A spectrum of jökulhlaup dynamics revealed by GPS measurements of glacier surface motion

Abstract

GPS campaigns on glaciers during jökulhlaups show how subglacial floods affect glacier motion and shed light on the dynamics of such floods. Three such campaigns have been carried out on southern and western Vatnajökull, southeast Iceland, over known jökulhlaup paths. Two slowly rising jökulhlaups from Grímsvötn and two rapidly rising jökulhlaups from the western and eastern Skaftá cauldrons were captured in these campaigns, with maximum discharge ranging from 240 to 3300 m3 s−1. Glacier surface movements measured in these campaigns are presented along with the corresponding discharge curves. The measurements are interpreted as indicating: (1) initiation of rapidly rising jökulhlaups with a propagating subglacial pressure wave, (2) decreased glacier basal friction during jökulhlaups, (3) subglacial accumulation of water in slowly rising jökulhlaups and (4) lifting of the glacier caused by subglacial water pressure exceeding overburden in both rapidly and slowly rising jökulhlaups. The latter two observations are inconsistent with assumptions that are typically made in theoretical and numerical modelling of jökulhlaups. Both viscous and elastic deformation of the glacier as well as turbulent hydraulic fracture at the ice/bedrock interface are important in the dynamics of the subglacial pressure wave at the front of rapidly rising jökulhlaups.

Keywords

• glacier flow
• glacier hydrology
• jökulhlaups (GLOFs)
• subglacial lakes