### State-of-the-Art Techniques  
Ground-penetrating radar (GPR) is based on radar technology with frequencies of 10-1000 MHz [6]. The frequency influences the measurement depth and resolution. Low frequencies offer a large measurement depth but have low resolution. High frequencies offer a higher resolution but a correspondingly lower penetration depth into materials. The measuring effect is based on the travel time of electromagnetic waves in different materials, which is mainly influenced by their permittivity. Due to the resulting differing travel times of waves in different materials, conclusions can be made about the ground conditions [7]. GPR can be used in many areas of geotechnics. Common applications are the investigation of the composition of soil near the surface, the detection of cavities or sinkholes and the inspection of foundations and roads [6]. However, GPR has also been used to detect erosion processes in dams. This requires two drill holes for transmitter and receiver. With this arrangement, a tomogram of the dam can be recorded, which can then be used to detect erosion [8]. GPR is not suitable for the planned application, because a measurement takes too long and boreholes are necessary. Even though it can be used to detect defects, it is not ideal for monitoring. Thus, it is not applicable for the planned laboratory evaluations. Another system for monitoring soil densities is the thermo-time domain reflectometry (T-TDR) [9; 10; 11]. This method combines the TDR technique with a heat pulse, which is used to determine the heat capacity and thermal conductivity of the soil. The heat impulse is emitted by one electrode, while the temperature change is measured over a distance by a second electrode. The heat capacity and thermal conductivity of the material can be determined using the known distance between the electrodes and the measured temperature increase. Moreover, TDR is utilised to determine the electrical conductivity of the material. Based on these three parameters, the density can be calculated. This method offers the advantage of non-destructive density measurement and monitoring. The measuring range is limited to the material between the electrodes, and the results become more accurate with increasing electrode length. If water flow in the measuring range is too strong, the heat dissipates through the flow and the measurement is distorted. Since this is the case for dikes in critical areas and also for the newly developed test cell for laboratory evaluations, this measuring method is not suitable for this application. A further method uses impedance spectroscopy in a soil density gauge system to determine soil characteristics such as soil density and soil moisture [12]. It consists of a transmitter electrode surrounded by a second ring-shaped receiver electrode. The flat electrodes are thereby placed on the material, and the impedance is measured between the two electrodes. The impedance spectrum is then used to determine the dielectric properties of the soil. Based on this, the soil density and moisture are determined. A comparison  
Figure 1: Erosion process during the experiment. with a nuclear density meter [13], which is a reference measuring method for density and moisture of soils, shows some disadvantages of the system [14]. A complex calibration with a known soil type, which must have similar properties to the measured soil, is needed. Therefore, a second measuring device is needed to carry out a field calibration. However, even after calibration, a large deviation of the density measurement remains, depending on the soil moisture. A known method for measuring soil moisture is impedance spectroscopy, whereby soil moisture is determined based on the dielectric constant of the soil [2]. To measure the impedance spectrum, two electrodes are positioned in the soil. When the water content changes, so does the dielectric constant, and therefore the impedance of the soil changes.