This manuscript discusses fortnightly (~14.8 day) variations in surface elevation along an estuary, as recorded in measurements and found in a model.  The focus is on tidal forcing with neglect of river discharge and atmospheric forcing.  The introduction and discussion indicate that these elevation variations can be significant in several contexts.  However, given considerable existing literature, some with river discharge, this is a simpler scenario and the results are in general unsurprising.  Good agreement between model and observations is obtained.  The degree of novelty can be questioned and one would expect more discussion of the relation with previous results including river discharge (which I guess has varied magnitude relative to the tidal prism in the various contexts treated in the literature).

Non-dimensional parameters are introduced but not sufficiently exploited in my opinion.  There is a focus on values pertaining to the observed Guadiana estuary.  What about an estuary near resonance with M2?

I am not convinced by the (repetitive rather than justified) attribution of the behaviour to friction rather than non-linearity in the advection terms.  This must depend on their relative magnitude which depend on the length and depth of the estuary.  I question the analysis in 4.2.  Non-linear effects on M2 usually show in M4 as a result of advection and M6 as a result of quadratic friction.  However, here the quarter-diurnal species (mainly M4) are attributed mainly to friction and sixth-diurnal species are not discussed.

In general the English is good and understandable, despite occasional strange usage which should be picked up in copy-editing.  However

• There is frequent reference to “equinoctial” which is incorrect and obscures the intended meaning, e.g. lines 199, 239, 240, 241, 244, 596. Omit and say precisely the time referred to.
• “spring” and “neap” should be used as adjectives e.g. “spring tide(s)” or perhaps as plurals e.g. “springs”.  Do not use “spring” alone; this is a season of the year.

Typos etc.

Lines 9, 49.  The direction of increase should be stated.

Line 34.  “metric order” should be “of order 1 m”?

Equation (1) and line 77.  There should be a reference for this form of friction, especially the depth-dependence.

Line 83.  The relative magnitude of the two terms on RHS(2) is independent of |U| so the relevance of Froude number is unclear.

Line 129.  “Equinoctial” can be omitted: not strictly true and made redundant by the other information given.

Line 138.  Why should a diurnal tide induce “jagged” fluctuations?  Specify their time scale and that of the smoothing.

Lines 139-141.  If another scientist were to check this work they would need more specification of the CWT.  Also line 187 “using the equations developed by . .” is too vague.  Either they should be repeated or at least equation numbers in the cited papers should be specified.

Line 190.  “Continuity” of volume flux?  Surface elevation?  Anything else?

Line 202.  This sentence is unclear.  Which period shows the greatest variations?  “equinoctial” occurs at the very end of the record.  Likewise lines 212-213.

Lines 245-246.  This sentence seems tautological; what exactly is being compared with what?

Lines 273-275.  This is not correctly expressed.  (8) relates K to Cd but (9) is an independent value for Cd, presumably coming from Soulsby (1997).

Line 337.  “opposed to observations” begs discussion which I don’t see.

Line 350.  15° is about 31 minutes for M2.

Line 355.  This is section 5.3!

Figures 7, 8.  It is better practice to include zero on the amplitude scale.

Figure 8.  The scales for distance from the mouth differ between (b) and (c) and do not include zero.

Table 1 expressions for damping numbers: if dη/dx and dv/dx are factors then they need to appear without other symbols interspersed.

The manuscript by Garel et al. is a very focused piece of research on the fortnightly variability in mean water level along a tide-dominated estuary with low river inflow. They apply an analytical model to determine the physical processes involved, validating their results against observations in the Guadiana estuary. The presentation of the research is well structured, has a good coverage of supporting literature to define existing knowledge - identifying the gaps to address, and has good use of figures to illustrate key points. I have a few minor comments, with the main one focusing on the implications of the research. Section 5.2 answers the initial research question posed, but it is not until this point (near to the end of the manuscript) that the reader has a good understanding of the importance of this research in terms of application to present-day management issues.  More detail is required earlier in the manuscript, for example L19 in the abstract more information about the impacts could be stated. On L56/57 more information is given but this could be summarised in the abstract. “Impacts on the estuarine environment” is very general and could be a positive or negative impact and may relate to the natural system and/or human influence on the system. While ecosystems are mentioned in the conclusions, flood hazard is not.

In the paragraph starting at L131, the influence of atmospheric pressure has been considered and removed. Could you comment on the influence that wind and waves may have on the water levels. I appreciate winds are mentioned later in the manuscript but a comment to acknowledge the limitations of not removing all meteorological (and wave) forcing on the water level would be of value.

I have a few other minor suggestions:

L34, “metric order”, I suggest providing a value the change in elevations may reach.

L35, “flood control” I would have thought flood hazard was related to the changes in mean high water rather than mean water levels. This point isn’t discussed later in 5.2. Are you really referring to inundation of the intertidal and the impacts on ecosystems as discussed later in section 5.2 or potential changes in saltmarsh that protect the coast from flooding? Please ensure the points in the introduction are directly linked to the discussion points in section 5.2 and vice versa.

L206 & L244, “species” is an unusual choice of term. I would have used component.

L209, Zf remains at a similar level but it is not perfectly constant.

L210,it  would be better to focus on the period where there are quality results stating the initial data are removed due to artefacts of the filtering techniques.

L235, both lines have a clear signal. One is larger than the other rather than clearer.

L272, how realistic is the use of a flat non-ripped sandy bed. How could the results vary for different conditions?

L396, clarify the study is numerical using a case study for validation. I suggest you start the conclusion saying “…in a numerical representation of tide-dominated estuaries”. Otherwise, it could be misinterpreted that the results are only applicable to the study site.

There are a few typos to correct including:

L16 and L46 need rewording for clarity.

L34, check punctuation.

L46, check gramma.

P6/P7, there is an unexpected break in the sentence across the pages.

L205, insert a space between “is0.8”

L208, depends on.

L250, nearly constant.

L403, in the upper estuary.

In the Conclusion both abbreviations (e.g. LWL) and full names (e.g. mean water level) are used. Be consistent.