Definitions, processes and models in morphology
This article introduces topics related to sediment transport and processes with direct links to corresponding articles available in the Coastal Wiki.
Contents
- 1 Sediment transport modes
- 2 Sediment classification
- 3 Sediment transport processes
- 4 Sediments and ecological processes in marine environments
- 5 Sediments and pollution
- 6 Mathematical models of sediment transport and morphology
- 7 Data Model Integration
- 8 Sediment transport measurement
- 9 External link
- 10 References
Sediment transport modes
Change in morphology is related to gradients of sediment transport processes as caused by either natural phenomena or by human interventions. Often, modification in morphological patterns can be traced back to the construction of engineering works.
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. In estuaries clay and silt concentrations are generally not uniformly distributed.
Sediment classification
Sediment is fragmented material, primarily formed by the physical and chemical disintegration 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, (detrital products of disintegration of pre-existing rocks), Biogeneous sediments (remains of organisms mainly carbonate, opal and calcium phosphate, and Hydrogeneous sediments (precipitates from seawater or from interstitial water).
Descriptive sediment classifications can also be used and are related to characteristics like colour, 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 63 um and smaller than 2000 um are usually referred to as sand particles.
Based on mineral and chemical composition, three types of sands can be distinguished: silicate sands, carbonate sands, and gypsum sands.
See Coastal and marine sediments for further details.
Sediment transport processes
Sand transport
Sand can be transported by gravity-, wind-, wave-, tide- and density-driven mean 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 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 intrusion).
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.
See for further information, Sand transport.
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. Also biological processes can lead to the formation of aggregates, e.g. through colloids. Mud is defined as a fluid-sediment mixture consisting of (salt) water, sands, silts, clays and organic materials.
See for further information, Dynamics of mud transport, Fluid mud and Flocculation cohesive sediments .
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. See: Sediment deposition and erosion processes.
Sediments and ecological processes in marine environments
Biogeomorphology concerns two-way interactions between organisms and landform development: geomorphology determines the physical habitat of organisms, while organisms modify hydrodynamics, sediment transport, erosion and deposition and thereby alter morphology. These interactions can generate feedbacks through which biological and morphological patterns develop together. See Biogeomorphology of coastal systems.
Sediments and pollution
Sediment deposits and dredged materials in fluvial, marine and estuarine conditions are becoming increasingly polluted with trace (heavy) metals, phosphorus, nutrients (dissolved chemical components vital to the health of plants and animals; nitrogen, phosphorus, organic carbon) and other contaminants. Human activities which have intensified the problem of polluted sediments are: channelization of rivers, closing of channels and lagoons, and extension and deepening of navigation channels and harbour basins. The resulting increased maintenance dredging yields enormous quantities of polluted sediments for which safe disposal areas on land or in the aquatic system have to be found. These topics are further discussed in Siltation in harbors and fairways and Nautical depth of harbors and fairways.
Mathematical models of sediment transport and morphology
When the natural system is largely disturbed due to human interference (closure of a channel, construction of a barrage or a harbour or the reclamation of new land), the morphological consequences should be studied on the basis of model predictions.
Two types of models can be distinguished:
- initial or sediment transport models which compute the sediment transport rates and the initial bed level changes for one time step or for one tidal cycle, resulting in a short-term prediction;
- dynamic morphological models which compute the flow velocities, the wave heights, the sediment transport rates, the bed level changes and again the new flow velocities, etc. in a continuous sequence (loop) resulting in long-term predictions.
Operational hydrodynamic models are available for one-dimensional (1D), two-dimensional vertical (2DV) and horizontal (2DH) simulations. The application of dynamic models for three-dimensional (3D) situations is not always possible because of excessive computer cost. General background information on hydrodynamic models is presented Modelling coastal hydrodynamics. Information on sediment transport models and morphological models is given in Sediment transport formulas for the coastal environment, Littoral drift and shoreline modelling, Estuarine morphological modelling and Physical processes and morphology of synchronous estuaries.
Data Model Integration
Application of techniques for data model integration are commonly used in hydrodynamic modelling (flows, water levels, waves) to improve the accuracy of model predictions. Besides a numerical process model, it requires the availability of observation records. Neither the model nor the data provide fully accurate predictions – they have uncertainties associated with them. If information on the (statistical) nature of these uncertainties is available, mathematical techniques can generate more accurate predictions by combining these two information sources. See Reduction of uncertainties through Data Model Integration (DMI) for explanations.
Sediment transport measurement
The Manual Sediment Transport Measurements in Rivers, Estuaries and Coastal Seas[1] provides general information on sediment transport measurement techniques. The following topics are discussed in Coastal Wiki articles:
- Principles, statistics and errors of measuring sediment transport
- Measuring instruments for sediment transport
- Measuring instruments for particle size and fall velocity
- Measuring instruments for bed material sampling
- In situ measurement of wet bulk density
- Instruments for bed level detection
- Measuring instruments for fluid velocity, pressure and wave height
External link
- [https://www.leovanrijn-sediment.com/ Pages on the following topics:
COASTS AND OFFSHORE (Coastal and beach erosion, Beach restoration (sand, gravel), Beach nourishment, Land reclamation, Sediment budget, Local scour), RIVERS AND ESTUARIES (Channel and bend migration, Reservoir siltation, Irrigation channels, Bed forms and navigability, Local scour near structures, Bed and bank protection), HARBOURS AND CHANNELS (Siltation and dredging, Fluid mud and navigability, Optimum channel dimensions, Required field surveys), TRAINING AND COURSES (Courses fluid and sediment, Courses erosion/sedimentation, Courses harbour/channel siltation, Training simple modelling tools)]
References
- ↑ Rijn, L. C. van (1986). Manual sediment transport measurements. Delft, The Netherlands: Delft Hydraulics Laboratory
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