Sliding of ice past an obstacle at Engabreen, Norway

At Engabreen, Norway, an instrumented panel containing a decimetric obstacl e was mounted flush With the bed surface beneath 210 m of ice. Simultaneous measurements of normal and shear stresses, ice velocity and temperature we re obtained as dirty basal ice flowed past the obstacle. Our measurements w ere broadly consistent with ice thickness, flow conditions and bedrock topo graphy near the site of the experiment. Ice speed 0.45 m above the bed was about 130 mm d(-1), much less than the surface velocity of 800 mm d(-1) Ave rage normal stress on the panel was 1.0-1.6 MPa, smaller than the expected ice overburden pressure. Normal stress was larger and temperature was lower on the stoss side than on the lee side, in accord with flow dynamics and e quilibrium thermodynamics. Annual differences in normal stresses were corre lated with changes in sliding speed and ice-flow direction. These temporal variations may have been caused by changes in ice rheology associated with changes in sediment concentration, water content or both. Temperature and n ormal stress were generally correlated, except when clasts presumably colli ded with the panel. Temperature gradients in the obstacle indicated that re gelation was negligible, consistent with the obstacle size. Melt rate was a bout 10% of the sliding speed. Despite high sliding speed, no significant i ce/bed separation was observed in the lee of the obstacle. Frictional force s between sediment particles in the ice and the panel, estimated from Halle t's (1981) model, indicated that friction accounted for about 5% of the mea sured bed-parallel force. This value is uncertain, as friction theories are largely untested. Some of these findings agree with sliding theories, othe rs do not.