the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Dec 2025
Vertical and temporal fluid mud dynamics during spring-neap tidal cycles
Abstract. The hyper-turbid Ems estuary has undergone extensive channel deepening since the 1980s resulting in distinct tidal asymmetry and substantial sediment accumulation which led to the occurrence of a fluid mud layer. This might cover up to 60 % of the water column, creating density-driven stratification that significantly affects hydrodynamics, ecology, and navigability. During a six-week monitoring period we investigated fluid mud dynamics with the focus on occurrence, thickness, density, and driving processes to obtain insights about formation and break-up of the fluid mud layer. We identified two superimposed recurring cycles of fluid mud occurrence. On a shorter timescale, we observed distinct differences over the semidiurnal tide while on a longer timescale fluid mud occurrence shows a distinct spring-neap tide relation. We conducted dedicated measurement campaigns around spring and neap tide to further investigate spring-neap variability of fluid mud occurrence. One main finding of those campaigns is that during neap tide, the majority of the water column is covered by a dense fluid mud layer, thus reducing the hydrodynamic cross-section. This leads to increased current velocity and entrainment at the lutocline, and reduced stability of the fluid mud layer. In contrast during spring tide, the hydrodynamic cross-section remains wider, leading to less friction at the lutocline and thus enhances stability of the fluid mud layer and thus longer persistence of fluid mud.
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Status: final response (author comments only)
- RC1: 'Review manuscript Slabon et al', Yoeri Dijkstra, 20 Jan 2026
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RC2: 'Comment on egusphere-2025-5706', Anonymous Referee #2, 02 Feb 2026
Manuscript ref: egusphere-2025-5706
Title: Vertical and temporal mud dynamics during spring-neap tidal cycles
Summary and recommendation:
The manuscript reports on in-situ data of fluid mud dynamics in the Ems Estuary on two different time scales: semi-diurnal tidal and fortnightly cycle. Two different data sets were analyzed: one of a 6 week-mooring period collected in 2019, and another one set of four times height hours profiling period collected in 2023. The datasets include measurements of velocity, turbidity, salinity and dissolved oxygen. Based on their observations, authors concluded that spring tides favor persistent and stable fluid mud layers, while neap tides enhance turbulence reducing the stability of the fluid mud layers due to a smaller hydrodynamic cross-section.
I am usually really interested by novel measurements of fluid mud dynamics in hyper-turbid estuaries and I am pleased to see that the authors have attempted to do so. Such studies could be of a great interest for the scientific community. However, I have major reservations about the data post-treatment and quality check, the interpretation of the results and generalization of the findings. I would not recommend the present manuscript for publication at this stage. A new submission should be encouraged after major revisions.
Major comments:
1) Scientific significance:
The scientific gap that justifies the present study is not clearly identified in the introduction. What is the main research question? What is the difference with previous studies focusing on fluid mud dynamics? The novelty of this study is not clear.
Moreover, the findings are presented without broaden significance. The contribution of the present study to global knowledge on fluid mud dynamics is missing.
2) Scientific quality:
The calibration of the instruments especially the turbidity sensors are not presented not even mentioned. No water sample from the field site is mentioned. Therefore, the question arises on what was the calibration procedure to convert turbidity values to SSC. Such conversion is highly dependent on the suspended sediment nature and should be done with field site water samples. Moreover, most of the turbidity sensors are not able to measure properly under concentration of several tens of grams per liter with natural sediments. The authors indicates that TSS probe has measured concentration up to 500g/L. This needs to be clarified.
For the velocity data, we have no information on the measurements (continuous sampling or burst, acoustic frequency, 2 or 3 velocity components...) and the post-treatment procedure (especially the quality control and filtering processes). Regarding the quality control, I am very surprised that authors were able to measure flow velocity under such high turbidity conditions (hundreds of grams per liter). Acoustic Doppler current meter are usually not able to make reliable measurements in more than a few tens of grams per liter. That’s why we are usually using electromagnetic technology to complement the measurements close to the bed (e.g. Becker et al, 2018). In addition, the current meters were placed on profiling/mooring lines and were sampling at 20Hz. Movements of profiling/mooring lines and vibrations from the motor of the ship should have been discarded from the data set with filtering processes. The full post-treatment procedure should be given.
Finally, I’m not sure to understand what “velocity” means: is it the magnitude of the velocity? If yes, could you provide the direction. Or is it the along channel velocity component? If yes, could you provide the across channel velocity component? Usually the velocity is projected on a local coordinate referential (with along channel velocity and lateral velocity component).
For the multi parameter probe, the acquisition frequency is very low and the profiling speed very high, it results in a very poor vertical resolution (10-20 points per profile). We usually used multi-parameter probes at 2-4Hz for vertical profiles or we strongly reduce the profiling speed. This should be clarified.
3) Interpretation of the results:
One of the major flaw in the interpretation of the results is that the mud dynamics is only considered along the vertical. However, fluid mud patches in estuaries are advected by the tidal current and the river flow, they moves back and forth along the estuarine channel. Along channel mechanisms such as advection should be considered and discussed when interpreting the results.
In addition, strong assumptions have been made that does not match the observations. In particular, the assumption that considers that the fluid mud does not contribute to the flow, while Fig.4 demonstrates the opposite. However, this could be linked to the fact that the velocity measured in the fluid mud layer is not reliable (see previous comment).
In addition, the luthocline is usually defined as the location where SSC gradient is maximum and not 10g/L.
Finally, the salinity and dissolved oxygen data set remains underutilized.
4) Presentation quality:
In the section “Results and discussion”, please introduce each figure before discussing the results. In addition, numerous citations are not appropriate in text. Please check your references.
In the interpretation of the results section, the reasoning is sometimes very difficult to follow and the information is often imprecise.
5) Discussion:
In order to broaden the significance of the results, comparison with other study sites should be given. Is the Ems Estuary an exception or similar results have been observed elsewhere? If so, why?
In addition, the limitations of the study should be acknowledged and discussed. For example, the fact that profile measurement campaigns do not cover the entire tidal cycle but only 8h, should be discussed and considered when interpreting the results.
Other comments:
L.31. References are normally ordered alphabetically or by date of publication. This comment applies to the whole manuscript.
Wang et al. (2020) is not an appropriated reference here.
References to other study sites than the Ems Estuary would underline the fact that hyper-turbidity occurs in estuaries worldwide.
L.32. High turbidity and Estuarine Turbidity Maxima are not caused by storms events or construction. Please clarify.
L.33. Reference is not appropriate.
L.36. Reference Zhou et al. (2019) is not given in the reference list.
Table.1. The table might be moved to supplementary material.
L.98. What is the novelty of your study? What is the research question here?
L.100. Are you investigating the effects on fluid mud on navigation and ecological functioning in this study?
L.101-102. The importance of permanent monitoring system in hyper-turbid estuaries is already acknowledged for decades and several European estuaries are already equipped with permanent monitoring systems for years.
L.121. Please introduce acronym when used for the first time.
L.124. “dry periods along with low upstream river discharge”. Strange wording: dry periods=low river discharge.
L.130. The ETM is where the turbidity is maximum, not where the strongest salinity gradient is reported. Please clarify.
L.134-138. This should be moved to Section3.
L.150. Exact dates of measurements should be given, as well as the forcing conditions (tidal range, river discharge...)
L.151. 8h/day only covers 2/3 of the tidal cycle. This could be justified and you should discuss the influence of missing 1/3 of the tidal cycle on your interpretation of the results. How did you choose the 8hrs to be sampled? Why around the LWS? And not the HWS?
L.154. Why a single point current meter? Why not a velocity profiler? What is the acoustic frequency of the instrument? Did you do continuous sampling? How did you filter out the vibration of the profiling line, the ship motion and motor vibration?
L.159. 10-20 points per vertical profile is insufficient. Water depth at the sampling site should be given.
L.162. “Vertical profiles were analyzed based on in situ measurements” Please clarify.
L.164. Low water slack is supposed to be time of minimum velocity, not maximum velocity.
L.168. Why do you consider the luthocline being where SSC>10g/L? Why not looking for the maximum of SSC gradient?
L.170. This is a strong assumption which seems not in line with your own observations. For example, in Fig.4b current velocity in the fluid mud layer seems to be up to 1m/s. Therefore, based on your data set this assumption does not stand. Please justify.
L.174. Again this is a strong assumption and should be justified. How is your cross section? Rectangular? Or with shallower banks?
Eq.1. The Richardson Number is generally expressed as Ri = -(g/ρ0).(∂ρ/∂z)/(∂u/∂z)²
Your way to express Ri seems to indicate that you have interpolated ρ and u on the same depth profile. If so, this should be mentioned.
L.184. “time-averaged velocities” Tidally averaged? Phase averaged? Please clarify over which period of time the averaged is made.
L.187. The Richardson Number is usually expressed as the squared ratio of the buoyancy frequency and the velocity shear: Ri = N²/S², with S=∂u/∂z.
What is the point of estimating Ri and N? The idea with Ri is to compare the stability of the water columns due to stratification with the instability produces by mixing.
L.193. In the introduction, the fluid mud is said non-Newtonian with thixotropic properties, and rightly so. Therefore, this assumption should be justified.
L.195. Define first u* and Rib and then give their expression.
L.198. Give references to justify this expression of the velocity shear.
L.202. ∆b is known as the reduced gravity and it is generally named g’.
L.212. “Approximately 1,95 m above the bed”. Instruments were not equipped with pressure sensor? Please provide total water depth at the mooring site. Does the river flow change during the 6 week mooring period?
L.217. Please provide calibration curves between 0,1-500 g/L with in-situ sediment coming from your field campaigns.
L.222. “vertical measuring transect”. Do you mean vertical profile? Because transect are measurements made along a cross-section of an estuary with a boat equipped with a winch. Here, you are talking about a mooring station, right?
L.237. Please introduce first the figure and the data presented before making conclusions.
Fig. 2. Please clarify “mean flow velocity”. Do you mean depth-averaged velocity?
Is it the velocity magnitude? Or the along channel component of the velocity? Why does the velocity is positive during all tidal cycle? Is it the absolute value?
Could you please express the velocity in m/s?
Time to LWS 0h should correspond to a zero velocity current (current reversal). Why does the velocity remains constant around 1m/s?
L.238. Around LWS+2h.
“near-continuous presence of fluid mud […] regardless of tidal conditions” This is not true. In Fig2, during spring tide, fluid mud is absent during hours, from HWS-1h to (we don’t really know because of missing data but we could expect) LWS-1h.
L.240. Only 0%-40%, there is period with no fluid mud at all.
L.241. The five stages defined here, do they correspond to those defined by Becker et al. (2018). If so, it should be mentioned. If not, it should be discussed.
L.241. Stage I seems not appropriate for spring tide where fluid mud is not persistent.
L.244. Stage V (same comments) fluid mud is not persistent during spring tide.
L.247. Reference to Wu et al. (2022) to clarify.
Are you presented your own data? or are you talking about Wu et al. (2022) observations? If so, clarify under which conditions (type of estuary, forcing conditions etc ….) other studies have shown the same behavior.
L.254-255. Maximum current velocity (and velocities in general) are very similar between spring and neap tides: around 1.40 m/s.
L.256. Neap tides rising currents are only slightly stronger than spring tide rising currents. Please provide numbers.
L.257. Where can we see this information?
L.258-259. This is during ebb phase (Stage I) not flood phase (Stage II, III, IV, V).
L.260. As per Fig4, Current velocities are really similar during Stage I, II and III, and spring tide velocities seem more important during Stage IV and V. Therefore, I don’t understand how you can conclude that the fluid mud modulates turbulence and current velocity from Fig2.
As previously mentioned, if you are presenting conclusion from your data set, do not add reference to another study at the end of the sentence. If you want to compare your results to another study findings, you have to clearly state it.
L.263. Is one data point sufficient to support a 1DV model?
This 1DV model could be applicable if the fluid mud was stagnant or moving very slowly compared to the upper layer, as it is shown in Becker et al. (2018) with one order of magnitude difference between the upper and the lower layer. In the present study, the lower layer flows at very high velocities (similar to the upper layer). Please justify.
L.270. Please introduce the figure first.
“average entrainment rate” Please clarify: is it tidally averaged, phase averaged, depth averaged?
L.273. During ebb-neap tide (Stage I) the buoyancy does not remain constant. Please clarify.
Tab. 2. Is this table necessary?
Entrainment rate, buoyancy frequency and hnorm have been average over the stage and depth, right? If so it should be clearly stated in the caption and in the text.
“Hnorm is the normalized height” of what? The luthocline height?
Please introduce the table in the text.
L.275. Again if you are presenting your data please do not refer to other studies. If you want to make comparison, do it in another sentence.
L.280. “quasi-stationary” Do your observations match this statement?
L.283. Ri values below and above the 50g/L horizontal line seems similar to me at Stage I. At Stage II, there is no data below 50g/L. And Ri remains below 0,25 (Fig. 4a, b), therefore your lower “fluid mud” layer seems unstable all along the tidal cycle. We could have expected quite the opposite. This should be mentioned and discussed.
Fig. 4. Please introduce Fig. 4 in the text.
Why is there no Richardson number calculated above 10g/L? if you want to compare the stability of the upper layer compared to the lower layer, you need to calculate it over the whole water column.
Shaded data should be mentioned in the caption.
Be consistent between Fig. 2 and Fig. 4 regarding the water depth or the height above the bed.
The unit of the buoyancy frequency seems erroneous. A line indicating N=0,4 s-1 could be added.
The density seems to be correlated to SSC only, i.e. the salinity seems very low. This could be mentioned in the text.
L.310-313. Could it be linked to the fact that the measurement site is located at the end of the mud patch? Therefore, the mud patch is advected upward with the strong flood currents.
In Fig. 4, we can see that at Stage II the lower layer is moving at a velocity of 0,8 m/s or more.
L.340. Please introduce Fig. 5 and 6. What does mean ebb tides for you? Only stage I? And flood tides means Stage II to V? Or did you average between HWS and LWS?
Why do you analyze ebb and flood separately? I don’t see the point here. From my point of view, Fig. 5 and Fig. 6 should be merged, and fluid mud occurrence should be analyzed along whole tidal cycles.
L.341. How could you state “persistent state” when the fluid mud disappears during flood?
L.342. Fig.6 presents flood tides data. It should not be mentioned here as the sentence begins with “Based on ebb tides”.
L.343. “50% of the tidal duration” Is it true? Or do you mean 50% of the tidal phase duration (ebb/flood)?
L.345. “throughout most of the tidal cycle” what does it means? Tidal cycle or tidal phase (ebb/flood)? And “most” means more than 50% of the time? Please clarify.
L.345. “intermediate to neap tides” Please give clear value of tidal range.
Fig. 5. and 6. The lower subplot should represent occurrence along the y axis, time along the x axis and with one colored line for upper probe and another colored line for lower probe.
As blue and orange were used to discriminate neap from spring tide in previous figures, it could be envisaged to use others colors to indicate the different probes.
L.380. Do not refer to other studies when presenting your own results.
L.382. “At mid TSS probe” At 2m above the bed.
L.384. How much tidal amplitude differences?
Fig. 7 please indicate the water level in meter.
Please expand y axis for water level to clearly display the tidal asymmetry.
Please use validated data set for water level (Fig. 7b and c) to avoid vertical lines.
Indicate date in addition of time, or give the date in the caption.
L.388. Give numbers.
L.392. Are we discussing the “absence state” (Fig. 7a) only? If so, clearly indicate it.
L.393-394. There is a clear vertical gradient in dissolved oxygen levels generated during ebb tide, with lower values close to the bed.
L.409. “continuous” the fluid mud occurrence is not continuous during the “build up phase”.
L.410. Replace “recorded by the mid TSS probe” by at 2 m above the bed.
L.414-415. Fluid mud formation can be produced in totally fresh water condition (in most estuaries). There is no necessity for salinity stratification. Why are you specifying that?
L.416. The fluid mud is absent during part of the “persistent state” (during hours around HWS). Please consider changing the name of this “state”.
L.419. Is “dissolution” appropriate wording?
L.421. What do you mean by “fluid mud layers modulates salinity stratification”?
L.422. “as dissipation is dampened”. Please clarify.
L.427. Uniform instead of consistent.
L.434-437. Have you compared river flow before and during both experiments (mooring vs profiles)? The river flow largely impacts the position of the fluid mud patches. That could probably explain the discrepancies between data set.
L.438-440. This is probably due to advection instead of resuspension.
L.443. Defontaine et al (2019) is not an appropriate reference regarding organic matter, microbial activity or nutrient cycling.
L.441-446. This is highly hypothetical.
L.450-454. In your data, the fluid mud layer disappears at every beginning of flood phase impeding “consolidation” and solid structure to develop. Therefore, I don’t see you point here. Please clarify.
L.456-457. As per Fig 4., velocity is similar or higher during spring than neap tides (not the opposite), and the fluid mud layer (>10g/L) is far from immobile (velocity up to 1 m/s). This reasoning seems quite inappropriate.
L.457-460. Does your current meter provides the lateral (cross sectional) velocity component? Could you have a look to it to investigate the potential lateral spreading of mud?
L.462. SOM content is not part of your data set.
Citation: https://doi.org/10.5194/egusphere-2025-5706-RC2
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Please see my full review in the attached pdf