Combined Measurements of Water and Sediment Discharges and Surface Velocity of Midtdalsbreen, Norway

Because high suspended sediment concentration (SSC) in melt waters may cause damage to hydro-electric powergenerating machinery, the ability to predict the suspended sediment loads of pro-glacial streams is important for economic reasons. The study to which this abstract refers examines how the sliding velocity of a glacier might help to account for some of the variance of SSC in a pro-glacial stream, which is otherwise accounted for neither by diurnal and seasonal fluctuations in melt-water discharge nor by variations in the supply and exhaustion of sediment. Anomalies in SSC of pro-glacial streams occur, often without any noticeable change in discharge. Pulses of turbid water during the mml-surges of Variegated Glacier (Humphrey and others, 1986) were attributed to the tapping of new local sediment sources beneath the glacier during the shifting of drainage routes. Such anomalies are significant in the transport of suspended sediment in pro-glacial streams, and cannot be estimated by using sediment-rating curves. Our study reports patterns of water discharge, SSC, and glacier velocity of Midtdalsbreen , a northern outlet glacier of Hardangerj0kul in south Norway. The glacier is I km wide and about , 5 km long, in the altitude range 1400-1830 m, and covers 77% of a catchment area of about 9 km 2. Between 22 June and 21 August 1987, water discharge and rainfall were recorded hourly at a site 600 m from the glacier snout. Water samples, collected every 2 h, were filtered in the field and returned to the laboratory where SSC was determined. Glacier-surface velocities were calculated by repeatedly surveying the position of four fixed markers on the centre line of the glacier at spaci ngs of about 350 m. Between 7 July and 29 August, when weather permitted, surveys were made five times daily and a 2 hourly velocity-time series was synthesized using cubic spline interpolation for each of the four markers. In order to explain as much as possible of the vartatlon in SSC, a statistical model was produced and residuals from the model were analysed to see whether any of the large posItIve residuals occurred at times of short-term increases in glacier velocity. After transforming the variables by taking log10' several models were produced. The suitability of each model was assessed on the basis of the significance of the model parameters, the magnitude of the residual sum of squares (RSS), and absence of pattern in the autocorrelation function (acf) and the partial acf of the residuals.