The geomorphological and sedimentological effects of jokulhlaups
The aim of this study was to test a predictive model of the geomorphological and sedimentological effects of floods resulting from the sudden drainage of ice-dammed lakes (jokulhlaups). A process-based, conceptual model for channel and sedimentary characteristics was tested within a jokulhlaup routeway near Sondre Stromfjord, west Greenland. River channel change resulting from a jokulhlaup monitored in 1987 was studied in three channel types along the flood routeway: deltaic, high gradient bedrock-controlled, and low gradient confined valley sandur. Flood powers, sediment supply and channel resistance to erosion were found to vary considerably between the channel types. Consequently, amounts of channel change and the character of the sedimentary record showed extreme variation. In addition, parts of the jokulhlaup channel which experienced backwater effects may have experienced two sediment transport peaks. Variations between the character of deposits found within each of the three channel types reflect local differences in sediment supply, stream power and channel resistance to erosion. Deposition in the delta and the sandur records sedimentation related to a number of jokulhlaups, whilst the spillway and the bedrock-confined channel provide a record of the action of the last jokulhlaup as low stage deposits, as well as that of the earliest flood events as high stage deposits. This study has shown that it is possible to predict the morphology of channel macroforms. The internal structure of these macroforms is strongly dependent upon both the amount and character of sediment supplied to the channel during a jokulhlaup. The more detailed the knowledge of the controlling variables for any one channel, the more detailed the resulting predictions are. At present, the model concerns only the products of turbulent, Newtonian, water floods. It may, however, be possible to predict thresholds marking the transition to non-Newtonian flows where sediment supply conditions and hydrograph characteristics are known. The model may also be used in reverse to reconstruct the magnitude and frequency of former jokulhlaups where sufficient geomorphological and sedimentary evidence is available.