Internal hydraulic control in the Little Belt, Denmark &ndash; observations of flow configurations and water mass formation

Nielsen, Morten Holtegaard; Vang, Torben; Lund-Hansen, Lars Chresten

doi:https://doi.org/10.5194/os-13-1061-2017

Articles | Volume 13, issue 6

https://doi.org/10.5194/os-13-1061-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/os-13-1061-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 13, issue 6

Research article

|

18 Dec 2017

Research article |

| 18 Dec 2017

Internal hydraulic control in the Little Belt, Denmark – observations of flow configurations and water mass formation

Morten Holtegaard Nielsen, Torben Vang, and Lars Chresten Lund-Hansen

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Final revised paper (published on 18 Dec 2017)
Preprint (discussion started on 18 Apr 2017)

Interactive discussion

Status: closed

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

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RC1: 'Review of "Internal hydraulic control in the Little Belt, Denmark. Observations of flow configurations and water mass formation"', Anonymous Referee #1, 23 May 2017
- AC1: 'Reply to Anonymous Referee #1', Morten Holtegaard Nielsen, 12 Jul 2017
RC2: 'Review of Manuscript os-2017-22 by Nielsen et al.', Anonymous Referee #2, 05 Jun 2017
- AC2: 'Reply to Anonymous Referee #2', Morten Holtegaard Nielsen, 12 Jul 2017
RC3: 'Review', Anonymous Referee #3, 09 Jun 2017
- AC3: 'Reply to Anonymous Referee #3', Morten Holtegaard Nielsen, 12 Jul 2017

Peer-review completion

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

AR by Morten Holtegaard Nielsen on behalf of the Authors (30 Aug 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (17 Sep 2017) by Markus Meier

RR by Anonymous Referee #1 (03 Oct 2017)

Suggestions for revision or reasons for rejection

I am happy with the modifications that give me more confidence that we are actually seeing quasi-stationary hydraulic controlls. However, it could be taken one step further by just considering mass conservation. In a steady system with fully mixed downstream water, the downstream salinity must be determined by the salinities and relative volume fluxes of the two upstream layers (e.g Arneborg, 2016, JGR, 121, 2035–2040). That could be mentioned at some parts of the text, as suggested in the detailed comments below. I also think that the discussion about Chl_a and primary production could be extended somewhat, as also specified in more detail below.

Detailed comments:
Line 96-97: I still don’t understand where this comes from. Prastewo et al (2009) find a mixing efficiency of about 0.11 which is less than the standard value of 0.2 used by most other researchers. Mixing may be small in sub-critical flow, but there is no reason to believe that the mixing efficiency is 10 times smaller than 0.11. In flows dominated by bottom friction, the bulk mixing efficiency may be smaller since most of the turbulent dissipation happens in the well mixed bottom boundary layer, but that is not really what you write.
Line 186: Is it reasonably to assume that 80% of the volume flux takes place in the upper layer, as these results indicate? I think it is, but I also think this extra constraint on the results should be mentioned and evaluated, even though the evaluation cannot be very precise without velocity measurements.
Lines 198-201: To me this is not that clear to see directly from Figs 1 and 2, but it may be an interpretation of the results. Please write some more explanation.
Lines 234-236: Same as above. Is it reasonable to assume that 60% of the volume flux takes place in the upper layer?
Line 251: I had some problems understanding this. Maybe add “of the strait” to make clear what “upper part” refers to.
Line 255: Change to “…top of the slope.”
Line 256: Change to “…in case of a relatively…”
Line 258: Change to “In nature, hydraulic controls…”
Lines 359-362: I understand this explanation for the upper layer in Fig. 8, where bottom water is mixed up into the upper layer. However, I would like some more text on how the authors interpret the enhanced Chl_a in the lower layers. Why does entrainment of upper layer water enhance primary production in the lower layer? The nutrients are already there and the light is probably not better than upstream. Is it because primary producing plankton are mixed down into the layer where they can grow on the limited light sources there?
Lines 404-408: In my way of seing this, it is the control that determines the downstream salinity through the relative volume fluxes in the two layers. The entrainment can do whatever it wants as long as it is strong enough to homogenize the fluid.
Line 437: Change to “..downstream of the sill.”
Lines 477-478: This follows from the control limiting mainly the volume flux in the upper, active, layer.

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ED: Publish subject to minor revisions (Editor review) (09 Oct 2017) by Markus Meier

AR by Anna Mirena Feist-Polner on behalf of the Authors (20 Oct 2017) Author's response

ED: Publish as is (06 Nov 2017) by Markus Meier

Short summary

Using field observations we studied the effects that the narrow contraction in the Little Belt, one of the three straits connecting the Baltic Sea and the North Sea, has on currents and water masses. We found the existence of internal hydraulic control, a phenomenon well-known from theoretical and laboratory studies, but surprisingly rarely observed in nature. This phenomenon plays an enormous role due to intense mixing between the water masses, which may drive a highly productive ecosystem.