Do landslides follow landslides? Insights in path dependency from a multi-temporal landslide inventory

Landslides are a major category of natural disasters, causing loss of lives, livelihoods and property. The critical roles played by triggering (such as extreme rainfall and earthquakes), and intrinsic factors (such as slope steepness, soil properties and lithology) have previously successfully been recognized and quantified using a variety of qualitative, quantitative and hybrid methods in a wide range of study sites. However, available data typically do not allow to investigate the effect that earlier landslides have on intrinsic factors and hence on follow-up landslides. Therefore, existing methods cannot account for the potentially complex susceptibility changes caused by landslide events. In this study, we used a substantially different alternative approach to shed light on the potential effect of earlier landslides using a multi-temporal dataset of landslide occurrence containing 17 time slices. Spatial overlap and the time interval between landslides play key roles in our work. We quantified the degree to which landslides preferentially occur in locations where landslides occurred previously, how long such an effect is noticeable, and how landslides are spatially associated over time. We also investigated whether overlap with previous landslides causes differences in landslide geometric properties. We found that overlap among landslides demonstrates a clear legacy effect (path dependency) that has influence on the landslide affected area. Landslides appear to cause greater susceptibility for follow-up landslides over a period of about 10  years. Follow-up landslides are on average larger and rounder than landslides that do not follow earlier slides. The effect of earlier landslides on follow-up landslides has implications for understanding of the landslides evolution and the assessment of landslide susceptibility.     

Soil water content vertical profiles under natural conditions: matching of experiments and simulations by a conceptual model

The prediction of soil moisture content, θ, as a function of depth, z, and time, t, is of fundamental importance for applications in many hydrological processes. The main objective of this paper is to provide an approach to solve this problem at a local scale in soils with vegetation. The matching of soil moisture vertical profiles observed under natural conditions in grassy plots and their simulations by a conceptual model is presented. Experimental measurements were performed in a plot located in Central Italy, complete with hydrometeorological sensors specifically set up and equipped with a time domain reflectometry system providing the water content, θ_e(z, t). A conceptual model framework earlier proposed for two‐layered soil vertical profiles was modified and adopted for simulations. The changes concern the incorporation of evapotranspiration, the reduction of the original model for applications also to homogeneous soil vertical profiles, and a correction for the differences existing between assumed and observed initial moisture contents. In the model calibration, it was found that the effects of vegetation could be represented adequately by a fictitious soil vertical profile with a more permeable upper layer of saturated hydraulic conductivity, K_s, independent of time. Then, for the validation events, the model simulations in the stages of both infiltration and redistribution/evapotranspiration reproduced appropriately θ_e(z, t) with typical values of root mean square error in the range 0.0017–0.0657. Similar results were obtained by applying the modified two‐layered model for simulations of experimental data observed in three other plots located in Northern Italy and Germany. For all four vegetated sites, the two‐layer profile better matched the experimental data than the assumption of a homogeneous profile. Thus, the conceptual approach based on a two‐layered scheme for representing θ(z, t) in soils with vegetation appears to be appropriate for many hydrological applications. Copyright © 2013 John Wiley & Sons, Ltd.     

Sensitivity analysis of CDOM spectral slope in artificial and natural samples: an application in the central eastern Mediterranean Basin

In the past two decades, optical properties of chromophoric dissolved organic matter (CDOM) in marine environments have been extensively studied. Many of these studies report CDOM properties for the offshore environment where this complex mixture of optically active compounds is strongly diluted. Nevertheless, autochthonous and allochthonous sources have been identified and sinks related to photodegradation and bacterial activity have been demonstrated. The calculation of the spectral slope of the CDOM absorption curve has been proven to be useful and is often reported. However, a rigorous uncertainty analysis of the slope calculation is rarely reported. In this paper, we propose a method to evaluate the uncertainty of CDOM spectral slope calculated between 270 and 400 nm, using both naturally sampled and artificial solutions. We use these results to study the ultra-oligotrophic waters of the Mediterranean Sea (central eastern basin), where little is known about CDOM spatial distribution. We show that dilutions of both artificial and natural samples produce a Gaussian distribution of spectral slopes, indicating that consistent values may be determined, with a typical uncertainty of ±0.0004 nm⁻¹ when absorption at 300 nm was greater then 0.1 m⁻¹ (0.1 m pathlength). Comparing the distribution of spectral slopes from central eastern basin samples to a Gaussian distribution, we show differences between measurements that were significantly different. These values allow us to distinguish possible sources (algal derived CDOM), sinks (e.g. photo-bleaching) at different depths. We propose a subdivision of CDOM compounds into refractory and semilabile/refractory pools and evaluate the CDOM spectral slope of algal derived CDOM released at or near deep chlorophyll maximum.     

Coastal defence through low crested breakwater structures: jumping out of the frying pan into the fire?

The Adriatic coast of Punta Marina (Ravenna) is protected by 3-km long low crested breakwater structures (LCSs). Through a 3-years long multidisciplinar study, we assessed the impact of such defensive structures on environmental and biological condition. LCSs create pools where conditions are very different from the surrounding nearshore system. Mechanical disturbance by currents and waves varied greatly in intensity and frequency between seaward and landward sides of the structures. Sedimentary budget was positive at the landward side, but it was due to a gain on the seafloor and not on the emerged beach. The budget at seaward was negative. LCSs determine differences in benthic assemblages, alter the seasonal pattern of communities, and modify seasonal fluctuations of animal assemblages. Landward sheltered areas can be seen as “lagoonal island” surrounded by a “sea of marine habitat”. Differences in ecological quality status, obtained through M-AMBI, are due to the sum of these factors.     

Degradation and Detoxification of Chlorophenols in Continuous‐Flow Fixed‐Bed Aerobic Reactors

An indigenous bacterial strain of Delftia sp. capable of degrading 2,4‐dicholorophenol and an indigenous bacterial community that degrades 2,4,6‐trichlorophenol (TCP) were employed to inoculate continuous down‐flow fixed‐bed reactors. Continuous‐reactors were constructed from PVC employing hollow PVC cylinders as support material. Synthetic wastewater was prepared by dissolving the corresponding chlorophenol in non‐sterile groundwater. Biodegradation was evaluated by spectrophotometry, chloride release, GC, and microbial growth. Detoxification was evaluated by using Daphnia magna as test organism. Delftia sp. was able to remove an average of 95.6% of DCP. Efficiency in terms of chemical oxygen demand (COD) was of 88.9%. The indigenous bacterial community that degrades TCP reached an average efficiency of 96.5 and 91.6% in terms of compound and COD removal, respectively. In both cases stoichiometric removal of chloride and detoxification was achieved. When synthetic wastewater feed was cut off for 7 days, both reactors showed a fast recovery after inflow restarting, reaching average outlet concentration values within 36 h. The promising behavior of the microorganisms and the low cost of the reactors tested allow us to suggest their possible application to remediation processes.     

Formation rates of Subantarctic mode water and Antarctic intermediate water within the South Pacific

The formation of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) significantly contributes to the total uptake and storage of anthropogenic gases, such as CO₂ and chlorofluorocarbons (CFCs), within the world's oceans. SAMW and AAIW formation rates in the South Pacific are quantified based on CFC-12 inventories using hydrographic data from WOCE, CLIVAR, and data collected in the austral winter of 2005. This study documents the first wintertime observations of CFC-11 and CFC-12 saturations with respect to the 2005 atmosphere in the formation region of the southeast Pacific for SAMW and AAIW. SAMW is 94% and 95% saturated for CFC-11 and CFC-12, respectively, and AAIW is 60% saturated for both CFC-11 and CFC-12. SAMW is defined from the Subantarctic Front to the equator between potential densities 26.80-27.06 kg m⁻³, and AAIW is defined from the Polar Front to 20°N between potential densities 27.06-27.40 kg m⁻³. CFC-12 inventories are 16.0×10⁶ moles for SAMW and 8.7×10⁶ moles for AAIW, corresponding to formation rates of 7.3±2.1 Sv for SAMW and 5.8±1.7 Sv for AAIW circulating within the South Pacific. Inter-ocean transports of SAMW from the South Pacific to the South Atlantic are estimated to be 4.4±0.6 Sv. Thus, the total formation of SAMW in the South Pacific is approximately 11.7±2.2 Sv. These formation rates represent the average formation rates over the major period of CFC input, from 1970 to 2005. The CFC-12 inventory maps provide direct evidence for two areas of formation of SAMW, one in the southeast Pacific and one in the central Pacific. Furthermore, eddies in the central Pacific containing high CFC concentrations may contribute to SAMW and to a lesser extent AAIW formation. These CFC-derived rates provide a baseline with which to compare past and future formation rates of SAMW and AAIW.     

Recovery of the macrozoobenthic community after severe dystrophic crises in a Mediterranean coastal Lagoon (Orbetello, Italy)

The Orbetello lagoon, one of the largest Western Mediterranean lagoons, was affected by high algal blooms and severe anoxic crises in 1992 and 1993, with fish mortality throughout most of the lagoon and a drastic reduction of benthic species. Many measures were undertaken between 1994 and 1996 to remove the severe eutrophication of this lagoon. Such measures included harvesting of the floating seaweed biomass, removal of all wastewater and sea channel enlargement by dredging activity. The aim of the present research was to study the macrozoobenthic assemblage in 1999 and to compare it with previous research in order to assess the recovery of the lagoon. A total of 106 taxa and 45,175 individuals were collected in three areas (sea inlets, western and eastern lagoon) having different organic matter loading during winter and summer. The results of multivariate and correlation analyses indicated that trophic status and its seasonal dynamics were crucial in determining species distribution among the different areas. Moreover, comparison of the macrozoobenthic structure between winter 1995 and 1999 showed differences in species composition and structural parameters in the western and eastern lagoon, where a clear recovery of benthic assemblages was observed in 1999. However, the dominance of opportunistic species in most of the lagoon and the decrease in some structural parameters during summer suggested that this basin still presented signs of disturbance in 1999, six years after the last extended anoxic crises and even after completion of the restoration measures carried out in the environment. It was hypothesized that a secondary disturbance, sustained by a high seasonal release of nutrients from the sediments, could create considerable deviations from the expected improvement in benthic conditions.