the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Synoptic- to meso-scale circulation connects fluvial and coastal gravel conveyors and directional deposition of coastal landforms in the Dead Sea basin
Moshe Armon
Yehouda Enzel
Nadav G. Lensky
Abstract. Streams convey coarse-clastic sediments towards coasts, where interactions with deltaic and coastal processes determine the resultant landscape morphology. Although extracting hydroclimatic signals from landscapes is a desired goal, many studies rely on interpreting paleoclimatic proxies and the link between depositional/geomorphic processes and the hydroclimate remains vague. This is a consequence of the challenge to link processes that often are studied separately, span across large spatial and temporal scales including synoptic-scale hydroclimatic forcing, stream flows, water body hydrodynamics, fluvial and coastal sediment transport, and sedimentation. Here, we explore this chain of connected processes in the unique setting of the Dead Sea basin, where present-day hydroclimatology is tied closely with geomorphic evolution and sediment transport of streams and coasts that rapidly respond to lake-level fall. We use a five-years-long (2018–2022) rich dataset of (i) high-resolution synoptic-scale circulation patterns, (ii) continuous wind-wave and rain-floods records, and (iii) storm-scale fluvial and coastal sediment transport of varied-mass, ‘smart’ and marked boulders. We show that Mediterranean cyclones approaching the eastern Mediterranean are the main circulation pattern that can provide sufficient rainfall and winds that concurrently activate two perpendicular sediment conveyors: fluvial (floods) and coastal (wind-waves). The synoptic-scale westerlies (>10 m s-1) are orographically funneled inside the Dead Sea rift valley, turning into surface southerlies. They generate 10–30 high-amplitude northward propagating storm waves per winter, with <4 m wave height. Such storms transport cobbles for hundreds of meters alongshore, north of the supplying channel mouths. Towards the decay of the storm wave, the high-altitude synoptic westerlies provide moisture to generate 4–9 flash-floods, delivering unsorted coarse gravels into the basin. These gravels are dispersed alongshore by waves only during subsequent storms. As storm waves dominates and are >five times more frequent than flash-floods, coarse-clastic beach berms and fan-deltas are deposited preferentially north of channel mouths. This depositional architecture, controlled by regional hydroclimate, is identified for both the modern and Late Pleistocene coast and delta environments, implying that the dominance of present-day Mediterranean cyclones has persisted in the region since the Late Pleistocene when Lake Lisan occupied the basin.
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Haggai Eyal et al.
Status: final response (author comments only)
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RC1: 'Comment on esurf-2022-59', Anonymous Referee #1, 19 Mar 2023
Overview & Recommendation
This manuscript presents hydrological, meteorological, and sedimentological data from over a five-year timespan along the west coast of the Dead Sea. In many ways it resembles a “source-to-sink” type study of two decades earlier: it traces the hydrometeorological conditions under which sediment can be exhumed from the landscape and delivered to the coast, where it is reworked alongshore by those same or subsequent storm events. One key aspect that allows this manuscript to stand out from those earlier efforts is that this study focuses on an interior basin with rapidly falling water levels; thus the coastal depocenter is one marked by forced regression and attendant preservation of falling-stage deposits (largely in the form of coarse to mixed clastic beach ridges), rather than a prograding delta or deep-water marine environment.
The manuscript is thorough and data-rich. It is structured well and provides adequate background and information, particular given how heavily it relies on previous work by these authors and others (all well-cited). Figures are generally very clear, well-constructed and add to the text by providing detail and context through examples from individual events (I provide some suggestions below for minor edits). The manuscript does, however, require some careful proofreading and editing: there are commas out place, missing words, noun-verb disagreement, inappropriate word choices, random capitalization, etc. These are all minor and do not detract from the science or paper, but readability would be improved if fixed. I have provided several examples in the “detailed” edits given below. Along with this, I would suggest that the authors reduce the use of acronyms as possible, and be sure to define all of them in use prior to first use (e.g., “ARST” is used on L172 but I believe first defined on L296, along with other acronyms which had already been defined).
The science presented appears sound and conclusions well-founded. Comparisons in the Discussion to published works from nearby river mouths extend and strengthen the inferences made here. However, section 5 and 6 are missing a discussion of the broader implications of this work. The role of storm-associated flood events in delivering sediment (including coarse gravels) to the coast, and of high wave energy events transporting those alongshore (particularly along wave-dominated coasts), is well known. Thus, as is, this remains somewhat of a site-specific study: certainly highly appropriate for publication, especially given that this is a relatively understudied region of the world (as are ephemeral fluvial systems in general). But, additional analysis placing these findings in the context or other river and coast dispersal systems would expand the likely audience.
Finally, I note one location where some additional analyses might strengthen the manuscript: in section 5.1, the authors discuss coastal change in terms of the length of beach ridges (berms), and conclude that these record an acceleration in sediment delivery to the basin attributed to enhanced incising due to base-level fall. The conclusions and inferences are sound, but the manuscript contains no evidence of quantification of these changes in sediment delivery. Figure 2d shows the locations of recent paleoshorelines. The authors might consider trying to calculate sediment volume fluxes into these shorelines through time. Subsurface data (ground penetrating radar would likely work well in this environment) could be used to map deposit thickness through time to estimate volumes and properly demonstrate the increase in sediment load. Such an approach would also consider shoreline orientation and thus plan-view estimates of shoreline area built over a given period of time. Moreover, in Lines 585-588, the authors posit that large gravel travel further in more recent years because there is simply more large gravel to move. But, the coast is building out as base level in the Dead Sea falls. So, does shallow nearshore bathymetry play any role here? If the nearshore bathymetry steepens offshore, then wave energy reaching the beach would be expected to increase, thereby increasing the rate of sediment transport and the size of clasts transported. These types of three-dimensional considerations are critical to understanding the processes shaping the coast, and thus be able to relate that to sediment inputs (and upstream incising). These are completely lacking at present. At a minimum, producing a map of progradation rates through time (and assuming progradation of ridges with similar thickness onto a planar antecedent surface) could help to quantify the (assumed?) increase in sediment delivery. This would also fill a substantial gap in the work, in that any depocenter geomorphic analyses are lacking.
Detailed Edits
Below are largely examples for improvements to writing clarity and precision noted above. However, several are questions and recommendations from specific parts of the manuscript.
Lines 79-80: provide examples
Line 97: “nature” is non-quantitative. What does this mean here?
Line 139: comma before “are fossilized” seems unnecessary
Line 167: what is the “heart” of the winter? Be precise.
Line 172: “ARSTs” is not previously defined
Line 225-227: wordy and complex. Consider breaking into multiple sentences and overall shortening.
Line 265: how many is “tens”? Be precise.
Lines 266-268: this is an incomplete sentence. There is no verb.
Lines 306-307: unclear writing and unclear how this conclusion was reached.
Line 478: “lagging” should probably be “lag”
Line 496: what is “L” before “60%”?
Line 562: why are some names in all caps?
Line 582: from where does the estimate that <50% of coarse sediment is submerged underwater originate? How was this calculated? This is important as it comes back to the poor or missing (as described above) estimates of beach volumes that seem to feed directly into the conclusion of accelerated sediment delivery due to downcutting.
Line 583: why is “advanced” in quotes?
Line 643: the name of the study site is misspelled.
Figures
Figures are generally excellent. Some minor suggestions are:
New Figure: the authors might consider a new figure to supplement section 2.1.1. A map showing predominant wind and storm conditions (as is common in many climate and paleoclimate studies) would help illustrate this complex system for the reader.
Figure 2: the keys and some other text in (a) and (c) are illegible (dark text on dark blue background). Also, is it accurate to call these geomorphic features along an exposed lake shoreline a “shelf” and “slope”? Those are typically reserved for large-scale continental margin features within marine environments.
Line 383 (Figure 5): it is not clear what the gradient fills illustrate, or why they are gradients of blue and gray. (same comment stands for figures 6, 7, 8, and 9, and associated captions).
Figure 10: some dark text is illegible on dark background. Otherwise, a very nice figure that helps illustrate the different forcings within the drainage basins and along the Sea coast
Figure 13: the light purple, green, and blue shading (0-100 percentile) is very difficult to see. Suggest darkening all.
Citation: https://doi.org/10.5194/esurf-2022-59-RC1 -
RC2: 'Comment on esurf-2022-59', Jaap Nienhuis, 21 Mar 2023
I reviewed the manuscript of Eyal et al., documenting coarse-grained fluvial and coastal sediment transport around river mouths in the Dead Sea basin, and their connection to atmospheric circulation patterns.
The paper presents new field data about a river delta in an arid environment subjected to base level fall -- conditions which I am not very familiar with. I found it interesting to read, but I noticed I had to go back to the maps a lot to better understand the local geography and the explanations in the text. A map showing the "berm" that is frequently discussed would be helpful.
I have also some general feedback that I think would me (as a coastal geomorphologist) better understand the paper;
1) I was wondering about the connection between atmospheric circulation patterns and wave transport direction. It seems that, given the location of the delta within the Dead Sea, littoral transport to the North is inevitable and virtually independent of atmospheric circulation. Wave height is determined to a large extent by the fetch, and the only significant fetch is toward the south, which then drives transport to the North. I don't think any other weather types would be visible in the (paleo) record. This does not make it a very good place for paleoclimate reconstructions, but perhaps I am missing something here?
2) The argument in section 5.1, about coastal berms, I found difficult to understand. The way I read it, it seems to me that the boulder are a tracer of local river-derived sediment. But: the delta geomorphology in this case is a result of river-derived sediment as well as updrift coastal sediment supplied by the waves. This delta seems strongly wave-dominated, such that most of the alongshore transport is updrift rather than river-derived. A figure could be helpful here to better understand the geography/time evolution of the berms, perhaps also with a mass balance to constrain the fluxes.
3) The relation between transport and orbital velocities is not very clear (around L568). Alongshore transport of sediment is commonly calculated with the CERC formula, or its equivalent. There is a dependence on wave height and wave approach angle. An increasing river flux (from a steepening channel bed) would change the shoreline orientation and thereby also the wave approach angle. An asymmetric wave climate would steer the delta in a downdrift direction (see a study of mine about this for some better explanations: Nienhuis et al., EPSL 2016).
Some small additional comments:
L19: "rise" and fall?
L23: "perpendicular" sounds odd to me. Up, down, left, right? Perhaps write cross-shore vs alongshore, or fluvial vs. alongcoast etc.
L29: "dominates"-> dominate
L32: this is the first time you mention that you studied paleo records as well; perhaps include that in your list of methods in L19-20?
L40: "also"? as opposed to what?
L43: "jointly" refers to basin and terrestrial controls?
L58: there is a lot of literature on beach change and climate signals (nao, enso) so I'd be careful with a statement like this if you're not citing every study
L79: inferred
L172: what are ARSTs?
L196: these are very steep wavesCitation: https://doi.org/10.5194/esurf-2022-59-RC2
Haggai Eyal et al.
Data sets
Synoptic- to meso-scale circulation connects fluvial and coastal gravel conveyors and directional deposition of coastal landforms in the Dead Sea basin Haggai Eyal, Moshe Armon, Yehouda Enzel, Nadav G. Lensky https://doi.org/10.17632/65bhpwftrh.1
Video supplement
Video supplement Haggai Eyal https://photos.app.goo.gl/rLysYEfoVSzyGdQo7
Haggai Eyal et al.
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