**2. General methodology for dike diagnosis**

The management of a dike involves many stakeholders and consists in surveying, maintaining and making a diagnosis (Mériaux & Royet, 2001). The diagnosis should identify the weaknesses of the structure (zoning) and provide the degree of safety. Thus, a general methodology (Fig. 1) has been proposed by (Fauchard & Mériaux, 2007). It concerns the levees running alongside French rivers (Loire, Cher, Isère, Aggly), where the dikes are not in a permanent hydraulic heading. The diagnosis is performed in dry condition. The methodology is based on several tests carried out in the framework of the French National Project "CriTerre" and the ERINOH (Internal Erosion in Hydraulic Earthworks) project. This methodology can be applied to dams, with slight differences during in-situ inspections.

This diagnosis begins with preliminary studies, before performing geophysical surveys. It goes on with geotechnical testing, before concluding on the safety level of the dike.

### **2.1 Preliminary study**

The preliminary study consists in gathering as much information as possible concerning the dike, the near environment and its history (Lino et al., 2000).


#### **III – Geotechnical studies**

264 Novel Approaches and Their Applications in Risk Assessment

data at a high efficient rate. The last part is a presentation of a current research work where the Electrical Resistivity Tomography (ERT) method is implemented and tested on an experimental test dam as well as real ones in order to monitor the effect of internal erosion

The management of a dike involves many stakeholders and consists in surveying, maintaining and making a diagnosis (Mériaux & Royet, 2001). The diagnosis should identify the weaknesses of the structure (zoning) and provide the degree of safety. Thus, a general methodology (Fig. 1) has been proposed by (Fauchard & Mériaux, 2007). It concerns the levees running alongside French rivers (Loire, Cher, Isère, Aggly), where the dikes are not in a permanent hydraulic heading. The diagnosis is performed in dry condition. The methodology is based on several tests carried out in the framework of the French National Project "CriTerre" and the ERINOH (Internal Erosion in Hydraulic Earthworks) project. This methodology can be applied to dams, with slight differences during in-situ inspections. This diagnosis begins with preliminary studies, before performing geophysical surveys. It

goes on with geotechnical testing, before concluding on the safety level of the dike.

dike, the near environment and its history (Lino et al., 2000).

The preliminary study consists in gathering as much information as possible concerning the

a. **The historical research** (Fig. 2) can establish the locations of old repaired breaches, material distribution and the way the dike was built. The study of historical archives gives clues wherefrom the materials were extracted so as to build and repair the dike. b. The **geological study** (map and in-situ observations) of the near area gives information about materials potentially used for building the dike and on the underlying substratum. c. The **topography** of the dike contains valuable information. From the longitudinal profile of the crest, we can assess the risk of overtopping during a flood by comparing it with the highest past flood. A map of the transverse profile is also required for stability studies and risks of piping, as well as for an accurate location of any structure (walls, crest water gates, crossing networks…) that can modify the interaction between water and dike in case of flood. Finally, the topography is useful for dike management and maintenance. It provides 3D coordinates for visual inspection, geophysical and geotechnical studies. The topographic map has usually a scale of 1:500 to 1:1000. Longitudinal profiles are performed on the crest every 20 to 25 m and transverse profiles are realized every 50 to 200 m, depending on the context. This is a critical point in the dike study, and it could be time and cost consuming for dike of long extent. In that case, **LiDAR** systems are an interesting alternative surveying technique and provide accurate 3D points along the dike with a high point density (see section

d. The **visual inspection** is performed after the historical research and the topographic work. This phase confirms, completes or invalidates any information previously collected. At least three inspectors are required: one on the crest, and two at the toes dike in the

riverside and landside. Any anomaly should be reported on the topographic map.

within the structure.

**2.1 Preliminary study** 

4).

**2. General methodology for dike diagnosis** 


**IV – Diagnosis, stability studies, improvement of dike model** 

e. The **morphodynamic study consists in** understanding the sedimentology, the hydrology and the morphometric characteristics of the waterway. It takes into account the temporal evolution of the watercourse channel. For instance, a sandy islet in the bed river modifies the water current: new parts of the dike could be threatened in case of flooding.

Methodology Applied to the Diagnosis and Monitoring of Dikes and Dams 267

**First approach** 

x

z

O

y

*Longitudinal a) First zoning : localization of anomalies*

Physical parameter

**Profiles Measurements results** 

*body*

*Local zoning: direct mapping of the whole dike body*

(Oz)

(Ox)

**Second approach**

**Zoning**

*Longitudinal and transverse* 

*Longitudinal and transverse profiles* 

Fig. 3. Two approaches for geophysical survey on dikes

**Slower Higher costs but Lower risks** 

**Rapid Cost effective But Higher risks** 

*b) Second zoning: local mapping of dike* 

*Colour scale of the measured physical parameter* 

(Oy)

(Oz)

(Ox)

(Oz) (Oz)

x

(Oy)

Fig. 2. Example of historical data of the dikes of the Authion river (France, Loire) (Dion, 1961)

#### **2.2 Geophysical studies**

#### **2.2.1 Introduction**

The geophysical exploration consists in mapping the dike body (nature and distribution of the material– the dike substratum is considered as a part of the dike body). Both the geometry (stretch and height) of the dike and the materials influence the choice of the methodology as well as the interpretation of the measurements.

Considering a typical study where the dike is a long structure of several kilometres, a classical approach (Fig. 3) starts with carrying out a rapid and cost-effective survey. It provides information on the homogeneity of the entire dike body. Then, heterogeneous areas that may weaken the dike body during a flood event are located.

Depending on the geophysical method, a physical parameter is measured according to different profile paths: along the crest (longitudinal profile), across the dike (transverse profile), at the toes of the dike (longitudinal profiles at the river side and the land side). The results of a geophysical survey must be **correlated** with the previous studies. This first survey helps to focus on interesting areas, which can be measured with appropriate geophysical or/and geotechnical methods. This global methodology is presented in Fig. 1.

Generally stakeholders are in charge of managing permanent critical structures like dams, multifunctional dikes in urban area or dikes in heading conditions. In this case, and regarding the potential damages that a breach could generate, the choice of a more efficient, but more time and cost consuming method may be considered. Indeed, the slightest breach during a flood event, either in urban or rural areas, leads to dramatic damages. It induces costs generally higher than the diagnosis does. As a result, some **stakeholders prefer a detailed zoning whatever the stretch length of the dike** (Fig. 3).

e. The **morphodynamic study consists in** understanding the sedimentology, the hydrology and the morphometric characteristics of the waterway. It takes into account the temporal evolution of the watercourse channel. For instance, a sandy islet in the bed river modifies

the water current: new parts of the dike could be threatened in case of flooding.

Fig. 2. Example of historical data of the dikes of the Authion river (France, Loire)

methodology as well as the interpretation of the measurements.

areas that may weaken the dike body during a flood event are located.

**detailed zoning whatever the stretch length of the dike** (Fig. 3).

The geophysical exploration consists in mapping the dike body (nature and distribution of the material– the dike substratum is considered as a part of the dike body). Both the geometry (stretch and height) of the dike and the materials influence the choice of the

Considering a typical study where the dike is a long structure of several kilometres, a classical approach (Fig. 3) starts with carrying out a rapid and cost-effective survey. It provides information on the homogeneity of the entire dike body. Then, heterogeneous

Depending on the geophysical method, a physical parameter is measured according to different profile paths: along the crest (longitudinal profile), across the dike (transverse profile), at the toes of the dike (longitudinal profiles at the river side and the land side). The results of a geophysical survey must be **correlated** with the previous studies. This first survey helps to focus on interesting areas, which can be measured with appropriate geophysical or/and geotechnical methods. This global methodology is presented in Fig. 1.

Generally stakeholders are in charge of managing permanent critical structures like dams, multifunctional dikes in urban area or dikes in heading conditions. In this case, and regarding the potential damages that a breach could generate, the choice of a more efficient, but more time and cost consuming method may be considered. Indeed, the slightest breach during a flood event, either in urban or rural areas, leads to dramatic damages. It induces costs generally higher than the diagnosis does. As a result, some **stakeholders prefer a** 

(Dion, 1961)

**2.2 Geophysical studies** 

**2.2.1 Introduction** 

Fig. 3. Two approaches for geophysical survey on dikes

Methodology Applied to the Diagnosis and Monitoring of Dikes and Dams 269

**A rapid and cost effective technique** is the electromagnetic method of **Slingram** (Mc Neill, 1980)**.** It works in low frequency domain and measures the apparent conductivity of the dike body. The depth of penetration of such a method can reach 50 m, but common devices are designed to reach a classical depth of 10 m (mean height of dikes). A car can track the device if the dike crest has a pavement structure. The measurements are represented as a curve showing the variations of the conductivity (or resistivity) with regard to the distance. A significant variation compared to an average value is interpreted as a potential variation of

Nevertheless, these methods are highly sensitive to metallic environment, and their

**Airborne Electromagnetic Method (AEM)** belongs to the Slingram family methods: the survey is performed from an airborne platform. It has been widely used for the levee in the region of New Orleans (Dunbar, 2003). Other high output methods were also carried out in the French context of the Loire Rivers. The **Radio MagnetoTelluric method,** identical to the Very Low Frequency in resistivity mode, but uses higher frequencies, has been designed for first zoning with good performances in the re-localization of repaired breaches. Today, this method has been discarded by most geophysicists mainly because its technical aspect is

Another popular technique is the **Spectral Analysis of Surface Waves (SASW)** initially developed for marine seismic exploration (Gimble sensors). It is implemented for studying the contact surface between the dike body and its substratum. It evaluates the shear modulus. This method was described and implemented on the Loire levees (Samyn et al., 2009) for the detection of sinkhole in karstic substratum in the region of Orléans,

**Ground Penetrating Radar** is sometimes used on particular studies – on paved dikes for instance, or on very resistive dike bodies. This electromagnetic method is based on the radiation of electromagnetic waves in time domain and the reception of waves reflecting on dielectric contrasts encountered in the soil. Most of dike bodies absorb this kind of waves,

The local zoning consists in carrying out directly a more precise geophysical method. It is for instance an internal map (or tomography) of the dike body and of the top of its substratum. The best-suited method for this approach is the **Electrical Resistivity Tomography (ERT).** This approach takes more time and is more expensive than the others. However, it provides more accurate data entailing a better detection of potentially weak

In case of heading conditions (for dams or earthen embankments), the ERT can be applied for mapping internal structure and can be implemented for a time lapse monitoring. But here, the most important is to detect seepages and/or leakages, through or under the dike body. For that purpose **the self potential** is more appropriate. It can be implemented directly on the top of the structure, or could be carried out on river, along the dike. This method also leads to the estimation of the seepage flow through the structure (Bolève et al.,

dike body properties: it defines the area for a more detailed survey.

getting older, and because of the poor quality of incoming waves.

and the provided information is often useless for a diagnosis.

areas and therefore a better understanding of the structure stability.

**2.2.3 Local zoning with geophysical methods** 

applications are still difficult in urban areas.

France.

#### **2.2.2 First zoning with geophysical methods**

As discussed earlier, depending on the dike characteristics two different approaches are currently carried out. The choice is more dependent on the available time and allocated means than on the dike length.

The first approach (Fig. 3 top) consists in measuring a **physical parameter** related to the type of material of the dike body. The **apparent resistivity** (or its inverse, the conductivity) is a common physical parameter measured for this purpose. The resistivity describes how materials resist to (or conduct) electricity. It strongly depends on the nature of the studied material, its water and clay contents. Other parameters like tortuosity or water salinity of soils are also of importance. The resistivity values of encountered materials in dikes spread in a large scale: few .m (ohm meter) in clays, from few .m to few hundreds .m for silty soils and from few hundreds .m to several thousands .m in sand, gravels and limestone. Fig. 4 shows the range of resistivity values of the main materials encountered in applied geophysics.

Fig. 4. Resistivity (and its inverse, conductivity) of the main earth materials (Palacky, 1991)

As discussed earlier, depending on the dike characteristics two different approaches are currently carried out. The choice is more dependent on the available time and allocated

The first approach (Fig. 3 top) consists in measuring a **physical parameter** related to the type of material of the dike body. The **apparent resistivity** (or its inverse, the conductivity) is a common physical parameter measured for this purpose. The resistivity describes how materials resist to (or conduct) electricity. It strongly depends on the nature of the studied material, its water and clay contents. Other parameters like tortuosity or water salinity of soils are also of importance. The resistivity values of encountered materials in dikes spread in a large scale: few .m (ohm meter) in clays, from few .m to few hundreds .m for silty soils and from few hundreds .m to several thousands .m in sand, gravels and limestone. Fig. 4 shows the range of resistivity values of the main materials encountered in applied

Fig. 4. Resistivity (and its inverse, conductivity) of the main earth materials (Palacky, 1991)

**2.2.2 First zoning with geophysical methods** 

means than on the dike length.

geophysics.

**A rapid and cost effective technique** is the electromagnetic method of **Slingram** (Mc Neill, 1980)**.** It works in low frequency domain and measures the apparent conductivity of the dike body. The depth of penetration of such a method can reach 50 m, but common devices are designed to reach a classical depth of 10 m (mean height of dikes). A car can track the device if the dike crest has a pavement structure. The measurements are represented as a curve showing the variations of the conductivity (or resistivity) with regard to the distance. A significant variation compared to an average value is interpreted as a potential variation of dike body properties: it defines the area for a more detailed survey.

Nevertheless, these methods are highly sensitive to metallic environment, and their applications are still difficult in urban areas.

**Airborne Electromagnetic Method (AEM)** belongs to the Slingram family methods: the survey is performed from an airborne platform. It has been widely used for the levee in the region of New Orleans (Dunbar, 2003). Other high output methods were also carried out in the French context of the Loire Rivers. The **Radio MagnetoTelluric method,** identical to the Very Low Frequency in resistivity mode, but uses higher frequencies, has been designed for first zoning with good performances in the re-localization of repaired breaches. Today, this method has been discarded by most geophysicists mainly because its technical aspect is getting older, and because of the poor quality of incoming waves.

Another popular technique is the **Spectral Analysis of Surface Waves (SASW)** initially developed for marine seismic exploration (Gimble sensors). It is implemented for studying the contact surface between the dike body and its substratum. It evaluates the shear modulus. This method was described and implemented on the Loire levees (Samyn et al., 2009) for the detection of sinkhole in karstic substratum in the region of Orléans, France.

**Ground Penetrating Radar** is sometimes used on particular studies – on paved dikes for instance, or on very resistive dike bodies. This electromagnetic method is based on the radiation of electromagnetic waves in time domain and the reception of waves reflecting on dielectric contrasts encountered in the soil. Most of dike bodies absorb this kind of waves, and the provided information is often useless for a diagnosis.

#### **2.2.3 Local zoning with geophysical methods**

The local zoning consists in carrying out directly a more precise geophysical method. It is for instance an internal map (or tomography) of the dike body and of the top of its substratum. The best-suited method for this approach is the **Electrical Resistivity Tomography (ERT).** This approach takes more time and is more expensive than the others. However, it provides more accurate data entailing a better detection of potentially weak areas and therefore a better understanding of the structure stability.

In case of heading conditions (for dams or earthen embankments), the ERT can be applied for mapping internal structure and can be implemented for a time lapse monitoring. But here, the most important is to detect seepages and/or leakages, through or under the dike body. For that purpose **the self potential** is more appropriate. It can be implemented directly on the top of the structure, or could be carried out on river, along the dike. This method also leads to the estimation of the seepage flow through the structure (Bolève et al.,

Methodology Applied to the Diagnosis and Monitoring of Dikes and Dams 271

Airborne laser scanning (also called ALS) or LiDAR (Light Detection And Ranging) is an active remote sensing technique that provides georeferenced distance measurements between an airborne platform and the surface. It measures the time-of flight of a short laser pulse once reflected on the Earth surface. Strips of several kilometres, with a high overlapping ratio, provide the surveyed topography. The attitude of the airborne platform is acquired by both a GPS and an inertial measurement system. Distance measurements are then transformed into georeferenced 3D points. A detailed description of the processing

The height accuracy (resp. horizontal accuracy), at the top end process, is less than 0.05 m (resp. about 0.40 m ) or less depending on the flying conditions as well as on the surveyed

Moreover, such active systems, called multiple echo LiDAR, allow detecting several return signals for a single laser shot. It is particularly relevant in case of vegetation areas since a single LiDAR pulse allows acquiring not only the canopy, but also points inside the

In recent years this technique has been applied over natural landscapes to extract terrain elevation (Kraus & Pfeifer, 1998; Bretar & Chehata, 2010) or to classify land cover

In the particular case of dike monitoring, we need a high flexibility in the flight planning in terms of altitude (100-300 m) and heading, and also a high accuracy because dikes are civil engineering structures with a relative low height (less than 7 m) and with a lot of small surface singularities. As a result, it is advised to use a corridor mapping system like FLI-MAP (Fast Laser Imaging and Mapping Airborne Platform) developed by Fugro-Geoid

Embedded in an helicopter, FLI-MAP can provide†, over a 105 m wide corridor at a fly height of 150 m, a point density of 80 pts/m2, with an absolute height accuracy (Z) of 0.03 m. The Pulse Frequency Rate (PRF) of the latest version can reach 250 kHz with a field of view of 60 ° in the cross track direction. The survey is done following three scan plans in the flight direction (vertical for 50% of the points, front 7 ° and rear 7 ° for 2 x 25%), which

The trajectory of the helicopter is recorded by two dual frequency GPS and an inertial measurement unit. A digital camera in nadiral position, synchronised with the LiDAR system, records the surveyed landscape and is used both to build a mosaic of georeferenced images and to colorize in real time the 3D point cloud so that a user should have a better understanding of the scene (Fig. 5). The system also includes two frontal and oblique cameras (photo and video). These data are particularly popular for dike managers who use

† Example based on a recent application of the FLI-MAP technique on the Loire levees near Orléans, in

**3. The airborne LiDAR as an efficient tool for topographical survey and** 

chain can be found in (Mallet & Bretar, 2008) and (Shan & Toth, 2009).

**detection of surface anomalies on dikes** 

vegetation layer and on the ground underneath.

(Gomes Pereira & Wicherson, 1999).

reduce the effects of shadows.

the context of the FloodProBE European research project.

(Antonarakis et al., 2008; Yoon et al., 2008, Bretar et al, 2009).

**3.1 Backgound on LiDAR systems** 

topography.

2007). In that case, ERT provides additional measurements for processing the data. Some temperature probes could also be buried in the dike and the temperature variation could be correlated to the presence of water in the dike body (Radzicki & Bonelli, 2010).
