Collaborative Research: Investigation of Englacial Conduit Formation and Evolution

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ABSTRACTOPP-00-97137FountainOPP-OO-97095JacobelThis is a collaborative proposal from Principal Investigators at Portland State University and St. Olaf College. The movement of water through glaciers profoundly affects ice motion by influencing the stress distribution at the bed of the glacier. Therefore, a great deal of research has focused on the hydraulic nature of subglacial water transport. For temperate glaciers, the main source of water is surface-generated melt water. How this water reaches the glacier bed is of critical importance. Surface and englacial conditions may preferentially direct water flow to certain parts of the glacier bed and starve other parts of the bed. Therefore, an understanding of the controls on the spatial pattern of subglacial hydraulic conditions will depend on an understanding of the surficial and englacial conditions routing the water to the bed. Existing knowledge of englacial conduits has largely been a by-product of investigations into subglacial conditions. The goal of this project is to investigate the location, size, and hydraulic character of englacial conduits in an alpine glacier. This project will take place on the Storglaciaren in Tarfala, Sweden, where an extensive background of scientific wok exists and an active research program continues. The Principal Investigators will undertake a two-year field program that directly detects englacial conduits and uses ice radar to identify and trace the conduit paths. Low frequency radar measurements (5 MHz) will locate regions with strong englacial scattering suggestive of englacial water pockets and potentially productive areas for drilling. The radar will also be used to map the local bed depth and topography. Conventional hot-water drilling will penetrate the glacier to intersect englacial conduits. After a conduit is encountered, they will measure conduit diameter, water pressure, and water flow speed using down-hole instruments. They will attempt to determine the interconnectivity between conduits to measure their integrated hydraulic properties over the flow path. The path of the conduits will be traced using a high-resolution ground penetrating radar operated at 100 MHz, a frequency chosen both resolve individual conduits, and to monitor changes in echo characteristics corresponding to hydrologic changes over time.The results from this study will further our understanding of conduit origin and the controls on water routing to the bed. Identification of high-resolution radar signals with conduits of known sizes and locations will make significant progress towards future use of ice radar for detecting and mapping the location of englacial conduit. NSF Proposal Number: 0097137PI: FountainStatement of WorkWe will directly measure the geometry and hydraulics of englacial conduits The principle method of detecting englacial conduits will be drilling into the glacier to directly intersect the conduits. We will employ a hot-water drill capable of penetrating at rates of 1 m min-1. We will attempt to drill 20 boreholes in 3 different arrays for a total of 60 holes. During drilling we will monitor both drill depth and water levels. If openings appear on both sides of the borehole and flow is detected, either by natural particulates moving in the water or by a deflection of thread hanging from the camera, no further drilling in the hole will occur. If flow is detected careful hydraulic measurements will be made. The shape of the opening will be recorded and flow velocities will be measured. Within each borehole array and between arrays, we will test for interconnectivity by displacing the water in one borehole and measuring the response in other boreholes. The data from this project will test a model for the origin and evolution of conduits. Another test will compare the measured depth of a water-filled conduit and its hydraulic characteristics to that predicted by theory. The flow data and geometry of the conduits will be used to test Rothlisberger's (1972) theory. If we are able to monitor conduits over time we will also test Spring's (1980) non-steady state theory. From the borehole arrays and the radar imaging (Jacobel) we will infer the 3-dimensional topology of the conduit network at 6 different locations (over 2 years) in the glacier. These results will help to resolve the differences between the models of Shreve (1972) and Fountain and Walder (1998). More generally, spatial differences in the conduit networks imply differences in the flux of water to different areas of the bed and therefore impose an external condition on the development of subglacial hydraulic systems. These results will be reported in several journal articles.