Quantifying seepage flow velocities in embankment dams from optical fibre distributed temperature measurements

Seepage flow velocities are a key parameter to assess the risk of internal erosion in embankment hydraulic structures, such as embankment dams, canal embankments or levees. Accurate orders of magnitude of these seepage flow velocities are difficult to assess via conventional monitoring technologies or numerical modelling. EDF and geophyConsult have developed for 15 years an innovative technology of seepage detection using optical fibre distributed temperature measurements. This technology has already demonstrated its capability to locate seepages through embankment hydraulic structures using passive temperature measurements without any constraint of the optical fibre cable location with respect to the ground water table elevation. The next step was to use this technology to quantify the seepage flow velocities after having located them. A specific numerical modelling and analysis of optical fibre temperature data was developed and applied to a canal embankment experiencing seepage. After having presented the main geometrical and geotechnical characteristics of this canal embankment and its foundation, this paper presents the optical fibre monitoring installation. Then, the hydraulic behaviour of this canal embankment is discussed, based on visual inspections and piezometric measurements. In its third part, this paper presents the optical fibre temperature measurement analysis, allowing seepage location detection and seepage velocity quantification. This innovative technology still needs to be tested and implemented on other case studies but it has already shown a promising potential to significantly improve internal erosion assessment from monitoring data.