Introduction
Sediment
measurement involves sampling the water-sediment mixture to
determine the mean suspended sediment concentration, particle size
distribution, specific gravity, temperature of the water sediment
mixture, and other physical and chemical properties of the
transported solids. Suspended sediment concentration in a natural
stream varies from the water surface to the streambed and laterally
across the stream. Concentration generally increases from a minimum
at the water surface to a maximum at or near the streambed. The
samplers available here have been designed by the U.S. Federal
Interagency Sedimentation Project (FISP).
Depth Integrating Samplers
Depth integrating samplers are designed to continuously extract a
sample as they are lowered from the water surface to the streambed
and returned at a constant rate of travel. Ascending and descending
speeds need not be the same, but the rate of travel must be constant
in each direction. As the sample is collected, air in the container
is compressed so that the pressure balances the hydrostatic pressure
at the air exhaust and the inflow velocity is approximately equal to
the stream velocity.
Point Integrating Samplers
Point integrating samplers are equipped with an electrically
controlled rotary valve which opens and closes the sampler on
command. They are designed to take a sample at any point in a stream
over a short time interval. With the control valve fixed in the open
position, these samplers are also used to obtain depth integrated
samples. One-way depth integrated samples may be obtained by opening
the valve with the sampler at the water surface and lowering it to
the streambed at a constant speed. This permits sampling to greater
depths.
Suspended-sediment samplers should be used only with the
specified nozzle to give a truly representative sample. All
US-series samplers are designed to sample isokinetically which means
that water is entering the nozzle at the same speed as the water
would be traveling if the sampler wasn't there.
Bedload Samplers
Suspended sediment samplers will only sample to a point about 0.3
foot (9.1 cm) above the streambed. The sediment transported in the
unsampled zone is composed of both suspended load and bedload. The
bedload portion is composed of particles having a density or grain
size which precludes movement far above or for long distances out of
contact with the streambed.
In wide sand-bed streams with shallow flow depths and high sand
concentrations, more sediment may be transported in the unsampled
zone than in the sampled zone. As flow depth increases, the
proportion of sediment in the unsampled zone becomes smaller, often
accounting for only a small fraction of the total sediment load.
The bedload portion of sediment discharge is primarily sampled
using two styles of bedload samplers, the FISP designed series uses
a 1.4 expansion ratio while the widely used Helley Smith series has
a 3.22 expansion ratio design.
Bed Material Samplers
Bed material samplers are designed to collect samples from the
bed of a stream, or from lake or reservoir deposits. These samplers
take an undisturbed consolidated bed sample and are designed to
prevent sample washout or disturbance during retrieval. This is an
important aspect of bed material sample integrity.
