STREAM SEDIMENT SAMPLING Hydrography Skills Set Training Course 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 3

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 accumulation Irrigation 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 4

Why Sediment Samples are Required? Quantity and particle size of sediment is required Quantity: Determines necessity for and type of sediment removal devices Particle Size: As sediments of different size act differently, data for particle size is important Samples should be taken regularly and at different gauge heights (full range) 5

Theory of Sediment Transport Contact Load: Material that rolls or slides along bed of stream Suspended Load: Material that moves along the stream in suspension 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 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) 6

Effects of Sedimentation 7

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 8

Depth Integrated Sampling 9

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 10

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 BLSH BLS30 / BLS48 Van Veen Grab 11

Bed Load Sampling 12

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 13

Sampling Equipment Grab Sample: Water ‘pumped’ into plastic bladders Water scooped up in bottle manually Water ‘pumped’ into plastic bladders Sample preservation and chain of custody 14

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 15

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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