Timing the time of concentration: shedding light on a paradox

From the origins of hydrology, the time of concentration, t_c, has conventionally been tackled as a constant quantity. However, theoretical proof and empirical evidence imply that t_c exhibits significant variability against rainfall, making its definition and estimation a hydrological paradox. Adopting the assumptions of the Rational method and the kinematic approach, an effective procedure in a GIS environment for estimating the travel time across a catchment’s longest flow path is provided. By application in 30 Mediterranean basins, it is illustrated that t_c is a negative power function of excess rainfall intensity. Regional formulas are established to infer its multiplier (unit time of concentration) and exponent from abstract geomorphological information, which are validated against observed data and theoretical literature outcomes. Besides offering a fast and easy solution to the paradox, we highlight the necessity of implementing the varying t_c concept within hydrological modelling, signalling a major shift from current engineering practices.     

Assessment of the sensitivity of the southern coast of the Gulf of Corinth (Peloponnese, Greece) to sea-level rise

The eustatic sea-level rise due to global warming is predicted to reach approximately 18?C59 cm by the year 2100, which necessitates the identification and protection of sensitive sections of coastline. In this study, the classification of the southern coast of the Gulf of Corinth according to the sensitivity to the anticipated future sealevel rise is attempted by applying the Coastal Sensitivity Index (CSI), with variable ranges specifically modified for the coastal environment of Greece, utilizing GIS technology. The studied coastline has a length of 148 km and is oriented along the WNW-ESE direction. CSI calculation involves the relation of the following physical variables, associated with the sensitivity to long-term sea-level rise, in a quantifiable manner: geomorphology, coastal slope, relative sea-level rise rate, shoreline erosion or accretion rate, mean tidal range and mean wave height. For each variable, a relative risk value is assigned according to the potential magnitude of its contribution to physical changes on the coast as the sea-level rises. Every section of the coastline is assigned a risk ranking based on each variable, and the CSI is calculated as the square root of the product of the ranked variables divided by the total number of variables. Subsequently, a CSI map is produced for the studied coastline. This map showed that an extensive length of the coast (57.0 km, corresponding to 38.7% of the entire coastline) is characterized as highly and very highly sensitive primarily due to the low topography, the presence of erosionsusceptible geological formations and landforms and fast relative sea-level rise rates. Areas of high and very high CSI values host socio-economically important land uses and activities.     

Simulation of the earthquake-induced collapse of a school building in Turkey in 2011 Van Earthquake

Collapses of school or dormitory buildings experienced in recent earthquakes raise the issue of safety as a major challenge for decision makers. A school building is ‘just another structure’ technically speaking, however, the consequences of a collapse in an earthquake could lead to social reactions in the complex aftermath of a seismic tremor more than any other type of structure may possibly cause. In this paper a school building that collapsed during 2011 Tabanli, Van Earthquake in eastern Turkey, is analysed in order to identify the possible reasons that led to collapse. Apart from the inherent deficiencies of RC buildings built in Turkey in the 80's and 90's, its structural design exhibits a strikingly high asymmetry. In the analyses conducted, much attention has been given to the direction of the earthquake load and its coincidence with the bi-axial structural response parameters. The failure of the structure to comply with the 1975 Code, in vigor at the time of construction, has also been evaluated with respect to the structure’s collapse. Among the parameters that controlled the collapse, the high plan asymmetry and the coincidence of the vulnerable directions with the dominant shaking direction were critical, as well as the underestimation of the seismic hazard and the lateral design force level, specified by the then Turkish Earthquake Code.     

North Atlantic Oscillation – Concepts And Studies

This paper aims to provide a comprehensive review of previous studies and concepts concerning the North Atlantic Oscillation. The North Atlantic Oscillation (NAO) and its recent homologue, the Arctic Oscillation/Northern Hemisphere annular mode (AO/NAM), are the most prominent modes of variability in the Northern Hemisphere winter climate. The NAO teleconnection is characterised by a meridional displacement of atmospheric mass over the North Atlantic area. Its state is usually expressed by the standardised air pressure difference between the Azores High and the Iceland Low. ThisNAO index is a measure of the strength of the westerly flow (positive with strong westerlies, and vice versa). Together with the El Niño/Southern Oscillation (ENSO) phenomenon, the NAO is a major source of seasonal to interdecadal variability in the global atmosphere. On interannual and shorter time scales, the NAO dynamics can be explained as a purely internal mode of variability of the atmospheric circulation. Interdecadal variability maybe influenced, however, by ocean and sea-ice processes.     

Liquid water uptake in unconfined Callovo Oxfordian clay-rock studied with neutron and X-ray imaging

The Callovo Oxfordian clay-rock (COx) is studied in France for the disposal of radioactive waste, because of its extremely low permeability. This host rock is governed by a hydromechanical coupling of high complexity. This paper presents an experimental study into the mechanisms of water uptake in small, unconfined, prismatic specimens of COx, motivated by the comprehension of cracking observed during concrete/COx interface sample preparation. Water uptake is monitored using both X-ray tomography and neutron radiography, the combination of these imaging techniques allowing material deformation and water arrival to be quantified, respectively. Given the speed of water entry and crack propagation, relatively fast imaging is required: 5-min X-ray tomographies and 10-s neutron radiographs are used. In this study, pairs of similar COx samples from the same core are tested separately with each imaging technique. Two different orientations with respect to the core are also investigated. Analysis of the resulting images yields with micro- and macro-scale insights into hydromechanical mechanisms to be obtained. This allows the cracking to be interpreted as a rapid breakdown in capillary suction (supposed large both to drying and rebound from in situ stress state) due to water arrival, which in turn causes a loss of effective stress, allowing cracks to propagate and deliver water further into the material.

     

Macrobenthic community changes due to dystrophic events and freshwater inflow: Changes in space and time in a Mediterranean lagoon (Gialova lagoon, SW Greece)

This paper considers the hypothesis that changes in community structure through the control of the larvae maintenance and of the biological traits of the species mostly contribute to the spatio-temporal community pattern. This is supported by the results of the study, the changes to the macrobenthic community pattern deriving from two sampling periods in Gialova lagoon (SW Greece), carried out on a seasonal basis for two yearly periods: 1994-95 and 1998-99. Cognetti’s hypothesis that populations of the tolerant species occupying coastal marine habitats may belong to different species is another alternative hypothesis still to be tested. The importance of alternative management plans aiming at the amelioration of the hydrodynamic conditions of the lagoons and supported by continuous scientific monitoring is highlighted. Two canals bringing fresh water were opened at the beginning of the second sampling period, an intervention among others, suggested by the management plan proposed after the end of the first sampling period. The results show considerable variations in the values of the key environmental variables, for instance long periods with negative Redox potential values, decreased salinity and increased concentrations of the particulate organic matter and of the nutrients. The variables were correlated with the spatio-temporal community pattern, characterized during the second sampling period by: (i) greater dissimilarities among stations/seasons; (ii) disruption of the periodic trend observed on the seasonal scale; (iii) larger relative dissimilarities among the patterns stemming from the macrobenthos and the most abundant groups (polychaetes, molluscs, crustaceans); (iv) significant decrease in abundance or even the disappearance of several marine origin species, along with increased abundance in a few brackish-water species. All of the above changes in the community pattern are considered as early warning signals leading towards degradation, which has not yet been registered in the phylogenetic/taxonomic structure of the macrobenthic community.     

Alien biodiversity in Mediterranean marine caves

The number of alien species in the Mediterranean Sea is increasing rapidly, but few attempts have been made to evaluate impacts on specific habitat types. The present study investigated alien biodiversity in Mediterranean marine caves, both by contributing new records of Eastern Mediterranean cave aliens, and by reviewing the scattered existing literature; the main goals were to highlight potential impacts and investigate the importance of cave environments for the expansion of alien species. Seven new alien species were found in marine caves of the Aegean and Ionian seas, raising the total number of aliens reported from Mediterranean marine caves to 56 species, classified as molluscs, cnidarians, bryozoans, polychaetes, crustaceans, macroalgae, fishes and tunicates. Most cave aliens (66%) were recorded from the Southeastern Levantine coasts, specifically from Lebanese caves. Shipping and Lessepsian migration have been suggested as the main pathways of alien introduction into caves of the Mediterranean Sea. The comparison of alien cave biodiversity with the updated Mediterranean alien inventory (32 species added to the latest inventory) showed similar patterns and trends for species richness, biogeographical origin and major introduction pathways, corroborating previous findings on the high local representativeness of Mediterranean cave biodiversity. Alien species seem to have invaded mostly the entrance and semi‐dark zones of shallow and/or semi‐submerged caves and tunnels, whereas only a few have reached the dark inner sectors or caves of the anchialine type; thus, the unfavourable cave environment seems to be naturally protected from impacts related to opportunistic invasive species, at least to a certain point. Currently there is no research confirming any direct impacts of alien biota on the native cavernicolous one. However, some issues have emerged, implying potential threats that need to be further explored: (i) the presence of a considerable proportion of aliens in most studied marine caves of the Southeastern Mediterranean basin, (ii) the recently observed population explosion of alien cave‐dwelling fishes in the same area, (iii) several indications that alien diversity in marine caves is much higher than we know today. Quantitative surveys and monitoring schemes are needed in order to evaluate potential effects of alien diversity on cave community structure and the role of marine caves as stepping stones for its expansion in the Mediterranean.     

Non-extensive statistical physics approach to fault population distribution. A case study from the southern Hellenic arc (Central Crete)

Fault population statistics play a key role in the understanding of any statistical seismicity approach. In the present work a non-extensive statistical physics approach is formulated and tested for the local fault length distribution. The approach is composed of the following parts: (i) Tsallis entropy, S _q , (ii) maximization of the Tsallis entropy under appropriate constrains, and (iii) derivation of the cumulative distribution function (CDF) of the fault length population. This model is tested using fault length data from the Central Crete graben in front of the Hellenic arc and estimated a thermodynamic q parameter equal to 1.16, which supports the conclusion that the fault system in Central Crete graben is a sub-extensive one.     

Deformation patterns in the southwestern part of the Mediterranean Ridge (South Matapan Trench,Western Greece)

Seismic reflection data and bathymetry analyses, together with geological information, are combined in the present work to identify seabed structural deformation and crustal structure in the Western Mediterranean Ridge (the backstop and the South Matapan Trench). As a first step, we apply bathymetric data and state of art methods of pattern recognition to automatically detect seabed lineaments, which are possibly related to the presence of tectonic structures (faults). The resulting pattern is tied to seismic reflection data, further assisting in the construction of a stratigraphic and structural model for this part of the Mediterranean Ridge. Structural elements and stratigraphic units in the final model are estimated based on: (a) the detected lineaments on the seabed, (b) the distribution of the interval velocities and the presence of velocity inversions, (c) the continuity and the amplitudes of the seismic reflections, the seismic structure of the units and (d) well and stratigraphic data as well as the main tectonic structures from the nearest onshore areas. Seabed morphology in the study area is probably related with the past and recent tectonics movements that result from African and European plates’ convergence. Backthrusts and reverse faults, flower structures and deep normal faults are among the most important extensional/compressional structures interpreted in the study area.     

Wavelet analysis for relating soil amplification and liquefaction effects with seismic performance of precast structures

Precast concrete structures are preferred for facilities with large open areas due to easiness in construction. Such structures are typically composed of individual columns and long‐span beams, and are quite flexible and of limited redundancy. In this paper, nonlinear dynamic analyses of a typical such structure are conducted using as excitation 54 ground motions recorded on top of a variety of soils (hard, soft, and liquefied soil sites). The results show that liquefaction‐affected level‐ground motions systematically impose a greater threat to precast‐concrete structures in terms of seismic demand, even when low values of elastic spectral acceleration prevail, as opposed to soft‐soil records and even more to hard‐soil ones. Thus, elastic spectral acceleration appears to be an insufficient engineering demand parameter for design. Soil effects, the “signature” of which is born on ground motions, are first uncovered using wavelet analysis to detect the evolution of the energy and frequency content of the ground motion in the time domain. From this, the changes in effective (“dominant”) excitation period are noted, persuasively attributed to the nature of the soil, and finally correlated with the observed structural behavior. Copyright © 2016 John Wiley & Sons, Ltd.