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Definitions, processes and models in morphology

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This article is a summary of chapter 2 of the Manual Sediment Transport Measurements in Rivers, Estuaries and Coastal Seas[1]. This articles describes a wide variety of topics related to sediment transport and processes.

Introduction

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.In estuaries clay and silt concentrations are generally not uniformly distributed.

Sand and mud transport are both discussed. Definitions of bed load, suspended load and wash load are also given.

Fluid flow and sediment properties

The topics presented, are:

  1. Sediment classification;
  2. Fluid and sediment properties (bed-shear stress, fluid density and viscosity, sediment density, sediment shape, size and fall velocity, critical bed-shear stress);

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.

Fluid and sediment properties

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. The topics are: sand transport and mud transport, sediments and ecology, sediments and pollution, mathematical models and data model integration.

Sediment transport processes

The following aspects of sediment transport processes are described: sand transport, sand transport in steady river flow, sand transport in non-steady flow, sand transport in combined non-steady flow and oscillatory flow and mud transport.

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.

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.

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.

Sediments and ecological processes in marine environments

The following topics are presented in the manual:

  1. overview of processes and impacts,
  2. ecology related to dredging, mining and dumping of sediment,
  3. results of field studies related to dredging and mining of sediment.

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. Information of sediments and pollution aspects are presented in the manual. See also theme 4.

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:

  1. 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;
  2. 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 dynamic 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 yet feasible because of excessive computer cost. General background information of flow models, wave-propagation models, sediment transport models and morphological models is presented. See also, e.g. Process-based Morphological Models – Applications to longer Time Frame.

Data Model Integration

Application of techniques for Data Model Integration (DMI) are increasingly used in many fields of science, finance, economics, etc. Every day examples are improvement of geophysical model descriptions (flows, water levels, waves), improvements and optimization of daily weather forecasts, detection of errors in data series, on-line identification of stolen credit card use, detection of malfunctioning components in manufacturing processes. The one common element is the prior knowledge of the behaviour of a process in the form of an explicit model description, or a set of characteristic data. The second common element is a set of independent or new data. Neither the description of the behaviour nor the data are 100% certain – they have uncertainties associated with them. If one has information on the (statistical) nature of these uncertainties, smart mathematical techniques can be used to combine these two information sources and generate new or improved information. See also Reduction of uncertainties through Data Model Integration (DMI).

See also

Summaries of the manual

Other internal links

External links

References

  1. Rijn, L. C. van (1986). Manual sediment transport measurements. Delft, The Netherlands: Delft Hydraulics Laboratory
The main authors of this article are Rijn, Leo van and Roberti, Hans
Please note that others may also have edited the contents of this article.