Constraining bedrock erosion during extreme flood events: case study of Jökulsá á Fjöllum, Iceland
Résumé
Extreme flood events are characterised by very high discharges, potentially exceeding 10^6 m^3/s, over a short period of time. They can result from varied causes including landslide dam failure, glacial lake outbursts, subglacial volcanic eruptions and powerful typhoons. While these events are rare on a human timescale, they are common over geological time. During these events, the energy transferred to the land surface has the potential to significantly modify the shape of the landscape. Understanding and quantifying the impact of such events is thus crucial to understand the genesis and evolution of landscapes that bear the evidence of shaping by flowing fluids, in either terrestrial or extra-terrestrial contexts. This study aims to constrain the amounts, spatial distribution and processes of bedrock erosion during extreme flood events using three complementary approaches: topographic analysis, terrestrial cosmogenic nuclide (3He, 21Ne) dating and experimental analogue modelling. The Jökulsá á Fjöllum is a major river that flows north from the Vatnajökull icecap, Iceland. This river has experienced numerous outburst floods, termed 'jökulhlaups', of varying magnitude during the Holocene, the largest one reaching a modelled peak discharge of ~0.9 x 10^6 m^3/s. The route of Jökulsá á Fjöllum, particularly the Jökulsárgljúfur gorge system and Ásbyrgi, a large dry canyon (3 km long, ~0.5 km wide, up to 90 m deep), exhibits numerous topographic features that testify to the action of such extreme flood events. Detailed topographic survey of the gorge and landforms associated with the jökulhlaups, such as cataracts, mega-dunes and scablands, combined with the dating of abandoned strath terrace surfaces will allow quantification of the amount and spatial distribution of erosion during jökulhlaups and thus give some insight into the mechanisms of gorge formation and knickpoint migration. Different strath terrace surfaces within the Jökulsárgljúfur gorge correspond to the tops of different basalt lava units; the surface exposure ages will determine whether the different knickpoints corresponding to these terrace levels migrated simultaneously during the flood events or whether each terrace level represents a given flood event. Samples are being processed at GFZ Potsdam at the time of writing and ages are expected for November 2013. A complementary experimental study has been designed to define the influence of a series of parameters on the dynamics of gorge formation in a controlled environment. Simulations will be run using a silica-paste bedrock flume at Géosciences Rennes in November 2013. Patterns of erosion, landform morphology and knickpoint dynamics will be compared to those of the natural object in Iceland to define the control exerted by the following factors on gorge formation: flow regime, initial bedrock slope, pre-existence of knickpoint within the system, sediment availability and the heterogeneity of the bedrock substrate.