A new levee control system based on geotechnical and geophysical surveys including active thermal sensing: A case study from Poland

Abstract

Abstract Specific features of levees, particularly their large lengths, cause significant problems in the early and precise detection of leaks as well as erosion zones in both the levee body and subsoil. These processes are the primary threats to levees safety. Fast and relatively cheap methods for the internal investigation of levees, i.e., non-invasive geophysical methods, are used only during non-flooding periods and do not have equivalent accuracy to sensors installed in the levee. Additionally, there is no optimal monitoring method for levee leak detection during a flood. Such a method, which use a fibre-optic thermal linear sensor is very effective in the case of earth dams, but it has significant limitations when to applied to existing levees. In consequence, the primary goals of our research were to improve the thermal method for levee investigation and to propose a coherent levee investigation methodology that considers the entirety of the currently used methods. We therefore developed a new type of sensor: the Multi Points Thermal Sensor (MPointS). In our solution, thermal sensors are mounted next to one another inside the levee or/and in its subsoil by hammering them from the surface. This procedure creates a quasi-continuous line of sensors along the levee. This technology s key feature is that each sensor has an integrated micro-heater that uses an active (heating) thermal method for leak detection and in situ measurements of the seepage velocity. A pilot section of the Vistula River levee in Poland was chosen to verify the MPointS solution in addition to a geophysical, geological, and geotechnical survey. Some leakage zones on this levee were previously identified during the flood in 2010. We next used a wide variety of methods to investigate the levee during a period of non-flooding. Dynamic penetration light (DPL) soundings and geological drillings were applied in addition to geophysical methods such as ground-penetrating radar (GPR), electrical resistivity tomography (ERT), electromagnetic profiling (EMP), capacitively-coupled resistivity (CCR), and high-resolution reflection seismic imaging and multichannel analysis of surface waves (MASW). Loose zones of higher porosity in the levee and its subsoil were identified using geophysical methods. These zones also included the local leakage zones detected during the 2010 flood. However, the leakage zones themselves were not clearly and unambiguously detected by these geophysical methods. The thermal monitoring system using MPointS was next installed in accordance with the locations of the loose zones indicated by the geophysical methods. During the flood in 2019, MPointS allowed us to detect precise leakage zones and monitor whether the seepage velocity exceeded the critical values that may lead to the development of internal erosion. The results showed the importance of using a comprehensive and optimised methodology for levee investigation. MPointS proved to be an effective solution for levee leakage monitoring; however, it was found to be primarily applicable to the most vulnerable levee sections. Overall, levee investigations must be performed with cheaper and faster geophysical methods and supplemented by geotechnical surveys, which are also able to choose MPointS system locations.