

Abstract-In this paper, we aim at improving the change detection techniques by introducing an adaptive thresholding with a sliding time window. In particular, a real-time optimal sliding time window length is implemented without any preliminary learning step as required in conventional sensorfault detection methods. Based on Shannon's entropy, our method improves the change detection techniques using an adaptive thresholding. The technique can be applied by any change detection technique based on the generalized likelihood ratio (GLR). To validate the robustness of our approach, two commonly used change detection techniques are considered: the cumulative sum (Cusum) and the exponentially weighted Moving average (EWMA) control charts. Experimental validation is experimentally shown considering real data in the context of collaborative mobile robots. In addition, this experiment leads to a fault-tolerant fusion methodology based on the use of an extended Kalman filter (EKF).

I. INTRODUCTION

Change detection is concerned with the identification of data points that differ from those previously observed in the shortest time delay between data change. This technique is also known under the terminology "quickest change detection" [START_REF] Lau | Quickest Change Detection in the Presence of a Nuisance Change[END_REF]. Commonly, change detection techniques are developed through on control charts, implementing statistics based on online tests. These control charts are divided into two categories: memory-less and memory-type [START_REF] Awais | New Shewhart-EWMA and Shewhart-CUSUM Control Charts for Monitoring Process Mean[END_REF].

The Shewhart type charts [START_REF] Shewhart | Statistical Method from the Viewpoint of Quality Control[END_REF] are considered in the category of memory-less control charts. They are entirely based on current information. One of the main drawbacks of the Shewhart control charts is the lack of sensitivity to small changes in the observed signal behavior compared to memorytype charts.

Memory-type control charts include the cumulative sum (Cusum) and the exponentially weighted moving average (EWMA). These methods allow the accumulation of information over time. Indeed, both control charts take into account past and current information to maintain their statistics. This feature of memory-type helps them react quickly to small changes in signal parameters.

The Cusum control chart was initiated in 1954 by Page [START_REF] Page | Continuous Inspection Schemes[END_REF], and has been widely studied in the literature. Examples include Basseville and Nikiforov [START_REF] Basseville | Detection of Abrupt Changes: Theory and Application[END_REF]; Grigg, Farewell and Nawal Alsaleh is with the University of Lille, CNRS, Centrale Lille, UMR 9189 CRIStAL -Centre de Recherche en Informatique, Signal et Automatique de Lille, F-59000 Lille, France (e-mail: nawal.alsaleh@univlille.fr).

Bilal Daass is with the University of Lille, CNRS, Centrale Lille, UMR 9189 CRIStAL -Centre de Recherche en Informatique, Signal et Automatique de Lille, F-59000 Lille, France (e-mail: bilaldaass@gmail.com).

Spiegelhalter [START_REF] Grigg | Use of Riskadjusted CUSUM and RSPRTcharts for Monitoring in Medical Contexts[END_REF] and Granjon [START_REF] Granjon | The CuSum Algorithm-A Small Review[END_REF]. The EWMA control chart was introduced and developed by Roberts [START_REF] Roberts | Control Chart Tests Based on Geometric Moving Averages[END_REF]. It was subsequently studied in many works. Examples are Hunter [START_REF] Hunter | The Exponentially Weighted Moving Average[END_REF]; and Ye, Vilbert and Chen [START_REF] Ye | Computer Intrusion Detection Through EWMA for Autocorrelated and Uncorrelated Data[END_REF]. Numerous comparative studies are also presented in [START_REF] Boudaoud | A Comparative Study of CUSUM and EWMA Charts: Detection of Incipient Drifts in a Mutlivariate Framework[END_REF], [START_REF] Hawkins | The CUSUM and the EWMA Head-tohead[END_REF]. In this respect, various tools, techniques, approaches and their applications in different fields have been developed [START_REF] Ottenstreuer | A Combined Shewhart-CUSUM Chart with Switching Limit[END_REF], [START_REF] Haq | Improved Fast Initial Response Features for Exponentially Weighted Moving Average and Cumulative Sum Control Charts[END_REF].

In this context, it is important to determine an appropriate value for the threshold used in these control charts. This value will determine the performance of the detection system. Indeed, by considering a too low threshold, we risk getting a large number of false alarms. On the other hand, a too high threshold leads to a detection delay in the best case, and a set of missed detections in the worst case. For example, if we consider the EWMA chart, the width of the control limits is usually defined as three times the standard deviation of the signal. The value of this threshold is somewhat arbitrary. In this sense, several studies have proposed an adaptation for change detection methods [START_REF] Verdier | Calcul d'un seuil adaptatif pour des algorithmes de type CUSUM[END_REF][START_REF] Cisar | EWMA Statistic in Adaptive Threshold Algorithm[END_REF][START_REF] Lee | An Improved Position Sensor Fault Detection and Algorithm Transition using Adaptive Threshold for Sensorless Control of IPMSM[END_REF].

In our previous studies [START_REF] Daass | Using an Adaptive Entropybased Threshold for Change Detection Methods -Application to Fault-Tolerant Fusion in Collaborative Mobile Robotics[END_REF][START_REF] Daass | An Adaptive Threshold for Change Detection Methods using a Windowed Entropy-based Criterion -Application to Fault-tolerant Fusion in Collaborative Mobile Robotics[END_REF][START_REF] Hoballah | An Information Theoretic Approach to the Distributed Detection Problem[END_REF][START_REF] Pomorski | Entropy-based Optimisation for Binary Detection Networks[END_REF], an entropy-based strategy was proposed in order to overcome the constraint of using a fixed predetermined threshold. Moreover, we have discussed in [START_REF] Daass | An Adaptive Threshold for Change Detection Methods using a Windowed Entropy-based Criterion -Application to Fault-tolerant Fusion in Collaborative Mobile Robotics[END_REF] that the entropy-based adaptive threshold takes into account all previous observations. Thus, when several errors have occurred, the threshold can be significantly influenced. This problem has been addressed in [START_REF] Daass | An Adaptive Threshold for Change Detection Methods using a Windowed Entropy-based Criterion -Application to Fault-tolerant Fusion in Collaborative Mobile Robotics[END_REF] and a sliding time windowing technique was proposed. This technique consists in considering, at each iteration, only a window of the most recent observations. The window of length '𝑛' has been determined in [START_REF] Daass | An Adaptive Threshold for Change Detection Methods using a Windowed Entropy-based Criterion -Application to Fault-tolerant Fusion in Collaborative Mobile Robotics[END_REF] according to a method based on the minimization of the sum of the squared errors between the observations and the corresponding smoothed values. However, this method cannot work in a real-time manner as a preliminary learning step is necessary.

To address the need of fast and real-time sensor fault detection method, we extend our previous work by introducing a technique based on the use of Shannon's minimum conditional entropy. Thus, giving similar performance to the first one, but allowing moreover real-time operation without the necessity of the learning step. This paper is organized as follows. Section II briefly recalls the detection methodology based on the entropy-based criterion associated to two commonly used change detection techniques (Cusum and EWMA). Then, an improvement of this methodology using the Denis Pomorski is with the University of Lille, CNRS, Centrale Lille, UMR 9189 CRIStAL -Centre de Recherche en Informatique, Signal et Automatique de Lille, F-59000 Lille, France (e-mail: denis.pomorski@univlille.fr).

Kamel Haddadi is with the University of Lille, CNRS, Centrale Lille, ISEN, University of Valenciennes, UMR 8520 -IEMN, F-59000 Lille, France (e-mail: kamel.haddadi@univ-lille.fr).

Fast and Real-Time Sensor-Fault Detection using Shannon's Entropy

Nawal Alsaleh, Bilal Daass, Denis Pomorski and Kamel Haddadi sliding time window technique is presented in section III: a new method for real-time window length determination is implemented. Finally, section IV presents an experimental validation on real data from a mobile robotics platform.

II. ADAPTIVE THRESHOLDING BASED ON THE ENTROPY CRITERION

A. Entropy-based criterion

From a given phenomenon, the detection theory addresses the problem of discriminating a finite number of hypotheses. The knowledge of the vector of measurement 𝑦 = (𝑦 1 , 𝑦 2 , … , 𝑦 𝑁 ) issued from a vector of sensors 𝑌 = (𝑌 1 , 𝑌 2 , … , 𝑌 𝑁 ) allows to take a final decision 𝑢. In the context of the binary detection, this decision has two possible values 𝑢 ∈ {0,1} , depending on whether true hypothesis 𝐻 is considered to be 𝐻 0 or 𝐻 1 "Fig. 1". The optimization criterion chosen in this work is the minimization of the part of 𝐻 which is not explained by the final decision 𝑢. This quantity is defined by Shannon's conditional entropy ℎ(𝐻/𝑢) [START_REF] Hoballah | An Information Theoretic Approach to the Distributed Detection Problem[END_REF], [START_REF] Pomorski | Entropy-based Optimisation for Binary Detection Networks[END_REF], and represents the uncertainty on 𝐻 knowing the decision 𝑢. It can be written as a function of the probability of detection '𝑃 𝐷 ' and the probability of false alarms '𝑃 𝐹 ' as follows:

ℎ(𝐻/𝑢) = -∑ [𝛼 𝑖 log 𝛼 𝑖 𝛼 𝑖 + 𝛽 𝑖 + 𝛽 𝑖 log 𝛽 𝑖 𝛼 𝑖 + 𝛽 𝑖 ] 𝑖∈{0,1} (1)
with:

{ 𝛼 1 = 𝑃 0 𝑃 𝐹 𝛼 0 = 𝑃 0 (1 -𝑃 𝐹 ) 𝛽 1 = (1 -𝑃 0 )𝑃 𝐷 𝛽 0 = (1 -𝑃 0 )(1 -𝑃 𝐷 ) (2)
𝑃 0 is the prior probability of the hypothesis 𝐻 0 . Minimizing ℎ(𝐻/𝑢) consists thus of minimizing a mean risk for which the costs are not constant, unlike the Bayesian criterion. Moreover, taking into account the following notation:

{ 𝐶 00 = log 𝛼 0 + 𝛽 0 𝛼 0 𝐶 10 = log 𝛼 1 + 𝛽 1 𝛼 1 𝐶 01 = log 𝛼 0 + 𝛽 0 𝛽 0 𝐶 11 = log 𝛼 1 + 𝛽 1 𝛽 1 (3)
And under the reasonable assumption that 𝐶 10 > 𝐶 00 and 𝐶 10 > 𝐶 11 , the decision rule can be expressed as a likelihood ratio:

𝑃(𝑢 /𝐻 1 ) 𝑃(𝑢 /𝐻 0 ) 𝑢 = 1 ≷ 𝑢 = 0 𝑃 0 1 -𝑃 0 × 𝐶 10 -𝐶 00 𝐶 01 -𝐶 11 = 𝑇ℎ * (4)
This structure is similar to that obtained using Bayes or Neyman-Pearson criterion. In this entropy-based approach, the costs are not constant but depend on the posterior probabilities.

In order to find the optimal value 𝑇ℎ * of the threshold, an exhaustive search could be proposed. It consists of testing all possible values and keeping only the value minimizing ℎ(𝐻/𝑢). In most cases, and especially in a real-time context, a gradient-based technique is preferable [START_REF] Pomorski | Entropy-based Optimisation for Binary Detection Networks[END_REF].

B. Control charts

In the context of change detection, the Shewhart chart is very efficient when the magnitude of the shift is 1.5 σ to 2 σ or larger. Conversely, in the case of small shifts, the Cusum and EWMA control charts are good alternatives. However, these methods use predefined fixed thresholds, without any real justifications. We apply the variable threshold technique, based on the entropy, to these two algorithms.

Cumulative sum -Cusum

The Cusum chart directly integrates all the information into the sequence of sample values by plotting the cumulative sum of the deviations of the sample values from a target value.

Let 𝑥 𝑘 be the 𝑘 𝑡ℎ measurement. When the process is under control, 𝑥 𝑘 is issued from a normal distribution with mean 𝜇 0 and standard deviation σ. The Cusum algorithm works with two statistics 𝐶 𝑘 + and 𝐶 𝑘 -. These statistics are called the Upper and Lower Cusum respectively:

{ 𝐶 𝑘 + = max[0, 𝑥 𝑘 -(𝜇 0 + 𝐾) + 𝐶 𝑘-1 + ] 𝐶 𝑘 -= max[0, -𝑥 𝑘 + (𝜇 0 -𝐾) + 𝐶 𝑘-1 -] (5)
with: 𝐶 0 + = 𝐶 0 -= 𝜇 0 .

𝐾 is usually called the reference value. It is often chosen halfway between the target 𝜇 0 and the mean 𝜇 1 :

𝐾 = |𝜇 1 -𝜇 0 | 2 (6)
The mean 𝜇 1 is the out-of-control value for which we consider that the process is out of control.

If either 𝐶 𝑘

+ or 𝐶 𝑘 -exceeds the threshold 𝑇ℎ, the process is considered out of control. A typical value of 𝑇ℎ is usually defined as five times the standard deviation σ of the process.

Exponentially Weighted Moving Average -EWMA

The EWMA decision depends on the EWMA statistic, which is an exponentially weighted average of all previous data, including the most recent measurements. This decision is based on the weighting of the observations, so that the most recent observations contribute significantly.

The EWMA is sensitive to a small or gradual drift of the process. It is defined as follows:

𝑧 𝑘 = 𝜆𝑥 𝑘 + (1 -𝜆)𝑧 𝑘-1 (7) With 𝑧 0 = 𝜇 0 is the target, and 𝜆 is the weighting factor. The upper and lower control limits of the EWMA algorithm are defined as follows:

{ 𝑈𝐶𝐿 𝑘 = 𝜇 0 + 𝐿𝜎√ 𝜆 (2 -𝜆) [1 -(1 -𝜆) 2𝑘 ] 𝐿𝐶𝐿 𝑘 = 𝜇 0 -𝐿𝜎√ 𝜆 (2 -𝜆) [1 -(1 -𝜆) 2𝑘 ] ( 8
)
where 𝐿 is the width of the control limits. A typical value of 𝐿 is usually defined as three times the standard deviation σ of the process.

III. THE ADAPTIVE THRESHOLDING -SLIDING WINDOW TECHNIQUE

The value of the threshold 𝑇ℎ used in the Cusum and EWMA charts is often determined without any real justification apart from the experiment. However, the value 𝑇ℎ retained chosen will condition the resulting performance.

The threshold can be learned automatically, at each step of the algorithm, using the entropy-based criterion [START_REF] Daass | Using an Adaptive Entropybased Threshold for Change Detection Methods -Application to Fault-Tolerant Fusion in Collaborative Mobile Robotics[END_REF]. We have demonstrated that this approach is much more robust (with respect to false alarms and missed detections) than the usual approach considering a fixed threshold. However, at each iteration, our adaptive threshold takes into consideration all previous observations. Consequently, when several model changes have occurred, the adaptive threshold can be significantly influenced. In order to solve this problem, we propose an improvement of our approach by using a sliding time windowing technique. The latter consists in defining a fixed window of length 𝑛. This implies that at each iteration, only the 𝑛 most recent observations are considered. In the following, we present two different methods that allow to determine the sliding time window length 𝑛.

A. Smoothed value-based method

The first method was proposed in [START_REF] Cisar | EWMA Statistic in Adaptive Threshold Algorithm[END_REF]. It consists in defining the weighting factor of EWMA (𝜆 * ) as a first step. This is done by minimizing the sum of the squared errors (SSE) between the observations 𝑥 𝑘 and the corresponding smoothed value 𝑆 𝑘 calculated as follows:

𝑆𝑆𝐸(𝜆) = ∑ [𝑆 𝑘 (𝜆) -𝑥 𝑘 ] 2
𝑁 𝑂𝐵𝑆 𝑘=1 [START_REF] Hunter | The Exponentially Weighted Moving Average[END_REF] where:

𝑆 𝑘 = 𝜆𝑥 𝑘-1 + (1 -𝜆)𝑆 𝑘-1 ( 10
)
with S 1 = 𝑥 1 , 𝜆 ∈ [0 1] and 𝑁 𝑂𝐵𝑆 is the total number of observations. The optimal value of 𝜆 is the one for which the sum of the squared error is minimal. The window length 𝑛 is therefore determined according to this formula:

𝑛 = (2/𝜆 * ) -1 (11)
In other words, this window is a standard moving average that approximates the exponential smoothing S.

B. Minimum conditional entropy based method

According to the previous method, a learning step is required to determine 𝑛. To avoid this constraint, we propose a real-time method based on Shannon's conditional entropy minimization. This quantity, calculated by Eq. ( 1), increases very quickly, then stabilizes. The stability of the minimal Shannon's conditional entropy ℎ * (𝐻/𝑢) seems to be a good criterion to determine the time-window needed to store a sufficient amount of information to minimize the uncertainty on 𝐻 knowing 𝑢.

The method is as follows: the time derivative of ℎ * (𝐻/𝑢) is calculated according to [START_REF] Hawkins | The CUSUM and the EWMA Head-tohead[END_REF] at each time step, and a violation occurs when this derivative at time step 𝑘 is less than a predefined precision value [START_REF] Ottenstreuer | A Combined Shewhart-CUSUM Chart with Switching Limit[END_REF].

𝐷ℎ 𝑘 = ℎ * (𝐻 𝑢 ⁄ ) 𝑘 -ℎ * (𝐻 𝑢 ⁄ ) 𝑘-1 2 (12) |𝐷ℎ 𝑘 | < (1 - 𝑝𝑟e𝑐𝑖𝑠𝑖𝑜𝑛 100 ) × 𝐷ℎ 1 → violation (13)
We set the value of the time-window length after a certain number of consecutive violations 𝐾 𝑣 .

∑ 1 {|𝐷ℎ 𝑝 |<(1-𝑝𝑟e𝑐𝑖𝑠𝑖𝑜𝑛 100 )×𝐷ℎ 1 } > 𝐾 𝑣 𝑘 𝑝=1 → 𝑛 (14)
Although we cannot determine the optimal values of 𝐾 𝑣 and precision, with some approximations, this method can give an acceptable result. This is done without waiting for the end of the procedure as in the smoothed value method. We can thus obtain an approximated value of the time-window length in real-time situations.

IV. APPLICATION IN COLLABORATIVE MOBILE ROBOTICS

To validate our approach proposed, we consider an application consisting of two collaborative mobile robots (Turtlebot TM ) [START_REF] Daass | An Adaptive Threshold for Change Detection Methods using a Windowed Entropy-based Criterion -Application to Fault-tolerant Fusion in Collaborative Mobile Robotics[END_REF]. The objective of the application is to accurately estimate the pose (i.e. position and orientation) of each robot. However, the information from the on-board sensors can be disturbed at any time (strong magnetic field, data loss, drift, etc.). Consquently, we need to detect and isolate faulty sensors in the most efficient way to exclude them from the pose estimation algorithm.

After describing the platform used, we define the residual on which the Cusum and EWMA algorithms are applied in order to detect sensor faults. And finally, the results obtained using the classical approach (with fixed thresholds) and those obtained by the approach proposed in section III (with adaptive thresholds without/with a sliding time-window) are compared.

A. Platform description

Each robot in the platform is equipped with two differential wheels and two freewheels to improve stability (Fig. 2). The angular and linear velocities of each robot are independently controlled by an on-board laptop computer using the Ubuntu/ROS system. This laptop communicates 𝑣𝑖𝑎 Wi-Fi with a workstation equipped with Matlab TM software.

Each robot has wheel encoders, a gyroscope and an indoor navigation system (Marvelmind TM ). In addition, robot 1 is also equipped with a 2D-laser scanner (RPlidar TM ) and a camera (Kinect TM ).

B. Residual definition

At each step 𝑘, the state vector is defined as the pose of the two robots in a fixed global coordinate system:

𝑋 𝑘 = ( 𝑋 1 𝑋 2 ) 𝑘 𝑇 (15)
Where 𝑋 1 = (𝑥 1 𝑦 1 𝜃 1 ) and 𝑋 2 = (𝑥 2 𝑦 2 𝜃 2 ) are the pose of the robot 1 and 2, respectively. An EKF, detailed in [START_REF] Grewal | Applications of Kalman Filtering in Aerospace 1960 to the Present [Historical Perspectives[END_REF], is applied to properly estimate the state vector. This filter operates in two steps: A prediction step based on an evolution model (e.g., an odometry model), and an update step based on sensor measurements.

When the robot moves, its wheel encoders are used to predict its pose using an odometry model. Moreover, at each instant, each robot gets observations from its own onboard sensors. In addition, robot 2 receives information from the Kinect camera embedded on robot 1. In case of nominal operation (i.e. without any fault), these information are coherent. Nevertheless, in the case of a sensor failure, a drift of the estimation of the robot state compared to the real state will be observed. After detecting and excluding the faulty sensors, only the correct observations will be taken into account in the update step of the EKF.

A commonly used mismatch measure is the difference between the sensor observation (𝑍 𝑘 ) and the state prediction provided by the used evolution model ℎ(𝑋 𝑘 ). This measure, called the "residual", and on which an appropriate detection methodology is applied, will allow to exclude any erroneous observation from the fusion and estimation procedures. At each time step 𝑘, and for each observation, a residual is defined as follows:

𝑅𝑒𝑠 𝑘 = |𝑍 𝑘 -ℎ(𝑋 𝑘 )| ( 16
)
The proposed change detection method applied to each component of 𝑅𝑒𝑠 𝑘 allows for the detection of a significant difference between the raw measurement given by the sensor and the data issued from the predicted state. This faulty sensor is then excluded from the EKF. For each robot, at least one fault-free observation is assumed available at any time.

C. Experimental results

The experiment lasts 80 seconds, with a sampling time of 50 milliseconds. The robots follow a specific and defined trajectory while maintaining a distance of one meter between them.

In order to highlight the proposed approach, we introduced three different types of default that occur successively during the Kinect TM observation (a bias of 10 cm between iteration 500 and 700; a Gaussian noise "𝒩(0; 𝜎 = 0.10)" between iteration 900 and 1100; and a drift between iteration 1250 and 1500). In the following figure, we show the corresponding residual. At each time step 𝑘, Cusum and EWMA techniques with/without adaptive threshold are applied to each component of the residual in order to detect the sensor faults. Fig. 4 shows the results obtained with the Cusum algorithm, using a fixed threshold (red) and using an adaptive threshold issued from the entropy-based criterion (green).

Similarly, Fig. 5 shows the obtained results with the EWMA method, using a fixed threshold (red) and using the adaptive threshold issued from entropy-based criterion (green). Note that the fixed threshold technique generates some missed detections (Fig. 4). We also note that after each default, the adaptive threshold is highly influenced, generating therefore false alarms and missed detections in Fig. 5. Indeed, this adaptive method needs time after each default in order to re-adjust the threshold. We apply therefore the sliding time-windowing technique of length 𝑛.

Since our application requires online, real-time operation without prior learning, the determination of the window length is performed using the minimum conditional entropy method. Fig. 6 presents the measure of Shannon's minimum conditional entropy applied to the residual andFig. 7" presents its temporal derivative 𝐷ℎ 𝑘 . This stability of ℎ * (𝐻/𝑢) is a good indication of how long it takes for the system to stop needing additional observations to refine the adaptive threshold. Fig. 8 presents the optimal (in the informational sense) length of the sliding window obtained as a function of precision (ranging from 0 to 99) and 𝐾 𝑣 (ranging from 1 to 15). The optimal window length is an increasing function of the precision and 𝐾 𝑣 . Moreover, when the precision tends to 100, it becomes difficult to have the required number of 𝐾 𝑣 violations, the window length increases significantly. Therefore, in this application we have chosen the following values: 𝐾 𝑣 = 5, 𝑝𝑟𝑒𝑐𝑖𝑠𝑖𝑜𝑛 = 95. The optimal length of the temporal window obtained is therefore equal to 39.

We can notice that the window length determined online with the entropy method is very close to that determined after experimentation by the offline method based on the exponential smoothing (𝑛 = 30 in this case). However, we note that the offline method relies on the minimization of the sum of squared errors, a criterion that is difficult to justify as being the most appropriate.

We therefore propose to apply the sliding window whose length is obtained by the entropy based method. This technique is performed online, without prior learning. Fig. 9 (resp. Fig. 10) shows the results obtained with the Cusum (resp. EWMA) method with a fixed threshold on the one hand, and with an adaptive threshold derived from the entropy criterion and using the time window technique on the other hand. We note that the adaptive threshold without timewindowing is highly influenced after each default, which may implies some false alarms and missed detections (Fig. 4 and Fig. 5). However, by using the sliding time-windowing technique, the adaptive threshold 𝑇ℎ makes it possible to avoid these false alarms and missed detections (Fig. 9 and Fig. 10). Furthermore, the new threshold is more adaptive with respect to the residuals and it is not influenced by the defaults occurred previously.

V. CONCLUSION

In this paper, we have proposed to use a criterion based on Shannon's entropy to detect a model change in a digital sequence, in a general way. This digital sequence can be any kind of information delivered by a sensor. Our approach consists to determine an adaptive threshold that can be applied by any change detection method based on the (generalized) likelihood ratio. To illustrate our approach, we have combined this adaptive thresholding technique with two commonly used change detection methods: the Cusum and EWMA control charts.

However, the proposed approach consists, at each iteration, to take into account all previous observations, which can lead to a loss of robustness when several model changes have been occurred. To solve this problem, an improvement is proposed by using a sliding time-windowing technique: at each iteration of the algorithm, we consider only the 𝑛 most recent observations. Two methods have been identified to determine the value of the time-window length. The first method is based on the minimization of the sum of the squared errors between the measurements and the corresponding smoothed value. However, this method cannot work in real-time situations, a learning step is required. Therefore, we have proposed in this paper a second method based on Shannon's conditional entropy suited for real-time operation.

Finally, our approach makes it possible to propose a faulttolerant fusion methodology, which is illustrated by the use of an EKF in the context of collaborative mobile robotics. Both the standard change detection methods and the proposed approach are applied and compared to detect and remove faulty sensors. Our strategy is much more robust with respect to false alarms and missed detections. The proposed strategy has a significant robustness with respect to rare events, allowing especially its use, completely in real-time, in any change detection algorithm.

Figure 1 .

 1 Figure 1. Principle of a binary detection system.

Figure 2 .

 2 Figure 2. Mobile robotic platform.

Figure 3 .

 3 Figure 3. Corresponding residual as function of iteration number.

Figure 4 .

 4 Figure 4. Cusum result using a fixed/adaptive threshold.

Figure 5 .

 5 Figure 5. EWMA result using a fixed/adaptive threshold.

Figure 6 .

 6 Figure 6. Shannon's minimum conditional entropy as a function of time.

Figure 7 .

 7 Figure 7. Time derivative of h * (H/u) as a function of time.

Figure 8 .

 8 Figure 8. Optimal sliding window length obtained from the entropy method as a function of precision and Kv.

Figure 9 .

 9 Figure 9. Cusum results using fixed / adaptive threshold with windowing -as a function of time.

Figure 10 .

 10 Figure 10. EWMA results using fixed / adaptive threshold with windowingas a function of time.


PhD Joana Guerrin
email: joana.guerrin@inrae.fr

Comby Emeline

Raphaël Morera

Dr Emeline Comby

History and politics of sediment management on the Rhône River Authors

Keywords: Sediment, Rhône, Hydropolitics, Perception, Hydrosocial cycle Sediment, Rhône, Hydropolitics, Perception, Hydrosocial cycle

Sediment is physical matter and, as such, is usually studied in the physical and natural sciences, such as chemistry, geomorphology, or hydraulics and hydrology. However, sediment is also deeply embedded in society and politics. Sediment flows are affected by river transformation and, in turn, affect river management. This article highlights the changing ways in which society has considered sediment in the Rhône River (France). In the early-modern historical period (16e-18e c.) sediment was generally considered a blessing. However, the contemporary evolution of the Rhône's industrialisation, river engineering, and governance has challenged this perspective. Today sediment is more likely to be viewed as a threat. The concept of the hydrosocial cycle is a useful framework within which to describe the variety of ways societies have understood river sediment. The variation is caused by, and in turn affects, the meaning and representation of sediment, the structure of land and water governance, the nature of sedimentary and water resources, and the materiality of sediment and water, mediated by technology. This article aims to open the issue of sediment to the social sciences through a dialogue between history, human geography, and political science.

Introduction

Sediment is an essential part of rivers, and they are also important for the society living near rivers.

On the Rhône River (France), if sediment is today rather considered as a threat regarding quality and quantity issues, this was not always the case. This article highlights the changing ways in which society has considered sediment in the Rhône River (France), comparing the early-modern historical period (16e-18e c.) and today.

The Rhône is a major river in France, rising in Switzerland and running towards south-eastern France, ending in the Mediterranean Sea by the Camargue delta (Figure 1). On many French maps the river appears to start at the French border. However, the issue of sediment transport does not stop at political boundaries. The Rhône drains a catchment area of 98,000 km 2 along its 718 km course through Switzerland and France. In France it has a 512 km main stem, flowing from Lake Geneva to the Mediterranean Sea. Several rivers contribute to the Rhône's considerable flow, including the Sâone, Isère, Drôme and Durance. Representing a natural boundary, the river crosses three administrative regions and eleven departments. The Museum of Ancient Arles bears testimony to the rich history of human settlement on the banks of the Rhône and the use of the river for navigation and commercial exchanges in the Gallo-Roman period. The Rhône's particularly high flow played a crucial role in the reconstruction and modernisation of post-war France [START_REF] Pritchard | Confluence. The Nature of Technology and the Remaking of the Rhône[END_REF]. Notoriously dangerous to navigate, the river was tamed through extensive modification over the course of the nineteenth and twentieth centuries, providing flood protection, easier navigation, hydropower and nuclear power. This altered the landscape significantly, as well as profoundly shifting the river's hydrology and sediment dynamics. The riparian inhabitants were also forced to adapt their relationship to the river. Today, the Rhône Valley is a major transit route (railway, motorway and waterway) and industrial axis in France, the riverbanks having been massively artificialised linking cities, industrial sites, and farmland. (Comby, 2020) The Rhône River is about much more than water, as shown by a rich social science literature. Sara Pritchard's passionate history of the Rhône's post-war development shows how political and economic factors have been presented as 'natural' in terms of how the Rhône should flow. Indeed, 'masking the cultural and political work behind these "natural" changes has helped make their political implications invisible' (Pritchard, 2004, p.798). From a similar perspective of river development, other authors have shown how power relations have contributed historically to the shaping of the Rhône's riverscape [START_REF] Comby | Power and Changing Riverscapes: The Socioecological Fix and Newspaper Discourse Concerning the Rhône River (France) Since 1945[END_REF]. Political science has shown how floods were related to political factors, the risk being made alternately invisible through the Rhône development strategy and visible through sociopolitical crises [START_REF] Guerrin | Une inondation négociée? Politisation d'un risque naturel sur le Rhône[END_REF]. Sociologists have analysed the drivers and impediments to the implementation of restoration projects on the Rhône River and shown how local protests around crises such as flooding and pollution could hamper or boost socio-natural changes along its banks [START_REF] Barthélémy | A Comparison of Social Processes at Three Sites of the French Rhône River Subjected to Ecological Restoration[END_REF]. Such examples testify to the diversity of socio-natural studies being carried out on the Rhône. Yet few specifically focus on the sediment itself, tending instead to focus on its connection to pollution. For example, [START_REF] Gramaglia | L'expertise à l'épreuve d'une controverse environnementale et sanitaire: la production des savoirs et des ignorances à propos des PCB du Rhône (France)[END_REF] deconstruct the sociopolitical crises that emerged on the Rhône with chemical pollution stored and dispersed by sediment. [START_REF] Comby | How chemical pollution becomes a social problem. Risk communication and assessment through regional newspapers during the management of PCB pollutions of the Rhône River (France)[END_REF] analysed the same crises through the media coverage. In this paper we focus on sediment to show how the idea of sediment and its role has evolved through history.

As Jamie [START_REF] Linton | What Is Water ? The History of a Modern Abstraction[END_REF] comprehensively explains, water is a lot more than H2O. Created by scientific discourse and developed by hydrology, this modern and abstract form has affected how we perceive water today. Linton differentiates H2O, water as a component we can scientifically know, with other ways of experiencing it, such as through our senses and beliefs. From this perspective, the works on the 'hydrosocial cycle' propose reintegrating water's material nature (expressed by H2O) with theories, discourses, and representations of water, including the power relations and the role of technology and infrastructure within the construction of water [START_REF] Linton | The hydrosocial cycle: Defining and mobilizing a relational-dialectical approach to water[END_REF]. What has barely been proposed, however, is giving sediment back to water.

Recent works have advocated a reconsideration of one of the 'foundational binaries' of geography -'that of land and water' (Lahiri-Dutt, 2014)and a reintegration of land, and sediment, within the hydrosocial cycle (Lafaye de [START_REF] De Micheaux | When hydrosociality encounters sediments: Transformed lives and livelihoods in the lower basin of the Ganges River[END_REF]. In the context of riverscapes, water has been separated from land and, therefore, from sediment through regulation and practices inherited from private land ownership and agriculture, as well as colonialism (Lahiri-Dutt, 2014). Interestingly, Lafaye de [START_REF] De Micheaux | When hydrosociality encounters sediments: Transformed lives and livelihoods in the lower basin of the Ganges River[END_REF] propose opening the hydrosocial cycle model to sediment through an analysis of: i) meanings and interpretations (including scientific and/or local knowledge) of land and water in so-called 'muddyscapes'; ii) land and water governance and power relations; iii) resource (water and sediment) use and exchange patterns; and iv) physical and biological processes, partly mediated or affected by technology. This research agenda allows land to be brought back into the picture when looking at water, turning riverscapes into muddyscapes and reminding us that water and land are not so separate.

Sediment cannot be separated from water in riverscapes. It is deeply embedded in society and politics because it is not just physical matter. It has a social life. This paper shows that sediment is part of the Rhône's hydrosocial cycle when extracted to repair dykes or produce cement, when it overflows (for better or worse), when fish are contaminated by chemical pollutants, or when sediment-charged water kills wildlife after dam flushing. However, sediment's role has evolved over time. Until the 18 th century, in early modern history, its management was part of the daily life of riparian Rhône inhabitants. Used in agriculture and construction, sediment was considered a valuable resource.

Throughout the nineteenth and twentieth centuries the development of the river served to separate water from land through infrastructural changes related mainly to the hydropower and nuclear industries. The riverscape was significantly modified, transforming its water and sediment regime.

Yet, while sediment tended to be ignored and separated from water, we show that contemporary disputes have put it back on the Rhône's water agenda, albeit now cast in a negative role.

Society and public policy have been adapting to the evolving definition of sediment. The idea that the same matter can evolve from an asset to a threata resource to a hazardis well documented in the literature of the sociology of risk (for a review of the French literature see [START_REF] Borraz | Risk Studies: The French Contribution[END_REF].

Risk is always defined in terms of specific social, political, and historical processes. This corresponds to a 'framing process' [START_REF] Goffman | Frame analysis: An essay on the organization of experience[END_REF] or 'schemata of interpretation that enable individuals to locate, perceive, identify and label occurrences within their life space and the world at large' (Snow et al., 1986 p.464). Framing refers to the understanding of issues in terms of specific contexts that can vary over time (e.g. floods can be framed as an environmental or a public security issue). This process is usually analysed through the role of actors [START_REF] Cobb | Participation in American Politics: The Dynamics of Agenda-Building[END_REF], social movements [START_REF] Benford | Framing Processes and Social Movements: An Overview and Assessment[END_REF], or crises and political opportunities that open a 'policy window' [START_REF] Kingdon | Agendas, alternatives and Public Policies[END_REF], enabling something to be seen as a problem. Within a long-term perspective sediment in the Rhône has undergone a drastic evolution: framed as an asset in the past, it is now considered a threat. This trajectory is the core of our paper: what are the factors, actors, and effects of such an evolution?

The paper is organised into three main parts. First, we explore the role of sediment in early modern and modern history when it was seen primarily as a blessing. We then explore the infrastructural change on the Rhône in the nineteenth century, which intensified in the twentieth century, and the impact it had on sediment dynamics. In the third part we focus on the current construction of sediment as a threat through different aspects: pollution, flooding, and dam flush

Sediment as an asset in early modern times

The history of sediment is inherently linked to that of the Rhône's development. In medieval and early modern times the river largely maintained its natural course, evolving following major floods.

Riparian communities used sediment dredged from the river to build dykes and protect themselves from floods. Nevertheless, given the magnitude and frequency of the flooding they could not prevent overflow into human settlements. Due to their agricultural value, however, overflows of water and sediment could be considered a blessing.

The profile of the Rhône River has changed greatly throughout history. Since Roman times, however, the Rhône has been a fundamental axis for inland navigation between Lyon and the Mediterranean Sea, and the river was an essential link within the Roman Empire, remaining so throughout the Middle Ages. In the Middle Ages the lower basin was formed of large wetlands, whose landscape evolved in accordance with the rhythm of the river [START_REF] Rossiaud | Le Rhône au Moyen Âge[END_REF]. From at least the twelfth century, cities located along the Rhône built levees both to protect against flooding and to ease navigation [START_REF] Stouff | La lutte contre les eaux dans les pays du bas Rhône XIIe -XVe siècle. L'exemple du pays d'Arles[END_REF]. The form of their construction, in earth and wood, is well documented in local archives. The levees in Arles (located at the delta of the Rhône, Figure 1) are the best known.

Levadiers, city officials with responsibility for dykes, were documented from the mid twelfth century.

Paid from landowners' taxes, they had the authority to order landowners to maintain or repair their dykes.

Despite the constant efforts to control it, the Rhône was still free to change its course and would regularly flood the surrounding land. In the fourteenth century, the surveyor Bertrand Boysset gave a detailed description of the lower Rhône area. Boysset's skills were essential to the social equilibrium of Arles [START_REF] Portet | Bertrand Boysset, la vie et les oeuvres techniques d'un arpenteur médiéval (1355-1416[END_REF]: with flood after flood, property boundaries became less and less visible. Only the surveyor was able to delineate parcels of land and secure properties by fixing boundaries between land and water. Each flood event changed the waterways and could even create new arable areas due to sedimentation. Hence, medieval records describe the lower Rhône as a rich hunting and fishing area [START_REF] Roucaute | Gestion et exploitation des marais arlésien au Moyen Âge[END_REF]. The Romans cultivated the entire area, but during the Middle Ages wheat and other cereals were mostly grown in the upper areas, such as the Crau (a vast alluvial plain at the original confluence of the Durance and Rhône, Figure 1).

Practices had begun to change by the end of the fifteenth century when rural and urban elites invested in new land. Associations were established to oversee land drainage and agriculture, the first being officially registered in 1543 in the delta of the Rhône, the Camargue [START_REF] Morera | Mise en valeur des zones humides et associations de gestion. Naissance et affirmation de nouveaux pouvoirs territoriaux (France, XVI e -XVIII e siècles)[END_REF]. Their role was to channel communal efforts to overcome the challenge of water management. They also installed hydraulic infrastructure, such as dykes and canals, as flood protection. Yet, as investors worked to separate land and water in order to develop arable land it remained important to maintain a link between the river and the cultivated soil, and at that time the link was fluvial sediment.

At the time, flooding was an ambiguous event. On one hand, a winter or spring flood was a disaster for the crops and promised a hard summer too: the loss of a harvest was terrible for the communities.

But on the other hand, floods always enriched the land with sediment. It could wipe out a year's crops but at the same time ensure high yields in subsequent years. In the middle of the sixteenth century the Arles aristocrat Pierre Quiqueran de Beaujeu summarised this complex relationship between a community and its river, comparing the Rhône to the Nile because of sediment:

Then the Rhône returns to its bed, leaving behind very fertile silt similar to that of the Nile, apart from the monstrous animals. They are comparable because the Rhône's silt is also thick and sticky. When this mud settles in the summer not only does it burn those ears of wheat that are still standing, but when the heat dries it out it turns into sterile sand.

These floods only occur in those areas of Arles where the fertile land is very low, forcing it to defend itself against this very dangerous river by building dykes, but this makes this area admirable. The flooding that the city of Arles undergoes every two or three years, and sometimes two or even three years in a row, takes with it seeds, the work of men and oxen and the hopes of fathers. But the year the river does not overflow it provides them with such crops that it repays the damage and removes their fear of the future. (Quiqueran de [START_REF] De Beaujeu | Louée soit la Provence [French translation by Véronique Autheman De laudibus Provenciae[END_REF][START_REF] De Beaujeu | Louée soit la Provence [French translation by Véronique Autheman De laudibus Provenciae[END_REF] 1 ) It is clear that sediment so enriched the arable land that farmers could see the Rhône's floods as blessings. Sediment transport and deposition could offer valuable additional income to the community.

At the time the Rhône's profile was not continuous and regular as it is today. Even in its lower part the riverbed was highly complex and shifting. Thanks to the sediment and the irregularities of the riverbed several islands divided the water flow. These offered very fertile land that benefited the community.

Such was the case in Arles, where the city authorities leased the larger islands to farmers (Archives municipales d'Arles, DD 57 and 58) and thus obtained regular incomes. It was similar in the city of Avignon, the main part of which was built on islands that had been conceded by the king (Archives municipales d'Avignon, DD 106).

The relationship between the Rhône's riverside communities and its sediment became more complicated when property-related pressure intensified in the seventeenth century [START_REF] Morera | L'assèchement des marais en France au XVIIe siècle[END_REF]. The high level of sedimentation began to be seen as a problem. Draining projects were hampered by sediment build-up in the hydraulic infrastructure due to a lack of maintenance. Several surveys in the 1630s described the lower Rhône valley as entirely under the influence of water. Filled in with sediment, canals were further obstructed by reeds and other tall grasses. Whether it came from the Rhône or not, sediment had taken possession of its main riverbed (Archives départementales des Bouches-du-Rhône, 108 E 97).

The major drainage projects of the 1640s were focused on the hydraulic infrastructure they had inherited. They were carried out in a relatively short period of time thanks to massive funding and land appropriation by investors in land drainage. The work mainly consisted of cleaning the drainage canals at great depths and removing the sediment and alluvium to ease the flow of water. But again sediment would create arable opportunities. Indeed, land was drained, and farmland was enriched by sediment.

The agricultural results were rapid: yields on the newly cultivated land were high, thanks mainly to the richness of the alluvial deposits from the Rhône. The works led to a reoccupation of the land bordering the river. But now the benefits of the floods were less noticeable. Indeed, the risk they posed to infrastructure and crops was clearer than the benefit of sedimentary deposits. Capital expenditure had changed the environment and the relationship between the Rhône and those living alongside it.

Continuing to cultivate the land despite the challenges of the river meant local residents had to continuously maintain the hydraulic infrastructure, whether landowners or not. This entailed continuous renegotiation between the river and its sediment on one hand and social structures on the other. The eighteenth century was marked by a relative decline in the cultivation of the land in the 1 Authors' translation.

with the loss of arable land. In 1733 Gaspard Bernardy, a royal engineer, produced a detailed study of this issue, according to which, from the Durance to Arles, the Rhône Valley was entirely submerged.

This study laid the blame on a lack of maintenance and the filling in of canals. To bring the land back into cultivation Bernardy recommended restoring the slope of the canals and that, 'the land resulting from the dredging be used to thicken and raise the dykes to the necessary height to contain the highest waters so they do not overflow into the lowest lands' [START_REF] Bernardy | Raport du Sieur Bernardy[END_REF]. Sediment needed to be taken out of the canals and could then be used to reinforce the existing dykes.

It was predominantly the towns of the Lower Rhône that had to deal with the issue of sediment accumulation, since that is where the alluvial material ended its journey. Yet, even in this area, society's relationship with the natural process of sedimentation was double-sided. On one hand the river enriched the soil for cultivation, but on the other it relentlessly weakened the infrastructure constructed to protect towns and land. Before being deposited, sediment must be gathered by the water. Communities feared the loss of sediment and the vanishing of profitable land. Moreover, along the Rhône, dykes and levees were made of alluvial sediment and stone (at certain points), and these had to be maintained to contain the river's greed. It was able to give with one hand and take with the other. Engineering surveys of eighteenth-century Avignon depict the erosion of hydraulic infrastructures. An example from 1747 shows a large section of the riverbank having collapsed. Those leading the survey urged the authorities to rebuild the levees of dykes in stone (Archives municipales d'Avignon, DD 105). In the face of this challenge, the city of Avignon, that was under the rule of the papacy, called on a large workforce to bring earth to renew the dykes, called palières (Archives municipales d'Avignon, DD 125). The city council allocated a substantial budget each year to prevent sediment loss.

Avignon's authorities saw the Rhône as a threat from which they needed to be protected. In a letter to the vice legate (the pope's representative) the engineer P. Thibaud defended his project to reinforce the town's hydraulic equipment by arguing that the Rhone was 'imperceptibly carrying away its land' (Archives municipales d'Avignon, DD 106, date unknown). Later, presumably in the late 1750s, another engineer urged the papacy to finance new dykes by stressing that, 'the water is often on the verge of taking land away' (Archives municipales d'Avignon, DD 106, date unknown). Thus, for the city of Avignon, sediment loss threatened the income from land cultivation and taxes.

Agricultural pressure on valley bottoms gave rise to highly ambiguous relationships with the water and sediment. The communities' interest in these areas arose from the arable value of sedimentation alongside the obstacle it posed to their use due to the risk of flooding from land loss. This contradiction was partly addressed by viewing sediment as raw material for large, technical infrastructure to raise watercourses, in particular building dykes along the Rhône and affluent canals from sediment deposited on the riverbed. However, the fragility of this infrastructure and the unavoidable dyke breaches offered some flexibility to the socio-technical system that had been established. But it was an unstable equilibrium that lasted beyond the French Revolution only to be shaken by the agricultural crisis of the 1840s and state intervention during the Second Empire [START_REF] Allard | La gestion des espaces humides, d'un mode à l'autre : changement et continuité dans le bas Rhône[END_REF]. From the nineteenth century, floods became a public issue in France, and higher dykes were erected around major cities and the Camargue [START_REF] Guerrin | Une inondation négociée? Politisation d'un risque naturel sur le Rhône[END_REF]. Along with the transportation and energy advances of the twentieth century, this infrastructure caused significant change to the river and, consequently, sediment dynamics.

Industrialisation and river engineering from the nineteenth century onwards: affecting sediment quantity and quality

Until the nineteenth century, the Rhône was a meandering river with several channels. Sediment was part of the daily life of inhabitants. But in the nineteenth and twentieth centuries, industrial development and river engineering led to significant change in the river's landscape and in sediment dynamics. The creation of high and secure dykes durably separated land from water and limited the sedimentation of land. Sediment flux was reduced by the development of river engineering, as industrialisation created pollution that could reduce the sediment's quality.

Environmental and social transformations affect sediment flow. In terms of quantity, the amount of sediment flowing along the Rhône was drastically reduced from 18.8 mm 3 per year in the late nineteenth century to 6.1 mm 3 per year in 2010 [START_REF] Provansal | The geomorphic evolution and sediment balance of the lower Rhône River (southern France) over the last 130 years: Hydropower dams versus other control factors[END_REF]. Geomorphologists attribute this to climatic conditions, i.e. the natural decrease in the frequency of major floods at the end of the Little Ice Age [START_REF] Bravard | Discontinuities in braided patterns: The River Rhône from Geneva to the Camargue delta before river training[END_REF], as well as to anthropogenic factors [START_REF] Provansal | The geomorphic evolution and sediment balance of the lower Rhône River (southern France) over the last 130 years: Hydropower dams versus other control factors[END_REF]. Indeed, the nineteenth century saw a change in the flood regime of the Rhône River. Sediment flux was reduced by a combination of a rural exodus associated with a decline in mountain agriculture, catchment reforestation to prevent flood events, and the engineered stabilisation of feeder torrents [START_REF] Provansal | The geomorphic evolution and sediment balance of the lower Rhône River (southern France) over the last 130 years: Hydropower dams versus other control factors[END_REF]. The century also witnessed two major floods on the Rhône (1840 and 1856). For various sociopolitical reasons, the latter was to inspire France's first flood-protection legislation [START_REF] Guerrin | Une inondation négociée? Politisation d'un risque naturel sur le Rhône[END_REF]. Under Napoleon III the state invested in protection measures for the main cities along the Rhône River and the Camargue. Civil engineers from the Ponts et Chaussées corps supervised all dyke-building and maintenance on navigable rivers such as the Rhône. Major levees, termed 'unsinkable', were built around Lyon, Avignon and the Camargue to prevent flooding. The resulting decrease in the frequency of major and minor floods meant reduced sediment transfer and fewer sedimentary deposits on riparian forest [START_REF] Provansal | The geomorphic evolution and sediment balance of the lower Rhône River (southern France) over the last 130 years: Hydropower dams versus other control factors[END_REF].

In the nineteenth century, navigation was the primary means of economic development and had geomorphological and ecological impacts that remain today [START_REF] Comby | Pour qui l'eau ? Les contrastes spatio-temporels des discours sur le Rhône (France) et le Sacramento (Etats-Unis)[END_REF]. Sediment was cleared to improve navigation on the Rhône, and the river's fluvial pattern changed between 1860 and 1870 when the first levees were built [START_REF] Bravard | Discontinuities in braided patterns: The River Rhône from Geneva to the Camargue delta before river training[END_REF]. At this time, structures known as casiers Girardon, submersible groynes resembling groyne fields, were built to narrow the Rhône's channel, concentrate its flow and ease navigation [START_REF] Räpple | Patrons de sédimentation et caractéristiques de la ripisylve dans les casiers Girardon du Rhône : approche comparative pour une analyse des facteurs de contrôle et une évaluation des potentialités écologiques[END_REF]. They consisted of a series of submerged dykes, perpendicular to the bank, to encourage sedimentation on the sides while the waterway became consequently deeper in the middle of the riverbed. The channelisation of the Rhône entailed the artificial disconnection of most of its secondary channels [START_REF] Dépret | Evolution of abandoned channels: Insights on controlling factors in a multi-pressure river system[END_REF] and a loss of connectivity between the floodplains and the channel [START_REF] Vauclin | Effects of river infrastructures on the floodplain sedimentary environment in the Rhône River[END_REF]. However, the infrastructure itself attracted sediment. By 1945, 42 per cent of the area it covered had become land, and today only 10 per cent remains aquatic [START_REF] Thorel | Socio-Environmental Implications of Process-Based Restoration Strategies in Large Rivers: Should We Remove Novel Ecosystems along the Rhône (France)?[END_REF]. The divisive casiers Girardon are now recognised as hosting interesting biodiversity but also as polluted sediment traps or for artificially separating the river from its bed.

From the nineteenth century onwards the landscape of the Rhône changed rapidly with Napoleon III dykes and the casiers Girardon. A river with braided channels in the eighteenth century, it was transformed into a simple channel, disconnected from its former meanders (Figure 2), as we can see from these two historical maps showing the landscape's evolution between 1832 and 1907. [START_REF] Pritchard | Confluence. The Nature of Technology and the Remaking of the Rhône[END_REF]. In 1933 the Compagnie Nationale du Rhône (National Rhone Company, CNR) was created and was given the concession for managing the Rhône with three objectives: hydropower, navigation, and irrigation. From 1945 to 1986, CNR built 19 hydropower dams along the river (Figure 1), between the Swiss border and the upstream part of the delta, associated with major dykes to channel the river. Press reports of the time compared the CNR's projects to those of the Tennessee Valley Authority in the US [START_REF] Comby | Power and Changing Riverscapes: The Socioecological Fix and Newspaper Discourse Concerning the Rhône River (France) Since 1945[END_REF]. In the 1970s and 1980s five nuclear plants were built near the Rhône (four are still in operation), drawing water from the Rhône to cool their reactors.

These major infrastructural changes in the landscape of the Rhône had impacts on sediment flux.

Dams reduced sediment transport within the river, and, in its catchment, major Alpine dams retain much of the sediment. The large Serre Ponçon dam, for example, was estimated to have been retaining around 200 mm 3 of sediment, or 4 mm 3 per year since the 1950s [START_REF] Provansal | The geomorphic evolution and sediment balance of the lower Rhône River (southern France) over the last 130 years: Hydropower dams versus other control factors[END_REF], creating riverbed incision downstream of the reservoirs. Although CNR hydropower plants (mostly run-of-theriver dams) retain less water and therefore less sediment, some coarser sediment is unable to travel through the bypassed channel as the usual managed discharge is insufficient. Therefore, much of the sediment is conveyed during large floods [START_REF] Provansal | The geomorphic evolution and sediment balance of the lower Rhône River (southern France) over the last 130 years: Hydropower dams versus other control factors[END_REF].

Gravel mining further reduced sediment transport in the Rhône River. The amount of gravel dredged from the river's main channel between Lyon and Arles was estimated at 13.5 mm 3 (0.83 mm 3 /y) between 1970 and 1990 and around 84 mm 3 for the entire basin, according to [START_REF] Provansal | The geomorphic evolution and sediment balance of the lower Rhône River (southern France) over the last 130 years: Hydropower dams versus other control factors[END_REF]. Mining also took place in the tributaries, resulting in less available bedload flux and exacerbating riverbed incision [START_REF] Provansal | The geomorphic evolution and sediment balance of the lower Rhône River (southern France) over the last 130 years: Hydropower dams versus other control factors[END_REF].

The significant transformation of the Rhône's riverscape in the nineteenth and twentieth centuries was illustrated by the use of the term 'river highway' in the national press in reference to the straightening of the river [START_REF] Comby | Power and Changing Riverscapes: The Socioecological Fix and Newspaper Discourse Concerning the Rhône River (France) Since 1945[END_REF]. In the space of 150 years the river formerly described as a 'furious bull' by the French historian Michelet in 1840 was tamed and transformed into a source of many utilities (nuclear power, hydraulic power, water for irrigation, navigation route), becoming a channelised river between two large banks for the majority of its course, flanked by a motorway and a high-speed railway. The development continued as part of a post-war modernisation project that sought national independence through hydraulic and nuclear power as well as strong trade links through navigation. These strategic goals, along with riparian interests and local authorities looking to develop irrigated agriculture, defined how the Rhône's water and sediment currently flow. According to geomorphologists, the principal threats to large rivers, especially the Rhône, are artificial modifications to their discharge and channel and floodplain morphology [START_REF] Lamouroux | The ecological restoration of large rivers needs science-based, predictive tools meeting public expectations: an overview of the Rhône project[END_REF]. Indeed, gravel mining, dyke and dam construction and channelisation for navigation have simplified the lateral pattern of the channel and incised and paved the riverbed [START_REF] Parrot | Analyse spatio-temporelle de la morphologie du chenal du Rhône du Léman à la Méditerranée[END_REF]. This has separated land and water and deeply impacted sediment flux in the Rhône.

In the same period of time industrialisation took hold in the Rhône Valley. A 25 km stretch from Lyon harbour to Loire-sur-Rhône was called 'chemical valley' [START_REF] Duchêne | Lyon, vallée de la chimie. Traversée d'un paysage industriel[END_REF]. In the second half of the nineteenth century various factories were established in the south of Lyon, and for almost a century companies grew up along the Rhône, particularly in connection with Lyon's textile and photographic industries. For example, Rhône-Poulenc produced dyes for the silk industry. Various economic crises led these businesses to be joined by other economic sectors, such as the pharmaceutical industry and petrochemicals, with the installation of a refinery in Feyzin and a pipeline between Fos (France) and Karlsruhe (Germany), which has run along the valley since 1962. The industrialisation extends downstream, beyond Lyon's urban area. Among the companies present in the Rhône Valley, Lafarge operates quarries, notably at Le Teil in Ardèche. In the 1960s and 1970s the Rhône was polluted by industrial and domestic waste, as the press reported: 'The Rhône flows, depending on the day, with nothing but masses of red or blue moss, waste from riverside factories' (Le Monde¸ 16 June 1972); 'Upstream from Lyon, the river is practically free of pollution. It is not the same downstream' (Le Monde, 14 May 1966). Indeed, the Rhône was considered a convenient sewer.

But industrial accidents (e.g. Pierre-Bénite in 1976) highlighted the problem of pollution and had a lasting impact on its perception.

The pollution of the Rhône was due to the valley's industrialisation and urbanisation, as well as industrialisation around tributaries that increased the contaminated discharge. For example, the current presence of PCBs (polychlorinated biphenyl) in the river is partly linked to industrialisation along its course. PCBs were produced in large quantities from the 1930s, mainly for use in capacitors 2 and transformers sold in France through the firm General Electrics. The production of PCBs in France was estimated at between 130,000 and 180,000 tons for domestic use [START_REF] Gramaglia | L'expertise à l'épreuve d'une controverse environnementale et sanitaire: la production des savoirs et des ignorances à propos des PCB du Rhône (France)[END_REF]. In particular, two industrial sites near Grenoble produced the majority of PCBs used in France from the 1950s until 1984 [START_REF] Gramaglia | L'expertise à l'épreuve d'une controverse environnementale et sanitaire: la production des savoirs et des ignorances à propos des PCB du Rhône (France)[END_REF]. In Saint-Vulbas, along the Rhône River, a company (Tredi) treats PCB waste [START_REF] Comby | How chemical pollution becomes a social problem. Risk communication and assessment through regional newspapers during the management of PCB pollutions of the Rhône River (France)[END_REF]. The pollution arising from such intense industrialisation ultimately affected the quality of sediment.

Major anthropogenic and technological changes had a huge impact on sediment flux and quality and therefore on the relationships that the Rhône's communities had with the river. There was a significant reduction in sediment import from the upstream catchment and tributaries. Moreover, sediment transported within the river was blocked by the many dams and levees, even creating artificial banks, particularly at the casiers Girardon. Today, bed mobility remains very low, and it is mainly fine sediment that is transported [START_REF] Vázquez-Tarrío | Effects of continuous embankments and successive run-of-the-river dams on bedload transport capacities along the Rhône River, France[END_REF]. Since 1999 various reaches of the Rhône have been restored as part of a programme to regain lateral connectivity by removing dykes, installing migratory fish passages, increasing minimum flow, and regenerating floodplain channels and habitats that are no longer maintained or created by fluvial processes by dredging alluviums [START_REF] Riquier | Hydromorphological conditions in eighteen restored floodplain channels of a large river: linking patterns to processes[END_REF]. However, the implementation of this programme was hampered by the politicisation of sediment through the issues of flooding and pollution. We will see more closely how sediment came to be pushed into politics.

The contemporary politics of sediment

Sediment is subject to politics. It becomes political when policies are devised to address specific problems framed around sediment (i.e. quantity or quality) or when actors use sediment to defend their interests. The concept of politicisation can been defined as the 'requalification of any social activity that comes from an agreement between social agents to question or transgress the existing separation of activities' 3 (Lagroye, 2003, pp.360-361). According to Lagroye, politicising an activity or matter would mean changing, or intending to change, the category (social, political) it belonged to. The process of politicisation is not limited to qualifying an activity or matter as political but extends to the 2 A capacitor is an electrical component that stores energy.

3 Authors' translation.

questioning of an institutionalised category. Hence, studying the politicisation of sediment means questioning the apparently natural origin of the existing view of sedimentas a blessing, for example.

The politicisation of the Rhône's sediment arose from various sectors, such as professional fishing and flood and dam management, each of which defines the issue in its own way. Having been viewed as a blessing or valuable resource in the past, over time sediment came to be seen as a natural hazard.

It can now be considered a threat for reasons of quantity, such as when it is blamed for exacerbating floods or destroying biodiversity during dam flushing, or quality, such as when it contaminates fish and ultimately humans through the bioaccumulation of pollutants or spoils agricultural soil due to its adverse quality. Focusing on the politicisation leads to a questioning of the legitimacy of a particular order. The politics of sediment, which involve new ways of viewing it or making it (in)visible, or even a new framing of the issue, usually arise at times of dispute. Indeed, modifying the existing order implies a transgression of accepted categories, but one that can be legitimised by a specific crisis.

Three forms of the politicisation of sediment on the Rhône arose during sociopolitical crises connected to pollution, flooding and dam flushing.

Sediment and pollution

Rivers receive many different persistent organic pollutants, including PCBs, that are released into the environment by improper disposal, leakage, incineration and volatilisation [START_REF] Mourier | Historical records, sources, and spatial trends of PCBs along the Rhône River (France)[END_REF].

PCBs attach to sediment, which is ingested by fish that are then unsuitable for human consumption.

Indeed, PCBs have an impact on aquatic biota and, ultimately, human health through bioaccumulation [START_REF] Lopes | Transfer of PCBs from bottom sediment to freshwater river fish: A food-web modelling approach in the Rhône River (France) in support of sediment management[END_REF]. The production, use, and sale of PCBs were prohibited in the 1980s; however, river dynamics, such as floods and dam flushes, have been contributing to the dissemination of PCBs in the environment [START_REF] Mourier | Historical records, sources, and spatial trends of PCBs along the Rhône River (France)[END_REF].

In the 1980s local scientists and NGOs highlighted the issue of the Rhône's fish being contaminated with chemicals such as PCBs, but the authorities did not allow the warnings to reach the public. With two European Directives, in 1976 and 1985, the use of PCBs was prohibited in France in 1987, giving the impression that the problem had been dealt with [START_REF] Gramaglia | L'expertise à l'épreuve d'une controverse environnementale et sanitaire: la production des savoirs et des ignorances à propos des PCB du Rhône (France)[END_REF]. In 1996 the Council of the European Union planned PCB decontamination in several countries, but by 2002 France was criticised for having done too little too late to confront the issue [START_REF] Comby | How chemical pollution becomes a social problem. Risk communication and assessment through regional newspapers during the management of PCB pollutions of the Rhône River (France)[END_REF]).

As a result, the problem came to light again in 2005 locally in the Lyon urban area, in 2007 along the Rhône Valley and then nationally through sediment. In 2005, in order to reassure his clients of the safety of Rhône fish, a professional angler submitted two sample breams to the local veterinary services for analysis. A high concentration of PCBs was discovered in them. As a result, the prefect of the Rhône department imposed a ban on the sale and consumption of fish from the area where the first two had been caught [START_REF] Meunier | Le Rhône et les PCB : une pollution au long cours[END_REF]. Following the initial discoveries, several surveys were conducted within the valley, at certain points along the Rhône River up to the delta. These led to further department-wide bans on fish from almost every point on the river. This raised the issue of public authority jurisdictions with regard to rivers. The ban on fish consumption was imposed by the prefect, meaning the measure was taken at the departmental level. As the Rhône marks the boundary between departments, a fish could not be eaten on the left bank (the Prefecture of Rhône and Isère prohibited the sale and consumption of fish in February 2007), yet the same ban did not come into force until several months later, in June 2007, in the Loire Department on the right bank. Many anglers criticised these measures, pointing out that fish and sediment do not recognise departmental boundaries.

The pollution crisis made sediment a political object, notably through the actions of a few local elected representatives upstream of Lyon where the first two contaminated breams had been fished.

They maintained pressure on the authorities to act on the problem and helped to frame the issue as affecting the entire river rather than just their local area, thereby highlighting the disseminating role of sediment [START_REF] Vionnet | La pollution du Rhône aux PCB : la mise à l'agenda d'un problème public et sa gestion à travers une politique publique environnementale partenariale[END_REF]. Prior to 2007 the main stakeholders had been the prefects, a few mayors and an environmental NGO, but by the end of 2007 the role of the state and the presidential majority had become prominent [START_REF] Comby | How chemical pollution becomes a social problem. Risk communication and assessment through regional newspapers during the management of PCB pollutions of the Rhône River (France)[END_REF]. Thus, on 10 October 2007 the Minister of Ecology Nathalie Koscisuko-Morizet appeared beside the Rhône River to announce a national response (a commission and research programme) to the pollution of French rivers by PCBs. A National Assembly commission ordered a parliamentary report, which was produced in 2008, entitled 'The Rhône and PCBs: long-term pollution' [START_REF] Meunier | Le Rhône et les PCB : une pollution au long cours[END_REF]. The member of parliament responsible for the report belonged to the same party as the president and represented the area where the first two contaminated breams had been caught. At the same time the association of professional anglers on the Rhône filed a complaint, demonstrating the degree of concern from those in the private sector who were dependent upon fish production [START_REF] Meunier | Le Rhône et les PCB : une pollution au long cours[END_REF]. Similarly, environmental NGOs highlighted the unwitting actions of the dam operators (EDF and CNR) during river dredging that would be spreading contaminated sediment [START_REF] Vionnet | La pollution du Rhône aux PCB : la mise à l'agenda d'un problème public et sa gestion à travers une politique publique environnementale partenariale[END_REF]. The local and national media broadcast alarming information about the dangerous nature of PCBs and the fact that a public health crisis had been allowed to happen despite the authorities having been aware of the dangers twenty years earlier. The term 'French Chernobyl' was used several times by bloggers and NGOs [START_REF] Meunier | Le Rhône et les PCB : une pollution au long cours[END_REF].

The broadening of the PCB debate to the full length of the river had the effect of widening the administrative responsibility from the prefect of the Rhône Department to the river basin prefect in 2007 [START_REF] Gramaglia | L'expertise à l'épreuve d'une controverse environnementale et sanitaire: la production des savoirs et des ignorances à propos des PCB du Rhône (France)[END_REF] and then to the national level. As a result of this local crisis, a national action plan was adopted in 2008 to improve scientific knowledge on the dissemination of PCBs through sediment and to introduce the widespread monitoring of fish intended for human consumption [START_REF] Gramaglia | L'expertise à l'épreuve d'une controverse environnementale et sanitaire: la production des savoirs et des ignorances à propos des PCB du Rhône (France)[END_REF]. From 2005 to 2007 studies concentrated primarily on sediment and fish. Since 2008, however, in addition to examining more and more species of fish and types of sediment, researchers have also examined invertebrates and sediment cores [START_REF] Comby | How chemical pollution becomes a social problem. Risk communication and assessment through regional newspapers during the management of PCB pollutions of the Rhône River (France)[END_REF]. Scientists are working to establish the broader role of sediment pollution throughout the hydrologic cycle. According to scientists, it will be at least another decade before PCB concentrations meet the regulatory level for fish consumption [START_REF] Mourier | Historical records, sources, and spatial trends of PCBs along the Rhône River (France)[END_REF]. Fluvial dynamics mean that PCB concentrations in sediment increase exponentially in a downstream direction, and that they are unlikely to diminish in the short term [START_REF] Mourier | Historical records, sources, and spatial trends of PCBs along the Rhône River (France)[END_REF]. The significance of contaminated sediment to local actors was illustrated by the nexus that was discovered between floods, dam flushes, and contaminated sediment. Since 2007, however, the Plan Rhône (a development contract linking riverbasin stakeholders, the state, and the EU) brought local actors into river management, encouraging studies on PCBs and the tackling of the issue at the basin level. Sediment spread PCBs, and PCBs in turn framed sediment as a threat, due to its contamination, via the human consumption of fish. This pollution crisis impacted the treatment of another major public concern along the Rhône: flood risk.

The nexus of sediment and flood

Subsequent events were to reactivate the issues of sediment quantity and quality, including floods and the restoration of the Rhône's meanders. Sediment was placed on the political agenda by flooding on the Rhône and its tributaries between 1990[START_REF] Lagroye | Les processus de politisation[END_REF]. Severe events occurred in 1993, 1994, 2002[START_REF] Lagroye | Les processus de politisation[END_REF], around the Rhône and/or its tributaries that caused extreme damage. In 1993 dyke breaches caused severe damage in the delta; the events of 1994 claimed 10 lives; in 2002 flooding on Rhône tributaries caused 23 deaths in the Gard department; and in 2003 the Rhône and its tributaries flooded a large area, causing 1 billion euros of damage [START_REF] Guerrin | Une inondation négociée? Politisation d'un risque naturel sur le Rhône[END_REF].

Sediment was accused of aggravating the impact of flooding by lowering river margins and narrowing riverbeds. Dredging the river and removing sediment, in particular from the meanders of the 'former Rhône', became a regular demand from flood victim associations and local representatives in order to improve flood protection [START_REF] Barthélémy | La restauration écologique du fleuve Rhône sous le double regard du sociologue et de l'écologue[END_REF][START_REF] Guerrin | Une inondation négociée? Politisation d'un risque naturel sur le Rhône[END_REF]. Debates organised after a flood on the Rhône River in 2003 revealed that those affected by the flooding who were in attendance tended to believe that the river was becoming filled up with sediment and that the answer, therefore, was to dredge the river. According to [START_REF] Barthélémy | La restauration écologique du fleuve Rhône sous le double regard du sociologue et de l'écologue[END_REF], these impressions are based on what locals can see, such as bridge piers and casiers Girardon filled with sediment. Others suggest that such observations would be reinforced by a sceptical view of the CNR, whose programme of river containment had been understood by locals to include levees that would protect them from flooding [START_REF] Guerrin | Une inondation négociée? Politisation d'un risque naturel sur le Rhône[END_REF]. Indeed, they had been encouraged in this view by the CNR's communication strategy while developing hydropower. Following the floods of the 1990s and 2000s the CNR, along with the state, was regularly held responsible for what was seen as the failure of the levee and dam system. As a consequence, the CNR was also blamed for the issue of sediment, which was seen as having been exacerbated by dams [START_REF] Guerrin | Une inondation négociée? Politisation d'un risque naturel sur le Rhône[END_REF].

The demand for sediment to be removed from the river was then incorporated into the ecological agenda of environmentalists calling for the restoration of the Girardon infrastructure and the meanders of the former Rhône. In 1999, when the environmentalist Dominique Voynet entered government, a 10-year programme for the Rhône was launched. It aimed to restore a 'fast and flowing river'. State authorities and scientists understood this as increased river flows to benefit specific migratory fishes, but for urban planners and riverside municipalities it meant more life in former meanders, more amenable landscapes and more space for water during flooding [START_REF] Barthélémy | La restauration écologique du fleuve Rhône sous le double regard du sociologue et de l'écologue[END_REF].

They requested that sediment be excavated in order to store floodwater and reduce damage. However, the PCB crisis raised concerns over the practice of putting sediment back into the water [START_REF] Bouleau | The social life of solid load in the Rhône River: friends and enemies of sediments. Dams, Sediment Discontinuity, and Management Responses in Mediterranean River Basins Seminar[END_REF], and the project to remove the casiers Girardon was blocked from 2005 to 2010 [START_REF] Thorel | Socio-Environmental Implications of Process-Based Restoration Strategies in Large Rivers: Should We Remove Novel Ecosystems along the Rhône (France)?[END_REF].

Sediment quality was also used as political leverage to oppose a floodplain restoration project on the downstream Rhône. The 1990s saw changes to state flood policy in France, and more broadly in Europe and the United States [START_REF] Guerrin | Remparts ou menaces ? Trajectoires politiques de l'endiguement en France, aux Pays Bas et aux Etats Unis[END_REF]. The public savings ideology proponents questioned the public expense of dyke building and flood damage compensation, and Western governments became less and less willing to fund them. This new approach opened an avenue for environmentalists who had long criticised the impact of dykes on freshwater ecosystems. They successfully claimed that flooding was beneficial for the managing of sediment, land fertility, and groundwater recharge, and thereby gained public support for the restoration of the Rhône [START_REF] Bouleau | The social life of solid load in the Rhône River: friends and enemies of sediments. Dams, Sediment Discontinuity, and Management Responses in Mediterranean River Basins Seminar[END_REF].

However, not everyone was ready to celebrate the benefit of sediment brought by flooding. The mayors of two municipalities located on the downstream Rhône, Piolenc and Mornas, overcame their former political rivalry and came together to oppose the project that had earmarked their areas for floodplain restoration [START_REF] Guerrin | A Floodplain Restoration Project on the River Rhône (France): Analyzing Challenges to Its Implementation[END_REF]. To counter the environmentalist narrative of the benefits of sediment, these elected officials drew the public's attention to the pollution of the Rhône's sediment (particularly with PCBs) and its potential effect on agriculture and drinking water. They pointed out that contaminated sediment would be transferred, and contrasted this with the high quality of the land along the river thanks to the long-standing separation of agricultural land from (contaminated) water by the CNR dykes. Indeed, the dispersal of PCBs is exacerbated by sediment flushing operations and flooding, since floods cause bank erosion, which results in 'inputs of less-contaminated sediment in some cases, or in inputs of more contaminated sediment in others' [START_REF] Mourier | Historical records, sources, and spatial trends of PCBs along the Rhône River (France)[END_REF]. The politicisation of Rhône pollution was expressed by the media with a familiar dramatic reference:

'Chernobyl risk in Provence' (La [START_REF] Provence | Perception of braided river landscapes: implications for public participation and sustainable management[END_REF]. Alongside other concerns and obstacles, the opposition led by Piolenc and Mornas saw the cancellation of the floodplain restoration project [START_REF] Guerrin | Une inondation négociée? Politisation d'un risque naturel sur le Rhône[END_REF].

Sediment is therefore seen as a flood-related problem in two ways. Following the recurrent, devastating floods of the 1990s and 2000s, flood victim associations blamed the quantity of sediment for increasing the degree of overflow at times of flooding. But when a former floodplain was identified for potential restoration to store floodwater again, sediment came back as an issue but with regard to its quality, in terms of pollution. Land and water had been separate for so long that people were no longer used to living with floods. Rather, they had come to see the Rhône as polluted due to the contamination of sediment with PCBs. Those who opposed the floodplain restoration programme used the new evidence and the media coverage generated by the PCB crisis to support their cause.

They even recycled the image of Chernobyl that had been used locally during the initial crisis. A plan to store water within a former floodplain, framed as an issue of water quantity, came to be reframed as one of quality by its opponents. Hence, the Rhône's hydrosocial cycle created a link between flooding and the quantity and quality of sediment that was to prevent state action. The question of dam flushes further highlighted the controversial nature of the river's sediment.

Sediment becomes transnational through the issue of dam flushing

Dams control the flow of sediment in the Rhône. Over time, sediment accumulates behind dams, lowering their water storage capacity and hence their power production. Flushing operations are carried out to mitigate this phenomenon. On the Rhône, dam flushing is managed by the Verbois dam in Switzerland, which has a large reservoir, produces electricity, and prevents flooding in the Geneva urban area. However, it also traps solid material which it must regularly discharge in a process taking place about every three years. Between World War II and 2016, 22 flushing operations were carried out on the Rhône. A flushing event is a temporary and planned disruption of the functioning of water and sediment flows. This dam-water-sediment interaction affects ecosystems and potential uses of the river. Flushing on the Rhône necessitates international cooperation between Switzerland and France, and sees collaboration between the Swiss dam operator (SIG -Geneva Industrial Services) and the CNR in France.

From the 1950s to the 1970s French mayors believed that flushing was an effective way to remove

pollutants. Yet, in the 1970s the procedure became a political issue as fishers raised objections locally.

Initially these concerned the flushing itself, which visibly changed the water by suddenly filling it with sediment. At the time, dam operators carried out the operations in summer, when the water level is low and electricity production less profitable. However, the low water level made the impact of the flushing more obvious (by the sight and smell), potentially affecting tourism and leisure activities (e.g. fishing). Therefore, from the mid-1970s flushing operations were moved from July or August to May to appease the locals.

Since 1976 the hydrobiology laboratory at the University of Lyon 1 has been studying the impact of dams on invertebrate fauna. In 1978 a technical company conducted a survey in Brégnier-Cordon (Ain department, Figure 1) and observed that massive amounts of fish would die during flushing due to the anoxic flow [START_REF] Bravard | Du Léman à Lyon[END_REF]. The findings drew the attention of anglers, environmental NGOs and scientists. Indeed, it turned out that the flushing operation of 1978 had been an ecological disaster.

The decline in river flow at the Chancy-Pougny dam caused a high concentration of coarse and fine material downstream, notably at the Génissiat dam. Fish mortality was very high up to 100 km downstream of the dam. The event shed light on the flow of suspended material, emphasising the roles of geomorphology and physical geography in understanding the hydrosystem. In June 1978 an association of anglers filed a complaint in court. Environmentalists, anglers and scientists called for new bilateral management between Switzerland and France of flushing operations and the sediment issue. The CNR adopted specific measures, for example monitoring the minimum flow and sedimentary material during the operation and the creation of a nature reserve. Further, as a result of this issue a specific quantity of suspended material was established with regard to any financial compensation [START_REF] Comby | Pour qui l'eau ? Les contrastes spatio-temporels des discours sur le Rhône (France) et le Sacramento (Etats-Unis)[END_REF]. Regulatory standards were set that the French and Swiss dam operators could use to negotiate and establish agreements. While the immediate controversy waned, stakeholders continued to pay greater attention to the issue of sediment.

Although flushing is usually carried out every three years on the Rhône, none took place between 2003 and 2012, in part due to a public inquiry that required bilateral authorisation for each operation [START_REF] Bréthaut | The shifting territorialities of the Rhone River's transboundary governance: a historical analysis of the evolution of the functions, uses and spatiality of river basin governance[END_REF]. The delay was also affected by protests mounted by anglers in The flushing operation eventually took place in June 2012 and garnered huge media coverage. The level of the Chancy-Pougny dam was lower than expected. There was a lot of rain. Facing very high concentrations of suspended material, the Swiss dam operator decided to stop the flushing operation before it was complete. A few months later the Swiss operator explained that it had not been 2.6 mm³ of sediment that had transited from Switzerland to France as expected but 4.3 mm³ (Le Progrès, 5

March 2013). An environmental group suspected that the SIG had done it on purpose. Rumours fuelled the tension. The events had economic as well as ecological consequences: the CNR announced an 8 million-euro loss due to the pause in hydroelectric production, having to pay staff in the field to ensure people's safety, save fish, and monitor the flushing operation, and running an awareness campaign to forewarn river users. The object of people's anger was thus twofold, namely the ecological and economic impact of having failed to comply with the legal level of suspended material, which had been central to the balance of the system of actors since the event of 1978.

As has been seen, flushing can fuel a negative perception of sediment due to the change in the colour of the water during the operation, the size of the sediment (Le Lay et al., 2013), the quantity of dead fish, and the issue of pollutants. The event of 2012 led some environmental groups to blame sediment, even calling it 'shit' (Le Progrès, 5 March 2013). Flushing operations disturb riverscapes.

They remind inhabitants and users that the river is made of water and sediment. Views of sediment are built both from residents' experiences of it and detailed scientific monitoring by geomorphologists, hydrologists, and ecologists. One feature of the Rhône River is the transnational character of its flushing operations, where sediment flows from Switzerland to France, and the multilevel governance of this flow has been crucial in minimising disputes.

Conclusion

Sediment has a social and political life. Once welcomed as a valuable resource, sediment has come to be criticised for its quality by impacting biodiversity and also fishing and its quantity, being blamed for exacerbating flood damage. Sediment is subject to evolving interpretations, meanings, uses, and means of knowledge production, depending on historical, socio-natural and technical factors. The changing governance structure within which the Rhône falls has also affected whether sediment has been viewed as a blessing or a curse. Indeed, the state engineers' monopoly of expertise and the modernist approach to the Rhône's development nullified the role of sediment, which only came back onto the agenda thanks to local residents and elected representatives through sociopolitical crises, conflicts and disputes, which altered the pre-existing power relations. The Rhône's sediment, once a blessing, is now seen largely as a threat, hampering any state action to remove it and implement floodplain restoration projects.

The Rhône is indeed about more than water. As historical and contemporaneous politics remind us, sediment is part of water, and their combination affects sociopolitical processes that have a dialectical impact on biological and physical sediment dynamics. Indeed, sediment is deeply embedded within the Rhône's hydrosocial cycle. The management of the Rhône's water as a resource affected sediment dynamics, highlighting the fact that water is not only about H2O but also about sediment transport.

Sediment dynamics on the Rhône were deeply affected by technological developments in terms of their quantity and quality. Today, technology is in turn affected by the social and political life of sediment, when dam flushing is the subject of dispute and local opposition. The Rhône may not look like a muddyscape, since water and land seem clearly separated, but conceptually it is. Sedimenta natural featurenow confronts hydraulic infrastructure, and the societies it supports, head on. The mud that engineers had hoped to hide has dramatically re-emerged as a critical issue.

Sediment is involved in socio-natural processes and is, therefore, an object of natural as well as social science. In 2009 the Rhône Sediment Observatory (OSR) was established. Its research teams, spread along the river, gather data on the quantity and quality of sediment, as well as the levels of contamination. There are currently no social scientists at the OSR. Yet it has enormous potential as a forum for the comprehensive exchange of ideas between those in the social and natural sciences about socio-natural sediment dynamics. The social sciences have a role to play in highlighting the effects of sediment dynamics on societies and of social dynamics on sediment.

Figure 1 :

 1 Figure1: Map of the Rhône River valley(Comby, 2020)

Figure 2 :

 2 Figure 2: Historical maps and changing riverscape at Pont-Saint-Esprit (Gard department, France)

  Switzerland in 2006 and France in 2010. All were calling for greater transparency in terms of sediment flow through dams, and this created many challenges for flood management in the Geneva area, questioning a usually technical subject: 'The anglers have played a decisive role. It was unimaginable to question Verbois flushing,' stated Robert Cramer, a councillor in the canton of Geneva (Le Progrès, 19 June 2006). The protestors focused not only on the quantity of suspended material but also on flow rates and river ecology. During the French public inquiry organised across 47 communes, mayors expressed concern about, or even openly opposed, flushing. In an interview with a regional newspaper, the mayor of Aoste explained: 'We are going to see our Rhône loaded with mud and many other things. We have been Switzerland's dump for years and it is still going on' (Le Progrès, 27 May 2012). In the same article the mayor of Creys-Mépieu said, 'we voted against the operation because of the nature reserve. It is a scandal for flora and fauna to flush at this time of year. The results are less harsh in winter'. An environmental group declared they 'would prefer that flushing operations take place in autumn because of breeding periods. It could be done another way; for example, by removing the sediment more gradually. Yet one must understand that the Swiss do as they want. They have the Lake Geneva tap that allows them to work on the river'. The Swiss stakeholders are perceived to be more powerful because they generate the Rhône's flow by controlling Lake Geneva. The issue of flushing, therefore, embodies the unbalanced relationship between upstream and downstream stretches. Because of local resistance on the Swiss and French sides, the dam operators had to postpone the operation that was scheduled for spring 2010.

Acknowledgements

Agence Nationale de la Recherche (ANR) (FLUIDGOV -ANR-18-EQUI-0001) Dr Joana Guerrin Agence Nationale de la Recherche (ANR) (FluidGov (ANR-18-EQUI-0001)) Dr Raphaël Morera Agence Nationale de la Recherche (ANR) (ANR-11-LABX-0010) is researcher at the French National Institute for Research in Agronomics and the Environment (INRAE) in water-related risks. She has been mainly researching on the role of the State in governing floods on the Rhône River (France) and on drought governance in São Paulo (Brazil). She is the French coordinator of FLUIDGOV Project funded by ANR and focusing on hydrosocial cycle as a framework to analyze flood governance in the Upper Rhône (Swtizerland), Rhône Delta (France) and Ganges Delta (India). She is one of the two coordinators of the Social observation of the river governorship (OSTF) This research was generously supported by two research projects funded by the French National Research Agency (ANR): The FluidGov project (Towards 'fluid' governance: hydrosocial analysis of flood paradigms and management practices in Rhone and Ganges basins (India, France and Switzerland): ANR-18-EQUI-0001) and the French programme 'Investissements d'Avenir' (ANR-11-LABX-0010), managed by the ANR, supported by the OHM Vallée du Rhône and part of a programme of Rhône Basin Long-Term Ecosystem Research (LTER, ZABR). The authors are grateful for insights gleaned at a seminar in Lyon in 2018 ('Dams, Sediment Discontinuity, and Management Responses in Mediterranean River Basins') and a workshop in Berkeley in 2019 ('The Social Life of the Sediment Balance'). The authors would also like to thank the anonymous reviewers for their