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[image details]
Fig. 2.13: The brown colour of this glacial meltwater stream in East Greenland shows that it is carrying
a high sediment load.
Any energy remaining after a river has overcome friction can be used to transport sediment. As
discharge, velocity and turbulence increase so does the
river's capacity to transport it load. This load is transported in solution (called solution
load), as very fine sediment (called wash load), as fine sediment
(called suspended load) and as coarse sediment (called bed load). There are more images
below showing of solute and sediment transport processes and
how fluvial geomorphologists measure them. You should be familiar with the Hjulstrom curve (eg. Stott,
2000b, p.118)
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Fig. 2.14: This flow proportional sampler admits water through a pressure sensitive valve
which admits more water as the flow increases. It may be fixed in the stream for several
weeks. The integrated sample is later analysed for solutes.
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Fig. 2.15: This is a USDH-48 point integrating suspended sediment sampler. It is slowly
lowered, intake nozzle pointing into the flow, from the surface to the bed and back. It
thus samples the water/sediment mixture from a range of depths and so collects and
'integrated' sample for analysis.
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Fig. 2.16: This stream, photographed during a rainstorm, drained a recently ploughed
area of land. A water sample taken at the time was later analysed and found to have a
suspended sediment concentration of 2 300 mg/l.
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Fig. 2.17: The stream in Fig. 2.16 (above) flowed in Loch Lubnaig, central Scotland, and
the plume of suspended sediment can be clearly seen here.
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Fig. 2.18: This photograph of a meltwater stream draining the Skelbrae glacier in East
Greenland was taken in the afternoon on 2nd August when discharge and sediment load
was high (see Fig. 2.19 below).
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Fig. 2.19: The diurnal fluctuation in air temperature (green, lower plot), discharge (blue,
upper plot) and suspended sediment concentration (red) for the stream in Fig. 2.18.
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Fig. 2.20: Portable filtration equipment for separating suspended sediment
from a water sample. Pre-weighed filter papers are placed in the funnel,
the water sample (known
volume) is poured onto it and the hand vacuum pump speeds up the flow of
water through the filter. Sediment coarser than the pore diameter of
the filter is retained. The
paper (+sediment) is then oven dried and re-weighed. Suspended sediment concentration
(SSG) of the sample is then computed: SSC (mg/l) = w (g) / v (l) where w
= dry weight
of sediment (g) and v = volume of sample (l).
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Fig. 2.21: This is an automatic vacuum sampler. 24 glass bottles (in the
white crate) are each connected to a tube which enters the river and
is fixed to a block on the bed. A
bicycle-type pump is then used to evacuate the bottles which are then sealed.
A pre-set timer (in the green housing) releases the seals at pre-determined
time intervals. The
water from the river is 'sucked' into the sample bottle due to the vacuum.
The filters from a sequence of samples through a flood event collected
by this instrument are shown in
Fig. 2.22.
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Fig. 2.22: Filter papers for suspended sediment samples collected at 30 minute intervals
during a flood event. time increases from left to right. Note how papers 3-5 have more
sediment, paper 6 is lighter while papers 7-10 have much less sediment, suggesting that
the peak in suspended sediment load had passed by sample 7-8.
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Fig. 2.23: This is a Partech photo-electric turbidity meter. The probe (lower right)
contains a light source and a photo-electric cell. It can be mounted in a stream channel
and readings of relative turbidity can be taken from the panel meter or recorded on a
separate data logger. It must always be calibrated with hand samples collected as in
Fig. 2.15 or Fig. 2.21.
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Fig. 2.24: The operator here is using a modified Helley-Smith bed load sampler.
The metal orifice (0.1 x 0.1 m) is held on the river bed facing the
flow. Bed load entering the
orifice it trapped in the net which is later removed. The contents are dried,
weighed and may be sieved to assess the relative sizes of bed load
being transported. Photograph
location: Gipsdalselva, Svalbard.
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Fig. 2.25: The operator here is deploying a larger Helley-Smith bed load sampler from a
bridge. It is much heavier than the one in Fig. 2.24 and has a direction fin to help keep
the orifice pointing upstream.
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Fig. 2.26: The vertical columns in the plot show the weights of bed load caught in the
modified Helley-Smith bed load sampler (shown in Fig. 2.24 above) sampled at 30-minute intervals
over two flood cycles in the Skelbrae River, East Greenland.
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Fig. 2.27: Rather than using samplers as shown in Figs. 2.24 and 2.25, a more reliable
way of measuring bed load transport is to trap the bed load by building a trap, preferable
across the entire width of the stream as shown here.
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Fig. 2.28: Bed load trapped being removed by a JCB.
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Fig. 2.29: The bed load trap on the Nant Tanllwyth, Plynlimon (shown in Fig. 2.27 above)
is emptied periodically by a JCB. The cone of sediment seen here is then sampled to
assess the density and size distribution of bed load being transported.
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Fig. 2.30: This simple check dam consists of a wooden barrier lined with 2 mm netting.
This allows water and fine sediment to pass through but traps all sediment coarser than
2 mm. Bed load trapped is removed by hand shovel.
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Fig. 2.31: An alternative to trapping bed load is to trace its movement. Clasts here have
been painted yellow and have a small magnet fixed into a hole drilled in each clast.
Their transport rate is determined by re-locating the clast after flood events (see Fig.
2.32).
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Fig. 2.32: Magnetic susceptibility probe used for re-locating magnetically tagged bed
load clasts.
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Essential Reading
** Gregory KJ, Walling DE. 1973. Drainage Basin Form and Process, London, Edward Arnold. Chapter
3 The Measurement of Drainage Basin
Processes,150-164.
** Knighton D. 1998. Fluvial Forms and Processes, London, Arnold. Chapter 4 Fluvial Processes
118-141.
*** Morisawa M. 1985. Rivers: Form and Process, London, Longman. Chapter 4 Entrainment and Transport,
37-53.
** Petts GE, Foster IDL. 1985. Rivers and Landscape, London, Edward Arnold. Chapter 4 Sediment Transport,
95-119.
*** Richards K. 1982. Rivers: Form and process in alluvial channels, London, Methuen. Chapter 4 Sediment
Transport Processes, 90-121.
* Stott TA. 2000a. Bedload transport in rivers: trapping and tracing, Geography Review 13
(5), 23-27.
* Stott TA. 2000b. The River and Waterway Environment for Small Boat Users:
An Environmental Guide for Recreational Users of Rivers and Inland
Waterways, Nottingham, British Canoe Union. Chapter 2.2, 117-119.
** Thornes J. 1979. River Channels, Aspects of Geography Series, London, Macmillan. Sediments in Rivers,
9-15.
Further Reading
*** Moore RJ, Newson MD. 1986. Production, storage and output of coarse upland sediments: natural and
artificial influences as revealed by research
catchment studies, Journal of the Geological Society 143, 1-6.
*** Reid I, Bathurst JC, Carling P A., Walling DE, Webb BW. 1997. Sediment Erosion, Transport and Deposition, in C. R. Thorne, R. D. Hey and M. D. Newson (Eds.)
Applied Fluvial Geomorphology for River Engineering and Management, 195-135.
*** Stott TA, Sawyer A. 2000. Clast travel distances and abrasion rates in coarse upland channels determined
using magnetically tagged bedload tracers, in I. D. L .
Foster (Ed.) Tracers in Geomorphology, John Wiley and Sons, Chichester, 389-399.
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