AN ANALYSIS OF SEDIMENT FLUSH EVENTS IN THE PROGLACIAL STREAM OF GLACIER DE TSIDJIORE NOUVE DURING A 22 DAY PERIOD

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1 Hydrology in Mountainous Regions. I '- Hydrological Measurements; the Water Cxte (Proceedings of two Lausanne Symposia, August 1990). IAHS Publ. no. 193,1990. The significance of suspended sediment pulses for estimating suspended sediment load and identifying suspended sediment sources in Alpine glacier basins A.M. GURNELL & J. WARBURTON* Department of Geography, University of Southampton, Southampton S09 5NH, England present address: Department of Natural Resources Engineering, Lincoln College, Canterbury, New Zealand. ABSTRACT Variability in the suspended sediment concentration of proglacial streams is often attributed to the existence of temporal lags between suspended sediment concentration and discharge; and to the existence of sediment supply/exhaustion effects which underly hysteresis in the relationship between the two variables at diumal, sub-seasonal, seasonal and interannual timescales. This paper considers another component of the variability in suspended sediment concentration; the occurrence of frequent pulses or flushes of suspended sediment. Although these flushes are small both in duration and magnitude, some events transport sufficient sediment to make a significant contribution to the sediment yield. They have characteristic shapes, which appear to be related to their source area; and their occurrence is associated with particular transport conditions. Subsampling of continuous turbidity records to simulate the effect of a discrete interval sampling strategy (e.g. pump sampling) 'smooths' the record so that the detail of the flushes is quickly lost. Turbidity measurements are thus required to give accurate estimates of suspended sediment and precise definition of suspended sediment flush characteristics on proglacial streams. INTRODUCTION Many studies have described and analyzed the relationship between suspended sediment concentration and discharge, illustrating the low level of linear correlation between the two variables (e.g. 0strem, 1975; Fenn et al. 1985). This is often attributed to temporal lags between the two series and sediment supply / exhaustion effects which underly hysteresis in the relationship between the two variables at diurnal, sub-seasonal, seasonal and interannual timescales (e.g. Gurnell, 1987). Variability in suspended sediment concentration is also caused by non-periodic events: meltwater flood events (high magnitude - low frequency) and pulses or flushes, short-lived bursts in suspended sediment concentration (low magnitude - high frequency). This paper considers the latter component of suspended sediment concentration variability. Flushes of suspended sediment which occur with no apparent variation in discharge of the proglacial stream have been attributed to tapping of sediment sources in a variety of glacial and proglacial locations (B0gen, 1980; Hammer & Smith, 1983; Richards, 1984; Gurnell, 1987) and their causes have been ascribed to rapid changes in glacial motion (Humphrey et al. 1986), failure of channel banks (Hammer and Smith, 1983), channel erosion (Gurnell, 1982), inputs from tributaries and hillslopes (Bathurst et al. 1986), and surges in discharge caused by fluid instabilities in steep channels (Heggen, 1986). Sediment flushes are usually small in duration and in the magnitude of sediment transported, but some events transport a significant proportion of the sediment yield from the glacier basin. Their brevity is of great significance in relation to suspended sediment concentration estimates from pumped water samples, since the highest frequency of 463

2 A. M. Gumell & J. Warburton 464 published pump sampling in the proglacial environment is one hour (Gumell, 1987, Table 12.2). Sediment pulses are of interest to studies of the hydrology and sediment yield from glacier basins since they may provide indications of glacial and fluvial activity both within the glacial and proglacial zones of the drainage basins. This paper addresses the significance of sediment flushes as a component of suspended sediment transport in proglacial streams in three ways. First, 571 flush events observed from 22 days of turbidity records for the proglacial stream of Glacier de Tsidjiore Nouve are analysed to identify their size, shape and frequency characteristics in relation to the background suspended sediment concentration and discharge regime. Second, the causes of flushes are explored in relation to parameters of their size and shape using: (1) field observations at Glacier de Tsidjiore Nouve; (2) pulses derived from simulated bank instability on the proglacial streams of the Bas Glacier d'arolla and Glacier de Tsidjiore Nouve; (3) observations of sediment pulses routed through the Bas Glacier d'arolla. Third, the significance of flushes as a component of total suspended sediment transport is investigated using turbidity records from the proglacial stream of the Bas Glacier d'arolla. The degree to which pumped/hand sampling of glacial meltwaters can account for sediment transport flushes is estimated by subsampling a continuous turbidity record according to various simulated sampling frequencies. THE STUDY BASINS AND INSTRUMENTATION Glacier de Tsidjiore Nouve and Bas Glacier d'arolla occupy adjacent valleys in the Val d'herens, Switzerland, have similar catchment characteristics (Gurnell et al. 1988), and have short proglacial streams which are diverted into meltwater intake structures, forming part of the Grande Dixence S.A. hydro-electricity scheme, within 300m of the glacier snouts. Flow gauging stations within the intake structures provide accurate discharge records. Suspended sediment concentration was monitored using turbidity meters (Partech RP MKII suspended solids monitors with a single gap (SDM-10) probe) located in the water intake structures, which provide an environment free from ambient light and with a stable power supply. Suspended sediment concentrations were estimated from the turbidity record using standard gravimetric calibration procedures (400ml USDH-48 samples filtered through Whatman 40 papers). The same instrumentation, was also used in artificial pulse experiments. AN ANALYSIS OF SEDIMENT FLUSH EVENTS IN THE PROGLACIAL STREAM OF GLACIER DE TSIDJIORE NOUVE DURING A 22 DAY PERIOD All flush events were abstracted from a turbidity record for the proglacial stream of the Glacier de Tsidjiore Nouve over a 22 day period, 26 July to 16 August Parameters of each flush were abstracted from the turbidity chart, including the time of rise of each flush (t), the turbidity immediately preceding the flush (Tb), turbidity at the flush peak (Tp), and the turbidity at a lag equivalent to the time of rise after the flush peak (Tf). A calibration curve converted the turbidity values Tb, Tp, Tf into suspended sediment concentration estimates Sb, Sp, Sf. This analysis yielded 571 flush events with a temporal resolution of 1.5 minutes and a calibrated suspended sediment concentration resolution of 90 mg l 1. Further flush parameters were derived including the peak suspended sediment concentration (Spk = Sp - Sb), the slope of the rising limb (Sri = (Sp-Sb)/t), the slope of the falling limb (Sfl = (Sp-Sf)/t), the shape of the flush (Sfl/Srl) and the time of occurrence of the flush (to the nearest hour). Descriptive statistics for the flush parameters, which were all strongly skewed, are presented in Table 1. Of particular interest is the very short time of rise of the flushes (maximum, 36 minutes; median, 6 minutes) which may be important in causing poor linear correlation between suspended sediment concentration and discharge in proglacial streams. Although many of the flush peak suspended sediment concentrations

3 * * * * 465 The significance of suspended sediment pulses are small, the upper quartile of events with peak concentrations in excess of 450 mg l" 1 could have a strong effect on the total suspended concentration and could introduce scatter to the suspended sediment - discharge relationship if intercepted by a pump sampler. TABLE 1 Descriptive statistics of measured and calculated flush parameters, Glacier de Tsidjiore Nouve, 26 July to 16 August t Spk Sri Sfl Shape Median Upper Quartile Lower Quartile Maximum Minimum ' * This event is not included in Figures 2 or 3 because it is almost double the size of the second largest event and so it would distort the graphs. t is expressed in minutes; Spk in mg l" 1, Sri and Sfl in mg l" 1 min" 1 ; Shape is dimensionless. > 0 w 3 a sab J 0- œ > k i 1 * 285 M \ i $ X % * % " * / *? i m t, t L ï \ '-i i * V «* V * ' * * " wx wx lé JULIfit! M FIG. 1 Flush frequency characteristics in association with a) Julian day, b) time of day, Glacier de Tsidjiore Nouve 26 July - 16 August The first 11 days of the study period were subject to considerably higher discharges and background suspended sediment concentrations than the final 11 days. Flush frequency was greatest in the first 11 days of the study period (Figure la) and was also greatest between ll.ooh and 20.00h (Figure lb). Suspended sediment concentration and discharge were relatively high prior to the flush events when compared with the frequency distributions of these variables for the whole study period. Small flushes occurred throughout the study period, but the upper quartile of flush peaks occurred mainly in the first 11 days and between ll.ooh and 20.00h, were associated with preceding suspended sediment concentration in excess of 1500 mg l" 1 and with discharges over 1 m 3 s" 1. Large flushes had shorter times of rise than smaller flushes so that their rising limbs were also steeper. Variations in the shape parameter (Sfl/Srl) show that large flushes have a steeper rising than falling limb (shape parameter < 1), whereas smaller flushes vary across the range from

4 A. M. Gurnell & J. Warburton 466 slightly steeper to substantially less steep rising than falling limbs. In order to explore the relationships between flushes and discharge in more detail, the discharge associated with each flush event was allocated to one of four flow classes. Class 1 was on the rising limb of the discharge hydrograph but at a flow less than the previous day's peak flow. Class 2 was on the rising limb and exceeded the preceding day's peak flow. Class 3 was on the falling limb but exceeded the previous day's peak flow. Class 4 was on the falling limb and at a flow less than the previous day's peak flow. Table 2 compares the frequency and flood peak magnitude characteristics of flushes occurring in the flow classes with the percentage of time over the 22 day period occupied by each of the classes. TABLE 2 Comparison of the frequency and magnitude characteristics of suspended sediment flushes, by flow class, Glacier de Tsidjiore Nouve, 26 July to 16 August Flow Class Spk for all flushes (mg.l 1 ) Median Upper Quartile Lower Quartile % total flushes Flushes with Spk >= 450 mg.l" 1 Median Upper Quartile Lower Quartile % flushes with Spk >= 450mgl % study period in flow class times more flushes occur in class 2 than would be expected with a uniform distribution of flushes with flow class. The observed/expected ratios for classes 3, 1 and 4 are 2.0, 1.0 and 0.6 respectively. In the case of the upper quartile of flushes (according to the magnitude of Spk), the ratios of observed to expected flush frequency for classes 2,3,1 and 4 are 4.0, 2.2, 0.9 and 0.5, respectively, showing a more marked contrast between classes. The median time of flush rise does not vary with flow class but the maximum time of rise is higher in classes 1 and 4 than in 2 and 3, providing some further evidence of the more attenuated shape of some of the flushes with smaller peak suspended sediment concentrations. Thus over the 22 day turbidity record flush events occurred preferentially during periods of high discharge and particularly during rising flow in excess of the previous day's peak flow. The largest flushes had steeper rising limbs and shorter times to peak than the majority of flushes and occurred between and 22.00h. Although individual flushes may not be particularly large in either their peak suspended sediment concentration or the magnitude of the sediment load they transport, together they form a very significant component of suspended sediment transport during the 22 day period: 46% (1891 tonnes) of the total suspended sediment load transported by the proglacial stream was transported

5 467 The significance of suspended sediment pulses during flush events and 9% (369 tonnes) of the total suspended sediment load was transported as part of a flush peak rather than as part of the background suspended sediment load underlying the peak. CAUSES OF FLUSHES Field Observations at Glacier de Tsidjiore Nouve During the early part of the 22 day study period in 1986 and during a hydrologically similar period in 1983 (29 July - 3 August), when large numbers of flushes were associated with a period of gradually increasing diurnal peak discharge, the progressive encroachment of the proglacial stream across its floodplain appeared to have an influence on flush generation. During 1983, hand sampling of meltwater was undertaken on tributary streams at the glacier snout and at various sites along the main proglacial stream. These were used to determine suspended sediment concentration at different points on the proglacial stream network and, if flushes were recorded on the turbidity chart during hand sampling, it was possible to attribute the flushes to approximate source areas. The proglacial stream of the Glacier de Tsidjiore Nouve is steep, has a boulder bed and step-pool sequences in its long profile. As discharge increases, distributaries develop as water spills from the pools to occupy previously dry areas of the valley train. These increases in the area of active stream flow are often associated with the flushing out of fine sediments. High flows are also associated with bank erosion and collapse, which can also produce pulses in the turbidity chart (as was observed on both the 31 July and 2 August 1983). None of these proglacial sources of suspended sediment have been observed to produce very large turbidity pulses and they have a characteristic timing, being most noticeable during rising discharge and particularly when flows exceed the previous day's peak discharge. Two types of event have been observed to generate large, asymmetrical, peaked sediment pulses with steep rising limbs. On 5 August 1984, a large boulder collapsed down the snout of Glacier de Tsidjiore Nouve revealing an area of fine sediments at the snout and releasing water from the ruptured ice to two small streams. The tributary streams became highly charged with suspended sediment (4600 and mg l* 1 ) and produced a very peaked sediment pulse on the turbidity chart. On 2 August 1983, high suspended sediment concentrations (2900 and mg l" 1 ) were observed in two different streams draining from the snout of the glacier and were associated with sediment flushes on the turbidity chart. There had been a heavy thunderstorm the previous night, major cracks had developed across the glacier snout and the two tributaries transporting high concentrations of sediment had moved their point of exit from the glacier snout since the preceding day. In this case the peaked flushes tapped sediment sources upstream of the glacier snout. These limited field observations indicate that major, asymmetrical, peaked sediment flush events can be generated from the glacier-covered part of the catchment and so support the results of hand sampling in the proglacial stream in 1981 reported in Gurnell (1982). Gurnell noted flush events from single tributaries and simultaneously from multiple tributaries but turbidity records were not available to describe the form of the 1981 flush events. No major peaked pulses have been observed to be generated proglacially in the Tsidjiore Nouve catchment, but this may be a function of insufficient field observations. Experiments to simulate the impact of proglacial bank collapse were, therefore, undertaken to investigate the role of proglacial sources in generating sediment flushes. Suspended sediment flushes produced by simulated bank collapse - a field experiment In the field the effects of individual sediment supply events can rarely be studied directly, therefore field experiments involving either the artificial generation of a supply event (e.g. use of explosives in initiating cliff collapse - Bathurst et al, 1986) or simulation of sediment supply (e.g. bed disturbance experiments - Lambert and Walling (1988)) are necessary. In the Bas Glacier d'arolla, Haut Glacier d'arolla and Glacier de Tsidjiore

6 A. M. Gurnell & J. Warburton 468 Nouve proglacial zones two major sediment sources may contribute to sediment flushes: valley train bluff erosion and eluviation of the valley floor. Sediment transport experiments were carried out to determine the nature of suspended sediment pulse transport in proglacial channels using sediment derived from these sources. Stream bank collapse and valley train eluviation were simulated by injecting a known weight of sediment (in units of approximately 15 kg) into the proglacial channel. Downstream of the injection point, turbidity was monitored using a Partech suspended solids monitor. The sediment was collected from proglacial streambanks and bluffs but some finer sediment was also injected to allow comparison between the passage of coarse sediment (bank material) and finer sediment pulses. The experiments showed that sediment introduced at increasing distances from the turbidity monitoring station induced an increased time to peak and an attenuation of the turbidity trace so that after a short distance pulses were symmetrical in shape and even tended towards a steeper falling than rising limb. Increasing the amount of injected sediment resulted, over a fixed distance, in an increase in peak turbidity; finer sediment induced larger and slightly earlier turbidity peaks, supporting the view that more peaked sediment flushes are likely to be subglacial in origin. However, variations in turbidity response by a factor of two occurred for apparently similar samples collected from the same source. Differences may be attributable to inadequate mixing or to variations in source material. These experiments demonstrate the variability of suspended sediment transport, but the small-scale of the experiments is also significant. The study reaches were short (less than 50 m), sediment volumes were small (less than 75kg) and the experiments were carried out over a restricted discharge range ( m 3 s" 1 ), giving very short flush durations ( seconds) which would only just be recognisable on the turbidity charts used in the analysis of prototype events. It is questionable whether these results can be scaled to prototype pulses. However, the results suggest that it is unlikely that large sediment pulses would be generated from valley train sediment sources and that if flushes are generated they are likely to have an attenuated shape. Given the coarse nature of valley train sediments (average silt/clay content of bank and bluff materials at Bas Glacier d'arolla is only 4% by weight) and that proglacial suspended sediment is dominantly silt-sized, it would be necessary to erode over 130 m 3 of valley train sediments to generate a flush transporting approximately 10 tonnes of suspended sediment. The identification of sediment pulses routed through the Bas Glacier d'arolla Field observations at Bas Glacier d'arolla suggested that large asymmetrical sediment flushes in the turbidity chart (with steeper rising than falling limbs) recorded on the proglacial stream, originate from source areas upstream from the glacier snout. The turbidity record for the Bas Glacier d'arolla for June to August 1987 was characterised by suspended sediment flushes. The times of purging of the Haut Glacier d'arolla meltwater intake sediment trap were found to closely correspond with the large asymmetrical sediment pulses. Haut Glacier d'arolla is a high-level glacier occupying an adjacent valley 1.5 km to the southeast of Bas Glacier d'arolla. Virtually every pulse or pulse complex is linked with a purge of the Haut Arolla sediment trap, suggesting that sediment charged meltwater purged from the upper glacierised catchment is very rapidly routed through the Bas Glacier d'arolla. Analysis of the full turbidity record for Bas Glacier d'arolla 1987 (25 May - 4 September), identified 256 pulses. 91 of these flushes had a steep rising limb and marked asymmetrical form, and 85% of these flushes were matched with Haut Glacier d' Arolla purges. The remaining asymmetrical flushes, when observed in the field, were seen to also come from the glacier. The nature and frequency of flush events at Bas Glacier d'arolla and Tsidjiore Nouve are very different. The Bas Glacier d'arolla turbidity record is relatively simple with large flushes dominating (2.5 flushes/day season). At Tsidjiore Nouve the record is much more complex with more, smaller pulses, the majority of which occur with negligible variations in discharge. It is interesting to speculate whether the differences in the frequency of flushes is indicative of processes acting at the bed of these glaciers. Could this mean that

7 469 The significance of suspended sediment pulses the glacier drainage system at Bas Glacier d'arolla is more stable and accesses few sediment stores? Bezinge et al. (1988) and Gurnell et al. (1988) suggested that at Glacier de Tsidjiore Nouve enhanced sediment yield is associated with glacier advance and Humphrey et al. (1986), for the Variegated Glacier, Alaska, showed that discharges of turbid water were related to mini-surge activity. SIGNIFICANCE OF PULSES The significance of suspended sediment flushes can be evaluated in terms of their contribution to the proglacial meltwater streamload and the loss of information that results if the flush events are not adequately characterised. The contribution of suspended sediment flushes to total suspended sediment yield has been determined for the Bas Glacier d'arolla catchment for the period 25 May - 30 July The total amount of sediment contributed by purging of the Haut Glacier d'arolla sediment trap was 1903 tonnes (i.e. 28.8% of the suspended sediment (turbidity) load and 8.8% of the total basin sediment output during this period) whilst other 'natural' flushes contributed 630 tonnes to the suspended sediment yield. Thus, a total of 37% of the suspended sediment yield was derived from flush events. This is probably an underestimate of the importance of these events since flushes in suspended sediment may also be accompanied by pulses in bedload transport. Identifying suspended sediment flushes is important in the design of glacio-hydrological sampling strategies. Simulated sampling experiments were conducted on two continuous suspended sediment series (23 June - 27 June 1986 and 11 July - 11 September 1986) from the Bas Glacier d'arolla proglacial stream. An hourly sampling strategy resulted in respectively, 84% and 64% reduction in the number of pulses identified; implying a potential underestimation of suspended sediment yields and the loss of diagnostic information regarding the sediment origins. The two series represent, respectively, a period of frequent small-scale flushes (11.4 flushes/day) and a longer period of less frequent but larger flushes (1.8 flushes/day). The loss of information for the two glaciers reported here is quite large. Continuous turbidity records are, therefore, required to give an accurate measure of suspended sediment yield and a precise definition of the nature of the suspended sediment transport events. This supports the conclusion of Olive and Rieger (1988). Where turbidity meters are not available, probability based sampling methods (Thomas, 1988) can improve estimates of sediment load and increase the representation of suspended sediment flushes which, at Tsidjiore Nouve, have an increased frequency with higher background discharges. CONCLUSIONS Proglacial suspended sediment concentration series are characterised by frequent short flushes. They form an important component of suspended sediment load (46% and 37% of suspended load was transported in association with sediment flush events during the study periods at Glacier de Tsidjiore Nouve and Bas Glacier d'arolla, respectively) but have different frequency - magnitude characteristics in the two basins. Detailed analysis of size and shape parameters of flushes at Tsidjiore Nouve, field observations of specific events and their causes in both glacier basins, and small-scale field experiments simulating proglaciauy-generated flushes, indicate that the size and shape of flushes may be indicative of their source area. Peaked, asymmetrical flushes with steeper rising than falling limbs appear to be predominantly glacial in origin, whereas smaller, more symmetrical flushes can be generated from proglacial sediment sources. However, a better understanding of the genesis of suspended sediment flushes would be gained if measurements of glacial motion, particle size determinations and sediment tracing studies were carried out concurrently with proglacial stream turbidity monitoring. Particle size characteristics offer valuable information on sediment source areas and pathways (B0gen, 1988; Fenn and Gomez, 1989)

8 A. M. Gurnell & J. Warburton 470 and a quantitative knowledge of source areas is essential to understanding suspended sediment transport variability (Van Sickle and Beschta, 1983). ACKNOWLEDGEMENTS NERC are gratefully acknowledged for the provision of a studentship to support J. Warburton during this research. The generous provision of discharge data and logistical support by Grande Dixence S.A. is also very gratefully acknowledged. REFERENCES Bathurst J.C., Leeks G.J.L. and Newson M.D. (1986) Relationship between sediment supply and sediment transport for the Roaring River, Colorado, USA. In Drainage Basin Sediment Delivery, (Proc. Albuquerque Symp, Aug. 1986) IHAS Publ. 159, Bezinge A., Clark M.J., Gurnell A.M. and Warburton J. (1988) The management of sediment transported by glacial meltwater streams and its significance for the estimation of sediment yield. Annals of Glaciology, 13, 1-5. B0gen J. (1980) The hysteresis effect of sediment transport systems. Norsk Geografisk Tidsskrift, 34, B0gen J. (1988) A monitoring programme of sediment transport in Norwegian rivers. In Sediment Budgets (Proc. Porto Alegre Symp., Dec. 1988) IAHS Publ. 174, Fenn C.R., Gurnell A.M. and Beecroft I. (1985) An evaluation of the use of suspended sediment rating curves for the prediction of suspended sediment concentration in a proglacial stream. Geografiska Annaler, 67A, Fenn C.R. and Gomez B.(1989) Particle size analysis of the sediment suspended in a proglacial stream: Glacier de Tsidjiore Nouve, Switzerland. Hvdrological Processes, 3, Gurnell A.M. (1982) The dynamics of suspended sediment concentration in a proglacial stream. In Hvdrological Aspects of Alpine and High Mountain Areas (Proc. Exeter Symp. July 1982) IAHS Publ. 138, Gurnell A.M. (1987) Suspended sediment. In A.M. Gurnell and M.J. Clark (Eds) Glaciofluvial Sediment Transfer: An Alpine Perspective, John Wiley & Sons, Chichester, Gurnell A.M., Warburton J. and Clark M.J. (1988) A comparison of sediment transport and yield characteristics of two adjacent glacier basins, Val d'hérens, Switzerland. In Sediment Budgets (Proc. Porto Alegre Symp., Dec. 1988) IAHS Publ. 174, Hammer K.M. and Smith N.D. (1983) Sediment production and transport in a proglacial stream: Hilda Creek, Alberta, Canada. Boreas, 12, Heggen R.J. (1986) Periodic surges and sediment mobilization. In Drainage Basin Sediment Delivery, (Proc. Albuquerque Symp, Aug. 1986) IHAS Publ. 159, Humphrey N., Raymond C. and Harrison. (1986) Discharges of turbid water during minisurges of Variegated glacier, Alaska, USA. Journal of Glaciology, 32, 111, Olive L.J. and Rieger W.A. (1988) An examination of the role of sampling strategies in the study of suspended sediment transport. In Sediment Budgets (Proc. Porto Alegre Symp., Dec. 1988) IAHS Publ. 174, strem G. (1975) Sediment transport in glacial meltwater streams. In A.V. Jopling and B.C. MacDonald (Eds) Glaciofluvial and Glaciolacustrine Sedimentation, Society of Economic Palaeotologists and Mineralogist Special Publication 20, Richards K.S. (1984) Some observations of suspended sediment dynamics in Storbregrova, Jotunheim. Earth Surface Processes and Landforms Thomas R.B. (1988) Measuring sediment yields in storms using PSALT. In Sediment Budgets (Proc. Porto Alegre Symp., Dec. 1988) IAHS Publ. 174, VanSickle J. and Beschta R.L. (1983) Supply-based models of suspended sediment transport in streams. Water Resources Research, 19, 3,