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How is sediment sampled?
Sediment quantity and quality are sampled in a variety of ways:
- Specially designed suspended-sediment samplers are used to collect water/sediment samples that are analyzed for sediment quantity and sometimes quality.
- Bed-load samples are usually taken by lowering a specially made sampler to the stream bed. Resting there, the sampler traps the material moving along the bottom.
- Bed-material samples may be taken simply by hand from exposed bars or stream banks, or by samplers from the stream bed. Some samplers scoop sediment by simply digging into the bed, while other kinds extract a core from the bed.
Once collected, suspended-sediment samples are analyzed for concentration and particle size. This is usually done in a laboratory. The concentration is the ratio of sediment (dry weight) to the total water-sediment mixture, expressed as milligrams per litre (mg/L). The particle size is simply the size of the sediment particles. Depending on their size, they are classified as sand, silt, or clay.
To find out how much material is transported by a river, one can combine the concentration with the stream discharge, or flow. This gives the sediment load, which indicates the total amount of sediment transported over a certain time period, whether an hour, a day, month, or year. In such a way, it has been estimated that at Montreal the St. Lawrence River transports 2.3 million tonnes of sediment in suspension each year, or the equivalent of 230 000 truck-loads of soil.
Water, and more specifically the hydrologic cycle, plays a major role in driving the sediment life cycle. The amount of water and its distribution over time influence how and when sediment is sampled.
In Canada, sampling is usually for suspended-sediment data. Most of this sampling is done during high-flow conditions (spring, summer, and fall rainstorms), when most of the sediment is transported through the river system. However, a few samples may be taken at other times throughout the year, to better define the sediment regime. Bed-load sampling is typically undertaken in the spring, when high discharge mobilizes the stream bed. Bed-material may be sampled during the summer, when low-flow conditions may expose parts of the stream bed, making sampling easier.
Once the samples have been analyzed, the data on concentrations, particle sizes, or loads can be applied to engineering and environmental questions.
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