Self‐accelerated development of salt karst during flash floods along the Dead Sea Coast, 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, which in turn increase the ponding areas of flood 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.

Natural versus human control on subsurface salt dissolution and development of thousands of sinkholes along the Dead Sea coast

“…Digital elevation models (DEMs) were produced from the ground targets (“point clouds”) at 0.5 m/pixel with vertical precisions of 10 to 20 cm and were used for annual to decadal elevation change detection and for sinkhole mapping and catalog creation. Filtering for vegetation was not necessary due to the scarcity of vegetation [ Fillin et al ., ; Avni et al ., ].…”

“…Their evolution is primarily controlled by the rock solubility and secondarily by the competence of the sediment cover above the dissolution cavity, which is in turn related to its thickness and rheological properties [e.g., Maimon et al ., ; Parise and Lollino , ; Shalev and Lyakhovsky , ]. The high solubility of evaporitic rocks, e.g., gypsum and salt, enhances sinkhole formation when exposed to undersaturated groundwater [e.g., Yechieli et al ., ; Galve et al ., ; Frumkin , ; Avni et al ., ].…”

“…The sinkholes appear in the two main sedimentary environments of the DS coast [ Arkin and Gilat , ], the mudflats, consisting of fine‐grained lacustrine sediments, and the alluvial fans, composed of coarse‐grained fluvial sediments (Figure ). The depth to the top of the salt layer in the alluvial fans is between 35 m and 65 m. Below the mudflats, the top of the salt is usually around 20 m deep [ Yechieli et al ., ] or even shallower (~5 m deep near Zeelim alluvial fan [ Avni et al ., ; see Figure for location). The sediments of these two environments differ also in their mechanical properties: the alluvial fan sediments deform mostly as brittle material attested by the steep walls of the sinkholes and stream channels, whereas the fine‐grained sediments in the mudflats deform in a more viscous manner [ Shalev and Lyakhovsky , ].…”

“…Another mode of sinkhole formation has recently been identified in some of the active tributaries of alluvial fans [ Shviro , ; Avni et al ., ]. There, sinkhole formation was accelerated by flood waters draining into the subsurface through existing sinkholes.…”

Salt dissolution and sinkhole formation: Results of laboratory experiments

“…In the Dead Sea, the instantaneous subsidence, and the formation of collapse sinkhole, is found mainly at the alluvial fans. This type of sinkhole are related to the more competent gravel sediments there, relative to the weaker, muddy sediments built of silt and clay which form the mudflats and are characterized by more gradual subsidence and sinkhole formation [ Abelson et al ., ; Avni et al ., ; Shalev and Lyakhovsky , ].…”