Sediment Yield and Channel Processes

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.

• 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.

Suspend Sediment and Bed Load

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)

Lane’s Equation: (SL * SS) α (Slope * Q)

Transport Capacity vs. Sediment Source