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Sediment Yield and Channel Processes. Definitions. Suspend Sediment – sediment (orgranic or inorganic) which remains in suspension in water for a considerable period of time without content to the bottom.
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Definitions • Suspend Sediment – sediment (orgranic or inorganic) which remains in suspension in water for a considerable period of time without content to the bottom. • Bed Load – soil, rock particles, or other debris rolled along the bottom of a channel by the movement of water. • Saltation – is the process by which sediment is skipped along the channel bottom by the movement of water. • Total Sediment Load (Sediment Yield) – all the orgranic and inorganic material carried past a sampling station for a given period of time.
Sampling • Depth Integrated Sample – across a vertical profile. Best to take multiple samples across the stream. Sampler type is the function of discharge. The best method for describing total load.
Sampling • Surface Grab Sample – no effort is made to make an integrated sample. Used as an index of sediment condition. • Point sample – made at a given point in the water column, usually integrated over time. Many automated samples are point samplers. May be stage dependent. • Acoustic Measurements of Suspended Sediments • Turbidity measurements have be used as a substitute for sediment measurements due to cost.
Stage suspended sediment sampler You will need to be recording stage
Sampling Procedures • A suspend sediment sample provides you an estimate of concentration (m/l3). You need discharge to estimate load (m/t = m/l3 * l3/t). • Sediment transport is function of velocity which changes with depth and width. You need to take depth integrated samples across the stream to get a good estimate. Sediment concentration is highly variable. Always take two samples or more. You can also loss samples in the lab. • Always get an estimate of discharge so you can compute load and develop a sediment rating curve.
Lab Work • Dry and weight the filter paper before use. • Filter the sample and then dry the filter paper. • Weight the filter paper with sediment, subtract the paper weight. The result is mass from sediment (mg). • Measure the liters of water in sample. • Divide the mass by the sample volume. The result is the sediment concentration in mg/l or ppm.
Turbidity • Is the optical property of a fluid that causes light to be scattered and absorbed rather than to be transmitted. Used to describe murky or cloudy appearance in water. • Light is beams into a water volume the intensity of light transmitted through the sample is recorded. Less light more turbid. • Turbidity has been used as a water quality standard as the criterion for judging the level of suspend material permitted in streams. • Cheap and Easy • Errors in measurement. • Measurements vary between instruments. • Correlation between suspend sediment and turbidity not constant or without error. Other factors can increase turbidity (i.e. algae). • U.S. EPA (and most states) is moving away from turbidity as a standard, but it has created a void in data series to assessing potential impairment.
Sampling • Bed Load – Harder to do. • Hand samples • Cable • Traps (portable and instream) • The above samplers all change the local hydraulics and have issues with opening size. • Barrier (dams)
