the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Apr 2026
High current speed events in a harbor channel driven by resonant sub-hourly sea level dynamics: an example from Varna, Black Sea
Abstract. Current and sea-level observations in the 2.4-km-long, 300-m-wide channel between Varna Lake and the Black Sea, conducted by three novel Hydromast stations with a 1-minute resolution, revealed that moderate sub-hourly dynamics were occasionally interrupted by high-amplitude oscillations with a period of a few tens of minutes. The resonant excitation of the events was studied using barotropic 1D analytical and numerical models. Basic features of high-intensity sub-hourly variations can be interpreted as linear barotropic long waves in a sea-channel-lake system with resonant and damped forcing. The observed 37-minute oscillations, identified from the power spectra, spectrogram, and wavelet analysis, can be explained as the first mode of the channel-lake system. Another period of 19 minutes resembles the zero, quarter-wave mode of the channel and the second mode of the channel-lake system. During the measurement period, two high-amplitude events were highlighted. One of the events is interpreted as having been caused by a meteotsunami; moderate meteorological conditions prevailed; strong channel oscillations began abruptly and lasted for a few cycles. The second event occurred more than 5 h after landward winds up to 20 m s−1. The observed maximum changes in current and sea level – up to 0.8 m s−1 and 0.8 m, respectively, over 10 minutes – are harmful to ship navigation, harbor operations, and coastal management. Building on these observations, approaches for detecting and forecasting strong sub-hourly oscillations are discussed.
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Status: open (until 23 Jun 2026)
- RC1: 'Comment on egusphere-2026-1731', Anonymous Referee #1, 03 May 2026 reply
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RC2: 'Comment on egusphere-2026-1731', Anonymous Referee #2, 16 Jun 2026
reply
Reviewer Comments
The manuscript entitled “High current speed events in a harbor channel driven by resonant sub-hourly sea level dynamics: an example from Varna, Black Sea” presents a valuable observational and numerical investigation of high-current-speed events associated with resonant sub-hourly sea-level oscillations in the Varna harbor–channel–lake system. The high-frequency measurements and the combination of spectral, coherence, wavelet, and modeling analyses provide a solid basis for identifying resonant barotropic modes and assessing their hydrodynamic consequences. The manuscript is generally well written and scientifically sound.
However, the study’s main objective, practical relevance, and broader applicability are not sufficiently emphasized. Although the authors briefly mention implications for navigation safety, harbor operations, sediment transport, and coastal management, these aspects remain largely generic and are not fully developed. As currently presented, the findings appear somewhat site-specific. I therefore recommend strengthening the Introduction and Conclusions by clearly articulating the study motivation, intended applications, and the extent to which the results may be transferable to other harbor–navigation channel–lagoon/lake systems.
Major Comments:
- Operational relevance of the study area
The operational context of the Varna harbor–channel–lake system should be described in greater detail. Information such as harbor traffic intensity, vessel types and drafts, navigation frequency through the channel, and the extent of traffic between Varna Lake and the harbor would help readers better assess the practical significance of the reported current events.
If available, the authors should also discuss ship-generated waves (typical amplitudes and frequencies) and their possible interaction with the observed water-level oscillations. Even a qualitative assessment would provide useful context regarding the relative importance of vessel-induced and naturally forced hydrodynamic variability.
- Hydraulic structure and forcing mechanisms
The manuscript would benefit from a short subsection, placed after the Study Area section, describing the hydraulic functioning of the channel and the dominant forcing mechanisms (barotropic and/or baroclinic).
In particular, the authors should clarify:
- Whether the channel flow is generally vertically homogeneous or stratified;
- The role of density gradients between the lake and the sea;
- Under which conditions stratification breaks down, if relevant;
- The relationship between water-level gradients and current velocities.
Since the water-level difference between the two ends of the channel appears to be the primary driving mechanism, it would be useful to present this parameter together with the measured currents. For example, the water-level difference (Stn3–Stn1) could be included as an additional axis in Figure 2 to facilitate interpretation of the current response.
- Numerical modeling
Several aspects of the numerical model require further clarification:
- The justification for representing the two channels as a single equivalent channel by summing their widths should be explained in greater detail. Potential impacts on frictional losses, local hydraulic effects, and resonance characteristics should be discussed.
- The rationale for examining the alternative geometries listed in Table 4 should be clarified. Why was the realistic geometry not used as the primary reference configuration?
- The assumption of a linear barotropic system should be supported with information on typical salinity conditions in both the lake and the sea.
- The choice of grid resolutions (12 m in the channel and 30 m in the lake) should be justified, preferably with reference to a grid-sensitivity analysis.
- The manuscript does not clearly indicate whether the model was calibrated and/or validated against observations. Additional information is needed to assess the reliability of the numerical results.
- The statement that oscillations did not propagate significantly into the lake requires further explanation. The authors should clarify whether this behavior reflects physical processes or numerical limitations.
Minor Comments:
- Instrumentation: Please provide additional technical specifications of the Hydromast instruments, including sampling frequency, blanking distance, number of measurement cells, and other relevant parameters.
- Velocity measurement range: The statement that the instruments provide reliable velocity estimates in the range of 0.06–0.6 m s⁻¹ appears inconsistent with reported current velocities reaching 0.8 m s⁻¹. Please clarify the instrument limitations and the uncertainty associated with measurements exceeding the nominal range.
- Current direction convention: The sign convention for current velocity should be explicitly defined in both the text and the figure captions (e.g., positive and negative flow directions).
- River inflows: Please indicate whether significant river inflows enter Varna Lake and discuss their potential influence on the lake–channel system. Although likely negligible at sub-hourly timescales, river discharge may contribute to seasonal water-level gradients and extreme conditions.
- Operational thresholds: The discussion of current velocities reaching 0.8 m s⁻¹ would benefit from comparison with existing navigation or harbor-operation criteria, if such standards or guidelines exist. This would help quantify the practical significance of the reported events.
Recommendation
The manuscript can present an interesting and valuable contribution to the understanding of high-frequency hydrodynamics in harbor–channel systems and is suitable for publication after revision. Addressing the comments above would significantly improve the manuscript’s clarity, practical relevance, and broader scientific impact.
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- 1
This paper presents a good and worthy study of sub-hourly sea-level oscillations and currents along channels in Varna Bay. The study implements novel data, measured with novel instruments alongside existing data. Furthermore, it studies the system via analytic and numeric methods proving the characteristics of specific events of interest. It highlights the importance of theses sub-hourly oscillations and presents a theoretical framework for future prediction system.
Although the study looks valuable, some issues need to addressed before the final decision on publication is made. Most are only of technical nature. I attach the pdf file with my in-text comments for various parts of the manuscript. Below I add my general comments which you can find scattered around the pdf as well. Because some comments will likely need a change in Figure, or change in some parts of paragraphs, I propose major revision.
Before the comments I want to make clear that if you disagree with any comment or my comment is just wrong, just say it and I will accept that. For most of the in-text comments I just need an explanation on why is something done a certain way.
First bigger comment is in regards to Figures. I don't know why, but most, if not all, Figures have really low quality. Maybe that is the rendering which is made after your submission and you have uploaded higher quality Figures, I don't know but that should be changed. Additionally, a lot of Figures have really small font values and are hard to read, if not impossible. Furthermore, most Figures have titles for subplot which are not needed if the y-axis says the same thing or the caption in good clarity. On top of that, a lot of times x-axis is shared between subplots which can be the reason to omit ticks on some subplots and leave them only on the bottom ones for creating extra space. The spaces between subplots are also large for most Figures. It is better to name subplot as follows: (a) than caption for (a); (b) than caption for (b). Not stating the caption and at the end the letter of the subplot.
Link for the data repository is not working. I do not know if that an error from my side or is it wrong.