrolysis sensu lato) to the sulphide assemblages prior to diagenesis and this results in large-scale modification to the primary VMS body, resulting in distinctive morphological and mineralogical sub-types of sulphide body superimposed upon the tectonic association classification.It is proposed that a better classification of seafloor VMS systems is thus achievable using a three stage classification based on (a) tectonic (hence bulk volcanic chemistry) association, (b) local volcanic chemical evolution within a single edifice and (c) seafloor reworking and halmyrolysis.     

Fragmentation during Rock Falls: Two Italian Case Studies of Hard and Soft Rocks

In recent years, rock fall phenomena in Italy have received considerable attention for risk mitigation through in situ observations and experimental data. This paper reports the study conducted at Camaldoli Hill, in the urban area of Naples, and at Monte Pellegrino, Palermo, Italy. The rocks involved are volcanic Neapolitan yellow tuff (NYT) in the former area and dolomitic limestone in the latter. Both rocks, even though with different strength characteristics, have shown a significant tendency towards rock fragmentation during run out. This behavior was first investigated by comparing the volumes of removable blocks on the cliff faces (V ₀) and fallen blocks on the slopes (V _f). It was assumed that the ratio V _f/V ₀ decreases with the distance (x _f) from the detachment area by an empirical law, which depends on a coefficient α, correlated with the geotechnical properties of the materials involved in the rock fall. Finally, this law was validated by observation of well-documented natural rock falls (Palermo) and by in situ full-scale tests (Naples). From the engineering perspective, consideration of fragmentation processes in rock fall modeling provides a means for designing low-cost mitigation measures.     

Study of an unexpected convective storm

The occurrence of a severe storm in the Argentine littoral has been analyzed. Although general atmospheric conditions were not favorable to the development of deep convection, this storm produced strong winds and hail of considerable intensity. Its occurrence was associated with the entrance of a pulse of humid air at low levels and could be related to the development of two vorticity centers in the region. Atmospheric conditions existing above a low-level inversion are used to simulate two possible clouds, with base altitudes at 2200 and 3000 m and updraft speeds reaching maximum values of 27 and 29 m s⁻¹ respectively. It is noted that the base temperature of about 13°C, found for the first cloud, can be correlated with the prevalence of frozen drop embryos, observed in an analyzed hailstone sample. Hailstone vertical trajectories are simulated in both modeled clouds, assuming spheroidal and spherical particles and assigning different values to the drag coefficient C_d.     

Deep uncertainty in long-term hurricane risk: Scenario generation and implications for future climate experiments

Current projections of long-term trends in Atlantic hurricane activity due to climate change are deeply uncertain, both in magnitude and sign. This creates challenges for adaptation planning in exposed coastal communities. We present a framework to support the interpretation of current long-term tropical cyclone projections, which accommodates the nature of the uncertainty and aims to facilitate robust decision making using the information that is available today. The framework is populated with projections taken from the recent literature to develop a set of scenarios of long-term hurricane hazard. Hazard scenarios are then used to generate risk scenarios for Florida using a coupled climate–catastrophe modeling approach. The scenarios represent a broad range of plausible futures; from wind-related hurricane losses in Florida halving by the end of the century to more than a four-fold increase due to climate change alone. We suggest that it is not possible, based on current evidence, to meaningfully quantify the relative confidence of each scenario. The analyses also suggest that natural variability is likely to be the dominant driver of the level and volatility of wind-related risk over the coming decade; however, under the highest scenario, the superposition of this natural variability and anthropogenic climate change could mean notably increased levels of risk within the decade. Finally, we present a series of analyses to better understand the relative adequacy of the different models that underpin the scenarios and draw conclusions for the design of future climate science and modeling experiments to be most informative for adaptation.     

The amphipod Talitrus saltator as a bioindicator of human trampling on sandy beaches

The present paper assesses the use of the supralittoral amphipod Talitrus saltator as a bioindicator of the effects of human trampling on the supralittoral sandy band. Samplings in delimited areas were carried out at sites subjected to different human impact. The results showed a strong negative correlation between the number of swimmers and the sandhopper population density, while there was no clear relationship between sandhopper abundance and the other factors considered: granulometry, compactness and organic carbon content of the sand, and trace metal contents in the sand and sandhoppers. A field test of trampling conducted in a confined space showed its direct negative effect on sandhopper survival. However, trace metal analysis confirmed the ability of T. saltator to bioaccumulate some elements (Hg, Zn, Cu, Cd). Our study demonstrates that T. saltator is a good bioindicator of human impact in the supralittoral zone of sandy shores.     

Burial metamorphism of a Cretaceous volcanic sequence west from Santiago,Chile

This report is a petrographic study of alteration phenomena in an area of 100 km² in the Coastal Range west of Santiago. The stratified sequence of the area is of Cretaceous age and belongs to the western monoclinal limb of the Andean Geosyncline. Two structural units are present, separated by an angular unconformity. The older is about 9,000 m thick, and the younger 300 m thick. The rock types are mostly altered andesitic flows and flow breccias, and keratophyric ignimbrites and lavas, with alternating marine, brackish-water and terrestrial interbeds. Stratified rocks are cut locally by acid and basic apophyses and dikes, probably feeders of their volcanic host rocks. Discordant Cretaceous granitic plutons intrude the older unit. Throughout the whole stratigraphic section there are alteration minerals, which selectively replace the primary minerals, or fill amygdules and open fractures, or form a cement in flows, dikes and sedimentary interbeds. Patterns of alteration are regular and persistent; they correlate on a large scale with stratigraphic level and on a smaller scale with position within each individual flow and situation within amygdules. The stratigraphically controlled pattern is as follows: $$\begin{gathered} 1.{\text{ Younger unit}}{\text{.}} \hfill \\ {\text{ }}\left. \begin{gathered} {\text{Lower portion: 30m: albite}}---{\text{pistacite}}---{\text{actinolite}}---{\text{chlorite}}---{\text{ }} \hfill \\ {\text{calcite}}---{\text{sphene}}---{\text{quartz}} \hfill \\ \end{gathered} \right\}{\text{greenschist facies}} \hfill \\ {\text{2}}{\text{. Older unit}}{\text{.}} \hfill \\ {\text{ }}\left. \begin{gathered} {\text{a) 0}}---{\text{1,280 m : albite}}---{\text{pumpellyite}}---{\text{prehnite}}---{\text{calcite}}---{\text{chlorite}}---{\text{ }} \hfill \\ {\text{ laumontite}} \hfill \\ {\text{b) 1,280}}---{\text{4,850 m: albite}}---{\text{adularia}}---{\text{calcite}}---{\text{prehnite}}---{\text{pumpellyite}}--- \hfill \\ {\text{pistacite}}---{\text{white mica}}---{\text{quartz}} \hfill \\ {\text{c) 4,850}}---{\text{8,110 m: albite}}---{\text{pistacite}}---{\text{quartz}}---{\text{chlorite}}---{\text{calcite}}--- \hfill \\ {\text{white mica}}---{\text{sphene}}---{\text{adularia prehnite}}---{\text{pumpellyite}} \hfill \\ \end{gathered} \right\}{\text{prehnite}}---{\text{pumpellyite facies}} \hfill \\ {\text{ d) 8,110}}---{\text{9,060 m: albite}}---{\text{pistacite}}---{\text{actinolite}}---{\text{sphene}}---{\text{calcite \} greenschist facies}} \hfill \\ \end{gathered} $$ The pattern of alteration in the older unit is comparable to that described for burial metamorphosed sequences in New Zealand and Australia. Reappearence of the greenschist facies at a higher level in the younger unit poses a problem for which several explanations are possible. The smaller scale pattern of alteration shows a persistent tendency —not without exception — for the “grade” of the alteration assemblage (as correlated with depth on the large scale) to increase: from the base of the flow (non-amygdaloidal part) upward (amygdaloidal part), and from the rim of each amygdule inward. Also recognizable on the scale of a single flow is a tendency for upward increase in: a) extent of alteration (the basal zone may be fresh andesite), and b) weight percent of Na₂O, K₂O (with complementary depletion in CaO), and of Fe₂O₃/FeO. Preliminary observations indicate that this alteration pattern persists for at least 400 km north of the area here described in rocks of similar lithology and age. It is unrelated to local granitic plutons.     

Stability at high-pressure,elastic behaviour and pressure-induced structural evolution of CsAlSi<Subscript>5</Subscript>O<Subscript>12</Subscript>, a potential host for nuclear waste

The elastic and structural behaviour of the synthetic zeolite CsAlSi₅O₁₂ (a = 16.753(4), b = 13.797(3) and c = 5.0235(17) Å, space group Ama2, Z = 2) were investigated up to 8.5 GPa by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. No phase-transition occurs within the P-range investigated. Fitting the volume data with a third-order Birch–Murnaghan equation-of-state gives: V ₀ = 1,155(4) ų, K _T0 = 20(1) GPa and K′ = 6.5(7). The “axial moduli” were calculated with a third-order “linearized” BM-EoS, substituting the cube of the individual lattice parameter (a ³, b ³, c ³) for the volume. The refined axial-EoS parameters are: a ₀ = 16.701(44) Å, K _T0a = 14(2) GPa (β_a = 0.024(3) GPa^?1), K′ _a = 6.2(8) for the a-axis; b ₀ = 13.778(20) Å, K _T0b = 21(3) GPa (β_b = 0.016(2) GPa^?1), K′ _b = 10(2) for the b-axis; c ₀ = 5.018(7) Å, K _T0c = 33(3) GPa (β_c = 0.010(1) GPa^?1), K′ _c = 3.2(8) for the c-axis (K _T0a:K _T0b:K _T0c = 1:1.50:2.36). The HP-crystal structure evolution was studied on the basis of several structural refinements at different pressures: 0.0001 GPa (with crystal in DAC without any pressure medium), 1.58(3), 1.75(4), 1.94(6), 3.25(4), 4.69(5), 7.36(6), 8.45(5) and 0.0001 GPa (after decompression). The main deformation mechanisms at high-pressure are basically driven by tetrahedral tilting, the tetrahedra behaving as rigid-units. A change in the compressional mechanisms was observed at P ≤ 2 GPa. The P-induced structural rearrangement up to 8.5 GPa is completely reversible. The high thermo-elastic stability of CsAlSi₅O_12, the immobility of Cs at HT/HP-conditions, the preservation of crystallinity at least up to 8.5 GPa and 1,000°C in elastic regime and the extremely low leaching rate of Cs from CsAlSi₅O₁₂ allow to consider this open-framework silicate as functional material potentially usable for fixation and deposition of Cs radioisotopes.