Dept. of Geosciences Colloquium: Fluvial and coastal geomorphology sediment transport in response to regional hydrometeorology and Dead Sea level fall

Zoom: https://tau-ac-il.zoom.us/j/83800936221?pwd=dHQ5b0pYdWV3SzN1amNPanRQUnc4QT09

​Abstract:

Regressive falling levels of oceans and lakes in response to climate changes are common in the geological past. They expose newly emerged landscapes and trigger geomorphic processes in tributaries and along coasts. Rare are field-scale studies that can coevally analyze stream incision, coarse sediment redistribution into the regressive basin, and associated coastal landforms. Thus, understanding of such processes relies mainly on scaled-down laboratory experiments and/or models that cannot be validated by observations. This gap hampers linking depositional/geomorphic processes with the controlling regional or local hydroclimatic processes. 

I use high-resolution, data-rich observational and monitoring setup along the regressive shores of the Dead Sea (DS). The rapid lake level fall over the past decades enables to analyze the accelerated fluvial and coastal geomorphic and sediment transport changes and respective sedimentology under diverse environmental forcing. Questions asked include (i) What controls connectivity of coarse sediment transport across the newly exposed shelf and incision of streams under base-fall? (ii) How does the fluvially-derived coastal gravel get sorted under stormy longshore transport? (iii) How does the regional hydrometeorology govern and connect sediment transport in streams and along coasts?

Results indicate that the fluvial connectivity with the receding-falling shores depends on basin-specific hydrology and shelf geometry. These parameters also generate the high spatiotemporal variability in sediment routing, even at adjacent streams. I quantify this first field-scale evolution of channel incision and onset and intensification of coarse gravel bypass across the shelf, as captured by the theoretical Shields stress mobility of various clasts sizes.
Gravels reaching the channel mouth are sorted by storm waves, forming seasonal elongated beach berms. Motion and transport of various clast sizes are recorded by ‘smart’ and marked boulders that yield new relationships between various clast mass and critical wave height for predicting mobilization and longshore transport distances under storm wave height distribution. A new model, based on the hydrodynamic pressure-induced wave force and impulse exerted on the clasts, predicts (and verified) coastal sorting as a direct manifestation of regional hydroclimatology; this becomes a predictive tool in coastal management.

I explored the atmospheric circulation that drives stream flows, lake hydrodynamics, and fluvial and coastal sediment transport, and its sedimentation. Eastern Mediterranean cyclones concurrently activate fluvial and coastal sediment conveyors by sufficient rainfall and winds. Synoptic-scale westerlies turn into surface southerlies as funneled orographically inside the DS rift valley. Seasonal storm waves are >five times more frequent than flash-floods; thus, coarse-clastic beach berms and fan-deltas are impacted by waves and are deposited asymmetrically north of channel mouths. This hydroclimatically controlled depositional architecture is identified for both modern and Late Pleistocene coastal and delta environments, implying that Mediterranean-cyclones dominated regional hydrometeorology and geomorphology of the DS and its coast also during Late Pleistocene.

Event Organizer: Dr. Roy Barkan