1. Title slide: Sediment monitoring techniques
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2. Definition slide showing the relative scale of plot, sub-catchment
and catchment studies required to obtain field information.
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3. A cascade of flow splitters and sediment traps at the outlet
from one of a series of erosion plots on a tea estate. Note
the cylindrical cover of a recording water-level recorder
(Sri Lanka). |
4. This slide shows a 6.31ha sub-catchment with a well defined
stream and single outlet point. The site was selected to represent
typical, un-grazed land use (Philippines). |
5. Here a small H-flume structure is shown discharging after
a storm. In the foreground can be seen security cages containing
the automatic sediment sampler boxes. This particular catchment
is 26.5ha in size and is monitoring the effects of a physical
conservation and land use system (Philippines). |
6. If the land gradient is very small, in this case 3%, specifically
designed flumes may be needed. An extended-drive motor/pump
system can clearly be seen. The blue rectangle next to the
pump is the water level switch which starts the sampling sequence
when triggered by rising water (Botswana). |
7. This version of the automatic sampler is powered by a 12
volt battery, and the pump takes 24 volts. The sampler controls
are housed in the grey box at the back. The rack containing
24 numbered, half-litre bottles is standing on the flume wall
(Malawi). |
8. To record the time when a sample is collected the control
box sends out a signal which can be used to trigger an event
marker on a water level recording device, as shown here, or
to allow an automatic logger to store the time (Malawi). |
9. For large catchments, the field team should monitor stream
discharge and a range of sediment sizes. The product of these
two values gives the sediment flux as shown on this slide
(Kenya). |
10. A simple crane can be used to monitor sediment from a
bridge. A pipe is lowered to known positions in the flow and
samples pumped back to the bridge. The nozzle is hindered
from drifting downstream by use of a large mass (sinkerweight).
Velocity measurements can also be taken as the samples are
collected (Philippines). |
11. Where a bridge is not available and the river is in a
well defined, stable channel a mast with a series of nozzles
on it can be used to pump samples from various positions in
a vertical (Kenya). |
12. Where trash and scour are problems a cableway can be used.
The limiting factors here are the length of the cableway and
supporting cable sag (Philippines). |
13. A nozzle can be lowered from the cableway allowing samples
to be collected from almost any position. The maximum head
a suction can work to is approximately 9 metres and in general,
the suction length should be limited to about 30 metres (Philippines).
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14. To keep the sampling nozzle at the required position in
the flow, particularly at high flows, a large sinkerweight
is used (here a 75kg unit is shown). To reduce the drag on
the supporting cable and the suction line, an aerofoil section
has been incorporated (Philippines). |
15. Where control structures exist the fixed mast/nozzle system
is feasible. Here such a unit is shown mounted in a sluice
channel. On the sluice channel a gaugeboard is also visible
(Nepal). |
16. Fixed masts can also be installed on broad weirs as this
picture shows. Note the second unit installed on the far side
and pumping to the far bank because of the limitations of
suction line length (Thailand). |
17. In channelled rivers and irrigation channels, fixed mast
systems can be used very successfully, particularly where
the channel section is lined (Thailand). |
18. Where many sites require monitoring and budgets do not
allow construction of masts or cableways at each, a crane
can be used from the stern of a boat. Here such a crane is
seen being used to obtain current meter data across a section
(Egypt). |
19. Where a channel is enclosed in a tunnel the outlets can
be successfully monitored by use of a crane as in this example
(Nepal). |
20. A typical pump/motor combination is seen here in use with
a crane suspension system. The motor runs off 234 volts d.c.,
supplied by two car batteries (Botswana). |
21. The water/sediment discharge is split by a 63 micron sieve.
The sand sizes are retained and the silt passes through. The
operator is timing a known volume to check on the pump discharge
rate and to calculate the sand concentration (Thailand). |
22. The quantity of silt is determined by standard laboratory
vacuum filtration techniques |
23. Much of the eroded material will be trapped behind dams.
It is readily seen at the upstream limit of impoundment, but
the whole reservoir must be surveyed if an accurate measure
of siltation rate is to be obtained (Kenya). |
24. The reservoir survey’s accuracy is dependent on the number
of range lines undertaken and the system used to calculate
the volume. The above water and shallow sections can be surveyed
by traditional methods whilst the deep water ranges will require
echo sounding equipment (Philippines). |
