Measuring instruments for bed material sampling
7. MEASURING INSTRUMENTS FOR BED MATERIAL SAMPLING
7.1 General Aspects
Broadly, there are four methods of bed material sampling: grab samplers, dredge samplers, scoop samplers, and core samplers.
7.2 Bed material samplers: grab, dredge and scoop samplers
Grab, dredge and scoop-type samplers are used to collect a bed-surface sample.
A grab sampler consists of two buckets or jaws which are in an open position during lowering of the sampler. After contact with the bed the buckets are closed by using a messenger system or by pulling the hoisting cable.
For coarse and/or firm bed material a dredge-type sampler should be used. Simple and good samplers are the SHIPEK grab sampler and the VAN VEEN grab and dredge samplers.
A scoop-type sampler consists of a single scoop-type bucket which swings out of the bottom of the sampler body. The bucket surrounds and encloses the bed material sample. An advantage of the US BM-54 scoop sampler is its streamlined body enabling sample collection in high-velocity conditions.
7.3 Bed material samplers: core samplers
Core sampling consists of driving a tube into the bed material through the use of manpower, gravity, hydrostatic pressure or vibration.
A simple hand corer can be used in shallow streams, which can be waded or on tidal flats. The lower end of the sampler contains a cylinder which is pressed into the bed. A piston with a handle on its upper end passes through the sampler frame. The piston is retracted when the cylinder is pressed into the bed material. The suction created by the piston holds the sample in the cylinder.
Box core samples can be taken by using a box corer of about 300 kg lowered to the bed by use of a cable- winch system. A bed surface core sampler (sand to clay) is taken by mechanical penetration (box is pressed into the bed mechanically). This device takes a core length of about 0.5 m.
The gravity (or free-fall) corer is allowed to fall freely through the water and is driven into the bed by its weight. Vibration corers are used when core samples with a length upto 10 metres are required in all types of bed material with exception of rock and stiff clay. The corer is driven into the bed by vibration equipment mounted on top of the corer.
7.4 Particle size of bed materials
7.4.1 Based on metallic trace elements (MEDUSA)
The MEDUSA system can be viewed as a small soil/sediment-sensor that determines soil composition in-situ (under water as well as in air). The system is capable of continuously measuring very low concentrations of a number of metallic trace elements (cesium, cobalt, potassium, uranium and thorium) to a depth of about 30 cm inside soil. Moreover, the system measures water depth and includes sensors to determine the intensity of friction sound, generated when the detector is dragged over the sediment bed. The gamma-radiation detector system (based on Berillium Germanium Oxyde crystals) is towed over the seabed behind a ship in lines with a spacing of about 500 m. Software performs on-line data logging and on-line creation of data maps. After completion of the survey, the measured data are converted to composition (percentage of clay, silt and sand) of the sediment at each measured position.
7.4.2 Based on acoustic reflection (ROXANN)
ROXANN is a remote sensing hydro-acoustic sensor providing seabed classification data to produce seabed bottom type maps. ROXANN uses a patented technique to extract data on bottom roughness and hardness from the first and second echosounder returns from the seabed. It interfaces with a Global Positioning System (GPS) and PC enabling real-time seabed classification and mapping of geological and biological features using RoxMap Software.
7.5 Movement of bed material particles
7.5.1 Critical bed-shear stress for initiation of motion
The beginning of movement of bed material particles (especially mixtures of clay, silt and sand) can be determined by using in-situ erosion flumes and erosion containers (small-scale perspex tube with a propeller).
7.5.2 Tracer studies
Increasingly, and necessarily, there is a need to describe sediment (and contaminant) transport pathways on dynamically variable and spatially distributed scales rather than at single point localities. 'Particle tracking', or as it is also known 'particle' or 'sediment tracing', providing certain assumptions are satisfied, offers a practical methodology for the assessment of transport pathways of a variety of sediments across wider temporal and spatial scales, and is available for silts, sands, granules, pebbles and cobbles.
Contents
References
See also
Other contributions of Leo van Rijn
- INTRODUCTION, PROBLEMS AND APPROACHES IN SEDIMENT TRANSPORT MEASUREMENTS
- DEFINITIONS, PROCESSES AND MODELS IN MORPHOLOGY
- PRINCIPLES, STATISTICS AND ERRORS OF MEASURING SEDIMENT TRANSPORT
- COMPUTATION OF SEDIMENT TRANSPORT AND PRESENTATION OF RESULTS
- MEASURING INSTRUMENTS FOR SEDIMENT TRANSPORT
- MEASURING INSTRUMENTS FOR PARTICLE SIZE AND FALL VELOCITY
- MEASURING INSTRUMENTS FOR BED MATERIAL SAMPLING
- LABORATORY AND IN-SITU ANALYSIS OF SAMPLES
- IN-SITU MEASUREMENT OF WET BULK DENSITY
- INSTRUMENTS FOR BED LEVEL DETECTION
- ARGUS VIDEO
- MEASURING INSTRUMENTS FOR FLUID VELOCITY, PRESSURE AND WAVE HEIGHT
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