PUBLICATIONS. Journal of Geophysical Research: Earth Surface

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1 PUBLICATIONS Journal of Geophysical Research: Earth Surface RESEARCH ARTICLE Key Points: A self-accelerated active salt karst system is formed along the Dead Sea coast Land subsidence and sinkhole formation documented at high temporal resolution (minute to annual) Groundwater levels and chemistry respond abruptly to recharge by floodwater through sinkholes Correspondence to: G. Baer, baer@gsi.gov.il Citation: Avni, Y., et al. (2016), Self-accelerated development of salt karst during flash floods along the Dead Sea Coast, Israel, J. Geophys. Res. Earth Surf., 121, 17 38, doi:. Received 24 SEP 2015 Accepted 30 NOV 2015 Accepted article online 7 DEC 2015 Published online 8 JAN American Geophysical Union. All Rights Reserved. Self-accelerated development of salt karst during flash floods along the Dead Sea Coast, Israel Yoav Avni 1, Nadav Lensky 1, Elad Dente 1,2, Maayan Shviro 1,3, Reuma Arav 4, Ittai Gavrieli 1, Yoseph Yechieli 1, Meir Abelson 1, Hallel Lutzky 1, Sagi Filin 4, Itai Haviv 3, and Gidon Baer 1 1 Geological Survey of Israel, Jerusalem, Israel, 2 Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem, Israel, 3 Geological and Environmental Sciences, Ben-Gurion University of the Negev, Beer Sheva, Israel, 4 Mapping and Geoinformation Engineering, Technion Israel Institute of Technology, Haifa, Israel Abstract We document and analyze the rapid development of a real-time karst system within the subsurface salt layers of the Ze elim Fan, Dead Sea, Israel by a multidisciplinary study that combines interferometric synthetic aperture radar and light detection and ranging measurements, sinkhole mapping, time-lapse camera monitoring, groundwater level measurements and chemical and isotopic analyses of surface runoff and groundwater. The >1 m/yr drop of Dead Sea water level and the subsequent change in the adjacent groundwater system since the 1960s resulted in flushing of the coastal aquifer by fresh groundwater, subsurface salt dissolution, gradual land subsidence and formation of sinkholes. Since 2010 this process accelerated dramatically as flash floods at the Ze elim Fan were drained by newly formed sinkholes. During and immediately after these flood events the dissolution rates of the subsurface salt layer increased dramatically, the overlying ground surface subsided, a large number of sinkholes developed over short time periods (hours to days), and salt-saturated water resurged downstream. Groundwater flow velocities increased by more than 2 orders of magnitudes compared to previously measured velocities along the Dead Sea. The process is self-accelerating as salt dissolution enhances subsidence and sinkhole formation, whichinturnincreasethepondingareasofflood water and generate additional draining conduits to the subsurface. The rapid terrain response is predominantly due to the highly soluble salt. It is enhanced by the shallow depth of the salt layer, the low competence of the newly exposed unconsolidated overburden and the moderate topographic gradients of the Ze elim Fan. 1. Introduction Karst landscapes and collapse sinkholes occur in several geologic environments around the globe [e.g., Martinez et al., 1998; Galloway et al., 1999]. Their most common formation mechanism is dissolution of soluble rocks and creation of subsurface cavities that collapse after becoming insufficiently supported [e.g., Waltham et al., 2005; Gutiérrez et al., 2008, 2014]. The rates of dissolution and surface lowering depend upon the solubility of the underlying rocks [e.g., Ford and Williams, 2007]. In carbonate environments these rates range from a few mm/kyr to ~150 mm/kyr [Gabrovsek, 2009; Ryb et al., 2014] and are thus too gradual to be documented in real time. In evaporitic rocks the dissolution, and thus the denudation rates are precipitation-dependent and significantly higher [e.g., Dreybrodt, 2004]. Gypsum dissolution rates vary from 0.1 mm/yr [Andrejchuk and Klimchouk, 1996] to about 10 mm/yr, increasing locally to a few hundred mm/yr along short and limited routes where fresh stream water has been recharged via swallow holes [Klimchouk and Aksem, 2005]. Salt karst yield values from about 0.5 mm/yr [Frumkin, 1994] to almost 200 mm/yr [Bruthans et al., 2008]. Thus, in evaporitic areas landscape development can be rapid enough to be documented in real time. Sinkholes are classified in the literature into three main types according to the processes involved in their formation [Gutiérrez et al., 2008, 2014]: collapse sinkholes which involve brittle gravitational deformation of cover and bedrock material, suffosion sinkholes in which cover deposits migrate downward through dissolution conduits accompanied with ductile settling, and sagging sinkholes where ductile sediments bend downward due to differential lowering of the soluble bedrock. In practice, many sinkholes form by a combination of these three processes. Collapse-dominated sinkholes are by far the most hazardous of the three types due to their unpredictable and sudden formation. Thus, to assess the potential danger from sinkholes it is important to identify, monitor, and understand the main factors contributing to the sinkhole formation process. AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 17

2 Previous studies quantified the relations between surface stream flow, groundwater levels, and groundwater contribution to stream flow in carbonate karst environments [Bonacci et al., 2006; Bailly-Comte et al., 2008]. Dissolution-induced subsidence above evaporite layers has been observed along active riverbeds in Neogene to Quaternary terrace deposits of the Ebro Basin, NE Spain, affecting long-term river response and thickness of alluvial deposits [Gutiérrez, 1996; Benito et al., 1998; Guerrero et al., 2008]. This subsidence, caused by underground waters that recharged into the subsurface of the valleys and flowed through alluvial aquifers, controlled the local river gradients and depositional history of the alluvial terraces [Guerrero et al., 2008]. Periods characterized by high-dissolution and high-subsidence/aggradation ratio (the ratio between elevation drop due to subsidence and elevation rise due to sediment deposition) along these streambeds resulted in generation of local depositional basins with thickened alluvial fill [Guerrero et al., 2008]. The real-time contribution of surface water and flash floods to the subsurface dissolution processes and to their surface response has been rarely documented [Klimchouk and Aksem, 2005]. The Dead Sea (DS) coast in Israel and Jordan offers a unique opportunity to observe and study real-time salt karstification. Sinkholes were first identified along the DS in the early 1980s [e.g., Wachs et al., 2000; Arkin and Gilat, 2000]. Their primary formation mechanism involves dissolution of a subsurface salt (halite, NaCl) layer, circa 10,000 years old, due to the replacement of hypersaline groundwater by undersaturated groundwater in response to the drop in the DS level. The sinkholes form as the overburden above the newly formed cavities collapse [Abelson et al., 2006; Yechieli et al., 2006]. The Dead Sea level currently drops by >1 m/yr as a result of major exploitation and diversion of water from the drainage basin of the Jordan River and due to pumping of Dead Sea brine by the Israeli and Jordanian chemical industries [e.g., Lensky et al., 2005; Parise et al., 2015]. The sinkholes are clustered along lineaments, which form above concealed faults that facilitate flow of fresh groundwater into the salt layers [Abelson et al., 2003] and along the western edge of the salt layer [Abelson et al., 2004; Frumkin et al., 2011]. In some locations the faults and salt boundaries coincide [e.g., Abelson et al., 2003]. The yearly occurrence rate of sinkholes has accelerated significantly from less than 50 new sinkholes a year before 1999 to 420 sinkholes in 2013 [Abelson et al., 2013]. Sinkholes are formed in two main sedimentary environments: mudflats, which consist mostly of laminated clay-silt-sized clastic sediments and authigenic aragonite and gypsum, where sinkholes are generally classified as suffusion and sagging types [Gutiérrez et al., 2008], and alluvial fans dominated by consolidated gravel, alternating in places with fine-grained clastic sediments, where sinkholes are generally classified as collapse sinkholes. The salt layer, which is present in the subsurface along most of the DS shores, is generally deeper under the alluvial fans than under the mudflats [Abelson et al., 2006]. The major source of groundwater responsible for the development of sinkholes along the western shorelines of the DS is the Judea Mountain aquifer, which provides a relatively constant year-round eastward flux of water [Yechieli et al., 2006]. During the years large volumes of floodwater were trapped in sinkholes and a significant increase in land subsidence and in the number of new sinkholes was noticed following individual flash flood events in the major riverbeds [Avni et al., 2012; Filin et al., 2012; Nof, 2013; Arav, 2013; Shviro et al., 2014]. This transition from aquifer-fed to runoff-fed processes and the interactions between surface runoff and near-instantaneous land subsidence, sinkhole formation, and landscape evolution are best demonstrated in the mudflats at the easternmost part of the Ze elim alluvial fan, southwestern DS (Figure 1). Here the declining water level of the DS has exposed a gently sloping (1 2%) ~1.5 km wide area built of young (<10000 years old), fine-grained, unconsolidated sediments that were water-saturated until very recently (20 30 years). The salt layer underlying these mudflats is relatively shallow (5 20 m deep) compared to >30 m in gravel-dominated alluvial fans along the Dead Sea. We hypothesize that the response of these incompetent (viscous) and unconsolidated overburden clay sediments to dissolution of an underlying salt layer should be more immediate and widespread than that of competent, brittle, and consolidated gravel. The latter resist subsidence and collapse for longer times before yielding, as proposed by Abelson et al. [2006] and predicted by viscoelastic modeling of sinkhole formation [Shalev and Lyakhovsky, 2012]. Furthermore, the fine-grained sediments are also expected to be more susceptible to gullying than consolidated gravel, enabling the development of numerous pathways for flash floods. As these develop they may encounter and drain into existing and newly formed sinkholes. The shallow depth of the cavities formed in the salt layer is also expected to promote a more immediate surface response compared to deeper cavities. Thus, the eastern Ze elim mudflats are expected to show higher landscape evolution rates compared to gravel-dominated alluvial fans along the Dead Sea. AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 18

3 Figure 1. Location map of the Ze elim Fan draped upon a hill-shaded lidar-based DEM from White dashed lines mark the location of cross sections (Figure 3). Gullies are numbered in black from north to south. R = Recharge (swallow hole) and D = Discharge (spring). Oblique aerial photo from west to east of the southern Ze elim sinkhole field (white rectangle on map; bottom left). Photo: Assaf Tsabar. AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 19

4 Figure 2. Schematic (not to scale) stratigraphic and hydrogeological cross section west of the Dead Sea, showing the Ze elim and Lisan formations and a circa 10 Kyr old salt layer at the lower part of the Ze elim Fm. Black dashed line marks the approximate transition between the two formations. Possible groundwater flow directions are marked by blue arrows (modified after Yechieli et al. [1995b]). To test this hypothesis under flash flood conditions and to explain and quantify the underlying relationships between flash floods, sinkholes, and subsidence, we selected the clay-dominated, low-gradient eastern part of the Ze elim Fan for a multidisciplinary study combining field surveys, time-lapse camera monitoring, Interferometric synthetic aperture radar (InSAR) measurements and light detection and ranging (lidar) mapping, spanning the period from the first occasions of floodwater inflow into the subsurface via sinkholes (acting as swallow holes) to advanced stages when considerable geomorphological changes have taken place. Groundwater level monitoring and chemical and isotopic analyses of surface- and groundwater during and after major flash flood events were used to resolve the salt dissolution process and trace the possible subsurface course of groundwater flow. The combined methodologies reveal an exceptionally rapid development of a real-time salt karst system that involves stream inflow into swallow holes during flash floods, subsurface salt dissolution, land subsidence, and sinkhole formation. 2. Site Description 2.1. Geology and Hydrogeology The DS is a terminal lake that was formed at the deepest part of the Dead Sea Rift system. The Dead Sea tectonic basin is bounded by normal faults that set Cretaceous carbonate rocks of the Judea Group against Quaternary alluvial and lacustrine sediments along the basin s western margin (Figure 2). Wadi Ze elim drainage basin is one of the largest along the western coast of the Dead Sea occupying an area of about AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 20

5 Figure 3. Cross sections showing the surface topography, sinkholes, subsurface salt units, and projections of nearby boreholes. See Figure 1 for locations. SH = Sinkhole; TLC = location of time-lapse cameras. Vertical exaggeration: 20. (a) Cross section A-A, made from the 2013 lidar DEM along the course of gully #3; upper (brown) profile follows the stream bank; lower (black) profile follows the stream bed. Dead Sea (DS) 2013 level is shown on the right. Question marks denote unknown extents of the salt layers. (b) Cross sections along B-B measured in years 2005, 2011, and Note the subsidence that occurred in pond (R5) between 2005, 2011, and AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 21

6 250 km 2. The Ze elim alluvial fan is incised into the fluvial-lacustrine sequence of the late Pleistocene Lisan Formation [Begin et al., 1974]. West of the 400 m elevation contour the fan is dominated by braided gravel channels. East of the 400 m contour (Figure 1), it is dominated by mudflats consisting of m of alternating layers of clay, aragonite, silt, sand, salt, and gravel of the Holocene Ze elim Formation, with a circa 10,000 years old salt layer at its base (Figure 2) [Yechieli et al., 1993; Stein et al., 2010]. This area was first exposed during the late 1970s in response to the DS level drop and is currently undergoing rapid gully incision (Figure 1). The depth, thickness, permeability, and extent of the subsurface salt layers constitute key components in understanding sinkhole and karst generation processes along the DS. A dense network of boreholes recently drilled in the Ze elim alluvial fan penetrated wedge-shaped salt layers at depths between 5 and 20 m below the surface (Figures 1 and 3). Salt layers are absent in boreholes drilled west of the 400 m contour. The salt layers are massive and are generally characterized by low permeability [Weisbrod et al., 2012], although higher porosity and permeability layers were encountered occasionally [Yechieli et al., 1995a]. The contrast in permeability is due to the degree of diagenetic processes that the salt has undergone since its deposition. With no diagenesis the salt is generally more porous and permeable, while dissolution and reprecipitation cause cementation of the salt and lowers its porosity and permeability. Flow of undersaturated water with respect to halite within the salt layer may also increase the permeability significantly due to the formation of dissolution channels and cavities [Shalev et al., 2006; Weisbrod et al., 2012]. As a result of rapid incision of gullies during recent years, the bed of the eastern gully lies only a few meters above the salt layers (Figure 3). Thus, future incision due to the ongoing DS level drop [Ben-Moshe et al., 2008] is expected to expose these shallow salt layers. The groundwater system draining into the western part of the DS consists of two main aquifers, the Upper Cretaceous Judea Group aquifer and the Quaternary alluvial coastal aquifer [Arad and Michaeli, 1967; Yechieli et al., 1995b]. Alternations between gravel and clay subdivide the coastal aquifer into several subaquifers that differ in their groundwater level and chemical composition. The recharge of this aquifer is mainly through lateral flow from the Judea Group aquifer, which is replenished in the highlands km to the west, vertical flow along faults, and to some extent by pulses of flash floods (Figure 2) [Yechieli et al., 1995b]. The Dead Sea and the adjoining groundwater system are hydraulically interconnected as expressed by a relatively fast (a few days) groundwater level response to lake level changes of the Dead Sea [Yechieli et al., 1995b]. The relative drop in groundwater level compared to the drop of the DS level decreases as the inland distance from the shoreline increases [Yechieli et al., 2009] Flash Floods and Water Drainage The annual precipitation in the upper drainage basin of Wadi Ze elim is ~350 mm; however, due to the rain shadow effect of the Judean Mountains, it is reduced to ~50 mm/yr above the DS and its coastal plain [Atlas of Israel, 1970]. Direct recharge of rain water over the Ze elim Fan is negligible because of the arid climate and high evaporation which is currently ~1.2 m/yr above the DS water body [Lensky et al., 2005] and ~4 m/yr above evaporation pans on land [Alpert et al., 1997]. The region is affected by two main synoptic systems, the Cyprus barometric depression, occurring typically during winter, and the Red Sea Trough, occurring typically during the fall and spring seasons. Both systems produce occasional high-intensity rain storms that generate flash floods. In practice, only storms exceeding 10 mm/h of rain over a considerable part of the drainage area generate sufficient runoff to trigger flash floods that reach the Ze elim Fan and the Dead Sea [Meirovich et al., 1998]. Annually, three to five significant rain storms generate flash flood events in the Ze elim Fan (Figure 4). The floods initiate in the upper Ze elim basin, flow eastward through the Ze elim bedrock gorge and continue downstream in the braided gravel bed channels to the Dead Sea. The long (~6 km) flow on coarse gravels enables some water percolation to the shallow subsurface, as indicated by seasonal springs and clusters of acacia trees at the eastern side of the gravelly fan. As the floodwater reach the eastern part of the fan (east of contour 400 m), they are channeled to discrete gullies incised in the mudflats (numbered 1 15 in Figures 1, 6, 7b, and 10c). The extent to which the floods continue to incise and interact with these gullies depend on their intensity and volumes. In recent years, due to processes in the braided gravel streams, most of the floodwater has been naturally diverted toward the northernmost and southernmost parts of the fan (gullies 3 5 and 14 15, respectively; Figure 1). AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 22

7 Figure 4. (top) Daily precipitation between 2010 and 2015 in Arad station, 20 km upstream from Wadi Ze elim (black bars; source: Israel Meteorological Survey). (bottom) Flash flood events in Ze elim Fan (blue bars). Colored rectangles mark the time intervals analyzed in Figures 9 and Land Subsidence and Sinkhole Formation Areas of gradual land subsidence (tens to hundreds of meters wide and up to a few decimeters deep) and discrete sinkholes (a few meters wide and up to 25 m deep) are closely related along the DS [Baer et al., 2002; Abelson et al., 2003]. In several cases, gradual subsidence was identified a few weeks to months before a specific sinkhole collapse event and has been used as a precursory tool [Nof et al., 2013]. In most sinkhole sites, gradual land subsidence continues for weeks to years after the collapse event. Thus, quantitative evaluation of the subsidence rates and their spatial and temporal extents provide important information about the subsurface sinkhole-forming processes and enable assessing the degree of activity in specific sites and distinguishing between active and inactive areas. 3. Methods and Data Processing The study incorporates a multidisciplinary set of methods including field, airborne, and spaceborne monitoring, time-lapse photography, water level measurements, surface water and groundwater geochemistry, and isotope analysis Airborne Lidar Airborne light detection and ranging (lidar), also known as airborne laser swath mapping, uses a laser ranging device, together with information on the position and orientation of the aircraft platform, to determine the x, y, andz coordinates (within a global geodetic reference frame) of ground targets [e.g., Glennie et al., 2013]. Lidar data of the Dead Sea area were acquired in December 2005, May 2011, May 2013, April 2014, and April 2015, at an average flight height of 700 m by Optech ALTM khz and Rigel AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 23

8 Figure 5. Temporal increase in the number of sinkholes. Note the gradual acceleration in sinkhole formation rate along the entire Dead Sea (red) and the distinct, significant rise that occurred at the Ze elim Fan in (blue). LMS-Q khz scanners. The densities of the ground measuring points were at least 4 points/m 2. Digital elevation models (DEMs) for the different years were produced from the ground targets (point clouds) at 0.5 m/pixel with vertical precisions of 10 to 20 cm. These DEMs were then used for annual to decadal elevation change detection and for sinkhole mapping and catalog creation. Due to the scarcity of vegetation, filtering for vegetation, commonly performed for separating off-terrain returns (e.g., tree canopies) was not carried out Ground-Based Lidar High-resolution elevation maps were made along gully #5 in December 2011 and March 2012, using a Leica ScanStation C10 terrestrial scanner, with ranging accuracy of ±4 mm and angular accuracy of ±12. The angular scanning resolution was (equivalent to approximately 20 points/cm 2 ), yielding approximately 5 M points per scan. For temporal change detection the data was resampled to 5 5 cm per pixel. Intracampaign coregistration was performed using target reflectors, with average accuracy of ±3 mm. Georegistration into the national reference coordinate system was performed using real-time kinematic GPS with an accuracy of ±3 cm. The total measurement accuracy is thus about ±3 cm (see [Barnhart and Crosby, 2013] for the computational approach). In order to detect temporal changes between terrestrial and airborne scans, we set our accuracy threshold by means of propagation of variances for both the airborne and terrestrial laser scanners, with confidence level of 95% Interferometric Synthetic Aperture Radar (InSAR) Over the last two decades, Interferometric synthetic aperture radar (InSAR) has become a widespread tool to measure subtle displacements at the ground surface [e.g., Massonnet and Feigl, 1998]. Radar images of the Italian Space Agency (ASI) COSMO-SkyMed satellites (3 m pixel size, 3.1 cm wavelength, 16 days revisit time, and ~2 mm vertical accuracy) were used to generate phase difference maps (interferograms) for three periods between November 2013 and February The airborne lidar DEM was used for georeferencing the interferograms and for removal of topographic phase artifacts that may occur when inaccurate DEMs are used. The interferograms were filtered using an adaptive filter function that is based on the local fringe spectrum [Goldstein and Werner, 1998], with a window size of pixels Time-Lapse Camera Time-lapse camera (TLC) allows direct observations of fast evolving landscapes in remote areas with limited access during flood events. TLC observations of the immediate ground and groundwater responses to flash flood events were conducted during the following flash flood events: March 2014, 9 10 January 2015, and February The observations were captured using a Brinno 200 Pro camera and were limited to daylight conditions Groundwater Levels and Analysis Groundwater levels in boreholes BH-2 and BH-14, located at the eastern and western parts of the fan, respectively (Figure 1), were monitored continuously at 15 min intervals since November Pressure measurements were carried out by a Solinst Levelogger Model 3001 (manufacturer accuracy of ±1 cm) and were density corrected to groundwater levels. Water samples for chemical and stable isotope (δ 18 O) analyses of winter 2013/2014 storms were collected from flood water, ponds (depressions and sinkholes), and briny springs. The pool spring (D10 in Figure 1) was sampled before and immediately after the February 2015 storm. Chemical and isotopic analyses were conducted at the laboratories of the Geological Survey of Israel (GSI). Density was measured with a portable density meter (Anton Paar DMA 35). AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 24

9 Figure 6. Sinkhole evolution map from the early 2000s to 2015, showing dominance of clustering until 2011 and transition to intercluster patterns thereafter (see also Figure 15). Note the linear sinkhole patterns within and east of the central sinkhole field and the NW-SE progression of sinkholes in the southern sinkhole field. AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 25

10 Journal of Geophysical Research: Earth Surface Figure 7. Surface elevation change maps of the Ze elim Fan draped on hill-shaded lidar DEMs showing the calculated subsidence rates between (a) 2005 and 2011 and (b) 2011 and Specific patterns of gradual subsidence and sinkholes are marked I VI (see text) and river incisions are marked VII. Flood recharge (R) and brine discharge (D) points are marked by yellow stars and white circles, respectively. Note the dramatic increase in subsidence rates after Results 4.1. Annual to Decadal Subsidence Rates and Sinkhole Evolution Sinkholes along the Israeli side of the Dead Sea were mapped and cataloged by the GSI since the 1990s using aerial-photographs and lidar DEMs from different years. The number and annual growth rate of sinkholes have constantly increased from less than 50 sinkholes a year before 2000 to more than 400 sinkholes a year in recent years [Abelson et al., 2013] (Figure 5). Sinkholes in Ze elim area first appeared in 2000, increasing steadily in number until Significant acceleration in the rate of new sinkhole appearances occurred in with further acceleration in 2012 (Figure 5). The Ze elim sinkhole occurrence map (Figure 6) shows sinkhole clustering in several fields until 2011, and formation of intercluster sinkholes along and around gully #4 and #5 beds and NW to SE sinkhole progression in the southern field since Annual to decadal subsidence rates were obtained by subtraction of airborne lidar DEMs. Elevation change maps between 2005 and 2011 and between 2011 and 2015 DEMs reveal subsidence and gully incision in the Ze elim area (green-blue areas and lines in Figure 7), as well as road construction and artificial mounds (gray patches and straight lines mostly in Figure 7b). While during the period subsidence was restricted to the eastern and western clusters of the central sinkhole field and to the northern field (Figure 7a), a significant increase in subsidence and in gully incision occurred during the years The most prominent changes occurred in the following locations listed from south to north: (1) an undulating belt at the western margins of the mudflats (marked I in Figure 7b), (2) the southern sinkhole field (marked II), (3) along gully #7 (III), (4) the area between the two clusters of the central sinkhole AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 26

11 Figure 8. Photographs showing water accumulation, stream inflow into sinkholes and brine springs discharge. (a) First appearance of a sinkhole on the bank of an active gully, July 2010 (see location in Figure 9a). (b) Ponded floodwater along gully #5, March 2012, a few days after the flood. (c) Stream inflow in gully #5, March 2014, a few days after the flood. (d) Stream inflow in gully #3, December 2013, 2 weeks after the flood. (e) Eastward flow of brines that discharged from spring D3 (Figure 1) after March 2012 floods. (f) Brine spring D10 (center of picture), September The width of the pool is ~10 m. field (IV), (5) a narrow N-S lineament connecting gully #4 with gully #3 (V), and (6) along gully #3 (VI). Gully incision is also clearly seen (e.g., VII). In many places the rate of gradual subsidence between 2011 and 2015 exceeds 50 cm/yr (Figure 7b) Short-Term Surface Changes (Minutes to Months) Seasonal Changes Floodwater inflow into sinkholes acting as swallow holes was first observed in winter after individual sinkholes were formed within and along the banks of active gullies [Avni et al., 2012] (Figure 8a). Since that winter, floodwater temporally accumulated in surface depressions and ponds associated with sinkhole fields along gullies #4 and #5 (Figure 8b). Following the November 2011 flash floods (Figure 4), a water volume of at least 9000 m 3, estimated by identifying water level contours on terrestrial lidar scans [Arav, 2013; Arav et al., 2014], was drained to the subsurface within 2 weeks, and following the March 2012 flood, an estimated water volume of at least 38,000 m 3 (Figures 8b) was drained to the subsurface in a single day. Swallow holes were also documented during the following winters (Figures 8c and 8d). Following these flood events, brines discharged from several springs at the bottom and banks of gully #3 and near the outlet of gully #10 (Figures 1, 8e, and 8f). The time lag between the flash flood events and the onset of the brine discharge decreased from a few days in 2011 to a few hours in 2015 (see section 5.2 below). Figure 9a delineates subsidence within the central depression in upper gully #5 (see location in Figure 6) for the period between the airborne lidar measurements of May 2011 and the terrestrial lidar scan of December On November 2011 two small floods were ponded in the central depression and were drained to the subsurface. The differential map for this period indicates a total subsidence volume of ~4000 m 3 (Figures 9a and 9c). The differential map of the following winter (December 2011 to March 2012) during which several flash floods events took place (see Figure 4) shows a significant increase in subsidence downstream (east) of the summer-autumn depression (Figure 9b). The estimated subsiding volume for this 4 month interval is ~29,000 m 3 (Figure 9c). AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 27

12 Figure 9. Ground subsidence maps within the central sinkhole field in upper gully #5 (see location in Figure 6), calculated by subtractions of (a) airborne lidar DEM of May 2011 from terrestrial lidar scan of December 2011 (pixel dimension m) and (b) terrestrial lidar scan of December 2011 from terrestrial lidar scan of March 2012 (pixel dimension 5 5 cm). (c) Cumulative subsidence volumes (gray line) and aerial extent (dashed black line) between December 2005 and May Volumes are calculated from the subtracted maps by multiplying pixel dimensions by their corresponding subsidence. Arrows a and b point to the subsiding volumes during the periods shown in Figures 9a (May 2011 to December 2011) and 9b (December 2011 to March 2012). Periods a and b are also marked on Figure 4. Note the significant increase in subsidence volume and aerial extent during the winter period (December 2011 to March 2012). Figure 10. InSAR measurements (unwrapped interferograms) showing displacements in the satellite to ground line of sight direction (negative values indicate motion away from the satellite). (a) One month before the 12 December 2013 flash flood event, (b) 2 weeks after the event, and (c) 6 weeks after the event (see location in Figure 6). Note the small displacements within the streambeds (white rectangles) before the flash flood, the abrupt increase immediately after the flash flood between the recharge and the discharge points, and the slight decay thereafter. The Roman letters in Figures 10c correspond to areas and lineaments with high-subsidence rate: I = the undulating belt at the southwestern part of the fan, II = the southern sinkhole field, III = gully #5, and IV = gully #3. AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 28

13 Figure 11. Time-lapse camera pictures taken downstream during the March 2014 flood event in gully #3 (R3 in Figure 1). Time is counted in hours from the first arrival of the flood. (a) One minute before the first arrival of the flood. (b) Ten minutes (0:10 h) after flood arrival. (c) 1:10 h floodwater is swallowed by a preflood sinkhole at the gully bed. (d) 1:17 h the stream banks surrounding the sinkhole collapse along circular concentric cracks. (e) 3:50 h a second sinkhole forms 2 m downstream, swallowing the floodwater for a period of more than 3 h, and followed by subsidence of the stream banks. (f) 8:30 h end of first flood. (g) 21:50 h arrival of second flood. (h) 30:50 h floodwater swallowed by a third sinkhole for about 3 h. The sinkhole is later filled by sediment. (i) 46:50 third flood, lasts about 3.5 days. (j) 144:05 h a forth sinkhole is formed ~15 m downstream, swallowing the remainder of the floodwater for a few more days. AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 29

14 Figure 12. Time-lapse camera picture pairs taken during the 20 February 2015 flood event. Left column shows westward looking pictures toward gully #5 (R5 in Figure 1); right column shows southward looking pictures of the brine spring (D10 in Figure 1). The time is shown in hours of the day. (a) 08:38 left: the first pulse of floodwater reaches gully #5. Right: discharge at D10 is minor, releasing black, reduced brine toward the Dead Sea. (b) 09:42 left: a floodwater pond is created at R5. Right: the spring shows no change in discharge. (c) 10:26 left: the pond is almost totally swallowed by a sinkhole. Right: pulses of black, reduced water start to appear within the spring pool (white arrow) and the discharge rate slightly increases. (d) 11:24 left: continued accumulation and swallow of floodwater. Right: a first pulse of brown, muddy water appears at the spring, accompanied by a significant increase in discharge rate and rise of the pool water level. (e) 14:12 left: termination of this flood episode. Right: discharge of muddy brine continues until 21 February evening. AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 30

15 Figure 13. (a) Rain intensity (mm/h; cumulative 10 min measurements) during the January 2015 storm at Ein Gedi and Arad stations. Data from Israel Meteorological Service. (b) Groundwater levels at boreholes BH-14 (orange) and BH-2 (black) during that period (see locations in Figure 1). (c) Zoom on 9 10 January event (green dashed rectangle in Figure 13b) with TLC frames depicting: first arrival of floodwater and its swallow near the stream bank (blue arrows for flow direction, SH for swallow holes) a few minutes before the groundwater level rise (left); channel-wide flood and continued flow into swallow holes (middle); post flood dry channel, with clear flow marks toward the swallow holes (right; dashed blue arrow) Post-Flood Changes Figure 10 shows unwrapped interferograms showing satellite to ground line of sight absolute displacements during 16 day intervals before and after the 12 December 2013 flash flood in the Ze elim Fan. While only minor displacement is detected in the preflood time interval (Figure 10a), a significant increase in the rates and spatial extents of land displacements away from the satellite (interpreted as subsidence) is evident in several areas along the major streambeds in the time intervals after the flood event (Figures 10b and 10c), reaching about 2 cm in each 16 day interval (i.e., >1 mm/d). High rates appear in lineaments and areas that were also noted for their high-subsidence rates during the long-term period (Figure 7b): the undulating belt at the southwestern part of the fan (marked I in Figure 10c), the southern sinkhole field (II), gully #5 (III), and gully #3 (IV) Syn-Flood Documentation In March 2014, a TLC was mounted above the major Ze elim gully #3 (Figure 1) several days before forecasted floods, with picture intervals set to 1 min. A series of flash flood pulses and sinkhole collapses (marked by white thick arrows in Figure 11) were documented during 6 days (see also watch?v=qj4lfzbd2j4). The selected frames in Figure 11 show multiple water inflow and gravel-filling episodes in swallow holes along a ~50 m stretch of the stream bed. Prior to the 20 February 2015 storm, two TLCs were mounted, the first looking westward (upstream) at gully #5, aimed at capturing the floodwater accumulation at pond R5 (Figure 1), and the second, about 1000 m to the south, focused on a brine spring at the Dead Sea shoreline (D10 in Figure 1). The two cameras documented the entire process of floodwater ponding and drainage by swallow holes in gully #5, and the discharge at AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 31

16 Table 1. Representative Analyses of Fresh and Saline Waters in the Ze elim Wadi Site δ 18 Na + K + Ca Mg Cl Br SO4 TDS O Density Na/Cl DSH a See Figure 1 for location Date Sample Description mg/l (at 20 C) Mole Ratio (at 20 C) 28/1/2015 DS brine /12/2013 diluted DS brine b West of site R3 16/12/2013 emerging from gravel , <0.01 West of site R3 16/12/2013 emerging from gravel , <0.01 Near DS shore 16/12/2013 massive flow, bottom of gully # , West of site R3 14/1/2014 emerging from gravel , <0.00 West of site R3 14/1/2014 flow head of gully # , <0.01 around S3 14/1/2014 spring in gully # , West of site R3 13/3/2014 flow head of gully # <0.01 Near DS shore 13/3/2014 massive flow bottom of gully # West of site R3 15/3/2014 pool head of gully # <0.01 West of site R3 15/3/2014 flow head of gully # <0.01 East of D3 15/3/2014 flow bottom of gully # D3 15/3/2014 spring within gully # D3 15/3/2014 spring within gully # East of D3 15/3/2014 Massive flow bottom of gully # Near DS shore 15/3/2014 Massive flow bottom of gully # D10 19/2/2015 spring pool preflood D10 23/2/2015 spring pool postflood a DSH: Degree of saturation with respect to halite. DSH = 1: Halite saturated solution. DSH < 1: undersaturated solution. Calculation using the PHREEQC code and database [Parkhurst and Appelo, 2013]. b Diluted Dead Sea brine sampled by the shore of Ze elim Fan following a flood event. AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 32

17 Figure 14. Salinity (TDS; total dissolved solids) versus Na/Cl ratio in selected water samples, showing increased salinity and high Na/Cl ratios due to the dissolution of the salt layer by fresh groundwater. Note that the Dead Sea brine has high salinity but low Na/Cl ratio. Oxygen isotopic composition values (δ 18 O) are marked next to each group. the saline spring (Figure 12; see also The 20 February 2015 flood encompassed all major gullies of the Ze elim Fan, and inflow at swallow holes also occurred at gullies #14 and #3 (R14 and R3, respectively, in Figures 1 and 7) during the same time interval Water Levels Water level variations in boreholes BH-2 and BH-14 (Figure 1) during the 7 15 January 2015 flash floods at Ze elim gully #3 are plotted in Figure 13 along with TLC pictures of the flash flood and rainfall measurements from Arad and Ein Gedi stations, representing upstream and local rainfall, respectively (Israel Meteorological Service Bulletin). The relationships between rainfall and groundwater levels are remarkably different at the two boreholes. While an abrupt water level rise of up to 4 m was observed in the eastern borehole (BH-2) a few hours after each significant rainfall pulse (>3 mm/h), the western borehole (BH-14) showed no response during the entire event (Figure 13b). On 9 January 2015, 09:16 A.M., a flash flood was pictured by the TLC, as it was swallowed into two sinkholes at site R3a, about 200 m south of borehole BH-2 (Figures 1 and 13c). Water level at the borehole rose abruptly and almost instantaneously thereafter, gaining 1 m in the following 6 h, while declining back to the ambient level after about 3 days (Figure 13b) Water Analysis The chemical analyses of the water samples (Table 1) have extremely high variability in both salinity and compositions, ranging from nearly fresh flood water (total dissolved solids, TDS < 500 mg/l) to brines (TDS > 35,000 mg/l) emerging as springs and flowing to the DS at the bottom of the gullies. The analyzed waters cluster to four major fields according to their chemical composition (Figure 14): (1) Fresh flood water with low TDS and low ( ) Na/Cl ratios, (2) Spring brines with high TDS and high ( ) Na/Cl ratios, (3) Dead Sea brine with high TDS and low ( ) Na/Cl ratios, and (4) Pool spring brines with high TDS and intermediate ( ) Na/Cl ratios. 5. Discussion Our observations show sinkholes, ground subsidence evolution, and groundwater levels and geochemical changes on scales of minutes to years, all in association with flash floods in the Ze elim channels. In the following discussion we explore the surface and subsurface processes that lead to this wide set of observations. We first discuss the geochemical evidence for subsurface salt dissolution and delineate the subsurface dissolution pathways. Then we discuss the onset and self-accelerating nature of this karst system, and finally, we elaborate on the implications of these processes for sinkhole hazard assessment. AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 33

18 5.1. Geochemical Evidence for Subsurface Salt Dissolution Three geochemical parameters indicate that the high salinities of the salt springs are derived from dissolution of halite (NaCl) rather than from the Dead Sea: 1. Their high sodium and chloride concentrations and high Na/Cl ratio (>0.8, often approaching 1.0), which characterize dissolution of halite by freshwater. In contrast, the Dead Sea brine is characterized by low Na/Cl ratio (~0.2), and relatively low sodium concentration. Figure 14 clearly demonstrates the increased Na/Cl ratio with increasing salinity of the water and the difference between the DS and the other brines encountered in Ze elim Fan. The figure demonstrates that the high salinity in the springs cannot be derived from the DS water. 2. The spring brines flowing in the lower parts of the gullies are mostly saturated or close to saturation with respect to halite (degree of saturation (DSH) = ~1) suggesting that these waters were in close contact with halite (Table 1). It should be noted that DS brine are also saturated with respect to halite and thus cannot dissolve additional halite unless diluted by freshwater [Gavrieli, 1997]. 3. The oxygen isotopic composition (δ 18 O) of the DS is positive (>2 ; Table 1), whereas that of the fresh runoff water is usually in the range of 5 to 7. Mixing of these two end-members to attain the observed salinities in the springs and flowing brines would result in enriched isotopic composition (positive and closer to the DS), which is not observed. In fact, the spring and the brines flowing in the gullies have isotopic composition similar to or slightly more enriched than that of the runoff water. The rapid dissolution of halite in the subsurface by the flash flood water was clearly demonstrated during the major storm event of December This storm had an oxygen isotopic composition that was exceptionally depleted (δ 18 O as low as 13 in Jerusalem; Avner Ayalon, personal communication, 2014). The flash flood that reached Wadi Ze elim Fan as well as the spring brines that discharged and flowed in the gullies during this event were accordingly exceptionally depleted, measuring 9.7 to 7.8 (surface runoff) and 6.5 (springs) (Table 1). The isotopically depleted (negative) values of oxygen isotopic composition in the spring brines are much lower than those previously analyzed in spring brines around the Dead Sea, thus lending additional support to their meteoric freshwater source. A slight deviation from the above described mechanism of salinization by halite dissolution is found in the spring pool (D10), which exhibits somewhat lower Na/Cl ratios than the other brines (0.54 and 0.72 before and after the flood, respectively). Its increased magnesium content suggests that it contains remnants from the Dead Sea brine which were washed out from the subsurface sediments. In fact, following the flood, the Na/Cl ratio rose, while the magnesium content decreased indicating dissolution of halite while diluting the remnants of Dead Sea brine Subsurface Dissolution Paths and Flow Velocity While fresh water enters the subsurface through sinkholes acting as swallow holes, the briny pool and springs are most likely the discharge points of the water after dissolving salt and creating subsurface channels and cavities. Subsidence patterns and sinkhole distribution could thus serve as possible surface manifestations of the subsurface dissolution channels. Figures 6, 7b, 9b, and 10c show three possible subsurface tracks. (1) A curvilinear NE trending sinkhole lineament and depression that marks the westernmost occurrence of sinkholes and the boundary of the salt layer and connects the R14 swallow hole with the western side of the southern sinkhole field. The southern sinkhole field follows a NW-SE trend displaying progressive sinkhole formation southeastward (Figure 6), with the pool spring located further to the SE along this trend. (2) A track that follows gullies #4 and #5 eastward from R5 swallow hole, and then northward along a straight sinkhole lineament that terminates at spring D3. (3) A short track of swallow holes and subsidence that follows gully #3 from swallow hole R3 to spring D3. During the 20 February 2015 storm, floodwater was swallowed simultaneously in three swallow holes R3, R5, and R14 (Figure 7b). Thus, the inflow of the floodwater that emerged at spring D10 (Figure 12) could have taken place at one of these three swallow holes. However, since an apparent subsurface path connects R14 to D10, we propose that the muddy water that resurged in D10 after the flood originated from swallow hole R14, about 1000 m to the west. Considering the 2 3h lag time between the swallowing initiation and water emergence at D10, a conservative estimate for the subsurface floodwater velocity is ~400 m/h (~10 cm/s). A possible connection between R5 and D10 is less likely as it crosses the Ze elim drainage network perpendicularly and has no linear subsidence expression. However, this possibility cannot be totally excluded. AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 34

19 Figure 15. Schematic illustration of sinkhole evolution along gully # 5 since the early 2000s. See text for details. AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 35

20 These groundwater velocities are quite similar to those found in a carbonate karstic aquifer system in northern Israel (~100 m/h) [Magal et al., 2013] but are extremely high compared to groundwater systems in granular soils. Previously measured groundwater velocities along the Dead Sea shorelines in alluvial, granular material range from 10 to 120 m/d (0.4 5 m/h) [Magal et al., 2010], about 2 orders of magnitudes lower than those of the drained floodwater in Ze elim. As the subsurface in the eastern part of the Ze elim alluvial fan is dominated by relatively impermeable clay, the observed high velocities suggest that the flow occurred in a more permeable media, most likely in dissolution channels within the salt. The increase in flood-induced groundwater velocities from 2011 to 2015 (which is expressed by the decreasing lag time between swallow and discharge over these years) suggest that the subsurface salt karst system is constantly growing following each flood event as existing conduits widen, additional conduits open and the permeability of the salt layer increases. Abrupt water level rise at borehole BH-2 after each flash flood, in contrast with the lack of response in the western borehole (Figure 13), provides an additional, independent indication for the rapid effect of swallow holes on the groundwater system. The water level changes reflect good hydraulic connection within the aquifer between these points, and not necessarily actual flow from R3 to BH-2. It should be emphasized, however, that since no flow occurred in any other nearby gullies at that time and swallow holes occurred only in gully #3, the immediate water level response in the borehole is most likely associated with the R3 swallow hole Onset and Self-Acceleration of Salt Karst in the Ze elim Fan Significant acceleration in sinkhole growth rate occurred in winters and (Figure 5). Our study provides detailed documentation of this accelerated process right from its onset, as illustrated in the conceptual model of salt karst evolution presented in Figure 15. The steady growth rate until 2010 correlates with the period before the first appearance of sinkholes along active gullies (Figures 7a and 15a). At that stage sinkhole clusters were formed due to salt dissolution by aquifer-fed groundwater that flowed eastward within the permeable sand units and ascended mainly along fault lines [see also Abelson et al., 2003]. Each sinkhole cluster was accompanied and surrounded by minor subsidence (Figure 15a). Following the ongoing gullying in the unconsolidated mudflats and the first appearance of sinkholes along active gullies in 2010 (Figure 8a), floodwater drainage at swallow holes took over as the dominant groundwater supply agents, enhancing subsurface salt dissolution, subsidence, and sinkhole formation in and between the sinkhole clusters (Figure 15b). During winter , the eastward drainage of gullies #4 and #5 was disconnected and reversed along a ~100 m long section due to subsidence and the rapidly growing depression along these gullies, and floodwater accumulated after each flash flood, forming long (~200 m) ponds that eventually infiltrated to the subsurface (Figures 8b and 15c). This dramatically increased salt dissolution which in turn triggered rapid formation of additional sinkholes and swallow holes in a positive feedback mechanism. We interpret the immediate subsidence that followed as evidence for rapid dissolution and channel flow through a karst system with increasing volumes of eastward flowing salt-saturated groundwater that eventually discharged in newly formed springs. The appearance of gully bed sinkholes (e.g., sinkhole D3 in Figures 3a and 11), may also be accelerated as the overburden above the dissolving salt layer under the incising gullies becomes thinner and weaker, improving the hydraulic connection between the gully beds and the salt layer. Future exposure of the salt layers by ongoing incision (Figures 3 and 7) could further enhance this self-accelerating process by direct transfer of undersaturated floodwater to the salt layers Implications for Sinkhole Hazard Assessment Flood-induced subsidence and sinkhole formation has been recently found in other streambed sinkhole sites along the western DS shorelines [Shviro et al., 2014]. Rapid increase in dissolution rates due to fresh stream water that has been recharged via swallow holes has also been documented in other evaporite environments [e.g., Klimchouk and Aksem, 2005] and is likely to form in any flood-prone area comprising highly soluble subsurface layers such as halite or gypsum. However, rapid landscape changes such as those observed in Ze elim were not observed in any of the consolidated gravelly fans along the Dead Sea nor reported from other locations. We thus suggest that the combination of a relatively shallow (<20 m deep) salt layer with fine-grained, low-competence clayey sediments are the major factors contributing to the rapid and immediate surface response to subsurface salt dissolution [see also Shalev and Lyakhovsky, 2012]. Together with the relatively moderate topographic gradients in Ze elim channels (less than 2%) compared to higher streambed AVNI ET AL. SELF-ACCELERATED DEVELOPMENT OF KARST 36