Investigating transient seepage flow and heat transfer using optical fiber distributed temperature sensors and hydrothermal modeling

Seepage during the first filling of a reservoir is a critical aspect for earth dams and embankments safety, which requires precise monitoring. The thermometric method has demonstrated significant potential for detecting seepage anomalies through continuous temperature measurements using optical fiber distributed temperature sensing (DTS). However, most previous research has primarily focused on thermal monitoring when seepage flow reached a steady-state condition, which highlights the need for more research on seepage and heat transfer in transient state, particularly in unsaturated soils during the reservoir's first filling. This paper addresses the transient seepage flow and heat transfer during the first filling of a laboratory sand model. Temperature variations within the sand were recorded using an optical fiber DTS, while seepage progression was tracked through digital imaging at regular intervals, followed by image processing. A coupled hydrothermal numerical model was also developed to simulate transient seepage flow and heat transfer within the unsaturated and variably saturated sand. In numerical modeling, heat dispersion and the thermal conductivity of sand were investigated through parameter calibration. Results indicate that thermal monitoring using optical fiber DTS is an effective method for estimating the development of the phreatic line during the first filling of the reservoir. Numerical simulations further revealed that seepage velocity plays a key role in the heat transfer process during transient seepage. Additionally, the results highlight that heat dispersion significantly influences heat transfer, particularly during transient seepage flow, whereas the effect of thermal conductivity is relatively minor as seepage progresses.