Gravity Monitoring of Underground Flash Flood Events to Study Their Impact on Groundwater Recharge and the Distribution of Karst Voids

Abstract

Flash flood events are expected to become increasingly common with the global increases in weather extremes. They are a significant natural hazard that affects karst landscapes, which host large resources of drinking water worldwide. The role played by underground flood events in the karst aquifer recharge is complex due to the heterogeneity of the basement which remains poorly understood. We present the analysis of 20 incave flash flood events affecting the Rochefort karst system (Belgium) using continuous gravity measurements at one single station and water level sensors installed in caves. Underground flood events typically produce a peak in the gravity signal, due to an increase in the associated mass change. After the flood, the gravity values drop but remain slightly increased compared to before the flood event. Via forward gravity modeling, we demonstrate that this remaining anomaly can be reasonably explained by the infiltration of local rainfall within the karst system rather than by allogenic recharge of the aquifer. Flash floods are mainly restricted to connected voids. This allows us to utilize them as proxies to investigate the distribution of cavities in the karst system. Forward modeling of the gravitational attraction induced by the mapped caves being flooded yields a gravity signal much smaller than the observed one. We conclude that at least 50% more cavities than those previously mapped are required to match the measured anomalies. This presents opportunities for implementing similar approaches in other diverse porous media, using gravity monitoring of hydrological processes (e.g., infiltration fronts, hydrothermalism, or tide effects in coastal aquifers) as proxies to characterize underground properties.

Karst and urban flood-induced solid discharges in Mediterranean coastal rivers: The case study of Las River (SE France)

“…This behaviour is yet typical of vauclusian karst aquifer and would be also observed at the Dardennes springs in natural conditions since several springs discharge at different levels, with the highest, the Ragas spring, overflowing 60 meters above the valley. The regulated karst water storage in the Dardennes Reservoir is a specific characteristic of the Las River directly impacting its discharge, but the effect of karst on floods is not unique and other case study showed that mitigation can be low (Sezen et al 2019, Watlet et al 2020. The highly variable discharge implies that a specific knowledge of Mediterranean rivers yield is required to better understand the coastal marine environment and develop management tools.…”

Hydrological signals in tilt and gravity residuals at Conrad Observatory (Austria)

“…Active pumping or injection experiments at different spatial scales have proven the high sensitivity of tilt to pore pressure changes (Weise, 1992;Kümpel et al, 1996;Weise et al, 1999;Fujimori et al, 2001;Jahr et al, 2008;Jahr, 2018). Within the framework of the large-scale injection experiment at the German Continental Deep Drilling Program (KTB) deep drilling site, Jahr et al (2006a, b; studied the surface deformation due to fluid-induced stress changes by borehole tiltmeter array observations.…”

Environmental And Anthropogenic Gravity Contributions At The Þeistareykir Geothermal Field, North Iceland

“…Many geoscienti c studies make use of continuous terrestrial gravity measurements to estimate subsurface mass changes associated with, for example, groundwater ow (Watlet et al 2020), volcanic activity (Jousset et al 2000;Carbone et al 2019) or exploitation of geothermal reservoirs (Portier et al 2018). Additional gravity effects from surrounding environmental processes, like Earth tides, atmospheric pressure changes, rain and snowfall superimpose onto the target signals of the gravity time series and have not been always considered, making interpretation inaccurate when gravity variations are small.…”