Quantification of water flow in permafrost rock walls

Here we present the first multi-annual study in periglacial environments quantifying and characterizing water accumulation in bedrock joints with the help of lysimeters, weather data, snowmelt modeling and gravimetric monitoring.

Continuous measurements allow to detect the timing and to estimate the quantity of water accumulations. These can easily generate significant hydrostatic pressures in sealed clefts and are one of the most important but less understood contributors to slope destabilization. Due to the recent increase of temperatures and the consequent deepening of active layers, it is expected that the influence of water will increase and potentially lead to bigger instabilities, dangerous for people and expensive for infrastructures.

Measurements have been conducted at Mount Zugspitze (Germany/Austria, 2962 m a.s.l.). Hourly cleft water discharge was recorded in a tunnel by two lysimeters-like loggers, high frequency weather data from the summit were provided by the German Meteorological Service and snow measurements from the plateau were obtained from the Bavarian Avalanche Service. Monthly measurements with a relative spring gravimeter Scintrex CG-5 were conducted in the tunnel together with the TUM Institute of Astronomical and Physical Geodesy to monitor water mass changes. Additionally, our temperature loggers and electrical resistivity tomographies recorded permafrost degradation, while a geological mapping provided a detailed cleft structure of the location.

Water flowing in the tunnel comes predominantly from clefts as the Wetterstein limestone exhibits very low porosity and permeability. Over the complete time of investigation, two repeating phases can be clearly distinguished. (i) Snowmelt from April to July provides the highest discharge rates, up to 800 l/d. These measures are well in agreement with the hourly melting rates obtained by the model Snowpack (SLF). Saturation of bedrock and clefts is at its maximum during this period and temperatures are constantly around 0°C, so that water-ice processes are expected to dominate slope stability. (ii) Rainfall events, normally present only from June to September, deliver smaller quantities of water since they mainly have high intensity but short duration. Nevertheless, due to a clear separation between events, it is possible to detect water flow continuing several days after the end of the rainfall, a clear evidence of water accumulation.

Although direct measure of hydrostatic pressures in single clefts remains an open challenge, this benchmark study provides measures on fluid flow and quantitative estimate on water accumulation leading to hydrostatic pressure in bedrock permafrost. Improving the knowledge of slope internal thermal-hydrological dynamics in periglacial environments can help understanding disastrous slope failures.