The origin,typology, spatial distribution and detrimental effects of the sinkholes developed in the alluvial evaporite karst of the Ebro River valley downstream of Zaragoza city (NE Spain)

Three types of sinkhole have been mapped in a 50 km² stretch of the Ebro River valley downstream of Zaragoza: large collapse sinkholes, large shallow subsidence depressions and small cover-collapse sinkholes. The sinkholes relate to the karstification of evaporitic bedrock that wedges out abruptly downstream, giving way to a shale substratum. Twenty-three collapse sinkholes, up to 50 m in diameter by 6 m deep, and commonly hosting saline ponds, have been identified in the floodplain. They have been attributed to the upward stoping of dissolutional cavities formed within the evaporitic bedrock by rising groundwater flows. Twenty-four large shallow subsidence depressions were mapped in the floodplain. These may reach 850 m in length and were formed by structurally controlled interstratal karstification of soluble beds (halite or glauberite? and gypsum) by rising groundwater flow and the progressive settlement of the overlying bedrock and overburden sediments. A total of 447 small cover-collapse, or dropout, sinkholes have been recognized in a perched alluvial level along the southern margin of the valley. These sinkholes result from the upward propagation of voids through the alluvial mantle caused by the downward migration of detrital sediments into dissolutional voids. The majority of these sinkholes, commonly 1·5–2 m in diameter, are induced by human activities. Over the karstic bedrock, there is a significant increase in sinkhole density downstream. This is interpreted as being a result of the evaporitic bedrock wedging out and the convergence of the groundwater flow lines in the karstic aquifer. The collapse sinkholes in this area, locally with a probability of occurrence higher than 140 sinkholes/km²/year, cause substantial damage to the linear infrastructures, buildings and agriculture, and they might eventually cause the loss of human lives. Copyright © 2006 John Wiley & Sons, Ltd.     

Evaluation of trenching,ground penetrating radar (GPR) and electrical resistivity tomography (ERT) for sinkhole characterization

This paper explores the suitability and advantages of combining the trenching technique with geophysical surveys [ground penetrating radar (GPR) and electrical resistivity tomography (ERT)] for sinkhole characterization in a mantled karst area. The approach is applied to two active sinkholes concealed by anthropogenic deposits and formed by contrasting subsidence mechanisms; collapse and sagging. The ERT section acquired across the collapse sinkhole images the clayey fill of the depression as an obvious low resistivity area, showing the approximate location of the sinkhole edges. Spatially dense GPR surveys provide information on the position of the boundaries of the concealed subsidence structures and their three‐dimensional (3D) internal geometry, revealing the dominant subsidence mechanism. We illustrate the impact of several factors on the quality of the GPR data such as sinkhole size, nominal frequency of the antennas, antenna shielding, and the presence of backfilled excavations and above‐surface objects. Trenches provided detailed information on the subsurface structure of the sinkhole, subsidence magnitude, partitioning of the strain, and the position of the sinkhole edges, especially when they are deep enough and excavated across the central sector and perpendicular to the boundaries. The stratigraphic and structural relationships observed in the trench were then used to infer the spatial evolution of the sinkholes (e.g. enlargement), their kinematic behavior (episodic versus progressive), and to differentiate discrete subsidence events and their associated magnitude. Numerical dates were used to estimate average subsidence rates and the recurrence of subsidence events. Such integrated data sets may be used as an objective basis to forecast the future behavior of potentially damaging sinkholes and to assess the associated hazard and risk. Copyright © 2013 John Wiley & Sons, Ltd.     

Quantifying potential recharge in mantled sinkholes using ERT

Potential recharge through thick soils in mantled sinkholes was quantified using differential electrical resistivity tomography (ERT). Conversion of time series two-dimensional (2D) ERT profiles into 2D volumetric water content profiles using a numerically optimized form of Archie's law allowed us to monitor temporal changes in water content in soil profiles up to 9 m in depth. Combining Penman-Monteith daily potential evapotranspiration (PET) and daily precipitation data with potential recharge calculations for three sinkhole transects indicates that potential recharge occurred only during brief intervals over the study period and ranged from 19% to 31% of cumulative precipitation. Spatial analysis of ERT-derived water content showed that infiltration occurred both on sinkhole flanks and in sinkhole bottoms. Results also demonstrate that mantled sinkholes can act as regions of both rapid and slow recharge. Rapid recharge is likely the result of flow through macropores (such as root casts and thin gravel layers), while slow recharge is the result of unsaturated flow through fine-grained sediments. In addition to developing a new method for quantifying potential recharge at the field scale in unsaturated conditions, we show that mantled sinkholes are an important component of storage in a karst system.     

Geoelectric structure of the Ein Gedi sinkhole occurrence site at the Dead Sea shore in Israel

Hazardous sinkholes started to appear in alluvial fans and unconsolidated sediments along the western Dead Sea coast in 1990. Since then hundreds of sinkholes have appeared from north to south along the shoreline. The Electrical Resistivity Tomography (ERT) method was used to achieve a better understanding of the subsurface geoelectric structure at the sinkhole development sites, taking into account that electric parameters (such as resistivity or conductivity) are very sensitive to formation properties and their variations in time. Fifteen image lines were surveyed at the Ein Gedi area during a period of active sinkhole development (in 2001–2002) over an area of 300 × 550 m². Resistivity cross-sections and maps were constructed from 2-D linear surveys. The process of sinkhole formation in the surveyed area is located in a strip 50–70 m wide and 300–500 m long, extending approximately in a north–south direction. The sinkholes are arranged along a tortuous line within this strip. On resistivity maps and sections this U-shaped zone appears as an alternation of high resistivity anomalies of 350–1000 Ωm (at sinkhole group locations) with narrow background resistivity zones of 50–100 Ωm. The large size of resistivity anomalies (250 × 300 m²), which are considerably greater than those of the sinkholes, form one of the features of the sinkhole sites in the Ein Gedi area. The anomalies continue down to the water table or even deeper (maximum of 25–35 m depth). A low resistivity layer of 1–8 Ωm underlies them. The combined analysis of the image results and other geophysical data shows that high resistivity anomalies are associated with the decompaction of the soil mass at the sinkhole development sites and surrounding areas. Recent studies have shown that sinkholes in the Ein Gedi area are developing along the salt western edge located at a depth of 50 m. The subsurface high resistivity anomaly conforms to the sinkhole line (and salt boundary). They are presumably located above the great dissolution caverns at the salt edge. The heterogeneity of the resistivity structure within the high resistivity anomaly (seen in both lateral and vertical planes) confirms that a disintegration of internal formation structure takes place. Away from the sinkhole sites the subsurface resistivity distribution is homogeneous.     

The application of GPR and ERI in combination with exposure logging and retrodeformation analysis to characterize sinkholes and reconstruct their impact on fluvial sedimentation

This work illustrates the practicality of investigating sinkholes integrating data gathered by ground penetrating radar (GPR), electrical resistivity imaging (ERI) and trenching or direct logging of the subsidence‐affected sediments in combination with retrodeformation analysis. This mutidisciplinary approach has been tested in a large paleosinkhole developed during the deposition of a Quaternary terrace on salt‐bearing evaporites. The subsidence structure, exposed in an artificial excavation, is located next to Puilatos, a village that was abandoned in the 1970s due to severe subsidence damage. Detailed logging of the exposure revealed that the subsidence structure corresponds to an asymmetric sagging and collapse paleosinkhole with no clear evidence of recent activity. The sedimentological and structural relationships together with the retrodeformation analysis indicate that synsedimentary subsidence controlled channel location, the development of a palustrine environment and local changes in the channel pattern. GPR profiles were acquired using an array of systems with different antenna frequencies, including some recently developed shielded antennas with improved vertical resolution and penetration depth. Although radargrams imaged the faulted sagging structure and provided valuable data on fault throw, they did not satisfactorily image the complex architecture of the fluvial deposit. ERI showed lower resolution but higher penetration depth when compared to GPR, roughly capturing the subsidence structure and yielding information on the thickness of the high‐resistivity alluvium and the nature of the underlying low‐resistivity karstic residue developed on top of the halite‐bearing evaporitic bedrock. Data comparison allows the assessment of the advantages and limitations of these complementary techniques, highly useful for site‐specific sinkhole risk management. Copyright © 2016 John Wiley & Sons, Ltd.     

离石北部北川河阶地特征及其新构造指示

详细研究了离石北部一带阶地的地层地貌特征,并尝试对吕梁山山体的隆升进行分析探讨。结果表明,晚更新世以来该区有过三次间歇性隆升,并且三级阶地形成以来即晚更新世早期山体隆升相对快速强烈,二级阶地形成以来即晚更新世晚期至全新世时期山体隆升处于相对缓慢的过程。     

TerEx Toolbox for semi‐automated selection of fluvial terrace and floodplain features from lidar

Terraces and floodplains are important indicators of near‐channel sediment dynamics, serving as potential sediment sources and sinks. Increasing availability of high resolution topography data over large areas calls for development of semi‐automated techniques for identification and measurement of these features. In this study we introduce a novel tool that accommodates user‐defined parameters including, a local‐relief threshold selected by a variable‐size moving window, minimum area threshold, and maximum distance from the channel to identify and map discrete terrace and floodplain surfaces. Each of the parameters can easily be calibrated for a given watershed or reach. Subsequently, the tool automatically measures planform area, absolute elevation, and height relative to the local river channel for each terrace polygon. We validate the tool in two locations where terrace maps were previously developed via manual digitization from lidar and extensive field mapping campaigns. The tool is also tested on six different types of rivers to provide examples of starting selection parameters, and to test effectiveness of the tool across a wide range of landscapes. Generally, the tool provides a high quality draft map of terrace and floodplain surfaces across the wide range of environmental conditions for which it has been tested. We find that the tool functions best in catchments where the terraces are spatially extensive, with distinct differences between the terrace and floodplain. The most challenging environments for semi‐automated terrace and floodplain mapping include steep catchments with dense riparian vegetation, and very small terraces (~10 m² in areal extent). We then apply the tool to map terraces and floodplains in the Root River watershed, southeastern Minnesota and generate exceedance plots for terrace heights. These plots provide a first pass analysis to indicate the tributaries and reaches of the river where terraces constitute a significant source of sediment. Copyright © 2013 John Wiley & Sons, Ltd.     

Assessment of sediment yield in a small karst catchment by using 137Cs tracer technique

There is very thin soil layer in karst rocky desertification areas in Southwest China,sediment deposition and sediment yield in the karst area affects the development of vegetation greatly.In the present study,the ¹³⁷Cs technique was used to assess the rate of sediment deposition and sediment yield in a small karst catchment.The ¹³⁷Cs inventory within the depression varied between 800 m^-2 and 2,200 Bq m^-2,with the mean value of 1,500.1 Bq m^-2.The ¹³⁷Cs reference inventory at a nearby reference site was 805.9 Bq m^-2.It could be inferred to that sediment deposition had occurred in the catchment.The mean depth of sediments deposition in the depression was 6 cm and the deposition rate was approximately 0.13 cm yr^-1.The analysis of the topographic characteristics of the catchment revealed that the sediment deposition occurred mainly at the lower part of the small catchment.Although,there was a sinkhole in the depression,little sediment had drained out with runoff through the sinkhole,because the local people built ridges around the sinkholes for storing water.According to this,sediment yield rate in the small catchment was estimated to be approximately 19.25 to 27.5 t km^-2 yr^-1,and the extremely low sediment yield was maybe the main obstacle to vegetation restoration in karst rocky desertification areas.     

TEM study of the geoelectrical structure and groundwater salinity of the Nahal Hever sinkhole site, Dead Sea shore, Israel

Since the 1990s a large number of sinkholes have appeared in the Dead Sea (DS) coastal area. Sinkhole development was triggered by the lowering of the DS level. In the literature the relationship between the sinkholes and the DS level is explained by intrusion of relatively fresh water into the aquifer thereby dramatically accelerating the salt dissolution with creation of subsurface caverns, which in turn cause sinkholes. The main goal of our project was detection and localization of relatively fresh groundwater. During our study we used the transient electromagnetic method (TEM) to measure the electrical resistivity of the subsurface. As a test site we selected Nahal Hever South which is typical for the DS coast. Our results show that resistivity of the shallow subsurface reflects its vertical and lateral structure, e.g., its main hydrogeological elements explain the inter-relations between geology, hydrogeology, and sinkholes. The TEM method has allowed detailed differentiation of layers (clay, salt, etc.) in the subsurface based on their bulk resistivity. The 10 m-thick salt layer composed of idiomorphic crystals of halite deposited during the earlier Holocene period was extrapolated from borehole HS-2 through the study area. It was found that in Nahal Hever the typical value of the bulk resistivity of clay saturated with the DS brine varies between 0.2 and 0.3 Ωm, whereas saturated gravel and sandy sediments are characterized by resistivity between 0.4 and 0.6 Ωm. The high water salinity of the aquifer (enveloping the salt layer) expressed in terms of resistivity is also an important characterization of the sinkhole development mechanism. The electrical resistivity of the aquifer in the vicinity of the salt unit and its western border did not exceed 1 Ωm (in most cases aquifer resistivity was 0.2-0.6 Ωm) proving that, in accordance with existing criteria, the pores of the alluvial sediments are filled with highly mineralized DS brine. However, we suggest that the criterion of the aquifer resistivity responsible for the salt dissolution should be decreased from 1 Ωm to 0.6 Ωm corresponding to the chloride concentration of approximately 100 g/l (the chloride saturation condition reaches 224 g/l in the northern DS basin and 280 g/l in the southern one).Based on TEM results we can reliably conclude that in 2005, when most of sinkholes had appeared at the surface, salt was located within a very low resistivity environment inside sediments saturated with DS brine. Intrusion of relatively fresh groundwater into the aquifer through the 600 × 600 m²area affected by sinkholes has not been observed.     

Using δ18O and δ2H to Detect Hydraulic Connection Between a Sinkhole Lake and a First-Magnitude Spring

Oxygen and hydrogen isotopes were used in this study to detect a hydraulic connection between a sinkhole lake and a karst spring. In karst areas, surface water that flows to a lake can drain through sinkholes in the lakebed to the underlying aquifer, and then flows in karst conduits and through aquifer matrix. At the study site located in northwest Florida, USA, Lake Miccosukee immediately drains into two sinkholes. Results from a dye tracing experiment indicate that lake water discharges at Natural Bridge Spring, a first-magnitude spring 32 km downgradient from the lake. By collecting weekly water samples from the lake, the spring, and a groundwater well 10 m away from the lake during the dry period between October 2019 and January 2020, it was found that, when rainfall effects on isotopic signature in spring water are removed, increased isotope ratios of spring water can be explained by mixing of heavy-isotope-enriched lake water into groundwater, indicating hydraulic connection between the lake and the spring. Such a detection of hydraulic connection at the scale of tens of kilometers and for a first-magnitude spring has not been previously reported in the literature. Based on the isotope ratio data, it was estimated that, during the study period, about 8.5% the spring discharge was the lake water that drained into the lake sinkholes.     

Landslide susceptibility analysis based on citizen reports

Landslide susceptibility estimates are essential for reducing the risk posed by landslides to social and economic well-being. However, estimates of landslide susceptibility depend on reliable landslide inventories whose production requires extensive field or remote sensing efforts. Further, most inventories are not updated through time and thus may not capture the influence of changes in climate and/or land use. Inventories based on citizen reports of landslide occurrence, have the potential to overcome these limitations. Such an inventory can be produced from citizen reports to a 311-phone and online system, a nationwide database that updates real-time and records reported landslides location and timing. Whereas this landslide inventory is promising, it has not used for landslide susceptibility analyses and may be associated with spatial uncertainties and reporting biases. In this study we explore the use of 311-based landslide inventory for landslide susceptibility estimates in Pittsburgh, PA, USA, where landslide risk is among the highest in the nation. We compare the 311-based inventory to field-validated inventories through a multi-pronged approach that combines field validation of 311-reported landslides, probabilistic analysis of the association between landslides and the underlying topographic and geologic factors, and spatial filtering. Our results show that: (a) approximately 70% of the 311-reported landslides are associated with an identifiable landslide in the field; (b) the spatial uncertainty of the 311-reported landslides is 104 ± 25 m; (c) 311-reported landslides differ from other inventories in that they are primarily associated with proximity to roads, however, field-correction of 311-reported landslide locations rectifies this anomaly; (d) a simple spatial filter, scaled by the uncertainty in location as determined from a subset of the 311-data, can increase the consistency between the 311-reported inventory and field-validated inventories. These results suggest that 311-based landslide inventories can improve susceptibility estimates at a relatively low cost and high temporal resolution.     

Detection of sinkholes using 2D electrical resistivity imaging

Sinkholes in dolomitic areas are notoriously difficult geophysical targets, and selecting an appropriate geophysical solution is not straightforward. Electrical resistivity imaging, or tomography (RESTOM) is well suited to mapping sinkholes because of the ability of the technique for detecting resistive features and discriminating subtle resistivity variations. RESTOM surveys were conducted at two sinkhole sites near Pretoria, South Africa. The survey areas are located in the dolomites of the Lyttelton Formation, which forms part of the Malamani Subgroup and Chuniespoort Group of the Transvaal Supergroup. The survey results suggest that RESTOM is an ideal geophysical tool to aid in the detection and monitoring of sinkholes and other subsurface cavities.     

Deep seated fault-related volcanogenic H2S as the key agent of high sinkhole concentration areas

Karst collapse sinkholes (KCS) are a peculiar karst morphology resulting from the collapse of the topographic surface caused by subsurface karstification. In the Southern Apennines these phenomena are not randomly distributed but concentrate in several zones, named High Sinkhole Concentration Areas (HSCA), showing peculiar geological, structural and hydrogeological conditions. Gas vents and mineral springs occur widely in these HSCA, and are often stricty related to the KCS. Starting from four representative areas, the aim of this study is to ascertain if there is a genetic link between peculiar mineralogical vs geochemical features of densely fractured/altered rocky masses in KCS, gas vents/springs occurrences and diffuse hypogenic karstification. By means of a multidisciplinary approach (geological/geomorphological, mineralogical and geochemical) we selected and analysed four different kinds of samples related to KCS, gas vent and spring occurrences: (i) altered to deeply altered limestones sampled in the sinkholes; (ii) unaltered limestones sampled in close proximity to sinkhole areas; (iii) gypsum-rich crusts/patinae, precipitated near to gas vents; (iv) gypsum- and calcite-rich precipitates occurring at springs or inside caves. Among neoformed non-carbonate minerals, gypsum is virtually ubiquitous, halides (fluorite, halite, sylvite) also occur in small to trace amounts. The mineralogical assemblages of the different samples show similarities and may also be compatible with hypogenic speleogenesis and with a process of alteration of the carbonate bedrock by means of uprising mineralizing fluids along structural discontinuities. Stable isotopic compositions (S, O) display strong variability in δ³⁴S and δ¹⁸O for sulfate in the different areas, but a deep-seated sulfur source can be hypothesized for many of the studied KCS-related samples. This study has important implications for the relationships between areas of high concentration of sinkholes, regional fault systems, mechanical characteristics of rocks and the high seismicity typical of these areas of the Southern Apennines. © 2018 John Wiley & Sons, Ltd.     

The chronology and stratigraphy of the alluvial terraces of the river Dane Valley,Cheshire, N.W. England

Three groups of alluvial terraces together with the modern floodplain mark the Postglacial development of the middle part of the Dane Valley, Cheshire. These are a High terrace group of late Pleistocene age, a Middle terrace group of late Pleistocene to early Holocene age, a Low terrace of mid–late Holocene age, and a modern (post ca. 1840 AD) floodplain. A chronology of erosion, deposition, and landform development since mid-Holocene times is established in this paper on the basis of terrace morphology, stratigraphy, sedimentology, soil analysis, magnetic mineral analysis, and four radiocarbon dates. After dissection of the Middle terrace during the early to mid-Holocene, a long period of lateral activity by the river was followed by a major aggradation phase, which formed the Low terrace surface. This was followed by dissection during the last ca. 300 years and the development of the modern floodplain since ca. 1840 AD. Various explanations for the changes during the Holocene are considered; the Low terrace aggradation appears to be related to a major phase of mediaeval soil erosion.     

Amazonian floodplain water balance based on modelling and analyses of hydrologic and electrical conductivity data

In the Amazon basin, floodplains form a complex mosaic of freshwater systems with differing morphologies, resulting in varied inundation patterns and heterogeneous chemical and ecological characteristics. In this study, we focused on the Janauacá floodplain, a medium‐sized system (786 km², including the local watershed) located along the Solimões River. Based on in situ and satellite observations acquired from November 2006 to November 2011, we computed water fluxes between the mainstream and the floodplain and examined the temporal dynamics of floodplain storage from river flooding, rainfall, runoff, and exchanges with groundwater through bank seepage for the 5 years from 2006 to 2011. The mainstream was the main input of water to the flooded area, accounting on average for 93% of total water inputs by the end of the water year. Direct precipitation and runoff from uplands contributed less than or equal to 5% and 10%, respectively. The seepage contribution was less than 1%. Model uncertainties, evaluated using Monte Carlo analysis of the input data and model parameters, showed that all water fluxes were relatively well constrained except for outflow through seepage, which had a standard deviation across simulations greater than 60%. The water balance computation was verified using electrical conductivity as an assumed non‐reactive tracer. Except during periods of very low water, the simulated and measured conductivities agreed well. Moreover, conductivity data analysis confirmed that the Janauacá system can be considered homogeneous in terms of electrical conductivity for filling percentages equal to or greater than 40% (i.e., when the water level is above 19.5 m, generally from April to August) but presented large heterogeneities during the rest of the hydrological cycle.     

Anthropogenic sinkholes in the territory of the city of Naples (Southern Italy)

About 190 anthropogenic sinkholes occurred within the territory of the city of Naples (Southern Italy) between 1915 and 2010.In the study area, the genesis of sinkholes can be ascribed to two major factors, often strongly interacting with each other: the existence of a complex network of underground man-made cavities, and the inadequacy of the sewage disposal system.Rainfall has been identified as the main triggering factor combined with anthropogenic activity. Based on such predisposing factors, in addition to the geological setting, a susceptibility map of the territory has been realised.     

SCALES: a large‐scale assessment model of soil erosion hazard in Basse‐Normandie (northern‐western France)

The cartography of erosion risk is mainly based on the development of models, which evaluate in a qualitative and quantitative manner the physical reproduction of the erosion processes (CORINE, EHU, INRA). These models are mainly semi‐quantitative but can be physically based and spatially distributed (the Pan‐European Soil Erosion Risk Assessment, PESERA). They are characterized by their simplicity and their applicability potential at large temporal and spatial scales. In developing our model SCALES (Spatialisation d'éChelle fine de l'ALéa Erosion des Sols/large‐scale assessment and mapping model of soil erosion hazard), we had in mind several objectives: (1) to map soil erosion at a regional scale with the guarantee of a large accuracy on the local level, (2) to envisage an applicability of the model in European oceanic areas, (3) to focus the erosion hazard estimation on the level of source areas (on‐site erosion), which are the agricultural parcels, (4) to take into account the weight of the temporality of agricultural practices (land‐use concept). Because of these objectives, the nature of variables, which characterize the erosion factors and because of its structure, SCALES differs from other models. Tested in Basse‐Normandie (Calvados 5500 km²) SCALES reveals a strong predisposition of the study area to the soil erosion which should require to be expressed in a wet year. Apart from an internal validation, we tried an intermediate one by comparing our results with those from INRA and PESERA. It appeared that these models under estimate medium erosion levels and differ in the spatial localization of areas with the highest erosion risks. SCALES underlines here the limitations in the use of pedo‐transfer functions and the interpolation of input data with a low resolution. One must not forget however that these models are mainly focused on an interregional comparative approach. Therefore the comparison of SCALES data with those of the INRA and PESERA models cannot result on a convincing validation of our model. For the moment the validation is based on the opinion of local experts, who agree with the qualitative indications delivered by our cartography. An external validation of SCALES is foreseen, which will be based on a thorough inventory of erosion signals in areas with different hazard levels. Copyright © 2010 John Wiley & Sons, Ltd.     

Late Holocene channel and floodplain development in a wandering gravel‐bed river: the River South Tyne at Lambley,northern England

Geomorphological analyses of the morphology, lithostratigraphy and chronology of Holocene alluvial fills in a 2·75 km long piedmont reach of the wandering gravel‐bed River South Tyne at Lambley in Northumberland, northern England, have identified spatial and temporal patterns of late Holocene channel and floodplain development and elucidated the relationship between reach‐ and subreach‐scale channel transformation and terrace formation. Five terraced alluvial fills have been dated to periods sometime between c. 1400 BC –AD 1100, AD 1100–1300, AD 1300–1700, AD 1700–1850 and from AD 1850 to the present. Palaeochannel morphology and lithofacies architecture of alluvial deposits indicate that the past 3000 years has been characterized by episodic channel and floodplain change associated with development and subsequent recovery of subreach‐scale zones of instability which have been fixed in neither time nor space. Cartographic and photographic evidence spanning the past 130 years suggests channel transformation can be accomplished in as little as 50 years. The localized and episodic nature of fluvial adjustment at Lambley points to the operation of subreach‐scale controls of coarse sediment transfers. These include downstream propagation of sediment waves, as well as internal controls imposed by differing valley floor morphology, gradient and boundary materials. However, the preservation of correlated terrace levels indicates that major phases of floodplain construction and entrenchment have been superimposed over locally complex patterns of sediment transfer. Reach‐scale lateral and vertical channel adjustments at Lambley appear to be closely related to climatically driven changes in flood frequency and magnitude, with clusters of extreme floods being particularly important for accomplishing entrenchment and reconfiguring the pattern of localized instability zones. Confinement of flood flows by valley entrenchment, and contamination of catchment river courses by metal‐rich fine sediments following recent historic mining operations, have combined to render the South Tyne at Lambley increasingly sensitive to changes in flood regimes over the past 1000 years. Copyright © 2000 John Wiley & Sons, Ltd.     

Salt karst and tectonics: sinkholes development along tension cracks between parallel strike‐slip faults,Dead Sea,Jordan

This work deals with the tectonic interpretation of an alignment of more than 300 sinkholes stretching along the Jordanian coast of the Dead Sea, Ghor Al Haditha area. Its dimensions are 6 km long with a width of 600 m. Sinkholes appeared during the last decades as a consequence of the very rapid lowering of the lake level. The linear shape was inferred from ground collapse inventories carried out between 1991 and 2008. The lineament is replaced and analyzed in its structural setting at regional and local scales. Its direction (N 24° E) is sub‐parallel to the ones displayed by many focal mechanisms, especially the one associated with the earthquake of the 23 April, 1979 (mb = 5·1; N 20° E ± 5°), which is representative of all focal mechanisms calculated on a fault plane compatible with the general direction of the Jordan‐Dead Sea Transform fault system for the east coast of the Dead Sea area. The alignment of sinkholes is constituted by 13 minor linear segments separated by as many empty spaces. Four minor linear units present an en‐echelon arrangement from which one can deduce the presence of a local extensional stress field. In this context, the sinkhole locations provide information of subsurface discontinuities interpreted as hidden fractures. In a close future, such results could support the work of decision‐makers and engineers in the projected development of the area. Copyright © 2009 John Wiley & Sons, Ltd.     

Impact of natural reforestation on floodplain sedimentation in the Dragonja basin,SW Slovenia

Changes in floodplain sediment dynamics have profound effects on riverine habitats and riparian biodiversity. Depopulation due to socio‐economic changes in the Dragonja catchment (91 km²) in southwestern Slovenia resulted in the abandonment of agricultural fields, followed by natural reforestation since 1945. This profoundly changed the water and sediment supply to the streams, as well as floodplain sediment deposition. This paper presents a reconstruction of the development of the Dragonja floodplain due to these land use changes during the last 60 years. The reconstruction is based on dating of floodplain sediments using ¹³⁷Cs profiles, measurement of actual sedimentation rates using artificial grass sedimentation mats, and linking this information to the present‐day hydrological behaviour of the river. The sedimentation mats showed that floodplain sedimentation was restricted to peak flows of considerable magnitude. Due to the reforestation, the return period of such high flows increased from 0·31 year in the period 1960–1985 to 0·81 year between 1986 and 2003, with commensurate changes in sedimentation rates. At the 1·5 m river terrace (formed about 60 years ago), ¹³⁷Cs‐based sedimentation rates (1960–1986) were roughly twice the rates inferred from the artificial grass mats (2001–2003). This finding matches the increase in the return period for larger peak events during the 1986–2003 period, which caused fewer major inundations at this level. Conversely, sedimentation rates determined for the lowest terrace at 0·5 m were similar for both techniques (and periods) because the return periods of the peak events responsible for sediment deposition at this lower level did not change much over the period 1986–2003. Copyright © 2006 John Wiley & Sons, Ltd.