Following the Sand Grains

Round 1

Reviewer 1 Report

In this paper, the authors investigated the pathways and related processes of sand transfer around a tidal inlet system. The topic is of interest to not only coastal researchers but also to practitioners. Based on the inspections of a variety of datasets covering different time scales, this study provided an important example of morphological processes in which the sand is transferred by parcels in discrete steps over a decadal time scale. The obtained findings could contribute to filling the gap in our knowledge of the processes that have been less well understood. Therefore, the reviewer considered that the paper is acceptable after a minor revision. The specific comments are described below:

<Moderate Items>

(1) In this paper, essential elements consisting of the sand transfer at the study area were first described in sequence in Sections 4 and 5 from the updrift toward the downdrift portion; Merrimack Delta, Plum Island, Sandy Point, Spit Planform, Main Ebb-Channel, Beach Protuberance at Castle Neck, Southern Spit, Essex Inlet, Essex Bay. The overall pathways were then discussed in Section 6. The reviewer suggests adding several schematic (conceptual) figures in Section 6 to illustrate the sequences and interrelations of these component processes. Such figures will help readers to understand the discrete steps in the sand transfer more easily and precisely.

(2) The reviewer wonders if the littoral cell has experienced severe impacts of human intervention during the last decades or not. Please provide a brief explanation of the characteristics of sediment supply to the littoral cell. For example, was there no significant change in sediment supply during the study period? Also, was no hard structure built in the study area?

(3) The present results indicated that the several morphological forms (spits, tidal channel, protuberance) undergo cycles of growth and decay on the order of longer than ten years. What determines the period? Does the change occur continuously and gradually? Or does it occur intermittently and abruptly only at the severe storm events?

(4) In section 4.2.4, a trend over 100 years was deduced. However, the data used in the analysis did not necessarily cover the whole period. Therefore, an aliasing error may appear because oscillatory changes with shorter time scales coexist here.

(5) Is it possible to roughly estimate the sand budget in the littoral cell?

(6) The tidal range in the study area reaches around 3 m. Please describe the method of tidal correction in the analysis.

(7) If possible, please add a brief explanation of how the present results can be utilized in the future projection of this site.

< Minor Points >

(8) Is it possible to add a wave rose figure over the whole year?

(9) A length scale and a direction symbol should be inserted into Fig. 3(a).

(10) What do the arrows in Fig. 4(a) mean?

(11) A direction symbol should be inserted into Fig. 5. Besides, the unit of d_50 should be shown. It seems that too many digits were used for the value of d_50 in the figure notation (e.g., 3.15 was used in the text, while it was written as 3.1464 in the figure).

(12) In Fig. 9, e.g., the red line indicates the time “1844-1897”. Does that indicate the mean shoreline between 1844 and 1897?

(13) In Fig. 9, it seems that the location of protuberance moves alongshore with time. Please add some explanation on this point.

(14) The lines in orange and purple colors are hard to recognize in Fig. 10(a).

(15) The reviewer suggests adding the shape of the spit (at 2006) in Fig. 13(b) as well as that of the Castle Neck River.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript presents the sand transport process in a tidal inlet based on field observations and numerical models. The complexity bathymetry along a section of the Merrimack Embayment barrier coast in northern Massachusetts, USA. includes two barrier islands, two tidal inlets, and a back barrier bay and marsh system. The study is interesting and the manuscript is well written. It is suggested to clarify some questions list below before publication:

1. Line 16-17: it is not clear what is the reason causing sediment starved. Is it related to the content discussed in 4.2.1 or 4.2.4?
2. Line 84: if the purpose is to clarify the sequential sequestration and connectivity of the pathways. It is suggested to provide a more solid conclusion/summary in the end of the manuscript (for ex, a schematic diagram to illustrate the transport process/pathway).
3. Line 349: (Figure 10b) indicate that little, if any, of the sand is transported to the backside of the spit. Further, although sand is likely moved onshore and offshore in the main channel of Essex Inlet, little of this sand is deposited permanently on the ebb-tidal delta:
4. It is not clear how the authors get the information from Figure 10 which shows little or none sand is transported to the backside. Is it possible to refer to the model results (shown in the next paragraph) or any other more quantitative measurements? In addition, how did the authors know the sand is moved onshore and offshore with little of deposition? Is it based on the calculation of net sediment flux? It is better to clarified.
5. Does the vectors in Figure 12 show the direction of residual sediment flux? It is suggested to clarify in the caption. In addition, what does it mean in the vicinity of Essex Inlet”?
6. Line 429: A major finding from this recent study emphasizes that sand is transferred along the coast, and especially at tidal inlets, as parcels, in discrete steps; and that the transferal process occurs over decadal timescales.
7. What does it mean “as parcels”? and what are discrete steps? Does it mean the transport is driven by different mechanisms or the process including transport-deposition-resuspension?
8. Line 483: Much emphasis in coastal studies today is based on the results of theoretical hydrodynamic and sediment transport modeling with passive attention given to testing these model’s ability to produce real coastal systems and their evolution.
9. If so, what are the suggestions for theoretical modelling based on the study? Please specify more clearly.

Author Response

Please see the attachment

Author Response File: Author Response.pdf