154
Erosion and Sediment Transport Measurement in Rivers: Technological and Methodological
(Proceedings o f l h e Oslo Workshop. June 2002). I A I IS Publ. 283, 2003.
Advances
Sediment monitoring of glacial rivers in Iceland:
new data on bed load transport
J Ô R U N N H A R D A R D Ô T T I R & ÂRNI S N O R R A S O N
Hydro-logical
Service,
Orkiistofnun,
Grensdsvegitr
9, IS-1
OS,
Iceland
[email protected]
Abstract Sediment monitoring of glacial rivers in Iceland has been carried out
at the Hydrological Service of the National Energy Authority during the past
five decades. This paper describes the expansion of the monitoring in recent
years, with special emphasis on recent bed load sampling. As an example, the
results from an extensive sediment sampling project in the harnessed glacial
river Thjôrsâ, south Iceland, are discussed and compared with results from
three other glacial rivers (Skaftâ in south Iceland and Jôkulsâ â Fjôllum and
Jôkulsâ â Dal in north and northeast Iceland) where bed load studies were
carried out in 2001. Great changes in "at-a-point" bed load rate measured with
a Helley-Smith sampler are seen among the four rivers, and variability within
individual channels and sampling campaigns at diverse river discharge is
prominent. Mean total bed load discharge for an individual sampling camp­
aign was lowest in Thjôrsâ (0.03 kg s" m" ) and highest in a June campaign in
Jôkulsâ â Dal (1.58 kg s"' m" ). The mean total bed load transport rates in this
study are higher than many published rates for arctic and alpine environments.
1
1
1
Key words
bed load transport; glacial rivers; Iceland; j o k u l h l a u p ; suspended sediment
load
INTRODUCTION
Nowhere in the world is freshwater of equal profusion per capita as in Iceland (over
two thirds of a million m per capita per year); hence, water resources are of special
importance to the country. Evaluation of this resource has been an integrated part of
environmental monitoring in Iceland, and carried out during the last 55 years at the
Hydrological Service of the National Energy Authority (HS). Most of this water, both
groundwater and surface water, originates from glaciers, which cover approximately
10% of Iceland's surface. Consequently, most of the largest rivers in Iceland are
glacially derived and transport great amounts of sediment within their course from the
glaciers towards the ocean.
Many processes effect the sediment load in these rivers, including both human
disturbances, such as land use and hydroelectric power construction, and natural insta­
bility caused by e.g. weather related flood events, glacier outburst floods (jôkulhlaups)
and glacier surges. Thorough knowledge of sediment load is thus vital for environ­
mental and engineering evaluation related to hydroelectric development of the rivers,
but also gives a basis to study processes such as glacial surging and melting, as well as
land erosion.
Sediment sampling in Icelandic rivers has been an integrated part of the river
monitoring carried out by the HS in Iceland since its establishment in 1947. Tômasson
has introduced the pre-1990 suspended sediment data set collected at HS in several
3
155
Sediment monitoring ofglacial rivers in Iceland: new data on bed load transport
papers (e.g. Tômasson, 1976, 1986a,b, 1991), and used it to evaluate geomorphic
processes such as shore development and glacier erosion, as well as the effect of
hydropower reservoirs on sediment load in rivers. In the last 15 years the sediment
monitoring of Icelandic rivers has extended greatly with increased studies at the HS,
including more detailed suspended analysis, and has recently initiated bed load studies.
Moreover, several studies have been focused on the variability of suspended sediment
load in Icelandic rivers (e.g. Lawler, 1991; Lawler et al., 1994; Old, 2000; Pâlsson et
al., 1998; Lawler & Wright, 1999). In addition, many studies have concentrated on the
sedimentological aspect of jôkulhlaups; especially following the great jôkulhlaup (peak
flow of about 50 000 m s" ) from the Vatnajôkull glacier in 1996 (e.g. Russell &
Knudsen, 2002; Snorrason et al., 2002).
J
1
In this paper we will briefly introduce the suspended sediment monitoring network
at HS, which has today produced data sets that are of supreme value for studying both
fluvial and glacial processes. However, our main focus will be on the new bed load
project, which has greatly extended during the last three years.
T H E S E D I M E N T S A M P L I N G P R O G R A M M E AT HS
Throughout the last five decades samples of suspended sediment have been obtained
predominantly from rivers with potential for hydroelectric power generation, or rivers
subject to jôkulhlaups. From 1963 to 2001 measurements of grain size and total dissol­
ved sediment have been made on over 11 000 suspended sediment samples, in addition
to measurements of total suspended sediment concentration that were started in 1949.
During the decades, the continuous data set has expanded greatly (Pâlsson &
Vigfusson, 1996) and today it comprises the basis of all evaluations of sediment trans­
port in Icelandic rivers (see e.g. Tômasson et al., 1996; Pâlsson et al., 2000, 2001).
Figure 1 shows the sampling locations of the suspended sediment programme in
operation during 2001. During that year, over 500 suspended samples were obtained
from 41 locations. The number of samples at each station (1-95) and quality of the
samples ranged greatly among the stations, as in some instances they were a part of a
detailed integrated sediment study, but in other cases a part of an annual sediment
monitoring programme integrated with the hydrometric network (operation). Most
samples are made of cumulative sub-samples taken from three or more locations across
the river channel. However, where conditions are unfavourable, sub-samples from only
two or even one location make up the suspended sample. The suspended sediment
samples are analysed for grain size, total suspended sediment concentration, and TDS
before a rating curve is constructed, which is used for correlation between water
discharge and sediment load. For pure glacier-fed rivers, the correlation coefficient is
usually good (r = 0.7-0.95), but differs greatly, depending on the number of used
samples and characteristics of the rivers.
Bed load sampling has, on the contrary, been a negligible part of the sediment
monitoring of Icelandic rivers (Pâlsson, 2000). A major step forward was taken in
2000 when a pilot bed load sampling programme was initiated in Jôkulsâ a Dal, a river
in northeast Iceland studied for possible hydroelectric development (Fig. 1) (VST &
Orkustofnun, 2003). Bed load sampling had not been employed in any detail at the
Hydrological Service before, as the great current speed in the Icelandic glacial rivers
156
Jôrunn
iras
Hardardôttir
& Ami
Snorrason
ileal Serylœ
....
.-.-••->
'
\\\--
V
«
v
a
J o k u l s à a Fjollum; Gnmsstaôïr
\\
A
*"
• :
"
A-A
*
' -'
A
.'-->":
*
A A.
A
A
A
. •-
A
A
Skaftà,
•
Svein|tindur' -,.
VA
' .
A
, l
•
./• ' V a i t n a j o.k u 11
J>\ôr$à
N
.'•A
4L
Hofs"
jekull
A
.<
A
.Jokulsa a Dal, Hjaroarhagi
A
A
A .
'
A
'
.--
A
A
Water gauge
Gauged drainage basins
! S t u d i e d b a s i n s in this p a p e r
0
50
100
200
km
ZI; G l a c i a t e d a r e a
Map of rivers and lakes by permission © Iceland Geodetic Survey
Helga P Finnsdôttir, June 2002
Fig. 1 Suspended sediment sampling sites in 2001. Shaded area represents the gauged
drainage basins at these sites.
Table 1 Characteristics of rivers subjected to bed load studies in 2001. Q
is the long-term discharge
average, O „ and Q
are the lowest and highest mean monthly discharge values.
m
m
i
m
River, sampling site
a
a
m
x
No. of trips
in 2001
Basin total
(km )*
2
Thjôrsâ, Krôkur**
7
7379
Jokulsa â Fjollum, Grimsstadir 2
5178
3
3322
Jokulsa â Dal, Hjardarhagi
Skaftâ, Sveinstindur
2
714
* basin area at water gauge.
** extensive hydroelectric development on the river.
Basin glacier
(km )
(mV)
(mV)
1014
448
1421
494
352
170
145
42.8
1459
2757
1180
1363
2
Qnv/x
Q „
f
,
44.1
43.9
7.6
14.5
was thought to prevent such measurements. Results from the pilot study in the River
Jokulsa â Dal proved satisfying and subsequently extensive total sediment programmes
were established in three additional glacial rivers. Those are the River Jokulsa â
Fjollum in north Iceland, and the rivers Thjôrsâ and Skaftâ in south Iceland (Fig. 1)
(Hardardôttir & J>orlaksdôttir, 2002a, 2002b; Hardardôttir & Gunnarsson, 2002),
which are all presently being studied as future or possible hydroelectric projects. The
characteristics of these rivers at the sampling sites differ greatly, e.g. in discharge
variables and size of their water basins (Table 1). Thjôrsâ is the only harnessed river
with several upstream reservoirs, whereas major jôkulhlaups affect Skaftâ on average
every year due to geothermal activity beneath the glacier it originates from. At the
sampling site Grimsstadir, Jokulsa â Fjollum includes a significant groundwater
157
Sediment monitoring ofglacial rivers in Iceland: new data on bed load transport
component in addition to the glacial water. Occasional glacier surges have been
monitored in the outlet glaciers of Vatnajôkull and Hofsjôkull glaciers (Fig. 1), from
which, the four rivers are derived.
METHODS OF BED LOAD SEDIMENT SAMPLING AND ANALYSES
The bed load samples have been collected with a Helley-Smith cable-suspended
sampler at 5-10 locations on each river transect. The number of sediment campaigns
carried out at each river in 2001 ranged from two in Skaftâ and Jôkulsâ â Fjollum, to
three in Jôkulsâ â Dal and nine in Thjôrsâ (Table 1). Most of the samples were collec­
ted with a 47.7-kg Helley-Smith sampler with a 0.0762 x 0.0762 m opening and a 3.22
expansion ratio, but a heavier 75.8 kg sampler with a 0.152 x 0.152 m opening but the
same expansion ratio was used in one sediment campaign for comparison.
All the bed load samples were weighed on location, but selected samples from
each river were dry-sieved to establish grain-size distribution of the bed load material.
After collecting 10 or more samples at each location on the river transect during each
trip, the total mean bed load transport was calculated in several steps. First the bed load
transport of each sample at each station was calculated by dividing the weight of each
sample (in grams) by the time interval the sampler sat on the river bed. The mean
transport at each station (J) was then calculated by:
1 AAf,
= —
**/ 2-77
where q j is in g s" m" , M-, is the mass of sample i (in grams), t-, is the sampling time
(in seconds) for sample i, d represents the width of sampler opening (0.0762 m), and w,
is the total number of samples at station j . Total transport through cross section Q :
1
1
b
b
^*
2
2
2
"
2
where Q is in g s" and x represents the distance between sampling points, between a
marginal point and the edge of the water surface, or that of the moving strip of stream
bed (World Meteorological Organization, 1994). If the discharge varied greatly over
the sampling period, the procedure above was carried out in steps for each discharge
interval.
For comparison of mean total bed load transport between rivers the values were
normalized by the active width of the channel.
1
b
CASE S T U D Y — T H J Ô R S Â RIVER, S O U T H E R N I C E L A N D
The most extensive study was done at Krôkur on Thjôrsâ River, southern Iceland
(Fig. 1), where bed load samples were taken at variable discharge values during nine
sediment campaigns in 2001, and during a 10-year flood event in January 2002
(Table 2). Close to 800 bed load samples were collected with the Helley-Smith 47.7-kg
cable-suspended sampler at 7-10 stations during each campaign.
158
Jôrunn
Hardardôttir
& Ami
Snorrason
Table 2 Results from bed load sampling at Thjôrsâ in 2001 and the January flood in 2002.
J
Campaign date
Mean discharge (m s" ')
Total integrated bed load transport (kg s"')
11-14 July 2001
23-25 July 2001
31-1 August 2001
8-9 August 2001
13-14 August 2001
23-24 August 2001
29-30 August 2001
18-19 September 2001
19-20 December 2001
10 January 2002
11 January 2002
351
320
326
327
326
442
342
352
352
1326
1066
4.7
5.0
8.7
9.5
9.3
13.1
9.8
8.7
5.6
134.3
37.0
Distance from reference point (m)
—
«
—
2 3 - 2 5 July 2 0 0 1
— B — 1 1 - 1 4
— A — 3 1 - 1
July 2 0 0 1
August 2001
- - t - - 8-9 August 2001
-•
+
— u -
• -
29-30 August
13-14 August 2001
- 23-24 August 2001
— &
••-
- -EB—
2001
18-19 September
19-20 December
2001
2001
Fig. 2 Mean bed load transport at each station in Thjôrsâ during sediment campaigns
in 2001. Also shown is a depth profile across the river channel from 2002. In this
depth survey the left bank was located at 14 m and the right bank at 200 m.
Great differences are seen in bed load transport among the stations within the river
channel (Fig. 2). Most of the bed load is transported in a narrow gully in the channel
between 50 and 70 m distance from the reference, about 18 m inland from the left river
bank. In contrast, only a minor fraction of the bed load is transported at other stations
closer to the river banks. Furthermore, grain size measurements of the bed load
samples show that the coarsest sediment is transported at the same locations (50-70 m
stations). Total bed load transport ranges from c. 4.7 to 13 kg s" during sediment
campaigns in 2001, but is greater than 134 kg s" during the peak of a 10-year-flood
event in January 2002 (Table 2, Fig. 3). The flood was triggered by a winter rainstorm
with heavy precipitation melting fresh snow on frozen ground.
1
1
159
Sediment monitoring ofglacial rivers in Iceland: new data on bed load transport
a
Jôkulsâ â Fjollum
• B — Thjôrsâ
+
Skaftâ
•
J o k u l s a à Dal
* y = 0.022 *
2
R = 0.95
200
m
Q (m s )
Fig. 3 Mean total bed load rate for individual bed load campaigns on rivers Thjôrsâ,
Jokulsa â Fjollum, Jokulsa â Dal and Skaftâ in 2001 and January flood in 2002. A
exponential curve is fitted to the Thjôrsâ data; however, additional samples have to be
taken before a reliable rating curve can be established.
The correlation between mean total bed load rate and the river discharge for the
sampling campaigns at Krôkur in 2001 is poor as we have a large distribution of total
transport at the same discharge interval (Fig. 3). The poor correlation is probably to
some extent related to the narrow distribution of discharge values during the year
2001, as well as intermittent reservoir operation upstream, and shows that we
especially lack data from low and high discharge periods. Hence, the three values
obtained in the January flood 2002 extended the curve considerably and suggest a
semi-exponential correlation between the discharge values and bed load transport. An
example of such an exponential curve is shown on Fig. 3; however, more transport
values at high and low discharges have to be determined before the correlation
between these parameters can be evaluated with greater certainty.
C O M P A R I S O N W I T H O T H E R BED L O A D STUDIES
The box plots in Fig. 4 compare the individual "at-a-point" bed load transport rate in
four rivers in Iceland; i.e. Thjôrsâ, Jokulsa â Fjollum at Grimsstadir, Jokulsa â Dal and
Skaftâ at Sveinstindur (Fig. 2; Table 1). The data include all measurements made in
each river in 2001 as well as data collected in Thjôrsâ during the January flood in
2002. As the data are independent of discharge, location on the river, as well as timing
of the sampling campaigns, a great scatter is observed in the data sets. Hence, the
values are here shown as logio values to allow comparable representation. Figure 4
shows that although the bed load transport of "at-a-point" samples is on average lowest
in Thjôrsâ, it is also most variable in that river, which is understandable due to the
160
Jorunn
Hardardôttir
& Ami
Snorrason
1
?J
-1
a)
o
-4
-5
-
1
-
L
1
1
'
log Thjôrsâ
1
1
1
1
'
log Jôkulsâ à Fjôllum
•
.
.
log Skaftâ
i
.
•
.
i
log Jôkulsâ â Dal
Fig. 4 Box plot showing the range of logm "at-a-point" bed load transport rates as
logio in the rivers Thjôrsâ, Jôkulsâ â Fjôllum, Jôkulsâ â Dal, and Skaftâ during
sediment campaigns in 2001 and January flood in 2002. The line within each box
shows the median value and the outer limits of the box contain 50% of the data.
Circles represent outliers.
great discharge difference during the campaigns and the great variability across the
river channel (Fig. 2). In contrast, the highest values are obtained in Jôkulsâ â Dal
(Fig. 1), although a certain scatter is observed in the data (Fig. 4) due to very variable
conditions during the three sediment campaigns. The wide distribution of bed load
transport values shows the stochastic nature of bed load transport well, as has been
recognized in numerous studies (e.g. Iseya & Ikeda, 1987; Pitlick 1988; Gomez et al.,
1989; Gomez, 1991; Hoey, 1992; Nicholas et ai, 1995). Consequently, one of the best
ways to minimize errors in bed load calculations is to sample frequently and calculate
the average transport. For this we need to collect numerous samples over a broad
discharge spectrum.
The different characteristics of the rivers Thjôrsâ, Jôkulsâ â Dal, Jôkulsâ â
Fjôllum, and Skaftâ is readily seen on Fig. 3, which compares the mean total bed load
transport rate in the Thjôrsâ campaigns with equivalent data for the other three rivers.
It is obvious that the rate is much less in Thjôrsâ than in the other three rivers at similar
discharge (0.03-0.07 kg s~' m" ) (Fig. 4), which is understandable because of the
upstream reservoirs in Thjôrsâ, where most of the coarse sediment accumulates. Only
during the January 2002 flood did the mean transport discharge reach values close to
those in the other rivers (0.08-0.96 kg s"' m"').
The extreme difference of mean bed load transport discharge in Jôkulsâ â Dal
during the three sampling campaigns in 2001 (0.03-1.58 kg s" u f ' ) shows the seasonal
variability of bed load transport well; with highly elevated transport in early summer
when glacial melting initiates and flushes out sediment in the channel, and minor bed
load transport in October when stream discharge had greatly decreased. The channel of
1
1
Sediment
monitoring
of glacial
rivers in Iceland:
new data on bed load
161
transport
Jôkulsâ â Dal from the glacier snout to the sampling site is steep, which in addition to
high discharge during the summer melting season (up to 1000 m s" ) hinders any
major deposition of sediment within the channel and causes the high and coarse bed
load transport compared to the other rivers.
The relatively low and comparable mean total bed load transport rate (0.22-0.25
kg s"' m" ) in the sampling campaigns in Jôkulsâ â Fjollum at Grimsstadir (Figs 1 and
3) is probably related to the great distance from the glacier to the sampling site, which
delimits bed load as it is deposited in sediment traps along the river course, as well as
the large component of sediment-deprived groundwater in the river, which moderates
the bed load transport at increasing discharge. On the contrary, the relatively high
mean total bed load transport rates in Skaftâ at Sveinstindur (0.24-0.67 kg s" m"')
reflects the shortest distance from glacier to sampling station, as well as the recurrent
jôkulhlaups in the upstream glacier. During the jôkulhlaups a great quantity of
sediment is transported downstream, which is being incorporated into the river channel
for months subsequent to the event.
3
1
1
1
It is also of major interest to compare the bed load transport in these Icelandic
rivers to bed load studies earned out in other parts of the world. It is, however,
essential to realize that a direct comparison may not always be practical as values for
bed load transport have been achieved using a variety of methods such as transport
formulae, morphological research, various indirect methods (e.g. tracer pebbles,
magnetic clasts, pebble transmitters, ultra sounding), as well as the direct measurement
of bed load with samplers or diverse trapping devices (see Gomez, 1991, for brief
summary). Most of these methods have limitations of some sort and do not always
produce data with the same spatial and temporal resolution which limits comparison.
Stott (2000) compared six alpine and six arctic/subarctic bed load studies and
suggested that bed load transport rate in alpine environments was significantly higher
(0.03^1 kg s" m" ) than in the arctic/subarctic locations of northern Norway,
Greenland, Svalbard, Iceland, and Antarctica (0.0002-3.5 kg s" n f ' ) . When data from
this study is compared with the summary by Stott it is seen that all the mean total tran­
sport rates greater than 0.03 kg s"' m" are higher than other arctic/subarctic bed load
transport rates, excluding extreme rain values in Lyngdalselva, Norway (3.5 kg s" m" )
and transport rates in the Onyx River, Antarctica (0.38-2.16 kg s" m~'). This includes
results from the glacial river Virkisâ in southeast Iceland (Nicholas & Sambrook
Smith, 1998).
1
1
1
1
1
1
1
This comparison indicates that the Icelandic total bed load transport rates are more
comparable with the alpine sites of White River, Washington, U S A (Fergusson et al,
1989), Sunwapta River in Canada (Goff & Ashmore, 1994), and Bas Glacier d'Arolla,
Switzerland (Warburton, 1992). Given the much higher discharge in the Icelandic
rivers than most of the other rivers, as well as the highly effective glacial erosion the
high values are not surprising.
The conclusions shown here are based on the initial results from bed load studies
of these four Icelandic rivers. The same rivers were sampled in 2002 and two more
locations were added for 2003. Although we are well aware of the limitations and
possible eiTors involved in such studies, we anticipate that these results show the great
range of bed load transport in Icelandic rivers and some of the possible causes for the
variability.
162
Jôritnn
Hardardôttir
& Ami
Snorrason
Acknowledgements For the largest part of the last 50 years, the sediment sampling
programme at the Hydrological Service has been funded through governmental
funding. However, during recent years the National Power Company (Landsvirkjun)
and The Energy Resources Division of the National Energy Authority (Auôlindadeild
OS) have funded the majority of the sediment monitoring programme at HS.
Landsvirkjun funded the bed load measurements at Thjôrsâ, Skaftâ, and Jôkulsâ â Dal,
whereas Auôlindadeild OS funded bed load measurements at Jôkulsâ â Fjôllum. The
manuscript improved greatly from valuable comments from an anonymous reviewer.
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