the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Aug 2024
Dynamics of salt intrusion in complex estuarine networks; an idealised model applied to the Rhine-Meuse Delta
Abstract. Many deltas in the world consist of a network of connected channels. We identify and quantify characteristics of salt intrusion in such systems with use of an idealised model. The Rhine-Meuse Delta is selected as a prototype example of a complex network with many channels. The model is able to capture the characteristics of the tide-dominated water level variations due to the main tidal component and the salinity time series for one year of observations. Quantification of tidally averaged salt transport components shows that transport related to exchange flow is dominant in the seaward, deep parts of the network, but tidal dispersion is dominant in shallower channels further inland. Close to the network junctions, a tidally averaged downgradient salt transport is generated by the tidal currents, which is explained from the phase differences between the tidal currents in the different channels. Salt overspill is confined to the most seaward part of the Rhine-Meuse Delta. The magnitudes of the response times of different channels to changes in discharge increases with distance to the estuary mouth, and with decreasing net water transport through the channel. In channels without a subtidal discharge, response times are a factor 2–4 larger than in the other channels. The effect of changes to the depth on the extent of salt intrusion strongly depends on where the change takes place. If the change is within the salt intrusion range, deepening will locally increase salt intrusion due to an increase in salt transport by the exchange flow. If the change is outside the salt intrusion range, changes to the net water transport dominate the response of the salt intrusion.
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Status: open (until 10 Oct 2024)
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RC1: 'Comment on egusphere-2024-2322', Tong Bo, 30 Aug 2024
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The manuscript by Biemond et al. investigates salt intrusion in complex estuarine networks, focusing on the Rhine-Meuse Delta as a prototype example. The authors develop and apply an idealized 2D model to understand the mechanisms contributing to salt fluxes. I believe this reduced-order model is a promising approach for understanding salt fluxes in multiple channel systems. The results provided by the model regarding the Rhine-Meuse Delta also offer valuable insights into the understanding of estuarine dynamics. My primary concern lies with the interpretation of the tidal dispersion term in the model. I believe the manuscript could be publishable after revisions to address this concern.Major comments:
The term T_D is referred to as both tidal dispersion and diffusion throughout the manuscript. It would be helpful to clarify the terminology. My understanding is that diffusion typically relates to turbulent diffusion and molecular diffusion, which are not necessarily isotropic but generally have similar magnitudes in the vertical and horizontal directions. In your case, the horizontal dispersion/diffusion coefficient K_h,st = 275 m^2/s is quite large, so I would call it a dispersion coefficient (resulting from the interaction of diffusion and advection processes rather than just diffusion).
I am particularly interested in the T_T term, the time correlation between velocity and salinity, although it seems not to be the main focus of the manuscript. This term is sometimes referred to as oscillatory tidal dispersion and is a dominant term in many shallow estuarine channels. Therefore, it is important to ensure that T_D and T_T do not overlap in their definitions in your model, as they can both be referred to as forms of dispersion. Please clarify this in the manuscript.
My concern is that the model appears limited in its ability to resolve the T_T term. In the model, exchange at network junctions is pretty much the only mechanism that can lead to T_T (I think the vertical advection of subtidal salinity that you mentioned in the manuscript is negligible in most estuaries, so its contribution to T_T should not be important). However, there are other mechanisms that can contribute to T_T, but not represented here, for example, jet-sink exchange at the river mouth, also referred to as tidal pumping (e.g., Chen et al., 2012). This mechanism depends on variations in horizontal flow structures, which cannot be resolved in the 2D width-averaged model. Also, other complex 3D topographic features can lead to oscillatory tidal dispersion (Dronkers and van de Kreeke, 1986; Garcia & Geyer, 2023). I am concerned that because some mechanisms contributing to T_T are not represented in the 2D model, their associated salt fluxes may be inaccurately captured within the T_D term when tuning the model.
In addition, it seems that only a single K_h,st is used for all the channel. In reality, K_h,st is likely to vary significantly between different channels due to varying dispersion mechanisms. Additionally, K_h,st is determined based on a specific set of observations, but it could change with varying forcing conditions like river discharge and with changing channel depth. This is a limitation as you discuss the responses to different forcing conditions by using a constant K_h,st.
Overall, the manuscript contains sufficient publishable material. It is understandable that the concerns raised may not be fully addressed within the framework of a 2D model. However, I think the manuscript would benefit from a more extensive discussion of the model's limitations, based on the concerns raised above:
(1) The relationship between terms T_T and T_D.
(2) The use of a single K_H,st value for all the channels.
(3) The potential variability of K_H,st with difference forcing conditions.Here are some references mentioned in the comments above:
Chen, S. N., Geyer, W. R., Ralston, D. K., & Lerczak, J. A. (2012). Estuarine exchange flow quantified with isohaline coordinates: Contrasting long and short estuaries. Journal of Physical Oceanography, 42(5), 748-763.
Dronkers, J., & Van de Kreeke, J. (1986). Experimental determination of salt intrusion mechanisms in the Volkerak estuary. Netherlands Journal of Sea Research, 20(1), 1-19.
Garcia, A. M. P., & Geyer, W. R. (2023). Tidal dispersion in short estuaries. Journal of Geophysical Research: Oceans, 128(2), e2022JC018883.
Minor comments:
1. Equation (2): What is the role of vertical velocity in your model? I am curious if it can significantly affect salt flux or it is mostly negligible.
2. Equation (3): K_h,st is introduced here, but is is not explained until Line164. Consider adding a brief explanation to it immediately following this equation.
3. Equation (4): Are boundary conditions like u_ti=0 and s_ti = 0 needed? Or could they make your equations overdetermined? And should s_riv be zero?
4. L118-121. What is meant by "away from the weir" and "toward the weir"? I would guess that it is landward when the discharge is away from the weir, and seaward when the discharge is toward the weir. If so, the boundary conditions seem reversed. I think seaward discharge should be based on a prescribed salinity, and landward discharge should be based on calculated salinity.
5. "The water level at this boundary is chosen in such a way that at the estuary mouth the imposed tidal water level is reproduced." This sounds like it requires quite some manual input. Given that factors such as geometry, friction, and tidal frequency can all affect how the water level propagates from sea boundary to the mouth. It would be helpful to include more detail on how this process is managed.
6. Figure 5. What are the black lines (T_O)?
7. Figure 7. The results in this figure are very interesting, but it took me quite a while to understand.
What does \Delta Q represent? Initially, I thought \Delta Q>0 indicated a high discharge event (wet) and \Delta Q<0 is a low discharge event (dry). But given that \Delta Q only has positive values, I was once confused how that represents both wet and dry conditions.
Also, \Delta S>0 for all the cases. But I would guess that salinity increases (\Delta S>0) with a decrease in discharge and decreases (\Delta S<0) with an increase in discharge.
Including more explanations on \Delta Q and \Delta S in the figure caption and main text would help readers understand it better. I hope this feedback is useful as you make these changes.
8. L348-383: These two paragraphs are lengthy and difficult to follow, particularly because they describe many specific locations. I suggest breaking them into shorter paragraphs to enhance readability.
9. L389-404. I am glad to see the discussion on the single K_H,st value here. So maybe expand on this discussion to include the other concerns raised above. Remember to split the text into shorter paragraphs for better readability.
Citation: https://doi.org/10.5194/egusphere-2024-2322-RC1
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