STREAM SEDIMENT SAMPLING

Hydrography Skills Set TrainingCourse No. 27743

June 2012

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

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

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

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)

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

Effects of Sedimentation

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

Depth Integrated Sampling

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

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

Bed Load Sampling

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

Sampling Equipment

Grab Sample:

• Water scooped up in bottle manually

• Water ‘pumped’ into plastic bladders

• Sample preservation and chain of custody

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

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.

Questions ?