Subglacial water storage in an Alpine glacier : including hydrometeorological and glaciological influences on flooding in Alpine glacierised basins
Glaciated catchments increasingly accommodate rising populations. As glaciers are capable of modifying peak flows and releasing floodwaters, understanding and developing models of subglacial water storage and release has significance to the safety of resident populations and land use decision-making. Glaciological and hydrometeorological processes play a critical role in determining water storage within the subglacial drainage system of Alpine glaciers. However, our understanding of spatial variations of these processes throughout the ablation season remains incomplete. Field results and modelling studies of the glacial hydrological system at Findelengletscher, Canton Valais, Switzerland are presented with a view to improving understanding of physical mechanisms controlling water flow within glacierised catchments. A physically-based model of surface runoff incorporating meltwater and precipitation has been developed. This model has limited data requirements using only air temperature, solar radiation, precipitation and elevation of the transient snow line in a simple, spatially distributed energy balance model. It has been used to predict surface runoff at an hourly resolution for the 1999 ablation season. Methodological advances have been made by creating conceptual models of water flow through the subglacial drainage system. Models are used for semi-quantitative interpretation of water level variations in boreholes, as surrogate measures of subglacial water pressures. The boreholes either directly intersect subglacial channels or hydraulically connect to subglacial channels through a subglacial sediment layer. Variations in borehole water levels are considered at both diurnal and seasonal timescales. Water storage has been calculated within the subglacial drainage network and interpretations are made of temporal variations in subglacial water storage. Borehole water levels indicate that the glacier subsole can be spatially separated into those areas that are hydraulically connected or unconnected to the subglacial drainage system. Hydraulically connected areas may further be subdivided into areas of efficient and inefficient subglacial drainage. These may intermittently connect and influence water balance within a glacier. Increasing and decreasing trends in water balance cycles are initiated by glaciohydrological mechanisms. These control the activity of intermittent hydraulic connections between efficient and inefficient areas of subglacial drainage. Connections form in response to two hydrometeorological factors: high elevation rainfall and short duration decreases in elevation of either the snowline or the 0°C isotherm. Increasing trends in water balance over successive days are associated with preferential routing of inputs into, and retention within, hydraulically inefficient areas of the subglacial drainage system. Occasionally the release of water from temporary subglacial storage is not synchronous with either hydrometeorological causal factor. Measurements of fall-line velocity and vertical displacement suggest that basal sliding may alter preferential subglacial flow pathways. However, uncertainty exists as to whether such changes may be the result of lagged effects of either high water pressures from rainfall or low water pressures from low daily surface runoff. These uncertainties are due to system response times affecting the time taken to transfer longitudinal strain within glacier ice. In the late ablation season the potential for rapid surface runoff over the annual maximum snowfree area within the catchment is high. In the event of a large rainfall event the capacity of a tunnel-conduit system to discharge may have decreased sufficiently to cause temporary retention of a large proportion of surface runoff, predominantly within distributed drainage. Temporary storage followed by re-integration of hydraulic connections formed earlier in the ablation season, increases the potential for proportionally large discharge events (relative to the volumes of inputs) in the late ablation season. Flooding in glacierised basins becomes more likely as a result.