Box 46 Predicting The Magnitude Of Jokulhlaups

Attempts have been made to predict the peak flow magnitude of jokulhlaups, both for theoretical purposes and for use in design of bridges or structures that may have to withstand jokulhlaup flows. Developing theoretical relationships to predict discharge from ice-dammed lakes has proved difficult, not least because the exact mechanisms of drainage are poorly understood in many cases. However, Clague and Mathews (1973) developed an empirical equation based on data from the jokulhlaups for which the peak discharge could be estimated accurately. They obtain a regression relationship between peak discharge and the volume of the ice-dammed lake.

where Qmax is peak discharge (m3 s 1) and V0 is the volume of the lake prior to discharge (m3).

This equation gives surprisingly good results and has been used widely to predict jokulhlaup flood magnitudes.

Source: Clague, J.J. and Mathews, W.H. (1973). The magnitude of jokulhlaups. Journal of Glaciology, 12, 501-4.

Glacial meltwater erosion beneath ice sheets and glaciers may result from either mechanical or chemical processes. The effectiveness of meltwater as an agent of erosion depends on: (i) the susceptibility of the bedrock involved, in particular the presence of structural weaknesses or its susceptibility to chemical attack; (ii) the discharge regime, in particular the water velocity and the level of turbulent flow; and (iii) the quantity of sediment in transport. Here we outline the main processes of glacial meltwater erosion. The landforms created by the flow of meltwater are described in Chapter 6.

Mechanical erosion occurs through two processes: (i) fluvial abrasion; and (ii) fluvial cavitation.

Fluvial abrasion occurs by the transport of both suspended sediment and sediment in traction within the meltwater. This sediment abrades the walls of rock channels and the bed of subglacial tunnels, striating and grooving the rock surface. The rate of meltwater abrasion is controlled by the following factors.

1. The properties of the sediment in transport. Of particular importance is the hardness of the sediment relative to the bedrock surface over which the melt-water is flowing. The harder the sediment in transport is relative to the bedrock beneath the meltwater the more erosion is achieved. The concentration of debris within the meltwater is also important. In general rates of abrasion increase with increasing concentrations of debris over the range of concentration normal in most meltwater.

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