Manual Sediment Transport Measurements in Rivers, Estuaries and Coastal Seas
MANUAL SEDIMENT TRANSPORT MEASUREMENTS
written by L.C. van Rijn Issued by Rijkswaterstaat, RIKZ, The Netherlands aand Aqua Publications (www.aquapublications.nl).
LIST OF CONTENTS
1. INTRODUCTION, PROBLEMS AND APPROACHES 2. MORPHOLOGICAL DEFINITIONS AND PROCESSES 3. MEASURING PRINCIPLES, STATISTICS AND ERRORS 4. COMPUTATION OF SEDIMENT TRANSPORT AND PRESENTATION OF RESULTS 5. MEASURING INSTRUMENTS FOR SEDIMENT TRANSPORT 6. MEASURING INSTRUMENTS FOR PARTICLE SIZE AND FALL VELOCITY 7. MEASURING INSTRUMENTS FOR BED MATERIAL SAMPLING 8. LABORATORY AND IN-SITU ANALYSIS OF SAMPLES 9. IN-SITU MEASUREMENT OF WET BULK DENSITY 10. INSTRUMENTS FOR BED LEVEL DETECTION ANNEX A: MEASURING INSTRUMENTS FOR FLUID VELOCITY, PRESSURE AND WAVE HEIGHT
1. INTRODUCTION, PROBLEMS AND APPROACHES
1.1 Introduction This manual provides information of all relevant aspects related to sediment transport measurements such as: - measuring principles and statistics, - type and accuracy of the instruments, - selection of the instruments, - analysis of the samples, - elaboration and presentation of the measuring results.
1.2 Sedimentation and erosion problems in rivers, estuaries and coastal seas 1.2.1 Introduction This Section addresses sedimentation and erosion engineering problems in rivers, estuaries and coastal seas and practical solutions of these problems based on the results of field measurements, laboratory scale models and numerical models. Often, the sedimentation problem is a critical element in the economic feasibility of a project, particularly when each year relatively large quantities of sediment material have to be dredged and disposed at far-field locations.
1.2.2 Sedimentation and erosion problems Human interference in hydraulic systems often is necessary to maintain and extend economic activities related to ports and associated navigation channels. In many situations engineering structures are required to stabilize the shoreline, shoals and inlets, to reduce sedimentation, to prevent or reduce erosion, or to increase the channel depth to allow larger vessels entering the harbour basin. Coastal protection against floods and navigability are the most basic problems in many estuaries in the world. Sedimentation problems generally occur at locations where the sediment transporting capacity of the hydraulic system is reduced due to the decrease of the steady (currents) and oscillatory (waves) flow velocities and related turbulent motions. Examples are: the expansion of the flow depth and width due to natural variations or artificial measures (dredging), the presence of vortex or eddy zones, flow separation zones, dead water zones and lee zones of structures. Expansions of the navigational depth will reduce velocities inducing shoaling. Similarly, the expansion of the width of turning and mooring basins inside a harbour area will reduce velocities stimulating shoaling conditions. Piers or piling structures create eddies resulting in increased shoaling.
1.2.3 Approach of sedimentation problems The general approach to solve sedimentation and erosion problems consists of the following major elements: A) Specification of the problem and definition of wider context (socio-economic, legal, political, environmental, administrative aspects, etc.). B) Formulation of general objectives and desired state of knowledge, C) Determination of problem dimensions and analysis of physical system (current state of knowledge), D) Formulation of hypotheses related to problem. E) Generation of alternative solutions and cost estimates, F) Selection of optimum solution. The tools available for solving problems are: - existing databases, - measurements and monitoring (field studies), - numerical and or physical modelling. The manual focusses on measurements and monitoring.
2. MORPHOLOGICAL DEFINITIONS AND PROCESSES
2.1 General Usually, the transport of particles by rolling, sliding and saltating is called bed-load transport, while the suspended particles are transported as suspended load transport. The suspended load may also include the fine silt particles brought into suspension from the catchment area rather than from the streambed material (bed material load) and is called the wash load. An important characteristic of wash load is that its concentration is approximately uniform for all points of the cross-section of a river. This implies that only a single point measurement is sufficient to determine the cross-section integrated wash-load transport by multiplying with discharge.
2.2 Definitions Definitions of bed load, suspended load and wash load are given.
2.3 Fluid flow and sediment properties 2.3.1 Introduction This Section presents an overview of : - Sediment classification, - Fluid and sediment properties (bed-shear stress, fluid density and viscosity, sediment density, sediment shape, size and fall velocity, critical bed-shear stress) - Sediment transport processes (sand transport, sand transport in steady river flow, sand transport in non-steady flow, sand transport in combined non-steady flow and oscillatory flow, mud transport).
2.3.2 Sediment classification Sediment is fragmental material, primarily formed by the physical and chemical desintegration of rocks from the earth's crust. Such particles range in size from large boulders to colloidal size fragments and vary in shape from rounded to angular. They also vary in specific gravity and mineral composition, the predominant materials being quartz mineral and clay minerals (kaolinite, illite, montmorillonite and chlorite). The latter have a sheet-like structure, which can easily change (flocculation) under the influence of electrostatic forces (cohesive forces) in a saline environment. Consequently, there is a fundamental difference in sedimentary behaviour between sand and clay materials. Sediments can be classified according to their genetic origin: - Lithogeneous sediments, which are detrital products of disintegration of pre-existing rocks, - Biogeneous sediments, which are remains of organisms mainly carbonate, opal and calcium phosphate, - Hydrogeneous sediments, which are precipates from seawater or from interstitial water. Descriptive sediment classifications can also be used and are related to characteristics like color, texture, grain size, organic content, etc. For example, a mixture of sand and clay is classified as a sandy clay when the percentage of sand is between 25% and 50%. Similarly, clayey sands, gravelly sands, sandy gravels, clayey gravels, and gravelly clays are distinguished. Sediment particles larger than 62 m and smaller than 2000 m are usually referred to as sand particles. Based on mineral and chemical composition, three types of sands can be distinguished: - silicate sands, - carbonate sands, - gypsum sands.
2.3.3 Fluid and sediment properties Description of the most important properties of fluids and sediments.
2.4 Sediment transport processes 2.4.1 Introduction Morphological problems are strongly related to gradients of sediment transport processes as caused by either natural phenomena or by human interference. Often, the sudden changes in morphological patterns can be traced back to the construction of engineering works.
2.4.2 Sand transport Sand can be transported by gravity-, wind-, wave-, tide- and density-driven currents (current-related transport), by the oscillatory water motion itself (wave-related transport) as caused by the deformation of short waves under the influence of decreasing water depth (wave asymmetry) or by a combination of currents and short waves. In rivers the gravity-induced flow generally is steady or quasi-steady generating bed load and suspended load transport of particles in conditions with an alluvial river bed. A typical feature of sediment transport along an alluvial bed is the generation of bed forms from small-scale ripples (order 0.1 m) up to large-scale dunes (order 100 m). In the lower reaches of the river (estuary or tidal river) the influence of the tidal motion may be become noticeable introducing non-steady effects with varying current velocities and water levels on a diurnal or semi-diurnal time scale. Furthermore, density-induced flow may be generated due to the interaction of fresh river water and saline sea water (salt wedge). In coastal waters the sediment transport processes are strongly affected by the high-frequency waves introducing oscillatory motions acting on the particles. The high-frequency (short) waves generally act as sediment stirring agents; the sediments are then transported by the mean current.
2.4.3 Mud transport Sediment mixtures with a fraction of clay particles larger than about 10% have cohesive properties because electro-statical forces comparable to or higher than the gravity forces are acting between the particles. Consequently, the sediment particles do not behave as individual particles but tend to stick together forming aggregates known as flocs whose size and settling velocity are much larger than those of the individual particles. Mud is defined as a fluid-sediment mixture consisting of (salt) water, sands, silts, clays and organic materials. In a natural environment there is a continuous transport cycle of mud material which consists of: erosion, settling, deposition, saturation, consolidation, erosion and so on.
3. MEASURING PRINCIPLES, STATISTICS AND ERRORS 3.1 Measuring principles for suspended load transport Samplers for suspended sediment transport were developed in the past according to two different principles: the direct and the indirect measuring of the sediment transport. The direct method is based on the direct measurement of the time-averaged sediment transport (u c ) in a certain point (point-integrating) or over a certain depth range (depth-integrating). This latter procedure implies vertical movement at a uniform speed of the sampler over a certain depth range. The indirect method is based on the simultaneous but separate measurement of the time-averaged fluid velocity and the time-averaged sediment concentration, which are multiplied to obtain the time-averaged sediment transport. This method implies two assumptions which introduce errors: (1) the turbulent flux terms are zero and (2) the fluid and sediment particle velocity are equal.
3.2 Measuring principles for bed load transport The most widely used method for the sampling of bed load is the direct method by means of mechanical trap-type samplers. Many versions of the trap-type sampler have been used with varying amount of success. The problems of the trap-type sampler are the lowering and raising of the sampler to and from the streambed and the efficiency of the sampler in collecting the particles. The most widely-known indirect measuring methods for bed-load transport are: bed-form migration studies (tracking), sediment deposition and erosion studies and tracer studies.
3.3 Measuring statistics 3.3.1 General aspects A critical aspect of any morphological study is the field survey during which the samples to be analysed are collected. It is important to be aware of the fact that the quality of the total study can only be as good as the quality of the information gained through sampling. Thus, any errors incurred during sampling will manifest themselves by limiting the accuracy of the study. The objective of a field survey is to obtain samples from the project area with the purpose of characterizing the area sampled. The sample size should be small enough to be conveniently handled and transported and yet sufficient to meet the requirements of accuracy. The quality of the sampling process and analysis is dependent upon: selecting representative sampling sites in the project area, collecting sufficient samples at each sampling site, using appropriate sampling methods, protecting the samples during the storage period (sample preservation), and flexibility of the sampling programme.
3.3.2 Sampling site The selected sites should be well-distributed over the project area and be representative for the (mean annual) prevailing hydraulic and morphologic conditions. Some general requirements are: - located in a straight reach, i.e. over five channel widths upstream from the measuring location, - located in a stable cross-section (no erosion or deposition), uniform bed topography, regular spatial velocity distributions (no converging or diverging streamlines, no vortices, backward flow or dead water zones), - located normal to the main flow direction, uniform wave characteristics, - sufficiently deep with respect to the dimensions of the sampling equipment, - accessible and clear of natural and/or artificial obstacles (trees, bridges, piers), - well-defined geometrical dimensions (local depth, width, position).
3.3.3 Number of measurements for suspended load transport The total load consists of bed material load and wash load. The bed-material load can be subdivided in bed load and suspended load transport. The wash load consists of sediment particles (fines < 62 m) with particle sizes smaller than those found in appreciable quantities in the bed material. The fine particles usually are uniformly distributed over the entire cross-section. The sediment discharge can simply be obtained by multiplication of the flow discharge and the concentration. Since the concentration is approximately constant over the corss-section, the number of samples can be limited to a few samples. Usually, the samples are taken in reference points which have been found to be representative for the entire cross-section. This should be checked regularly (dry and wet season; low and high discharge). The suspended sediment discharge (particles > 62 m) usually is measured by taking a number of samples over the depth and over the width of the river. The cross-section is divided into a number of subsections. The sediment discharge passing through each subsection is determined by taking (point or depth-integrated) samples along one vertical within each subsection. The accuracy of the suspended sediment discharge depends on: -the number of points over the depth, - the number of verticals over the bed-form length in each subsection, - the number of verticals over the width (cross-section), - the number of verticals over time (flood period, ebb period). Guidelines are provided to find the optimum number of measurements and the accuracy involved.
3.3.4 Number of measurements for bed-load transport The number of measurements of mechanical trap type bed-load samplers is presented. Typical sampling problems related to the variability of the physical processes involved are: - sampling duration of individual measurements, - number of samples at each location, - number of sampling locations along the bed form length, - number of locations over the width of the cross-section.
4. COMPUTATION OF SEDIMENT TRANSPORT AND PRESENTATION OF RESULTS
When the suspended sediment samples are collected as point-integrated samples, there are two methods to compute the depth-integrated suspended load transport. First, there is the so-called partial method which gives the suspended load transport between the bed and the highest sampling point using a linear interpolation between adjacent (measured) values. Second, there is the so-called integral method, which gives the total suspended load transport between the bed and the water surface by fitting a theoretical distribution to the measured flow velocity and concentration profiles. Applying this latter method, the suspended load in the unsampled zone is taken into account. The transport rate of the suspended silt (<50 m) and suspended sand particles (>50 m) should be computed separately. If necessary, more fractions can be used The total load transport can be obtained by summation of of bed load and suspended load transport.
5. MEASURING INSTRUMENTS FOR SEDIMENT TRANSPORT
5.1 General aspects 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. There are the simple mechanical trap-type samplers, that collect the sediment particles transported close to the bed. Another possibility is the recording of the bed profile as a function of time (bed-formtracking). To measure the suspended load transport, a wide range of instruments is available from simple mechanical samplers to sophisticated optical and acoustical samplers. Most samplers are used as point-integrating samplers 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.
5.2 Instrument characteristics The most important characteristics of the point-integrating suspended load samplers are summarized: sampling period, minimum cycle period and overall accuracy.
5.3 Selection of sediment transport samplers 5.3.1 Guidelines for selection of sediment transport samplers The selection of the most appropriate sampling technique should be based on a number of criteria, as follows: A) type of process/parameters to be measured, B) type of sampling environment, C) type of sampling, D) type of project and required accuracy, E) available instruments and available budget.
5.3.2 Sediment transport measurements in 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 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.
5.3.3 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.
5.3.4 Sediment transport measurements in coastal seas
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 datalogger. 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:1) on-line sampling from piers connected to shore, platforms resting on seabed or sledges/trailers towed by vehicles (only in shallow surf zone) and 2) stand-alone sampling from frames/tripods/poles on/in the seabed or from drift bouys (profiling mode from surface to bed) using a package of sophisticated electronic sensors (electromagnetic and acoustic flowmeters, optical and acoustic backscattering sediment concentration meters).
5.4 Comparison of suspended load samplers Results of various instrument comparisons are presented: trap, bottle and pmp samplers as well as optical and acoustical instruments.
5.5 Descripton 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 are: Helley-Smith sampler and Delft Nile sampler. 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).
5.6 Description of suspended load samplers 5.6.1 Classification of samplers Direct method: Delft-Bottle sampler and Acoustic samplers Indirect method: Trap and bottle samplers, pump samplers, optical samplers, impact samplers
5.6.2 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.
18.104.22.168 USP-61 point-integrating sampler The 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.
22.214.171.124 Delft Bottle sampler The Delft Bottle 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 ym settle inside the bottle. Using this instrument, the local average sand transport is measured directly.
126.96.36.199 USD-49 depth-integrating sampler The USD-49 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 pint-bottle sample container is enclosed. The head of the sampler is hinged to permit access to the sample container.
188.8.131.52 Collapsible-Bag depth-integrating sampler The Collapsible-Bag 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 m) 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.
5.6.3 Pump sampler Usually a pump sampler consists of a submergible carrier (with intake nozzle, current meter and echo-sounder), 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 0.003 to 0.016 m, 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.
184.108.40.206 Pump-Filter sampler The water-sediment sample is pumped through a filter which separates 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 m 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.
220.127.116.11 Pump-Sedimentation sampler The method 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 m 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 m), a small water sample can be tapped during emptying of the container.
18.104.22.168 Pump-Bottle sampler This simple method 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 = 0.5-1 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.
5.6.4 Optical and Acoustical sampling methods 22.214.171.124 General principles 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.
126.96.36.199 Optical backscatter point sensor (OBS) The OBS is an optical sensor for measuring turbidity and suspended solids concentrations by detecting infrared 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 m is 50 times greater than to sand of 100 m 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.
188.8.131.52 Optical Laser diffraction point sensors (LISST) Various Laser diffraction instruments 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.
184.108.40.206 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.
220.127.116.11 Acoustic point sensors (ASTM, UHCM, ADV) Delft Hydraulics has developed an instrument (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 m). 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).
18.104.22.168 Acoustic backscatter profiling sensors (ABS and ADCP) Acoustic backscatter (ABS) measurement is a non-intrusive technique 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 s) 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.
5.6.5 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. The height of the voltage pulse is proportional to the momentum of the sand grains so that a crude grain size distribution can be obtained by pulse height discrimination. The impact transfers some of the sand particle momentum and energy to an impact sensor which converts it to an electric signal. The output is a count of frequency of impacts. The IOS-impact probe consists of a 1 x 10 mm stainless steel strip coupled to a slab of piezo-electric ceramic. The ceramic slab is totally enclosed, protected and screened from electrical pick-up. The body of the probe is formed from a stainless steel/glassepoxy laminate/stainless steel sandwich, covered in a 2-part epoxy coating. The sand concentration can be derived from the impact rate and the (independently) measured velocity. Thus, a separate instrument for measuring the fluid velocity is necessary.
5.6.6 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. In China the nuclear sampler consists of a PU238 X-ray source. Other sources are Cs137, Am241 and Cd109. 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.
5.6.7 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.
6. MEASURING INSTRUMENTS FOR PARTICLE SIZE AND FALL VELOCITY
6.1 General aspects As the sizes of sediment particles vary over extremely wide ranges, sediment particles are therefore measured in very large numbers and grouped into specific, but arbitrary size classes according to various analysis methods and definitions. Sediment particles not only vary widely with respect to size, but also with respect to specific weight and shape. Therefore, different particles of a given physical size will behave different in the hydraulic environment as though they are larger or smaller, depending on how their shape and specific weight vary from the defined size class. Because of the wide range of particle characteristics, particle size usually needs to be defined in terms of the method of analysis. Large sizes including boulders and cobbles can be measured directly by immersion and weighing. Intermediate sizes of gravel and sand are measured semi-directly by sieving resulting in sieve diameters. Small sizes of silts and clays are measured hydraulically by sedimentation or settling methods resulting in the particle fall velocity and the standard fall diameter. The relationship between the median sieve diameter" and the standard fall diameter is a measure of the effect of shape, roughness and specific gravity on the settling velocity of a particle.
This leads to the fact that there are essentially two types of measurements: - size- or volume-measurements - fall velocity measurements (sedimentation method).
The size- or volume-measurements include the determination of the: - diameter by means of photographs, sieves or the diffraction of coherent light beams (Laser granulometer); - volume by means of immersion or conductivity (Coulter Counter).
The fall velocity measurements, usually, consists of the determination of sediment accumulation as a function of time using a: - dispersed suspension for silt particles (pipet-withdrawal tube, bottom-withdrawal tube, balance-accumulation tube); - stratified suspension for sand particles (visual accumulation tube, manual accumulation tube, balance accumulation tube).
6.1.1 In-situ sampling Suspended sediment particles in estuaries and coastal seas generally consist of solid and aggregated (flocs) materials with densities as low as 1050 kg/m3. Particle surfaces may be coated with absorbed humuc molecules. In-situ measurements of sediment particles and flocs in these conditions is essential as natural flocs are disrupted easily by physical manipulation such as sampling by bottles or pumps. True particle size distributions of natural suspended sediments can only be achieved by in-situ systems. Most optical particle size methods are potentially non-disruptive.
6.2 Instrument characteristics Some characteristics (size range, required sample quantity and analysis period) of the various measuring methods for particle size and fall velocity are summarized.
6.3 Selection of instruments A summary of the most appropriate instruments for a specific sediment sample is given. Settling velocity of silt particles should be determined by means of an in-situ instrument only, using the field pipet-withdrawal tube or the field bottom-withdrawal tube.
6.4 Comparison of instruments Results of various instrument comparisons are presented: BAT and VAT for sand particles; BAT, PWT, Wet-sieving and Coulter-Counter for fine particles; PWT, BWT and BAT for fine particles
6.5 Description of instruments The following instruments are described: - photographic instrument, - sieving instruments, - sedimentation instruments, - Coulter Counter, - Laser diffraction, - Laser reflectance, - video camera.
6.5.1 Photographic instrument The method is based on taking photographs of the (dry) stream bed. The height of the camera depends on the size of the bed material and the lens system. A reference scale must appear in the photograph. The photograph is printed on thin paper to be inspected on a lightbox with special optical equipment. By adjusting the optical equipment, the diameter of a sharply defined circular lightspot appearing on the photo-graph can be changed and its area made equal to that of the individual particles. An automatic counting system can be used for registration of the particles. After registration each particle must be marked on the photograph.
6.5.2 Sieving instruments 22.214.171.124 General aspects Sieve analysis is one of the simplest, most widely used methods of particle size analysis, that covers the approximate size range from 50 m to 50000 m using standard woven wire sieves. Micromesh sieves extend the range down to 5 m and punched plate sieves extend the upper range. Sieve results can be highly reproducable (within 5%). Inaccuracies may be caused by: - size of total sample and size of particle fractions on each sieve - presence of aggregated lumps of particles - inaccuracies in size and shape of the sieve openings - the duration of the sieving operation.
126.96.36.199 Dry sieving The sieving analysis is carried out by stacking the sieves in ascending order of aperture size and placing the sediment sample of the top sieve. A closed pan (receiver) is placed at the bottom of the stack to collect the fines and a lid is placed on top of the stack of sieves to prevent loss of particles. A stack usually consists of five or six sieves in a root-two progression of aperture size. The stack is vibrated for a fixed time (20 min) and the residual weight of particles on each sieve determined.
188.8.131.52 Wet sieving The method can be used for particles in the range of 10-100 m. The sieves consist of nickel plates in which electrolytic holes are made with an accuracy of 2 m (micro-precision sieves). Usually, the sieves are stacked on top of each other and the sample is placed on the top sieve and washed with a liquid (water with a detergent, 0.1% Teepol) while the stack of sieves is being vibrated. The vibration can be accomplished by placing the stack of sieves in an ultrasonic bath. Before the sieving operation the sieves are dried and weighed.
184.108.40.206 Air-jet sieving The air-jet sieve is an instrument using an air-current to agitate the dry sediment particles on the sieve. A single sieve is placed above a rotating vane in an airtight container. The air-jet is blown through the rotating vane and the sieve above the vane. The air and the particles then passes down the sieve on both sides of the vane. The particles are collected on a filter paper. Firstly, the finest of the sieves is used and so on untill all sieves have been used. The method has been found useful for sieving low density materials and very fine sediments. Materials such as coal, wood and polystyrene particles can be sieved more efficiently.
6.5.3 Sedimentation instruments 220.127.116.11 General aspects Basically, two methods are used for particle size analysis: - stratified suspensions, - dispersed suspensions.
Stratified systems In a stratified system the particles start from a common source and become stratified at the bottom of the tube according to the settling velocities. Generally, this method is only used for sand particles. The stratified sediment layers at the bottom of the tube can be measured by means of a small capillary tube (Visual Accumulation Tube, VAT). Another possibility is to weigh the settled sediment particles directly by means of an under-water balance or to extract the settled sediment particles at pre-fixed time intervals by means of a mechanical method. The latter two methods produce the accumulated sediment weight as a function of time. Using the known settling height, the weight percentage of the particles with a certain fall velocity can be determined.
Dispersed system In a dispersed system the particles begin to settle from an initially uniform dispersion (equal concentration). Generally, this method is only used for silt or fine sand particles (5 to 150 m). Usually, the sediment weight is determined as a function of time by means of an under-water balance.
18.104.22.168 Accumulation tube An accumulation tube can be operated as a stratified system for sand particles in the range 50-2000 m or as a dispersed system for silt and fine sand particles smaller than 150 m. Typical examples of the accumulation tube method are: - Visual Accumulation Tube (VAT), - Manual Accumulation Tube (MAT), - Balance Accumulation Tube (BAT). The VAT, which operates as a stratified system, consists of a settling tube with a length of about 2 m and a diameter of about 0.03 m. The sample can be released on top of the tube by means of a simple clamp device or by means of rotating lamellae. Under the settling tube a small capillary tube is suspended in which the deposit height can be determined as a function of time. Based on this information, the fall velocity distribution can be determined. The capillary tube has a length of about 150 mm and an internal diameter of 4 mm for particles of 50-500 m and 10 mm for particles of 500-2000m. The method may not be very accurate due to hindered settling of the particles in the contracted section and the capillary tube. To minimize this effect, the length of the settling tube should be relatively large compared with that of the capillary. The settling is not properly defined and can be calibrated using samples of known diameter and settling velocity.
The MAT is quite similar to the VAT, but another method is applied to determine the weight increase of the settled sediment particles as a function of time. The particles are collected in small cups placed under the settling tube at pre-set times using a manual slide mechanism. The slide mechanism may be designed as a waterproof system or as an under-water system. In the latter case the tube must be closed directly after release of the sample by means of a rubber stop. By drying and weighting of the particles, the cumulative weight curve can be determined.
The BAT is based on the weighting of the settled particles by means of an under-water balance. The method can be operated as a stratified system using a long tube of about 2 m or as a dispersed system using a short tube of about 0.5 m for silt particles. In the latter case the tube should be equipped with a temperature control system because the measuring period may be rather large (24 hours) to determine the total weight of all particles.
22.214.171.124 Bottom Withdrawal Tube (BWT) The instrument is based on the sedimentation of sediment particles from an uniform suspension (dispersed system). The bottom withdrawal tube method can be used for the fall velocity analysis in the laboratory, but also for the in-situ determination of the fall velocity distribution. This latter possibility offers the advantage of using an undisturbed suspension sample and native water as settling medium, which is essential for flocculated sediments. The laboratory instrument consists of a tube with a length of about 1 m and an internal diameter of 0.05 m (or 0.025 m). The lower end of the tube is contracted into a nozzle. A pinch-clamp on a short piece of rubber is used to enable quick withdrawals. The field instrument consists of a stainless steel tube with a length of about 1 m and an internal diameter of 0.05 m. The tube is used for the collection of the sample as well as for the determination of the fall velocity distribution by means of a settling test. Therefore, the tube is equipped with two valves on both ends and a double wall for temperature control. The tube is lowered to the sample location in a horizontal position with opened valves. After closing the valves, the tube is placed in an upright position (start of settling process) and hoisted on board of the survey vessel.
126.96.36.199 Pipet-Withdrawal Tube (PWT) The fundamental principle of the pipet method is to determine the sediment concentrations of an initially uniform suspension (dispersed system) at a pre-fixed depth below the water surface as a function of settling time. Particles having a settling velocity greater than the ratio of the depth and the elapsed time period will settle below the point of withdrawal after the elapsed time period. The sediment concentration at a certain depth can be determined by withdrawing samples at that height. Usually, eight or nine samples are withdrawn. The pipet method can be used for the laboratory analysis of a silt sample but also for the in-situ analysis of a silt suspension. This latter possibility offers the advantage of using an undisturbed suspension sample and native water as settlingmedium, which is essential for flocculated sediments.
6.5.4 Coulter Counter The method is based on an electrical conductivity difference between particles and common diluent. Particles act as insulators and diluents as good conductors. The particles suspended in an electrolyte are made to pass through a small aperture through which an electrical current path has been established. As each particle displaces electrolyte in the aperture, a pulse essentially proportional to the particle volume is produced. Particles in the range of 1 to 500 m can be counted and measured volumetrically.
6.5.5 Particle size and concentration by Laser Diffraction (LISST, COULTER, PARTEC) The Laser diffraction method (Fraunhofer diffraction) offers a fundamentally superior basis for in-situ measuring the sizes of suspended sediment particles in a point in the water column. Unlike other and simpler optical or acoustic methods, the diffraction method does not suffer from a change in calibration with changing sediment colour, composition or size. When a parallel light wave strikes a particle, part of the wave enters the particle, and part is blocked by it. The wave entering the particle senses particle composition (e.g. colour, absorption). However, this part is scattered into a wide range at angles, very little of which appears in the original light wave direction. In contrast, light blockage produces a diffraction pattern that dominates the light intensity in the original direction. This pattern is bright and it is identical to the diffraction through an aperture familiar to optical physicists (analogous to the diffraction of waves on water surface by a jetty). When a lens gathers the scattered plus diffracted light, diffraction shows up on the lens axis. The diffraction pattern is weaker and wider for small particles, but tall and narrow for large particles. The width helps to distinguish particle size while the magnitude delivers concentration. Recent instruments (LISST) can derive the particle size distributions and also the particle volumes (volume concentration) from the measured data with an accuracy of the order of 20%.
6.5.6 In-situ photo and video camera An in-situ photocamera (and image-analysis software) for in-situ measurement of solid particles and aggregates (flocs) larger 4 m is avaialble. It can be used in depths up to 4000 m with concentrations up to 200 mg/l. In very clear ocean waters the system is not effcient because of the large number of photographs that have to be taken to obtain a reliable size distribution. The camera system consists of a steel frame (1.8 x 2 m) in which 3 cameras are mounted in such a way that there is a minimum disturbance of the water flow through the frame. The in-situ video camera VIS (Delft Hydraulics) is available to determine both the size and the settling velocity of the solid particles and the flocculated sediments. The in-situ video camera consists of a small vertical tube with a closed end at the bottom in which particles are settling down in still water. Two small windows are present in the tube for enlighting (light beam) and for video-recordings. The in-situ settling velocity instrument INSSEV (University of Plymouth) also is based on video camera recordings. The instrument comprises a computer controlled chamber (decelerator) with closing doors to slowly collect a sample of water and sediments, from which some of the suspended materials is allowed to enter the top of a settling tube (settling length of 110 mm). The settling flocs are viewed using a miniature video system.
6.5.7 Particle size and velocity by Phase Doppler Anemometry (PDA) PDA is an extension of Laser Doppler anemometry (LDA) and can determine not only the Doppler shift frequency of light refracted by a particle within the flow (hence its velocity) but also the phase shift as observed at three different receiving locations which can be utilized to derive the diameter of the scattering particle. Assuming constant density and spherical particles, the volume concentration can be determined. Hence, the simultaneous measurement of particle size, velocity and concentration can be obtained using phase Doppler anemometry as an extension of the principles of LDA.
6.5.8 Particle size by Laser Reflectance (PARTEC Laser) In-situ Laser diffraction techniques are severely limited in their use by the presence of high sediment concentrations larger than about 0.5 to 1 g/l. This limitation can be overcome by sing in-situ Laser reflectance techniques. The PARTEC 100 is a commercially available, Laser reflectance particle-sizing instrument which was initially designed for process control in the grinding and milling industries with concentrations in the range of 10 to 100 g/l. The sensor is computer-operated and the output of the PARTEC 100 consists of a histogram of 38 logarithmic size intervals over the size range 2 to 1000 m. The measuring principle employs an optical beam which is directed through a lens located eccentrically on a rotating disc within the reflectance probe such that the focal point describes circles of 8.4 mm in diameter. The light source is a semi-conducting Laser diode. As the focal point is typically smaller than the suspended particles and moving with a greater velocity, reflected light signals are assumed to be related to individual particles. When the sensor probe is immersed in a sample, measurements of reflected pulses are accumulated for a set period, typically 3 to 25 s depending upon particle numbers, and a particle chord size distribution is calculated. A correction algorithm, which assumes the particles are spheres, allows a distribution of spherical equivalent diameters to be calculated.
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.
8. LABORATORY AND IN-SITU ANALYSIS OF SAMPLES
Sample analysis usually consists of determining the following parameters: sediment concentration, sediment composition, sediment density, and chemical analysis. Samples for chemical or bioassay analysis should be immediately chilled and stored at 4 oC after collection.
8.1 Sediment concentration The two most commonly used methods are evaporation and filtration. The filtration method may be somewhat faster for samples of small concentrations. However, large concentrations tend to clog the filter material (silt concentrations > 100 mg/l).
8.2 Bed material composition Preparation of samples prior to analysis is of the utmost importance if accurate and reproducible results are to be obtained. Samples containing clay minerals or organic material are very liable to cracking on drying and care should always be taken to avoid samples drying out prior to analysis. However, when samples may have dried out naturally when collected on a mudflat or a riverbank, then the aggregates should be broken down (hydrogen peroxide treatment). Organic materials ranging from macroscopic plant and coal to microscopic colloidal humus does affect average specific weight and greatly affects the particle size and/or fall velocity, if present in sufficient quantities. Quantitative determination of organic material, usually is recommended if the sample consists of 10% or more of organic material. Complete removal of organic material is necessary for all samples to be analyzed for particle size or fall velocity when other than native water is used because the organic material may bind together the sediment particles. Samples having a size range from pebbles or cobbles down to fine sands will require hand separation of the largest particles. If possible, the size-distribution of the large particles (cobbles) should be determined in-situ by manual measurement of the nominal diameter or by means of photographic methods.
8.3 Suspended sediment composition The physical analysis of suspended sediment samples should be focussed on the determination of the particle fall velocity distribution because this latter parameter is of essential importance in sedimentation studies. Therefore, the sedimentation methods (settling tests) must be preferred above the other methods such as sieving or the Coulter counter. These latter two methods may be used to check the results of the sedimentation tests.
9. IN-SITU MEASUREMENT OF WET BULK DENSITY
9.1 General aspects In deposition and navigation depth studies of muddy areas the wet (bulk) density defined as the mass of the water-sediment mixture per unit volume is an important parameter. The position of the surface of consolidated mudlayers can be determined by means of echo-sounding instruments. Good penetration can be obtained with 30 kHz-instruments, see Fig. 1A. Higher frequencies (210 KHz) do not have sufficient energy to penetrate into the bed. Various methods are available to determine the wet bulk density: mechanical core sampler, acoustic probe, nuclear radiation probe, electric conductivity probe, vibration transducer probe, and pressure transducer probe. Electric conductivity probes and pressure transducer probes are not generally applicable. Electric conductivity probes are very sensitive to the fluid salinity which should be known beforehand. Pressure and vibration transducer probes can only be used in unconsolidated fluid muds (low density < 1200 kg/m3).
9.2 Mechanical core sampler A basic requirement is undisturbed sampling of bed material. Various mechanical core-samplers are available to take undisturbed bed material samples of the surface layers (upper 0.5 m of the bed). Most samplers can only be used during low velocity conditions to ensure vertical penetration of the bed. After sampling, it is common practice to make slices by a machined ring of the same internal diameter as the core. The core content is extruded into the ring until it is full of the water-sediment mixture. A thin plate is then introduced between the ring and the core to isolate the sample. As the core diameter is known and fixed and the slice thickness is fixed by the ring, the volume can be calculated. After weighing (and drying) of the sample, the wet and dry density can be determined.
9.3 Acoustic sensor The principle is based on measuring the attenuation of the intensity of monochromatic ultra-sonic waves through the (fluid) mud layer.
9.4 Nuclear radiation sensor The principle is based on measuring the attenuation or scattering of the radiation intensity through a water-sediment mixture.
10. INSTRUMENTS FOR BED LEVEL DETECTION
10.1 Introduction The management of rivers, estuaries and coastal seas always involves the production of bathymetric maps for evaluation of navigationable depths, shoaling and erosion volumes, etc. Hence, accurate measuring instruments for bed level detection are required. Herein, the following methods and accuracy involved are discussed: mechanical bed level detection in combination with DGPS; acoustic bed level detectors (single and multi-beam echo sounders); and optical bed level detection.
10.2 Mechanical bed level detection in combination with DGPS In coastal environments the bed level soundings are often performed by use of a vehicle moving through the surf zone. Rijkwaterstaat (The Netherlands) uses the WESP in combination with DGPS. The CRAB vehicle is in use at the Duck site (USA). The WESP is an approximately 15 m high amphibious 3-wheel vehicle, which can be used for bed level soundings in the surf zone in depths up to -6 m with waves upto 2 m. It is equipped with a DGPS positioning system. Small vehicles with DGPS can be used on the dry beach.
10.3 Acoustic bed level detection (Echo-sounding instruments) The most common system for measuring water depth is the single-beam echo sounder. This sonar instrument uses a transducer that is usually mounted on the bottom of a ship. Sound pulses (usually 210 KHz for surface detection) are sent from the transducer straight down into the water. The sound reflects off the seafloor and returns to the transducer. Acoustic penetration into the bed increases with decreasing frequency (usually 10 to 15 KHz for subsurface detection). The time the sound takes to travel to the bottom and back is used to calculate the distance to the seafloor. Water depth is estimated by using the speed of sound through the water (approximately 1500 meters per second) and a simple calculation: distance = speed x time. The faster the sound pulses return to the transducer from the ocean floor, the shallower the water depth is and the higher the elevation of the sea floor. The sound pulses are sent out regularly as the ship moves along the surface, which produces a line showing the depth of the ocean beneath the ship. This continuous depth data is used to create bathymetry maps of the survey area. Multibeam bathymetry sonar (Figure 2) is the relatively recent successor to single-beam echo sounding. About 30 years ago, a new technology has been developed that uses many beams of sound at the same time to cover a large fan-shaped area of the ocean floor rather than just the small patch of seafloor that echo sounders cover. These multibeam systems can have more than 100 transducers, arranged in precise geometrical patterns, sending out a swath of sound that covers a distance on either side of the ship that is equal to about two times the water depth. All of the signals that are sent out reach the seafloor and return at slightly different times. These signals are received and converted to water depths by computers, and then automatically plotted as bathymetric maps. One of the best systems for imaging large areas of the ocean floor is side scan sonar (Figure 3A), either ship-mounted or bottom-mounted. The basic concept is much the same as the basic echo sounder; however, side scan sonar instruments are towed behind ships and often called towfish or tow vehicles. This technology uses a specially shaped acoustic beam, which pulses out 90 degrees from the path that it is towed, and also out to each side. Each pulse provides a detailed image of a narrow strip directly below and to either side of the instrument. Seismic reflection uses a stronger sound signal and lower sound frequencies than echosounding. The sound pulse is often sent from an airgun towed behind the ship. An airgun uses the sudden release of compressed air to form bubbles. The bubble formation produces a loud sound. The sound from the airgun travels down to the seafloor. Some of the sound reflects off the seafloor but some of the sound penetrates the seafloor. The sound that penetrates the seafloor may also reflect off layers of material within the seafloor. The reflected sounds travel back up to the surface. The ship also tows a number of hydrophones (called a towed array or streamer) which detects the reflected sound signal when it reaches the surface. The time it takes the sound to return to the ship can be used to find the thickness of the layers in the seafloor and their position (sloped, level, etc). It also gives some information about the composition of the layers.
10.4 Optical bed level detection This instrument consists of a steel pole (diameter of 32 or 40 mm; lengths of 1.8, 2.4 and 2.9 m), which can be driven into the bed. The pole is supplied with many infra-red light sources/receivers (backscattering sensors) at spacings of 10 mm (100 sensors per meter; sampling volume of 0.5 cm3). The instrument measures: - vertical distribution of the turbidity levels in the water column; - transition from water column to bed based on the scattering of light from the suspended particles and the bed material particles; - transition from water column to air (if pole end is above the water surface).
ANNEX A: MEASURING INSTRUMENTS FOR FLUID VELOCITY, PRESSURE AND WAVE HEIGHT
A1 Introduction The electronic equipment commonly consist of : - electromagnetic velocity sensors, - acoustic velocity sensors (point sensors and profilers), - optical sediment concentration point sensors, - acoustic sediment concentration point sensors and profilers, - optical particle tracking sensors (size and fall velocity), - acoustic bed level sensors (altimeters, single/multi beam echo sounders; bed profilers, side scan sonar), - data storage discs.
A2 Velocity sensors A2.1 Velocities and bed-shear stresses, instrument characteristics and accuracies Electro-magnetic velocitymeters (EMV) are among the best instrumentation available for studying the structure of the bottom boundary layer where sediment transport takes place. The EMV’s are robust, resistant to fouling, moderately intrusive, and reasonably inexpensive, but they also suffer from severe limitations including offset drift, limited frequency response and relatively large sampling volume. Within the last few years, acoustic instruments have become increasingly available for coastal conditions. These instruments are also reasonably robust, resistant to fouling, and increasingly affordable. In addition, acoustic instruments are less intrusive, have better frequency responses and smaller sampling volumes. Examples are, the Acoustic Doppler Current profiler (ADCP), Ultrasonic Velocity Profiler (UVP) and the Acoustic Doppler Velocitymeter (ADV).
A2.2 Electro-Magnetic Velocitymeter (EMV) These instruments are based on the principle that a conducting fluid will generate a voltage proportional to the flow velocity as it passes through the magnetic field created by the sensor.
A2.3 Acoustic Doppler Velocitymeter (ADV) Basically, the ADV measures the velocity of particles (sediments) at a point in the water column from the Doppler shift in frequency of the emitted and received acoustic signals (without calibration) in 2 or 3-directions, depending on the sensor arrangement. The system includes three modules: sensor, signal conditioning module and signal processing module. The measurement probe consists of four ultrasonic transducers: a transmit transducer located at the bottom end of the stem and three receive transducers, slanted about 30o from the axis of the transmit transducer and pointed at the sampling volume, which is located about 0.1 m below the probed tip. Hence, the flow velocity in the sampling volume is not disturbed by the presence of the probe. The acoustic frequency is of the order of 10 MHz. The accuracy is of the order of 1% of the reading,
A2.4 Acoustic Doppler Current Profiler (ADCP, UVP) ADCP instruments are being used as: - bottom-mounted (big-size upward-looking for velocity profiles over the water column; or small-size downward-looking for near-bed velocity profiles), - ship-mounted (big-size downward-looking). The ADCP profiler measures the current profile in water using Acoustic Doppler technology. It is designed for stationary and non-stationairy (ship’s hull mounted) applications. It can be deployed on the bottom, on a mooring rig, on a buoy or on any other fixed structure. It is a complete instrument and includes all the parts required for a self-contained deployment with data stored to an internal data logger. Typical applications include coastal studies, online monitoring and scientific studies in rivers, lakes, estuaries and tidal channels.
A2.5 Phased Array Doppler Sonar (PADS) An acoustic signal from a bottom-mounted sonar is projected in a wide horizontal fan, radiating outward in the water from the instrument package and filling the water column in shallow water. The sound scatters off particles in the water (especially bubbles) and off the bottom. Some backscattered sound returns to the sonar, where the signal is received on an array, beamformed into returns from various directions, and analyzed for frequency shift versus direction and elapsed time since transmission. For direct-path transmission and return, the time-delay since transmission translates to distance from the sonar. The frequency shift of the backscattered signal (Doppler shift) is proportional to the radial component of the velocity of scatterers at the sample volume. A2.6 Coherent Doppler Velocity Profiler (CDVP) and Cross-Correlation Velocity Profiler (CCVP) The backscattered signal from suspended particles in the flow can be utilised to determine the velocities of the particles. The two techniques are: coherent Doppler method, and cross-correlation method. The coherent Doppler method is based upon pulse-to-pulse phase coherence between consecutive transmissions to measure the radial component of the velocity along the beam axis. This instrument uses Doppler shift to obtain the Doppler flow velocity. The Doppler frequency is obtained from the pulse-to pulse coherence (phase coherence). Averages over pulse pairs are taken. The correlation method employs a pair of horizontal separated transducers directed vertically downward and cross-correlation of the backscattered signals from the transucer pairs is used to obtain the velocity. Unlike the coherent Doppler system, the correlation method is incoherent, as it is the signal intensity that is used. The basic requirement is that there are fluctuations in the suspension field, which have spatial scales greater than the distance between the transducers and that these fluctuations can be cross-correlated. Small-scale turbulent fluctuations cannot be measured.
A3 Comparison of measured velocities Results of various instrument comparisons are given: - Electro-Magnetic Velocitymeter (EMV) and Laser Doppler Velocitymeter (LDV) - Acoustic Doppler Velocitymeter (ASTM) and Electro-Magnetic Velocitymeter (EMV) - Acoustic Doppler Velocitymeters (ADV) - Ultra-sonic Velocity Profiler (UPV) and Particle Image Velocitymeter (PIV)
A4 Fluid pressure and wave height instruments A4.1 General instrument characteristics, accuracies and selection criteria Water level fluctuations in deeper water generally are measured by pressure sensors or by wave bouys. Water level fluctuations can also be measured by capacitance wires/rods attached to poles jetted into the bed. Bottom-mounted applications can, in principle, also be used to determine the instantaneous wave height by using the horizontal velocity measured in the near-surface region and linear wave theory. The wave rider bouy (hull diameters up to 1 m) is a spherical bouy, which measures wave height and direction. The wave height measurement is based on the principle of measuring vertical accelerations. The direction measurement is based on the translational principle which means that horizontal motions instead of wave slopes are measured. Various types of pressure sensors are commercially available. Generally, piezo-electric transducers are used. Piezo-electric materials such as quartz crystals produce an electric field under deformation by pressure forces. The instrument offset can be determined in the laboratory prior to deployment and taken into account by the calibration curve. The ADCP instruments can also be used for wave field measurements. The basic principle behind wave measurement is that the wave orbital velocities below the surface can be measured by the highly accurate ADCP. The ADCP (with 2 Hz data recording and waves upgrade of software) measures the subsurface orbital velocities created by the wave field. This raw data is averaged to create a mean current profile, and is accumulated into time series for waves processing. Each time series of data is called a burst. From this burst, velocity power spectra, directional spectra, and mean water levels are calculated. The ADCP should be bottom mounted, upward facing (within 5 degrees of the vertical) with a pressure sensor for measuring tide and mean water depth.
A5 Comparison of measured wave heights Results of various instrument comparisons are given: - Pressure sensor and capacity wire - Pressure sensor and surface following wave gauge - Pressure sensors - Velocity sensor, fluid pressure sensor and capacity wires - Pressure sensor and resistance wave staff - Accelerometer and DGPS on wave rider bouy