On Internal Erosion of the Pervious Foundation of Flood Protection Dikes

Round 1

Reviewer 1 Report

1. Where is the innovation of the methods used in this manuscript?

2. Please list the implementation process in detail in section 3, preferably with a flow chart.

3. How to test the rationality of the results in section 4 needs to discussed.

4. In the Conclusion part, it is necessary to point out the shortcomings of the research methods and the future research direction. 

Minor editing of English language required

Author Response

Answer:

We thanks the Referee for his/her positive assessment of our paper and comments that helped us to clarify some points. Below, we provide detailed responses to all comments and explain how they have been accounted for.

All changes made to the manuscript are marked in blue. Please note that the responses are here numbered in the Referee’s original order.

Detailed remarks :

1- Where the innovation in the methods used in the manuscript ?

The novelty of this approach does not lie in the electrical methods used, which are classic geophysical observation methods. Currently, with regard to sand-boils and sinkholes, literature analysis shows that current analyses are based on a simplified description only: a broad majority of studies consider that the sand layer is horizontal within the foundation, and that such processes typically start along natural weaknesses or holes in the ground where flows are concentrated. Local geological conditions are rarely considered. The novelty of this work is to show that local geological conditions (in terms of geometry and position of layer interfaces) can be an important element in understanding the origin of erosion signatures (i.e. leaks, sand boils, sinkholes) observed, and that they are accessible with classical geophysical and geotechnical methods.

To clarify the scientific issue of this study, the following sentence has been added on introduction :

‘‘This study provides new information on the subsurface soils, under the dike and in the underlying protected zone (soil type and position of interfaces), making it possible to formulate new hypotheses concerning the causes of leaks, sand boils and sinkholes. The importance of knowing local geological conditions is illustrated by the case of the Agly dikes, where numerous leaks, sand boils and sinkholes have been frequently observed after floods, but have not yet been explained [7,8,26]. The results obtained being explained by the presence of a paleo-valley and paleo-channels under the river bed and under the dike, classic situations for river dikes, the methodology presented and the results obtained are likely to concern numerous dikes.’’

2. Please list the implementation process in detail in section 3, preferably with a flow chart.

We thank the referee for this suggestion. The purpose of the article is not to propose a method valid for all diking systems, with an implementation plan. The objective is to show the importance of local geological conditions to have elements of information to explain the presence of sand-boils and sinkholes. We show that this information can be obtained in the case studied with two classic and well-known geophysical methods. In another case, other geophysical methods may possibly be used: it is necessary to adapt to the site conditions. To clarify this part, we chose to not add an additional figure, especially given the short format of the manuscript, but to clarify the simple procedure used by adding the following paragraphs to parts 3 and 4:

‘‘The aim was to show the importance of local geological conditions in providing in-formation to explain the presence of sand-boils and sinkholes. At the site studied, we obtained this information using two classical geophysical methods [24]: Electromagnetic Induction Method (EMI) and Electric Resistivity Tomography (ERT).

" On the site studied, the method was finally as follows: (i) Horizontal mapping of average soil conductivity (based on EMI) in the protected plains. This mapping showed the presence of permeable soils and identifies vertical interfaces between high permeability and low permeability soils. (ii) Local cross-section mapping (based on ERT). This mapping confirmed the presence of permeable soils and positions the interfaces. It was carried out at different scales to confirm the results, bearing in mind that ERT alone only provides electrical resistivity gradients, not material interfaces. The key point here was that the relationship between electrical resistivity and permeability was deduced from the analysis of available soil cores."

3. How to test the rationality of the results in section 4 needs to discussed.

In order to better support the results obtained, two finite element numerical modeling results were added:

‘‘Two-dimensional numerical modeling using finite elements was carried out on simplified geometries deduced from the analysis of geophysical results and available soil cores. The mesh comprises 7357 linear elements. Dimensions, boundary conditions and permeabilities are given in Figure 10. The dike is 2 m high. The water level on the river side is at crest level. The water level on the protected area side is 5 m below natural ground level. The surface layer of sandy silt is e=1 m thick. The two cases of Figure 9 were modeled. Figure 10 plots the flow vectors q, and the pressure p(x) under the sandy silt surface layer. In both cases, the pressure under the surface layer of sandy silt is very high and can exceed 1.5 m: this is a favorable condition for the appearance of a resurgence likely to cause erosion. However, in case (a) (Figure 10.b), the curve p(x) quickly tends towards zero after the vertical interface: erosion signatures can only appear between the dike and the vertical interface. In case (b), the values of p(x) remain significant after the vertical interface: the erosion signatures can appear over a wider area, going beyond the vertical interface. The flow intensity in the gravelly sand of case (b) (Figure 10.c) is an order of magnitude greater than case (a) (Figure 10.b). However, in both cases flow velocities in gravelly sand can be locally of the same order of magnitude as suffusion initiation velocities (10^-6 to 10^-5 m/s). Carrying out modeling integrating the different types of erosion requires a finer description, which is outside the scope of this work."

Figure 10. Finite elements modeling on a simplified geometry: (a) Mesh, flow vectors q, isocolor of log||q||, and pressure p(x)=H(x)-e (m) under the sandy silt surface layer (thickness e) for the simplified geometries of Figure 9.a (b) and Figure 9.b (c), where H is the hydraulic head.

4. In the Conclusion part, it is necessary to point out the shortcomings of the research methods and the future research direction.

We thank the referee for these remarks and suggestions. We chosed to rewrite the concluding part in order to briefly remind to readers the main findings of the present study. Concerning the shortcomings of the methods used, the previous conclusion has been modified as follows:

‘‘Visual observations of leaks, sand boils and sinkholes in the protected area provide evidence of internal erosion processes in the underground soil. Local geological conditions are part of the information to be sought to explain these processes: presence of permeable soils and position of interfaces. This information was obtained on the site studied by combining two classic geophysical methods (EMI and ERT). The results were interpreted using the analysis of cored soils, in order to relate the electrical resistivity values, the soil types, the orders of magnitude of permeabilities, and the position of interfaces.

The results highlighted the presence of a layer of gravelly sand under the river bed and the dikes, approximately 300 meters wide and 25 meters deep, which is comparable to a paleo-valley resulting from the geomorphological history. The important characteristics observed were: the presence of the surface layer of low permeability sandy silt (0.5 to 1 meter thick), the extent towards the protected zone of the layer of gravelly sand (from 10 to 25 meters of thickness), which defines a vertical interface which mainly redirects the flows towards the surface. In one case, this permeable layer extends into the protected area with a band of 1 to 2 meters thick. In the other case, the interface continues to the surface. These local geological conditions control flows and provide elements to explain the spatial distribution of erosion signatures observed near the dikes.

Given the type of soil in place and the heterogeneity of flow, simply analyzing the occurrence of backward erosion piping is not enough. The possibility of internal erosion such as suffusion or contact erosion must also be considered as the cause of sand boils and sinkholes. It is then necessary to take granulometry into account, and to model the initiation and evolution of internal erosion processes through further work.

The aim of this work was to show the importance of local geological conditions in providing information to explain the presence of sand-boils and sinkholes. The results obtained being explained by the presence of a paleo-valley and paleo-channels under the river bed and under the dike, classic situations for river dikes, the methodology presented and the results obtained are likely to concern numerous dikes. The EMI and ERT methods proved suitable for the site studied. In other cases, other geophysical methods may have to be considered: site conditions have to be adapted. Whatever geophysical methods are used, imaging results are often affected by a lack of resolution, which means that interfaces cannot be accurately located. Two conditions are necessary for success: i) a strong contrast between permeable and low-permeability zones, ii) direct analysis in imaged zones by geotechnical investigations (Cone Penetration Test (CPT), core sampling and borehole drilling logs).’’

Author Response File: Author Response.pdf

Reviewer 2 Report

Comment

The type of work proposed by the authors (letter or brief report) seems to be a good form of presenting the results of field research, which were presented and described in an interesting and clear form. At the same time, it should be noted that the work does not end with presenting the final explanation of the problem of the occurrence of the described phenomena. Moreover, conclusions about the possible causes of the formation of specific geotechnical forms near the embankments after the flood remain at the stage of speculation, because they have not been verified, for example, by mathematical modeling. However, the authors are aware of this, which allows us to count on the fact that this research problem will be further analyzed by the authors. As a preliminary analysis of the problem, the report seems to me sufficiently complete and worthy of publication.

Technical Comments

·        The authors use the wrong method of citing articles by entering their full titles into the text of their manuscript. This makes the work extremely difficult to read and unnecessarily lengthens the text. This should be changed according to the publisher's requirements.

·        The wrong way of writing units is used in manuscript.

·        In Figure 2a, there is no cross-section L1, but L2 is marked twice.

·        The legend (color dots) from the Figure 2a should be also shown in Figure 3

·        Figure 10 should has number 9.

Author Response

Response to Reviewer 2 :

This work focuses on the mechanisms that trigger internal erosion in the pervious foundation of flood protection dikes.The origin of such permeable layers beneath dikes are commonly attributed to the presence of a paleo-valley and paleo-channels filled with sandy sediments, beneath and beside the present river bed, that partly extend into the soils of the protected area. The case of Agly dikes exposes that several internal erosion processes must be considered, namely suffusion, and contact erosion. The combination of both Electromagnetic Induction (EMI) and Electric Resistivity Tomography (ERT) methods, gained with sediment sampling analyses, represents a quick and cost-effective solution allowing to map the subsurface by providing the geometry of such geological layers. Results obtained highlight the origin of erosion signatures (i.e. leaks, sand-boils, and sinkholes) and assumes different scenarios to explain their spatial distribution in the subsurface soil of the protected area.

Comment :

The type of work proposed by the authors (letter or brief report) seems to be a good form of presenting the results of field research, which were presented and described in an interesting and clear form. At the same time, it should be noted that the work does not end with presenting the final explanation of the problem of the occurrence of the described phenomena. Moreover, conclusions about the possible causes of the formation of specific geotechnical forms near the embankments after the flood remain at the stage of speculation, because they have not been verified, for example, by mathematical modeling. However, the authors are aware of this, which allows us to count on the fact that this research problem will be further analyzed by the authors. As a preliminary analysis of the problem, the report seems to me sufficiently complete and worthy of publication.

Technical Comments

- The authors use the wrong method of citing articles by entering their full titles into the text of their manuscript. This makes the work extremely difficult to read and unnecessarily lengthens the text. This should be changed according to the publisher's requirements.

- The wrong way of writing units is used in manuscript.

- In Figure 2a, there is no cross-section L1, but L2 is marked twice.

- The legend (color dots) from the Figure 2a should be also shown in Figure 3

- Figure 10 should has number 9.

Answer:

All changes made to the manuscript are marked in blue. Please note that the responses are here numbered in the Referee’s original order, but sometimes gathered for a more suitable explanation.

1- At the same time, it should be noted that the work does not end with presenting the final explanation of the problem of the occurrence of the described phenomena.

We thank the Referee for this remark and chose to rewrite the conclusion in order to clarify the different findings of this short communication. The previous conclusion has been replaced by :

‘‘Visual observations of leaks, sand boils and sinkholes in the protected area provide evidence of internal erosion processes in the underground soil. Local geological conditions are part of the information to be sought to explain these processes: presence of permeable soils and position of interfaces. This information was obtained on the site studied by combining two classic geophysical methods (EMI and ERT). The results were interpreted using the analysis of cored soils, in order to relate the electrical resistivity values, the soil types, the orders of magnitude of permeabilities, and the position of interfaces.

The results highlighted the presence of a layer of gravelly sand under the river bed and the dikes, approximately 300 meters wide and 25 meters deep, which is comparable to a paleo-valley resulting from the geomorphological history. The important characteristics observed were: the presence of the surface layer of low permeability sandy silt (0.5 to 1 meter thick), the extent towards the protected zone of the layer of gravelly sand (from 10 to 25 meters of thickness), which defines a vertical interface which mainly redirects the flows towards the surface. In one case, this permeable layer extends into the protected area with a band of 1 to 2 meters thick. In the other case, the interface continues to the surface. These local geological conditions control flows and provide elements to explain the spatial distribution of erosion signatures observed near the dikes.

Given the type of soil in place and the heterogeneity of flow, simply analyzing the occurrence of backward erosion piping is not enough. The possibility of internal erosion such as suffusion or contact erosion must also be considered as the cause of sand boils and sinkholes. It is then necessary to take granulometry into account, and to model the initiation and evolution of internal erosion processes through further work.

The aim of this work was to show the importance of local geological conditions in providing information to explain the presence of sand-boils and sinkholes. The results obtained being explained by the presence of a paleo-valley and paleo-channels under the river bed and under the dike, classic situations for river dikes, the methodology presented and the results obtained are likely to concern numerous dikes. The EMI and ERT methods proved suitable for the site studied. In other cases, other geophysical methods may have to be considered: site conditions have to be adapted. Whatever geophysical methods are used, imaging results are often affected by a lack of resolution, which means that interfaces cannot be accurately located. Two conditions are necessary for success: i) a strong contrast between permeable and low-permeability zones, ii) direct analysis in imaged zones by geotechnical investigations (Cone Penetration Test (CPT), core sampling and borehole drilling logs).’’

2- Moreover, conclusions about the possible causes of the formation of specific geotechnical forms near the embankments after the flood remain at the stage of speculation, because they have not been verified, for example, by mathematical modeling. However, the authors are aware of this, which allows us to count on the fact that this research problem will be further analyzed by the authors.

In order to better support the results obtained, two finite element numerical modeling results were added:

‘‘Two-dimensional numerical modeling using finite elements was carried out on simplified geometries deduced from the analysis of geophysical results and available soil cores. The mesh comprises 7357 linear elements. Dimensions, boundary conditions and permeabilities are given in Figure 10. The dike is 2 m high. The water level on the river side is at crest level. The water level on the protected area side is 5 m below natural ground level. The surface layer of sandy silt is e=1 m thick. The two cases of Figure 9 were modeled. Figure 10 plots the flow vectors q, and the pressure p(x) under the sandy silt surface layer. In both cases, the pressure under the surface layer of sandy silt is very high and can exceed 1.5 m: this is a favorable condition for the appearance of a resurgence likely to cause erosion. However, in case (a) (Figure 10.b), the curve p(x) quickly tends towards zero after the vertical interface: erosion signatures can only appear between the dike and the vertical interface. In case (b), the values of p(x) remain significant after the vertical interface: the erosion signatures can appear over a wider area, going beyond the vertical interface. The flow intensity in the gravelly sand of case (b) (Figure 10.c) is an order of magnitude greater than case (a) (Figure 10.b). However, in both cases flow velocities in gravelly sand can be locally of the same order of magnitude as suffusion initiation velocities (10^-6 to 10^-5 m/s). Carrying out modeling integrating the different types of erosion requires a finer description, which is outside the scope of this work."

Figure 10. Finite elements modeling on a simplified geometry: (a) Mesh, flow vectors q, isocolor of log||q||, and pressure p(x)=H(x)-e (m) under the sandy silt surface layer (thickness e) for the simplified geometries of Figure 9.a (b) and Figure 9.b (c), where H is the hydraulic head.

3- The authors use the wrong method of citing articles by entering their full titles into the text of their manuscript. This makes the work extremely difficult to read and unnecessarily lengthens the text. This should be changed according to the publisher's requirements.

We thank the Referee for this remark and have now corrected this error.

4- The wrong way of writing units is used in manuscript.

All units mentionned in this paper use the International System of Units (SI). Tomograms both expose the soils conductivity (in S/m) and the soils resistivity (in Ohm.m) using a logarithmic color scale Veridis, since data can vary over several orders of magnitude on the same figure.

5- In Figure 2a, there is no cross-section L1, but L2 is marked twice.

We thank the Referee for this remark and have now corrected this error.

6- The legend (color dots) from the Figure 2a should be also shown in Figure 3.

We thank the Referee for this suggestion and have now added the legend on Figure 3.

7- Figure 10 should has number 9.

We thank the Referee for this remark and have now corrected this error.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Accept in present form

Author Response

We would like to sincerely thank the reviewers for the time taken to deeply read and correct the manuscript. 

We have taken into account all remarks and included corrections in the revised version which has been greatly improved from the submitted one. 

Author Response File: Author Response.pdf