Coupled CFD-DEM Modelling of subsidence and canyon formation in an evaporite karst system

Surface stream-channels and subsurface conduits form the connecting interface between on- and offshore groundwater in the coastal transition zone. Rapid canyon formation occurs due to erosion and dissolution of material rapid retrogressive growth, slope failure, and subsidence, thus posing important geohazards in coastal areas.

We here focus on the formation of canyon systems with theatre shaped heads as found along the Dead Sea. Underlying their recent development is a dynamic evaporite karst system fed by channelized groundwater flow with subrosion processes and subsequent discharge into the lake. We use a 3D hydromechanical modelling approach to derive information on the hydromechanics and feedback between changing fluid pathways, deformation and the formation of stream-channel morphologies under varying conditions. We use a hydrogeological setup consisting of (A) a layered alluvial fan system alternating between mechanically weak, salt/rich clay-silt material and mechanically stronger, compound alluvial sandy-gravel sediments, (B) a pronounced lateral border between the former Dead Sea lakebed and the alluvial fans, (C) a Darcy-flow type fresh-water inflow through tubes at different depths and (D) a natural hydraulic gradient of 30 m/km. We hereby couple simple computational fluid dynamics with distinct elements to simulate subrosion processes as observed for the Dead Sea shore.

We found that the shape of the canyon, and particularly the morphology of canyon heads, is (1) intrinsically linked to the geologic material conditions, i.e. the stratigraphy of the subsurface, (2) the nature (3D tube network) of the karst system and (3) the hydraulic gradient conditions. This study hence gives further insight into the role of the hydromechanical conditions that drive the formation of canyons and subsidence in unconsolidated material and shows the applicability of this approach to derive morphometrics in similar coastal environments.