Difference between revisions of "Measuring instruments for sediment transport"

Latest revision as of 21:17, 19 August 2020

This article is a summary of chapter 5 of the Manual Sediment Transport Measurements in Rivers, Estuaries and Coastal Seas^[1]. This article describes different measurement instruments available to measure sediment transport in rivers, coastal seas and estuaries. Many of these instruments are also described in separate articles (see text for links to these articles).

Introduction

Various instruments for measuring the sediment transport rate are described. Usually the sediment transport is represented as the summation of the bed load and suspended load transport.

To measure the bed load transport, two measuring methods are available: simple mechanical trap-type samplers (collecting the sediment particles transported close to the bed) and the recording of the bed profile as a function of time (bed form tracking).

To measure the suspended load transport, a wide range of instruments is available from simple mechanical samplers to sophisticated optical and acoustical (electronic) sensors. Most instruments are used as point-integrating instruments which means the measurement of the relevant parameters in a specific point above the bed as a function of time. Some instruments are used as depth-integrating samplers, which means continuous sampling over the water depth by lowering and raising the instrument at a constant transit rate.

All instruments are described in terms of their measuring principle, ractical operation, inaccuracy and technical specifications. To get a better understanding of the accuracy of the various instruments, special attention is given to comparative measurements.

Selection of sediment transport samplers

Guidelines for selection of sediment transport samplers

Guidelines for the selection of the most appropriate sampling technique for a certain environment are given, based on the following criteria:

type of process/parameters to be measured,
type of sampling environment,
type of sampling,
type of project and required accuracy,
available instruments and available budget.

For more information on guidelines, see Guidelines for selection of sediment transport samplers.

Sediment transport measurements in rivers

Figure 1: Pump sampler for rivers

Simple mechanical instruments such as the bottle-type, the trap-type and the pump-type samplers are still very attractive because of their robustness and easy handling, particularly when used at isolated field sites. The accuracy of the measured parameters involved can be increased by increasing the number of samples collected. Analysis costs of all samples involved may be critical with respect to the available budget. Optical and acoustic instruments are attractive when large numbers of data have to be collected. Since calibration is involved, the accuracy strongly depends on the quality/reliability of the calibration curves. Hence, many calibration samples are required using a pump sampler with the nozzle as close as possible to the optical/acoustic sensor.

A major technological advance for measuring suspended load transport is the in situ Laser diffraction instrument (LISST). This instrument can measure the particle size distribution and sediment concentration simultaneously. For more information on instruments for measurements in rivers, see Measuring instruments for rivers.

Sediment transport measurements in estuaries

Simple mechanical instruments such as the bottle-type and the trap-type samplers are not attractive because of the very short sampling times involved. Accuracy cannot be improved by increasing number of samples due to time-variation of sediment concentrations within the tidal cycle.

Point-samples should be taken over the entire water column in strong tidal flows as the sediments will be mixed over the water column by turbulent eddies. Data sampling can be confined to the bottom region in weak tidal flows. Flocculation often is a dominant process in muddy estuaries. The LISST-ST which is an in-situ Laser diffraction instrument in combination with a settling tube offers a powerful solution to measure particle sizes, concentrations and densities of the individual particles as well as the flocculated aggregates (see also Optical Laser diffraction instruments (LISST)). For more information on instruments for measurements in estuaries, see: Measuring instruments for estuaries.

Sediment transport measurements in coastal seas

Figure 2: Wesp placing tripod in coastal zone

Instruments available for measuring suspended sediment concentrations and transport in coastal environments are: mechanical traps (streamer traps in shallow surf zone <1 m), pump samplers, optical samplers and acoustic samplers. Many samples at the same location are required to eliminate the random fluctuations.

Pump samplers have been used by many researchers to measure time-averaged sediment concentrations. These types of samplers can only be used from a pier or platform. The intake nozzles should be directed downwards.

Optical and acoustic probes are available to measure instantaneous sediment concentrations from a pier or platform or from a stand-alone tripod. Data transmission can take place by telemetry or on-line to a computer or data logger (see e.g. application of data loggers to seabirds. Optical probes cannot be used in conditions with both sand and silt particles in suspension. The optical instruments are relatively sensitive to fine mud particles. Hence, the mud background concentration must be small (<50 mg/1). Otherwise, the sand concentrations cannot be measured accurately. Acoustic probes cannot be used in plunging breaking wave conditions due to the presence of air bubbles.

Nuclear probes which have been used in Russia and in China, cannot be used in low-energy conditions where the concentrations are relatively small. The threshold concentration is of the order of 500 mg/1.

Suspended sediment transport measurements in conditions with combined current and wave conditions cannot be performed from moored or sailing survey ships. Two options are possible:

On-line sampling from piers connected to shore, platforms resting on seabed or sledges/trailers towed by vehicles (only in shallow surf zone)
Stand-alone sampling (see Figure 2) from frames/tripods/poles on/in the seabed or from drift buoys (profiling mode from surface to bed) using a package of sophisticated electronic sensors (electromagnetic and acoustic flow-meters, optical and acoustic backscattering sediment concentration meters). For more information on instruments for measurements in coastal seas, see Measuring instruments for coasts.

Description of bed load samplers

The basic principle of mechanical trap-type bed-load samplers is the interception of the sediment particles which are in transport close to the bed over a small incremental width of the channel bed. Most of the particles close to the bed are transported as bed load but the sampler will inherently collect a small part of the suspended load (related to vertical size of intake mouth).

Popular instruments to sample bed load transport are: Bed load transportmeter Arnhem (BTMA), Helley-Smith sampler (HS) and Delft Nile bed load and suspended load sampler (DNS).

The bed-load transport measured by a mechanical sampler is dependent on its efficiency (instrumental errors), on its location with respect to the bed form geometry (spatial variability) and on the near-bed turbulence structure (temporal variability).

The efficiency of the bed-load sampler depends on the hydraulic coefficient, the percentage of width of the sampler nozzle in contact with the bed during sampling and on sampling disturbances generated at the beginning and the end of the sampling period.

Typical instrumental problems of a (bag-type) bed-load sampler are:

the initial effect; sand particles of the bed may be stirred up and trapped when the instrument is placed on the bed (oversampling),
the gap effect; a gap between the bed and the sampler mouth may be present initially or generated at a later stage under the mouth of the sampler due to migrating ripples or erosion processes (undersampling),
the blocking effect; blocking of the bag material by sand, silt, clay particles and organic materials will reduce the hydraulic coefficient and thus the sampling efficiency (undersampling),
the scooping effect; the instrument may drift downstream from the survey boat during lowering to the bed and it may be pulled forward (scoop) over the bed when it is raised again so that it acts as a grab sampler (oversampling).

Bed load transport can also be determined by bed form tracking.

Description of suspended load samplers

This section describes different samplers to measure the suspended load. For a comparison of different suspended load samplers, see the manual ^[1], which compares trap, bottle and pump samplers as well as optical and acoustical instruments. Suspended load samplers can be classified as a direct method (Delft Bottle sampler and acoustic samplers) or and indirect method. Indirect methods may be point-integrating (trap/bottle samplers, pump samplers, optical samplers, impact samplers) or depth-integrating (USD-49 and collapsible bag sampler). The most important characteristics of the point-integrating samplers (sampling period, minimum cycle period and overall accuracy) are summarized in the article Instrument Characteristics of point-integrating suspended load samplers.

Bottle and Trap samplers

The basic principle of all mechanical bottle and trap samplers is the collection of a water-sediment sample to determine the local sediment concentration, transport and/or particle size by physical laboratory analysis.

Optimal sampling of a water-sediment volume by means of a mechanical instrument requires an intake velocity equal to the local flow velocity (iso-kinetic sampling) or a hydraulic coefficient, defined as the ratio of the intake velocity and local flow velocity, equal to unity. Differences between the intake velocity and local flow velocity result in sampling errors.

USP-61 point-integrating sampler
The USP-61 suspended load sampler consists of a streamlined bronze casting (= 50 kg), which encloses a small bottle (= 500 ml). The sampler head is hinged to provide access to the bottle. The intake nozzle, which can be opened or closed by means of an electrically operated valve, points directly into the approaching flow.

Delft Bottle sampler
The Delft Bottle suspended load sampler is based on the flow-through principle, which means that the water entering the intake nozzle leaves the bottle at the backside. As a result of a strong reduction of the flow velocity due to the bottle geometry, the sand particles larger than about 100 um settle inside the bottle. Using this instrument, the local average sand transport is measured directly.

USD-49 depth-integrating sampler
The USD-49 depth-integrating sampler is a depth integrating sampler. The sampler is lowered at a uniform rate from the water surface to the streambed, instantly reversed, and then raised again to the water surface. The sampler continues to take its sample throughout the time of submergence. At least one sample should be taken at each vertical selected in the cross-section of the stream. A clean bottle is used for each sample. The USD-49 sampler has a cast bronze streamlined body in which a round or square bottle sample container is enclosed. The head of the sampler is hinged to permit access to the sample container.

Collapsible-Bag depth-integrating sampler
The Collapsible-Bag depth-integrating sampler is based on the principle that the static pressure acting on the outside surface of the flexible bag (devoid of air) creates at the nozzle exit a pressure equal to the hydrostatic pressure at the nozzle entrance. Using this method, samples can be collected throughout any depth. The sampler consists of a wide-mouth, perforated, rigid plastic container enclosed in a cage-like metal frame. The head of the frame supports a plastic intake nozzle (6 or 13 mm) and swings open to permit the plastic container to be removed. When the head is closed, the end of the nozzle extends slightly into the mouth of the container. Perforations in the container allows the air in the container to escape during submergence. For sampling, a collapsed flexible plastic bag is placed inside the rigid container.

Pump sampler

Usually a pump sampler consists of a sub-mergible carrier (with intake nozzle, current meter and echo-sounder; see example 2), a deck-mounted pump and a flexible hose connecting the intake nozzle and the pump. The hose diameter should be as small as possible to reduce the stream drag on the hose. Using a hose diameter (bore) in the range of 3 to 16 mm, the pump discharge will be in the range of 1 to 30 litres per minute. In case a deck-mounted pump is used the maximum suction lift will be about 7 m. Assuming a static lift (= height of pump above water level) of about 2 m, the suction lift available for operation of the pump will be about 5 m resulting in a maximum hose length of about 50 m. In extreme deep waters an underwater pump must be used. Operation of a pump sampler is limited to flow conditions with velocities smaller than 2 m/s because of excessive stream drag on the pumphose and carrier. To obtain a reliable average sediment concentration, the sampling or measuring period should be rather large (about 300 seconds). Furthermore, the collection of a large sediment sample for size-determination by sieving or settling tests requires the sampling of a relatively large water volume (about 25 to 50 litres).

Pump sampling also is an attractive method for concentration measurements in coastal conditions because a relatively long sampling period can be used which is of essential importance to obtain a reliable time-averaged value. The sampling period should be rather long (15 min) in irregular wave conditions (at least 100 waves). A problem of sampling in conditions with irregular waves is that the magnitude and direction of the fluid velocity is changing continuously. This complicates the principle of isokinetic sampling in the flow direction. A workable alternative may be the method of normal (or transverse) sampling, which means that the intake nozzle of the sampler is situated normal to the plane of fluid velocity.

The collection of a large sediment sample for size-determination by sieving or settling tests requires the sampling of a relatively large water volume (about 25 to 50 litres). Both requirements can be satisfied by collecting water samples by means of a pump in combination with an in situ separation of water and sediment particles. See also pump sampling in unidirectional and oscillatory flow and pump samplers.

Pump filter sampler
The pump-filter sampler takes a water-sediment sample which is pumped through a filter to separate all particles larger than the mesh size of the applied filter material. To separate the sand fraction, nylon filter material with a mesh size of 50 um can be used. The water volume is recorded by means of a (simple) volume meter. After taking a sample, the filter system is opened and the filter material with the sand catch is removed and returned to the laboratory for drying, weighing and size analysis. During removal of the filter, the pumping is continued using a bypass system. The filtration method cannot be used in a silty environment with silt concentrations larger than about 50 mg/1 because of rapid filter blocking by the fine silt particles.

Pump-Sedimentation sampler
The pump-sedimentation sampler is based on the filling of a large calibrated container (= 50 liters), in which the sand particles can settle. Using a settling height of about 0.75 m, the sand particles larger than 50 a 60 um can be separated in about 5 minutes. A high separation efficiency can be obtained by using a conical container and a vibrator to avoid settlement of the sand particles on the inside of the container. To determine the silt concentration (particles smaller than 50 um), a small water sample can be tapped during emptying of the container.

Pump-Bottle sampler
The pump-bottle sampler is based on the continuous pumping (propeller type pump) of a water-sediment mixture. On board of the survey vessel a small part of the pump discharge is used to fill a 1 liter-bottle or 2 liter-bottle in 3 to 5 minutes by using a small siphon tube. Using this method, a relatively long sampling period and hence a (statistically) reliable concentration measurement can be obtained.

When a peristaltic pump is used (discharge of 0.5 to 1 1/min), the bottle can be filled directly. An optical sensor can be used to determine the silt concentration in the bottle after settling of the sand particles.

Optical and acoustical sampling methods

Optical and acoustical sampling methods enable the continuous and contactless measurement of sediment concentrations, which is an important advantage compared to the mechanical sampling methods. Although based on different physical phenomena, optical and acoustical sampling methods are very similar in a macroscopic sense. For both methods the measuring principles can be classified in: transmission, scattering, and transmission-scattering. For more information, see general principles of optical and acoustical instruments.

Optical backscatter point sensor (OBS)
The Optical backscatter point sensor (OBS) is an optical sensor for measuring turbidity and suspended solids concentrations by detecting infra-red light scattered from suspended matter. The response of the OBS sensors strongly depends on the size, composition and shape of the suspended particles. The OBS response to clay of 2 um is 50 times greater than to sand of 100 um of the same concentration. Hence, each sensor has to be calibrated using sediment from the site of interest. The measurement range for sand particles (in water free of silt and mud) is about 1 to 100 kg/m3. See also Optical backscatter point sensor (OBS).

Optical Laser diffraction point sensors (LISST)
Various Optical Laser diffraction instruments (LISST) are commercially available to measure the particle size and concentration of suspended sediments.

LISST-100: This instrument is the most widely used Laser diffraction instrument, which delivers the size distribution by inversion of the 32-angle scattering measurements.
LISST-ST: This instrument has been designed to obtain the settling velocity distribution of sediments of different sizes. In this case, a sample of water is trapped and particles are allowed to settle in a 30 cm tall settling column at the end of the instrument-housing.
LISST-25A and 25X: This instrument is a simpler, less expensive version of the LISST-100.
LISST-SL: This instrument is a streamlined body that draws a sediment-laden stream into it for Laser measurements. It incorporates a Laser, optics, multi-ring detector identical to the LISST-100 and electronics for signal amplification and data scheduling and transmission. A pump is also built-in to ensure isokinetic withdrawal rates. See also Optical Laser diffraction instruments (LISST).

Various other Optical point sensors
Various types of optical samplers were and are commercially available. Herein, the following types of optical instruments are discussed: Eur Control Mex 2, Partech Twin-Gap, Metrawatt GTU 702 and Monitek 230/134.

Acoustic point sensors (ASTM, UHCM, ADV)
Various acoustic point sensors (ASTM, UHCM, ADV) are commercially available. Delft Hydraulics has developed acoustic point sensors (ASTM or USTM; Acoustic or Ultrasonic Sand Transport Meter; in Dutch: Acoustische Zand Transport Meter) for measuring the velocity and sand concentration in a point. The USTM or ASTM is an acoustic instrument for measuring the flow velocity in 1 or 2 horizontal dimensions and the sand concentration.

The Acoustic Sand Transport Monitor (ASTM) is based on the transmission and scattering of ultrasound waves by the suspended sand particles in the measuring volume. Using the amplitude and frequency shift of the scattered signal, the concentration and velocity and hence the transport of the sand particles can be determined simultaneously and continuously. The ASTM consists of a sensor with a pre-amplifier unit mounted on a submersible carrier and a separate converter with panel instruments and switches. The velocity measurement if mounted on a carrier is one-dimensional and related to the carrier orientation, which is measured by means of a magnetic compass. The vertical position is measured by a pressure gauge (height beneath water surface) and an echosounder (height above bed) mounted on the carrier.A transmitting frequency of 4.5 Mhz has been chosen to minimize the particle size dependency and to make the instrument insensitive to silt particles (< 50 um). The influence of temperature and salinity variations is also negligible.

The UHCM-instrument (only concentration) is a small-sized instrument which has been developed for the high concentration range of 1 to 100 kg/m3 near the bed. This instrument is based on the measurement of the attenuation of ultra-sound by the sediment particles. The transducer heads are close together at a distance of about 10 to 20 mm (depending on application; user-specified). See also Acoustic point sensors (ASTM, UHCM, ADV).

Acoustic backscatter profiling sensors (ABS and ADCP)
Acoustic backscatter profiling sensors (ABS) are non-intrusive techniques for the monitoring of suspended sediment particles in the water column and changing sea bed characteristics. An acoustic backscatter instrumentation package comprises acoustic sensors, data acquisition, storage and control electronics, and data extraction and reduction software. The basic principle of the acoustic backscatter approach is as follows. A short pulse (10 us) of acoustic energy is emitted by a sonar transducer (1 to 5 MHz). As the sound pulse spreads away from the transducer it insonifies any suspended material in the water column. This scatters the sound energy, reflecting some of it back towards the sonar transducer, which also acts as a sound receptor. With knowledge of the speed of sound in water, the scattering strength of the suspended material and the sound propagation characteristics, a relationship may be developed between the intensity of the received echoes and the characteristics of the suspended material. See also Acoustic backscatter profiling sensors (ABS).

Impact sensor
Impact probes are based on the momentum-transfer principle. The high density of sediment particles gives them excess momentum over the surrounding water so that they tend to strike a transducer placed in the stream rather than follow the path of the water particles. This effect discriminates between sand and silt particles. Silt particles do not possess sufficient excess momentum to impact. The sand concentration can be determined from the impact rate and the independently measured water velocity.

Nuclear sensor

Nuclear samplers for suspended sediment concentrations have been used in Russia, Hungary, Poland and China. The principle is based on the absorption of radio-active energy by the sediment particles. The radio-activity is measured by (radiation) counters. Calibration is required. The concentration range is 0.3 to 1000 kg/m3 with an inaccuracy of 20% for low concentrations and 5% for high concentrations.

Conductivity sensor

Delft Hydraulics has developed a small-scale conductivity sensor (CCM) for measuring sand concentrations in the high concentration regime (100 to 2000 kg/m3). The sensor (size of 0.01 m) measures the conductivity of the fluid sediment mixture near the sensor points. The sensor has been used to measure sand concentrations in the sheet flow layer close to the bed.

[R-1] 1.0 ^1.1 Rijn, L. C. van (1986). Manual sediment transport measurements. Delft, The Netherlands: Delft Hydraulics Laboratory

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