TRATAMIENTO QUIRÚRGICO

During 1998 and 1999, the glacier was drained by two principal meltwater streams with distinct glacial sub-catchments (Figure 2.5). Dye-tracing investigations from moulins on the glacier surface were used to determine the boundary o f the two sub-catchments during 1998 (Section 2.2.4.3; Figure 3.9). Monitoring was confined to a single sub­

catchment during each season since sufficient equipment was not available to allow the monitoring o f both sub-catchments simultaneously. In 1998, monitoring was concentrated on the western proglacial stream, which in previous years had been shown to drain the majority o f the glaciated area (Sharp et al., 1993). The eastern proglacial stream, which drained the majority o f the glaciated area during 1998 (Figure 3.9; Section 3.4.4), was monitored during the 1999 melt season. Monitoring o f both sub-catchments from downstream o f the confluence between the eastern and western proglacial streams was also impossible, since this would: 1) require a well-mixed sampling point downstream o f the confluence, such that the gauging station could not be located near to the glacier snout; 2) integrate changes in suspended sediment concentration and quality from separate sub-catchments, thereby masking processes of sediment evacuation that may be temporally or spatially variable within each catchment; and 3) generate practical problems with regards to the maintenance and calibration o f the gauging station due to high discharges and flow velocities.

Proglacial

\ stream

2 800---C ontour with elevation (m)

99a 1 km

[99b]

E a ste rn stre a m

[98a] 2 7 0 0 '

[98b]

98c

W estern stre am

F igure 2.5 Detail o f the proglacial area at Haut Glacier d ’Arolla, showing the Eastern and Western proglacial streams and the position o f the gauging stations during 1998 (98a-c) and 1999 (99a and 99b).

Monitoring was conducted from a fixed gauging station located near to the glacier snout in order to eliminate suspended sediment contributions from proglacial sources. A typical arrangement o f sensors at the gauging site is shown in Figure 2.6. The sensors were positioned at roughly half the mean flow depth, as recommended for at-a-point measurements (Rooseboom and Annandale, 1981), although in practice such a position was difficult to maintain (see below). Care was taken to ensure that sensors were located away from the channel banks and within the main flow (cf. Gumell et. al., 1992b), and that turbidity measurements were not influenced by excessive turbulence that may have introduced bubbles or large proportions o f saltating material high into the flow.

2.2.3.1 1998 melt season

During 1998, monitoring began on Julian Day (JD) 135 (May 15th). Thick snow cover in the proglacial area required excavation o f the western proglacial stream ~ 50m from the glacier snout in order to position the gauging station (gauging station 98a, Figure 2.4).

The gauging station was subsequently moved to within ~ 40 and ~ 20m o f the snout on JD 158 and 156 respectively (gauging stations 98b and c) as the proglacial snowcover retreated. Turbidity was monitored using a single sensor (T l) from JD 135 and from two sensors (Tl and T2) after JD 139. Turbidity was recorded separately for each sensor using a Campbell CR23x datalogger; measurements were taken every 10 seconds and averaged over 5-minute intervals. The sensors required frequent maintenance to remove

V —

o Turbidity * ! ■

h ISCO

Intake

Hose _o Turbidity

Sensors

X X >

0.4 m 0.2 m 0.3 m

F igure 2.6 A typical arrange m e nt o f the turbidity sensors and Isco intake hose d uring the 1998 and 1999 m elt seasons. The turbidity sensors w ere positioned at depth d, roughly equal to h a lf the m ean flow depth (h). C hannel w idth w a s typically ~ 3 - 4 m.

saltating debris that b ec am e trapped b etw e en the sensor heads. Occasionally, the position o f the sensors also had to be adjusted to take account o f changes in bed elevation and m ean daily flow depth. Sensor T2 failed on JD 187 and was replaced with a new sensor ( T 3 ) that operated during the period JD 2 1 6 -2 1 7 .

A uto m ated sam pling w as initiated on JD 135. Since suspended sedim ent co ncentrations varied during the m elt season, sam ple volu m es w ere adjusted in order to obtain ad equate sedim ent content w hilst m aintaining good tem poral co verage and acceptable filtration times (Table 2.5). A n u m b e r o f significant gaps (> 12 h) occur during 1998, tw o o f w hich were due to p o w e r failures and a third occurred w h e n the intake hose b ec am e iced during cold weather. S am pling was also im paired if the intake hose bec am e su rrounde d by sedim ent or large rocks, frozen or em erged above the w a te r surface. C onsequently, very small sam ples (typically < 200 m l) and sam ples with excessive quantities o f sedim ent (usually w h e n the intake hose had been found to be buried by rocks or sedim ent) w ere discarded. O nly 35 % o f days had com plete tem poral coverage at the desired interval due to loss o f sam ples for reasons described above.

U S D H 4 8 sam ples w ere obtained daily for later asse ssm e nt o f sam pling p ro g ra m m e representivity. A w id th and depth-integrated sam ple w as obtained im m e d ia tely d o w nstre am o f the g a uging station whilst the IS C O w a s sampling. Norm al practice is to

T able 2.5 Approximate volume and interval o f samples collected for analysis o f suspended sediment concentration and quality during 1998 and 1999.

Period (JD) Volume (1) Interval (h) Notes 1998 Ablation Season

135-136 4 4

137-138 0.25 1

139-140 ^{- -} Power failure

141-143 ~ 1 ~ 4

144-147 ~ 1 2

148-149 - - Power failure

150-153 ~ 1 2

154-162 © oo 1

163 - - Hose iced

164-170 1 2

171-208 0.9 1

1999 Ablation Season

169-175 5 6

176-179 4 4

180-185 2 2

186-187 - - Spring flood event

188-204 2 2

obtain a separate depth-integrated sample for each o f 10 equal sections across the stream (cf. Edwards and Glysson, 1988); however, the ISCO sampled too rapidly and for consistency a single sample was obtained from 10 consecutive depth-integrations o f 10 equal sections. Samples obtained using the USDH48 (full volume 470 ml) should be between 375 and 420 ml to be representative o f mean suspended sediment characteristics (Federal Interagency Sedimentation Project, no date). In practice, it was difficult to obtain samples within this range in the turbulent, rapidly flowing proglacial stream, and hence only samples that were very low (< 350 ml) or almost full (> 460 ml) were discarded. A total o f 52 USDH samples were obtained between JD 142 and 206 during 1998 at times ranging from 11:00 to 19:00, thus providing good seasonal coverage over a range o f flow conditions.

ISCO and USDH samples were filtered in the field through 2.7 pm papers. Samples were screened for particles > 2 mm that may have biased measurements o f sediment concentration and quality (cf. Fenn and Gomez, 1989) using a standard laboratory sieve.

The filtrate from one sample per day (typically a USDH48 sample) was used to assess the representivity o f the filtration method. This involved collecting any sediment remaining around the sides o f the Buchner funnel and gasket and re-filtering using pre­

weighed 0.1 pm papers.

2.2.3.2 1999 melt season

During 1999, monitoring began on JD168 (17th June) in the eastern proglacial stream.

The gauging station was initially located within a well-defined channel ~ 20 m from the glacier snout (gauging station 99a) and moved to a more optimum location ~ 10 m from the snout on JD 186 (99b; Figure 2.4). Turbidity sensors T2 and T3 (see above) were installed on JD 168 and automated sampling was initiated on JD 169. As in 1998, the volume and frequency o f sample collection was varied with respect to changing levels of suspended sediment in the eastern stream (Table 2.5). Automated sampling was used to obtain slightly larger samples than during 1998 of 2x 1 litre throughout most o f the monitored period; one or both o f the containers were filtered depending upon the sediment concentration. USDH48 samples were collected frequently as per the 1998 melt season. A total o f 26 USDH48 samples were obtained; only one was discarded due to the volume being below 350 ml. A second automatic sampler was used to obtain composite daily samples from JD 171 to 205 for analysis o f seasonal changes in sediment quality. Composite samples consisted o f 120 ml samples collected every 3hrs from 00:00. All samples were filtered as per 1998.

Turbidity measurement and automated sampling suffered similar difficulties to those experienced during 1998, although some unique difficulties were experienced. From JD 181 to 189, frequent gaps occur in the turbidity records due to a faulty battery connected to a solar panel that resulted in the failure o f the sensors at night. A logger failure later prevented the downloading o f data from JD 191 to 195. Problems also occurred due to the dynamic nature o f the eastern stream, which at different times occupied up to three channels as it flowed from the glacier snout. Following ~ JD 203, the stream once more

became entrenched in a single channel. High and variable discharge through the single channel prevented the collection o f USDH48 samples and made the gauging station very difficult to maintain. Consequently, automatic sampling and turbidity measurement were stopped on JD 205 and 206, respectively, to avoid damage to and loss o f equipment.