stream sediment sampling hydrography skills set training course no. 27743 june 2012
TRANSCRIPT
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STREAM SEDIMENT SAMPLING
Hydrography Skills Set TrainingCourse No. 27743
June 2012
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WHAT IS SEDIMENT?
Definition: “Sediment is any solid matter eroded, transported or deposited by flowing water”
‘Silt’ denotes a size range of material 0.004 to 0.062mm
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Sedimentation Processes
Erosion: The wearing away of the earth’s surface by surface water runoff
Transportation: Movement of eroded particles through stream channels to the point of deposition
Deposition: Deposition of eroded particles in stream channels, flood plains, reservoirs, harbours, canals etc
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Why is Sediment a Problem? Reservoirs:• Loss of potential water storage due to sediment
accumulation• Further reduction by evaporation due to change in lake
surface area from sediment accumulationIrrigation Canals:• Generally constructed in erodable material, so scour
and sediment deposition can occur
Degradation Below Dams:• Potential dam failure due to undermining causing
lowering of bed level downstream of wall
Power Station Turbines:• Sediment can damage blades and bearings
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Why Sediment Samples are Required?
Quantity and particle size of sediment is required
Particle Size:• As sediments of different size act differently, data
for particle size is important
Quantity:• Determines necessity for and type of
sediment removal devices
Samples should be taken regularly and at different gauge heights (full range)
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Theory of Sediment Transport
Suspended Load:Material that moves along the stream in suspension
Contact Load: Material that rolls or slides along bed of stream
Bed Load = Coarse material moving near the bed (i.e. Contact Load and Saltation Load and other particles that cannot be sampled by a
suspended load sampler)
Saltation Load:Material ‘bounding’ along the stream bed, in contact with bouncing particles which may force other particles upwards for a short period of time
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Effects of Sedimentation
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Sampling Equipment Depth Integrating Type:• By traversing the stream at a uniform speed the sampler is
moved through each vertical to receive a sample proportional to stream velocity
• Sampler is designed to fill at a rate proportional to the velocity of the approaching flow
• One sample per vertical is taken by lowering and raising the sampler to and from the stream bed to achieve a representation of mean concentration and particle size in each vertical
• Attached to a gauging rod or handline
DH-59 DH-48
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Depth Integrated Sampling
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Sampling Equipment Point Integrating Type:• Sampler remains stationary at each point on the
vertical with electrical activation of sampler at specific point on vertical
• Samples are integrated or ‘added-up’ on each vertical over a period of time
• Allows mean concentration of sediment to be determined
• Usually suspended from a gauging winch
P 61
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Sampling Equipment Bed Load Samplers:
• Designed to sample sand, silt, gravel or rock in close proximity to stream bed
• Samples collected in a polyester/monofilament bag or scoop
• Suspended from either a gauging rod or winch
BLS30 / BLS48BLSHVan Veen Grab
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Bed Load Sampling
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Sampling Equipment Static Samplers:
• Free standing or mounted on trees, bridge piers, gauge posts to obtain discreet samples at set gauge heights
• Sample bottles are replaced after flood events
Rising Stage Sampler
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Sampling Equipment
Grab Sample:
• Water scooped up in bottle manually
• Water ‘pumped’ into plastic bladders
• Sample preservation and chain of custody
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Sampling Equipment
Automatic Sampler:
• Water pumped from stream and bottles filled
• Available as refrigerative or non-refrigerative
• Configured to sample at critical gauge heights or flow proportional
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References
• Snowy Mountains Hydro-Electric Authority – Hydrographic Course (September 1966), Volume 3
• United States Geological Survey ‘ Field Methods for Measurement of Fluvial Sediment’, Book 3, 1982.
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Questions ?