Hydrodynamics of an allogenic karstic system from coupled gravimetric and hydrologic observations

The Classical Karst region, a region shared between Italy and Slovenia, hosts one of the most archetypical karstic aquifers in the world. Here, the large limestone plateau has been continuously dissolved by meteoric waters during the past 5 million years, leading to the formation of large cavities interconnected by a well-developed network of conduits and shafts. The system is also fed by an important allogenic contribution, the Reka river, which enters the Classical Karst aquifer through the Škocjan cave system and flows underground for more than 30 km, finally outflowing in the Adriatic Sea. The Reka river experiences large flow variations during the recharge process, resulting in fast and large water accumulation in several cave systems along its underground water path. The Škocjan caves are able to store up to 3 million m³ of water during one of these flood events and represent just one example of these allogenic dominated karstic systems in the Classical Karst.In 2018 a continuously recording gravimeter (gPhone gravimeter) was installed nearby the Škocjan caves to get more insights into the water mass balance of the system during these flood events; the instrument is presently still operating. In February 2019 the gravimeter recorded one of the largest events in the past 50 years  (peak discharge > 300 m³/s) that caused flooding of the cave system with a recorded water level increase >80 m and gravity variations >400 nm/s². Modelling of both gravimetric and hydraulic responses allowed to obtain a new hydraulic model of the cave system and a refined mass flux estimate during the flood (Pivetta et al., 2021). Apart from this event, the gravimeter was able to record the response to a few smaller flood events with peak flows of less than 250 m³/s. The gravity and hydraulic response to smaller floods differs dramatically from the 2019 event both in magnitude and time difference between peak flood and peak gravity. In this contribution we aim to describe in more detail the different response of the coupled gravimetric hydrologic observations to different flood events, evidencing the complex non-linear response of this karstic system to the recharge process. By discussing this case we show the potential of terrestrial gravity observation to depict the hydro-dynamics of this system and the potential of a remote monitoring of the storage units. In future an array of MEMs gravimeters in the Classical Karst could be an excellent tool to fill the gaps of the sparse hydrologic observations, helping to obtain a full 4D model of the hydrodynamics of the system.

Pivetta, T., Braitenberg, C., Gabrovšek, F., Gabriel, G., and Meurers, B.: Gravity as a tool to improve the hydrologic mass budget in karstic areas, Hydrol. Earth Syst. Sci., 25, 6001–6021, https://doi.org/10.5194/hess-25-6001-2021, 2021.