Subsurface flow mixing in coarse, braided river deposits

Huber, Emanuel; Huggenberger, Peter

doi:https://doi.org/10.5194/hess-20-2035-2016

Articles | Volume 20, issue 5

https://doi.org/10.5194/hess-20-2035-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/hess-20-2035-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 20, issue 5

Research article

|

23 May 2016

Research article |

| 23 May 2016

Subsurface flow mixing in coarse, braided river deposits

Emanuel Huber and Peter Huggenberger

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Final revised paper (published on 23 May 2016)
Preprint (discussion started on 10 Sep 2015)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC C3958: 'Interesting study, but it's not really addressing mixing', Anonymous Referee #1, 30 Sep 2015
- AC C5394: 'Response to referee #1', Emanuel Huber, 04 Dec 2015
RC C3998: 'An interesting work that could better focus on the hierarchy of heterogeneity', Anonymous Referee #2, 03 Oct 2015
- AC C5408: 'Response to referee #2', Emanuel Huber, 04 Dec 2015

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (08 Dec 2015) by Mauro Giudici

AR by Emanuel Huber on behalf of the Authors (01 Mar 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (13 Mar 2016) by Mauro Giudici

RR by Anonymous Referee #1 (28 Mar 2016)

RR by Anonymous Referee #2 (04 Apr 2016)

Suggestions for revision or reasons for rejection

This is my second revision of this paper. In this second version of the manuscript the Authors have introduced some major changes. Firstly they define the concept of “advective mixing” accounting for the existing literature. Secondly they propose a visualization of advective mixing based on particle/streamline tracking and an estimate of particle deviation, particle divergence, and particle intertwining, discussing the interplay between hydraulic conductivity and hydraulic heads.
In its present form the paper suggests, more clearly than in the first draft, some very interesting ideas about how advective mixing is influenced by any kind of large-scale curved sedimentary feature, characterized by internal strong layered K contrasts, embedded within an homogeneous “matrix” of sediments with a planar anisotropy of K. In my opinion, within these boundaries (i.e. the study of a plausible conceptual geometry that could exist), the paper represents a very useful contribution that I recommend for publishing as it stands.

Nevertheless, I would like to feel free from any reviewer’s commitment and clarify the points of my first revision, that in my opinion are still inherent the second version of the paper.
In both the versions the Authors never define the concepts of heterogeneity and hierarchy they use. In absence of these definitions, to understand the set-up of the “geologically realistic representation of high-permeable trough-fills” (lines 76-77) the reader must go through the whole text, combining many sparse statements throughout the paper. Doing so, it might be inferred that the scheme adopted by the Authors to describe the hierarchical arrangement of heterogeneity includes (in order of increasing rank) texture-structure-depositional element (see lines 47-49, 80-82, 92-96, 114-117, 126-130, 143-156, 299-300, for instance). In my opinion, the components of hydrostratigraphic heterogeneity (i.e. texture, structure and composition, the latter being non influent on this study might be temporarily disregarded) should be considered separately. In this way the textural and structural components of the heterogeneity of the so called “background matrix” could be compared with the features of the troughs that have a comparable hierarchic order (external geometry and internal architecture on the side of the structural component, grain-size distribution, sorting and fabric, concerning the textural component). By assigning a planar anisotropy with a Kh/Kv ratio of 6 to the poorly sorted gravel outside the troughs (“to reflect the layered structure that hinders vertical flow”, line 156) an hydraulic behavior is assigned a priori to the entire set of, let’s say, non-trough fill facies associations, independently from the hierarchy and arrangement/orientation of structural vs. textural components of heterogeneity. Even if the Authors state several times that “the contribution of thin, high-permeable structures to advective mixing is expected to be negligible compared to that of the trough-fills” (for instance lines 291-292), they also discovered that “The vertical extent of the convex hull zone downstream from the trough fills as well as the vertical particle deviation are inversely proportional to the vertical anisotropy (Kh=Kv) of the poorly-sorted gravel texture (matrix) because a large vertical anisotropy of the poorly sorted gravel texture hampers vertical flow” (lines 266-269). I agree with this observation, that is very interesting indeed, also because it shows the importance of resolving the heterogeneity of the entire depositional system, without assigning “a priori” the behavior to a part of the association and trying to estimate the mutual influence of the heterogeneities of equivalent ranks. Which would have been the response of the Authors’ model with a different orientation of anisotropy and/or with a variable Kh/Kv ratio through the sediments outside the troughs? From this point of view, it is also worth mentioning that there is still some literature on heterogeneity of braided aquifers with scours that has not been considered by the Authors.
Another point of my concern was, and still is, the relation between the Tagliamento case history and the “geologically realistic representation of high-permeable trough-fills”. I agree that the model proposed by the Authors is plausible and that somewhere some comparable aquifer heterogeneity exists. However, its relation with the Tagliamento case is still too loose to make it a true analogue of this kind of braided river case histories. In my opinion the Authors have generated a sort of second order synthetic analogue, combining the high-order features obtained from the Tagliamento GPR image (the external shape of the scour-pools) with the low-order heterogeneities observed within the scour-pools at several different quarry sites (let me say that they produced something like the analogue of an analogue). Examples of this loose relation between the Tagliamento site and the synthetic model are sparse in the manuscript, like the statement at lines 120-121: “The GPR data indicate that the trough fills are elongated in the main flow direction (i.e., the valley orientation) with cross-tangential reflector.” Neither in the main text, nor in Figures (i.e. Fig.2 and Fig.3) there is any indication about the setting of the investigated “structures” (either “depositional elements” or “textures” in other parts of the paper, see for instance lines 292-293 and 298-300). Please note that this is not just a formal point. In my opinion this is the starting point for a reliable generalization of the results of modeling, that should provide support to the very conclusive statements of the paper.

Typewriting highlights

Line 31: “this phenomena”, this phenomenon? these phenomena?
Line 264: “extend”, extent?
Line 275: “45deg angle”, 45° angle
Line 276: “Adjective”, Advective
Line 289: “carries”, quarries?

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ED: Publish subject to minor revisions (Editor review) (12 Apr 2016) by Mauro Giudici

AR by Emanuel Huber on behalf of the Authors (23 Apr 2016) Author's response Manuscript

ED: Publish as is (25 Apr 2016) by Mauro Giudici

AR by Emanuel Huber on behalf of the Authors (04 May 2016) Manuscript

Short summary

This study puts the hydraulic heterogeneity of coarse, braided river deposits in concrete terms, combining geophysics and sedimentological observations; shows the possible impact of the heterogeneity on the three-dimensional flow field in terms of subsurface flow mixing; and demonstrates that not only the fast subsurface flow paths are important but also the hydraulic head field.