SCALES OF VARIATION OF SUSPENDED SEDIMENT CONCENTRATION AND TURBIDITY IN A GLACIAL MELTWATER STREAM

A wide range of temporal scales of variability exists in suspended sediment and turbidity records from proglacial meltwater streams. These variations and their causes are illustrated in this paper using data from the stream draining the Haut Glacier d'Arolla, Valais, Switzerland. The data include hourly pump-sampler suspended sediment data from June to September 1990; a quasi-continuous 24 hour record of turbidity and velocity sampled at 5 second intervals in July 1992; and 3-minute series of turbidity and velocity at various heights in the boundary layer sampled at 10Hz, also obtained in July 1992. The high-frequency measurements made in 1992 employed an active head, infra-red suspended solids monitor deployed in close proximity to a twin-axis, discoidal electromagnetic current meter. Data were analysed using a combination of statistical and deterministic ('event structure') modelling. Particular emphasis is placed on the parallel variation of turbidity and velocity on time scales of less than 1 second. Variability at this scale has been relatively neglected, but must be understood before turbidity monitoring programmes can be designed that involve sampling from the continuous variation of turbidity by averaging over 10-60 seconds every 30-120 minutes, logging the data automatically, and calibrating the record using pump or hand held samplers. Fluctuations in suspended sediment concentration and turbidity are accounted for in terms of glacial-hydrological, channel-marginal, and flow-related controls whose significance varies with timescale. from the seasonal through the diurnal to the instantaneous. The results have implications for research designs employing turbidity meters as indicators of suspended sediment variation. Representative turbidities, and comparisons between meters, must consider time-averages scaled with respect to turbulence characteristics at particular locations. The possibility of interaction between more or less discrete transport events results in turbidity variation which cannot, at present, be satisfactorily predicted.