Reply on RC2

The first three sentences of the abstract are not well connected to the storyline of the paper. Therefore, I recommend the authors team to re-write them, showing the significance of the study.


Overall:
The manuscript is well written and focused on five major variables. I have concerns that are they enough? The relative elevation difference between riverbeds at the inlet and average height of selected beel, river slope, and others may be important variables. Another concern is that whether the calibrated model for a particular beel is scalable to the entire domain. Maybe authors can explore the viability of TRM supported by dredging and excavation in future work. Also, since the TRM is participatory, please try to incorporate some socialeconomic indicators.

Authors' response
Authors thank the reviewer for the suggestions. As we explained in the method section, we selected those variables that are the primary controls of the sediment trapping within a beel: In the area without tides (river area during monsoon) the inundation depth, and in the lower reaches and during low flow seasons the tidal range determine the volume of water that may enter a beel. The sediment concentration in the river water then linearly determines the sediment load conveyed into the beel. The inlet size and beel size are controls of the actual volumes entering a beel, and the residence time of the waterdetermining the time for sediment settling. We expect that additional variables suggested by the reviewer certainly will correlate with the amount of sediment trapping, but we do not expect much added value from these. This is because the presumed effects of suggested additional variables may not be direct controls of sedimentation ("river slope") or are already incorporated by the variables we used, or spatially correlate to these. Specifically addressing suggested variables: "the relative elevation difference between riverbeds at the inlet and average height of selected beel, river slope" We agree that this might be a useful variable, but we did not consider this elevation difference, mainly since it is difficult to determine (particularly "river bed elevation" is hard to determine). Alternatively, we considered "Inundation depth" which is the difference between the water level and average land surface elevation inside the beel. We expect this inundation depth to some extent to correlate to the difference with channel depth (suggested by the reviewer), since a higher channel bottom level will at the same time lead to higher water levels in the feeding river, and hence to a smaller inundation depth of the beel. This Inundation depth at the same time includes the suggested "beel elevation". The variable "river slope" will likely correlate with deposition, but is not a direct control of deposition, as our variables do. Tidal range will directly (negatively) correlate to river slope (increasing tidal range for decreasing river slopes) in this area, so we do not expect that adding river slope will provide better predictions of sedimentation.
We agree that the question if a calibrated model can be used to understand a broad range of sedimentation inside the beels is important, which is the reason why we have studied this. We conclude that our method to produce a priori estimation of sedimentation inside the beel using understanding of morphodynamic changes of one of the beels of southwest Bangladesh due to operation of tidal river management (TRM) is valid. But we also conclude that for more precise estimation, location specific detailed study will be required, see discussion section.
Here we explored physical controls and morphodynamic changes inside the beel; socioeconomic indicators were beyond the scope of this study. We indicate this focus in our introduction, and in our discussion we refer to the socio-economic aspects that need to be considered for full assessment of impacts and feasibility of TRM in practice. The following sentence has been added after line 469: "Socio-economic aspects such as conflict between the stakeholders, lack of proper compensation, lack of livelihood opportunities, lack of stakeholders' participation during planning and implementation phase of the TRM, lack of cooperation between stakeholders and government agencies, divergence in common interest, imbalance between expectation and outcome and unequal distribution of benefits of TRM challenged the effective implementation of previous TRM operations ( van Minnen, 2013;van Staveren et al., 2017;Gain et al., 2017;Mutahara et al., 2018)." Authors agree with the reviewer on the suggestion for future study including the effect of dredging or excavation of riverbed. Authors appreciate this suggestion and the following sentences has been added in the discussion after line 469: "Implementation of several previous TRM operation included excavation of the adjacent river supplying sediments inside the beel (IWM 2017a; 2017b). Water and sediment dynamics of the river are affected by the change in river bathymetry. Therefore, the effect of dredging and exaction of the river should be explored in the future."

Abstract:
The first three sentences of the abstract are not well connected to the storyline of the paper. Therefore, I recommend the authors team to re-write them, showing the significance of the study.

Authors' response
The lines are adjusted as: Following the suggestion of the reviewer the Authors have revisited the introduction and have taken a closer look. We have adjusted the lines and concluded that at current state the lines are appropriate and fits the storyline as these state the dangers of relative sea level rise (RSLR) to the delta, TRM being a potential solution and knowledge in the form of determining the potential of TRM. Therefore, we have decided not to alter these sentences. Line 12 to 13 are adjusted as: "The potential of TRM application in different beels across southwestern Bangladesh has been estimated previously (based on geomorphic factors) but requires further exploration as seasonal and spatial variations (in physical drivers) were not taken into account, nor the non-linear character of physical drivers and several sensitive parameters for sediment deposition."

Reviewer's comments:
Is it true that the potential of the TRM application is yet to be determined? I could see the paper has cited Adnan et al. (2020), which explored the potential of TRM for 234 beels using different variables. However, it would be better to write in abstract that the potential of TRM remains to be explored, considering several sensitive parameters.
Line 12 -13: The potential of TRM application in different beels across southwestern Bangladesh, however, still remains to be determined.

Authors' response
Authors thank the reviewer for the Line 12-13: has been adjusted as: "The potential of TRM application in different beels across southwestern Bangladesh has been estimated previously (based on geomorphic factors) but requires further exploration as seasonal and spatial variations (in physical drivers) were not taken into account, nor the non-linear character of physical drivers and several sensitive parameters for sediment deposition. "

Reviewer's comments:
Line 24 -26: I believe the streamflow would be high from the upstream during the monsoon. And also, the sediment transport from the upstream would be substantial. So how effective would it be to operate the TRM during the monsoon season? I assume the TRM process takes full advantage of tidal flow from the sea in bringing the sediment deposited into the selected beel. And for the effective operation of the TRM, minimum upstream flow is recommended. Therefore, how challenging would it be to manage the upstream flow during the monsoon season?

Authors' response
The reviewer has rightly stated that the flow from upstream will be larger during monsoon and tide is one of the governing factors for sediment deposition inside the beel. Islam et al. (2021) indicated that seasonal variation of tidal range isn't large for tide dominated flow region, even during monsoon season and most of the polders of Bangladesh are located within the tide dominated flow region. Moreover, higher water level during monsoon and larger water and sediment flow from upstream results in larger sediment volume available in these rivers during monsoon (Islam et al., 2021). This results in largest sediment deposition during monsoon inside the beel for the regions of tide dominated and mixed flow (Islam et al., 2021) where all the polders and the beels of southwest Bangladesh are located. These are in line with the findings of this study.
In the discussion section we have now explained this point, and we added a reference to a paper in which we explored this issue.

Introduction:
Are five variables enough? Are any variables proposed by Adnan et al. (2020) important to be included, as additional physical controls?
Authors' response Adnan et al. (2020) considered geomorphic variables only, that indeed likely correlate with potential sediment accumulation in beels, but which are no direct controls. We have explored the physical drivers. As explained in the reply to the first reviewer, the variables considered by us take into account geomorphic conditions as well. The variable "Inundation depth" is the difference between the average land elevation and water level, as such it takes geomorphic variables proposed by Adnan et al. (2020) into account. Additionally, two of our proposed variables "tidal range" and "suspended sediment concentration" take water and sediment dynamics of the river into account. In the method section we added the following sentences after line 163 to be more precise to define our variables, and also make connections to the geomorphological variables as proposed by Adnan et al. (2020): "We explored five variables which affect the total sediment deposition inside a beel: river tidal range (TR), river suspended sediment concentration (SSC) are the two physical controls related to hydrodynamics of the river, inundation depth (ID), the width of the inlet (IW) and the surface area of the beel (retention basin, BA) are the three physical controls related to geo-morphodynamics of the beel. We have selected the variables that are the primary controls of the sediment trapping within a beel. The ID and the TR determine the volume of water and sediment that may enter a beel. The SSC in the river water then linearly determines the sediment load conveyed into the beel. The IW and BA are controls of the actual volumes entering a beel, and the residence time of the waterdetermining the time for sediment settling. Adnan et al. (2020) considered geomorphic variables that likely correlate with potential sediment accumulation in beels, but which are not direct controls whereas we have explored the physical drivers and the variables considered by us take into account geomorphic conditions as well. The variable ID is the difference between the average land elevation and water level, as such it takes geomorphic variables proposed by Adnan et al. (2020) such as land elevation, topographic slope and curvature of the land into account. Additionally, two of our proposed variables TR and SSC take water and sediment dynamics of the river into account. Analysis of variance (ANOVA) and Pearson correlation matrix indicate that the variables and their interactions are significant for sediment deposition (Table 3) and have strong correlation with sediment deposition (Table 4)."

Authors' response
The authors agree with the reviewer for pointing these out. This is demonstrated by our results, and we explain this in our discussion.

How will be the effect of saline sediment is deposited on the selected beel? Do we need to consider the quantification of salinity?
Authors' response This issue was reported as part of our broader study by de Bruin (2019): https: //livingpolders.sites.uu.nl/wp-content/uploads/sites/308/2020/08/Final-Thesis-Jeroen-M-de-Bruin-15072019.pdf. To understand how the sediments delivered by the tide inside the beels affects the soil fertility and food productivity, we collected samples of new and old sediment deposits from three different locations. The locations were: The beel where TRM was operated in the past (Khukshia Beel), the beel where TRM operation was ongoing (Pakhimara Beel) and an area which received large sediment during a dike breach as a result of cyclone induced storm surge (Polder 32). Sediment samples were also collected from the areas which are protected from tidal flooding. Rice variety preferred by local farmers was cultivated in the collected sediment samples following traditional and local practices. The results indicate that the sediments delivered by tide had similar fertility and food productivity as the old sediment deposits which were not flooded in recent years.
In the discussion section we have added the following sentences after line 470 on this quality issue, and indicate that salinity does not negatively affect the deposited sediment as soil for crop production, with reference to the study of De Bruin (2019)."To understand how the sediments delivered by the tide inside the beels affects the soil fertility and food productivity, we collected samples of new and old sediment deposits from three different locations. The locations were: The beel where TRM was operated in the past (Khukshia Beel), the beel where TRM operation was ongoing (Pakhimara Beel) and an area which received large sediment during a dike breach as a result of cyclone induced storm surge (Polder 32). Sediment samples were also collected from the areas which are protected from tidal flooding. Rice variety preferred by local farmers was cultivated in the collected sediment samples following traditional and local practices. The results indicate that the sediments delivered by tide had similar fertility and food productivity as the old sediment deposits which were not flooded in recent years. Therefore, the sediments delivered by the rivers of southwest Bangladesh is as good as old sediment deposits if not better."

Methods
Is the modeling approach calibrated for Pakhimara Beel applicable and scalable to entire southwestern Bangladesh?

Authors' response
The objective of this research is to present a method to explore physical controls, provide a priori estimation of sedimentation and explore the potential to counterbalance the yearly relative sea level rise (RSLR) through sediment accumulation in the beels of southwest Bangladesh. The beels located in southwest Bangladesh are within the same, tide dominated, flow region with similar ranges in geomorphological variables. Therefore, the calibrated 2D morphodynamic model and the resulting understanding of morphodynamic changes in Pakhimara Beels was assumed suitable to explore the general potential of the beels of the southwest Bangladesh using the location specific variables. For more precise estimation, location specific detailed study will be required, which is also suggested by the authors in the discussion section.

Reviewer's comments:
The operation and completion of TRM on adjacent beel play an important role in the sedimentation of current ongoing TRM-operated beel. Because the successful operation of one TRM would decrease the river profile considerably. I could see the paper has cited Talchabhadel et al. (2020). How will you consider such a dynamic effect on sedimentation?
Authors' response Authors agree with the reviewer that operation of TRM on adjacent beels will play an important role on the sediment availability in the river section. Our model takes into account the tidal range (TR) and suspended sediment concentration (SSC) of the river section. In this way we consider any changes in the sediment load potentially to be delivered by the tidal river, for example resulting from operating TRM in multiple beels along the same channel. However, our model explores the potential considering operation of TRM in one beel within a river reach at once. The following lines are added in the discussion section to reflect this: "Our study also considers operation of TRM in one beel within a river reach at once. More detailed study will be required to reflect on the changes in sediment availability in the river section during a basin wide operation and shifting of TRM in several beels as suggested by Talchabhadel et al. (2016 and2020). As our study considers TR and SSC of the river sections, the changes in sediment availability for such TRM operation can be taken into account. Moreover, similar to our study Talchabhadel et al. (2016 and2020) also explored the shifting of TRM operation considering TRM in one beel at once." In an ongoing study we investigate the effect of simultaneously opening multiple beels along the same channel on sediment deposition in a beel. This will be reported in a forthcoming paper.

Are there any calibrations of beel sedimentation and river bed changes on seasonal levels (monsoon and nonmonsoon) with ground-based observations?
Authors' response Calibration and uncertainty of the 2D morphodynamic model was done in a previous part of the project, and reported in (Islam et al. 2020). This was based on observations of sediment deposition in Beel Pakhimara in the period 2015-2017, complemented with measured SSC in the river and feeding channel (IWM 2017b). We added this information to our explanation in lines 156 to 158 as, that read: "The related R 2 for water level, discharge and sediment concentration were 0.87, 0.88 and 0.84, respectively. The NRMSE (%) for water level, discharge and sediment concentration were 9.7, 16.6 and 18.3, respectively." The uncertainty of estimated sediment deposition using NLMs against sedimentation data presented in previous literatures for different beels are explained in line 332 to 337 as:  Authors' response Authors agree with the reviewer that the IW and BA will be constant for a beel; the other variables represent time variable dynamics (tide, inundation depth, sediment concentrations), but the variable themselves are constant during the seasonal time periods we considered (tidal range, SSC during flood, inundation depth during maximum inundation). Therefore, we can analyse them with the other 'constant' variables using the Pearson coefficient. We will indicate this in more detail in our methods section. Inundation depth was introduced as it is a primary control in the upstream reaches during monsoon, when there is no tidal effect. During monsoon the water level in the river is higher resulting in larger inundation depth (ID) and during dry and pre-monsoon seasons water level in the river is lower resulting in lower ID. In contrast, the tidal range (TR) is also higher during the dry and pre-monsoon seasons. During the monsoon season, the flow from upstream rivers dampens the effect of tide, resulting in lower tidal range. Therefore, the correlation between TR and ID is negative. This is explained in line 292 to 295 as: "The correlation matrix also indicates that TR and ID have a very strong negative correlation. Tidal range (TR) is lowest during the monsoon season when water level in the river is highest, whereas TR is high during the dry and the pre-monsoon seasons when water level in the river is low."

Discussion
Overall, discussion on the calibrated model on one particular beel to the whole study domain is insufficient. It would be better to highlight some of the results on earlier TRM operated beels like Khuksia, Bhaina, Kedaria, etc.