Predicting the equilibrium bed slope in natural streams using a stochastic model for incipient sediment motion

In this paper, a method to predict the equilibrium bed slope in natural streams based on the incipient motion criterion is proposed. The method is based on the criterion suggested by Gessler to simulate the grain size distribution of the armour coat using the concept of critical shear stress of a sediment mixture. In particular, a different expression of the probability for a single particle size to be part of the armour coat is firstly proposed; then, a simple two‐steps criterion is suggested to estimate the safety factor required by the proposed approach. The method is applied in three different Italian regions (Calabria, Basilicata, and Tuscany) and required several field campaigns involving the survey of 251 stream reaches upstream of grade‐control structures. The area including five Calabrian streams is firstly used to test the suitability of the two‐steps approach. In this area, considering the detailed information about the particle size of bed material, the effect of using a simplified grain size distribution for each torrent is also checked. The method is then applied within the two additional areas (Basilicata and Tuscany) in order to check the geographic influence on the prediction of the stable longitudinal profile. A comparison between measured and estimated values of the equilibrium bed slope showed a good agreement for the Calabrian streams, where the number of the investigated stream reaches is greater, and satisfactory results for the torrents located in Basilicata and Tuscany where the field data set is more limited. The overall results encourage the extension of the proposed method to additional field data and suggest the use of this approach by hydraulic designers in order to stabilize the longitudinal profile of natural streams. Copyright © 2011 John Wiley & Sons, Ltd.     

Middle-Late Pleistocene polycyclic evolution of a stable coastal area (southern Apulia, Italy)

The Otranto–Leuca coastal tract is marked by the presence of numerous sea caves placed close to present sea level. They are located generally at the back of a shore platform covered by a sequence of breccia deposits, marine sediments and speleothems. At Grotta di Masseria dell'Orte, marine cemented sands rest on a narrow shore platform at about 6.2 m above mean sea level and are covered by speleothems older than 185 ka. At Grotta del Diavolo, which is mostly filled by breccia deposits, three beach levels have been detected at about 3.0, 3.5 and 5.9 m above msl. They are either covered by or overlie speleothems that yield an U/Th age of 340, 78 ka and between 170.3 and 146.5, respectively. Geomorphological evidence and radiometric ages indicate that the area after a period of uplift has been tectonically stable since the last part of the Middle Pleistocene so that marine landforms close to the present shoreline underwent a polycyclic evolution. The sedimentary fills of sea caves formed during Middle-Late Pleistocene glacial stages, when arid or semiarid conditions promoted the removal of regolith and the development of thick breccia deposits. During Marine Isotope Stages (MIS) 9.3, 5.5 and 5.1, cave sediments were partially eroded whereas beach layers and related speleothems developed. These are, in fact, the only marine isotope stages marked by a sea level position which in this Mediterranean region was either close to, or slightly higher than, the present one.     

Effect of the dilution process on the attenuation of contaminants in aquifers

This paper discusses the effects of the dilution process on levels of aquifer contamination attenuation, developing analytical equations for application in theoretical and practical cases. The key finding is that, in aquifers, the dilution process causes a reduction of the contaminant concentration. Analytical equations for the assessment of the contaminant attenuation capacity, by means of the dilution process in an aquifer, are put forward. In particular, the attenuation of the contaminant concentration in an aquifer, and thus the dilution, is described by the volumetric flow rate per unit width perpendicular to the flow direction (q _u). Moreover, a classification of q _u was developed, considering six classes of attenuation capacity due to dilution. For the equations validation, nitrate contamination attenuation due to dilution was analysed in two sample areas in Piedmont (North-West Italy). The results confirmed the hypothesis, evidencing high nitrate concentrations in areas with low q _u values, connected to low dilution rate of the contaminant in the groundwater; vice versa, low nitrate concentrations were evidenced in areas with high q _u values, due to a high nitrate dilution rate.     

Improved 2-D attenuation analysis for Northern Italy using a merged dataset from selected regional seismic networks

A merged, high-quality waveform dataset from different seismic networks has been used to improve our understanding of lateral seismic attenuation for Northern Italy. In a previous study on the same region, Morasca et al. (Bull Seismol Soc Am 98:1936–1946, 2008) were able to resolve only a small area due to limited data coverage. For this reason, the interpretation of the attenuation anomalies was difficult given the complexity of the region and the poor resolution of the available data. In order to better understand the lateral changes in the crustal structure and thickness of this region, we selected 770 earthquakes recorded by 54 stations for a total of almost 16,000 waveforms derived from seismic networks operating totally or partially in Northern Italy. Direct S-wave and coda attenuation images were obtained using an amplitude ratio technique that eliminates source terms from the formulation. Both direct and early-coda amplitudes are used as input for the inversions, and the results are compared. Results were obtained for various frequency bands ranging between 0.3 and 25.0 Hz and in all cases show significant improvement with respect to the previous study since the resolved area has been extended and more crossing paths have been used to image smaller scale anomalies. Quality-factor estimates are consistent with the regional tectonic structure exhibiting a general trend of low attenuation under the Po Plain basin and higher values for the Western Alps and Northern Apennines. The interpretation of the results for the Eastern Alps is not simple, possibly because our resolution for this area is still not adequate to resolve small-scale structures.     

Effect of subsurface water level on gully headcut retreat in tropical highlands of Ethiopia

Gully erosion is a major cause of soil loss and severe land degradation in sub-humid Ethiopia. The objective of this study was to investigate the role and the effect of subsurface water level change on gully headcut retreat, gully formation and expansion in high rainfall tropical regions in the Ethiopian highlands. During the rainy seasons of 2017–2019, the expansion rate of 16 fixed gullies was measured and subsurface water levels were measured by piezometers installed near gully heads. During the study period, headcut retreats ranged from 0.70 to 2.35 m, with a mean value of 1.49 ± 0.56 m year⁻¹, and average depth of the surface water level varied between 1.12 and 2.82 m, with a mean value of 2.62 m. Gully cross-section areas ranged from 2.90 to 20.90 m², with an average of 9.31 ± 4.80 m². Volumetric retreat of gully headcuts ranged from 4.49 to 40.55 m³ and averaged 13.34 ± 9.10 m³. Soil loss from individual gullies ranged from 5.79 to 52.31 t year⁻¹ and averaged 17.21 ± 11.74 t year⁻¹. The headcut retreat rate and sediment yield were closely related over the three study seasons. Elevated subsurface water levels facilitated the slumping of gully banks and heads, causing high sediment yield. When the soil was saturated, bank collapse and headcut retreat were favoured by the combination of elevated subsurface water and high rainfall. This study indicates that area exclosures are effective in controlling subsurface water level, thus reducing gully headcut retreat and associated soil loss.     

Structural trends and palaeogeography of the central and western Sicily belt: new insights

The geology of the Sicilian mainland is summarized by N–S geological sections. A continuous late Cenozoic orogenic belt through central and western Sicily resulted from a complex deformative history, recorded by several tectonic events. The deformation mainly involved the sedimentary cover of the old African continental margin, formed in a large basinal area, bordered at its southern margin by a shallow-water carbonate environment attached to Gondwana. The orogenic belt involves a complex architecture of thrust systems, of different size, geometry and palaeogeographical origin. Deformation, which mainly developed in the earlier stages of thrusting in the basinal rock assemblages, mainly gave rise to a stack of three different duplex structures, respectively, composed of Palaeozoic, Mesozoic–Palaeogene and Neogene strata. Large-scale clockwise rotation of the thrusts predated transpressional movements in the hinterland during the latest Miocene to Pliocene. High- angle reverse faults, with lateral components, modified earlier tectonic contacts within the allochthons. Contemporaneous southwards- directed imbrications affected the external southern areas, progressively incorporating foreland and piggyback basirts. The stratigraphic relationships of basin-fills to the tectonic structures reveals that reactivation processes have been active during the last Plio-Pleistocene.     

Dynamical evolution of NEAs: Close encounters, secular perturbations and resonances

We discuss the main mechanisms affecting the dynamical evolution of Near-Earth Asteroids (NEAs) by analyzing the results of three numerical integrations over 1 Myr of the NEA (4179) Toutatis. In the first integration the only perturbing planet is the Earth. So the evolution is dominated by close encounters and looks like a random walk in semimajor axis and a correlated random walk in eccentricity, keeping almost constant the perihelion distance and the Tisserand invariant. In the second integration Jupiter and Saturn are present instead of the Earth, and the 3/1 (mean motion) and v ₆ (secular) resonances substantially change the eccentricity but not the semimajor axis. The third, most realistic, integration including all the three planets together shows a complex interplay of effects, with close encounters switching the orbit between different resonant states and no approximate conservation of the Tisserand invariant. This shows that simplified 3-body or 4-body models cannot be used to predict the typical evolution patterns and time scales of NEAs, and in particular that resonances provide some fast-track dynamical routes from low-eccentricity to very eccentric, planet-crossing orbits.On leave from the Department of Mathematics, University of Pisa, Via Buonarroti 2, 56127 Pisa, Italy, thanks to the G. Colombo fellowships of the European Space Agency.     

Introspection on improper seismic retrofit of Basilica Santa Maria di Collemaggio after 2009 Italian earthquake

The 2009 L’Aquila, Italy earthquake highlighted the seismic vulnerability of historic masonry building structures due to improper "strengthening" retrofit work that has been done in the last 50 years. Italian seismic standards recommend the use of traditional reinforcement techniques such as replacing the original wooden roof structure with new reinforced concrete (RC) or steel elements, inserting RC tie-beams in the masonry and new RC floors, and using RC jacketing on the shear walls. The L’Aquila earthquake revealed the numerous limitations of these interventions, because they led to increased seismic forces (due to greater additional weight) and to deformation incompatibilities of the incorporated elements with the existing masonry walls. This paper provides a discussion of technical issues pertaining to the seismic retrofit of the Santa Maria di Collemaggio Basilica and in particular, the limitations of the last (2000) retrofit intervention. Considerable damage was caused to the church because of questionable actions and incorrect and improper technical choices.