Introductory Chapter: Sediment Transport

1. Introduction

Sediment transport has been, and still is today, one of the most fascinating and challenging research topics in coastal, hydraulic, and environmental engineering. Probably first studies on sediment transport problems can be tracked back to ancient historical periods (i.e., Roman Empire, Egypt etc.) in relation to the sizing and maintenance of irrigation canals e.g. [1]. Obviously in this periods problems have been solved only by empirical trial and error method. First theoretical concepts can be referred to DuBuat (1734–1809) who was the first to talk about the concept of shear-resistance e.g. [1]. In the following years, there have been many researchers who have contributed to the topic from a theoretical, empirical or experimental point of view (e.g. Du Puit, DuBoys, Reynolds, Forchheimer, Schoklitsch Shields, kennedy, Einstein, and Bagnold) e.g. [1]. In the past, the most of the research is devoted to understand and model the physical processes in order to estimate, for example, the shear stress, the order of magnitude of the sediment transport, bed shear stress and forms.

At present, the change in land use, dams constructions, exploitation of coastal areas, deforestation and in general human activities, have expanded the pool of topics referable to sediment transport area.

Indeed, as recently underlined by [2] one of the most important component of global change can be related to the soil erosion (namely the sediment transport). Last decades have seen the decreasing trend (i.e. a decrease has been found in almost 50% of the world’s rivers) in river sediment loads [2], hence the need to study possible causes investigating the mechanism behind these problems.

Therefore, the policies and strategies for future adaptations to climate change, must contain the information about the future scenarios in order to plan future actions to manage catchments and rivers e.g. [2].

Moreover, the attention for water quality and the interest, that is, beginning to be paid to natural systems (e.g. protected areas) has encouraged a holistic and multidisciplinary approach to the study of sediments transport.

There are a lot of papers, books and manuals in literature in this field. The purpose of this volume is not to replace existing literature.

On the other hand, this book is intended to collect original works and review concerning numerical and experimental investigation, theoretical works, methodological approaches, and any other technique that allows giving the actual state-of-the-art in the field of sediment transport.

2. Erosion: a natural and human-induced process

In general erosion (and sedimentation) phenomenon can be considered as a natural process referred to the particles motion e.g. [3, 4]. As a seek of example, the sediment can be eroded by the river current acting on the movable bed, or it can be moved by the raindrop impact from eroding slopes (see Figure 1) and conveyed downstream reaching a river or a valley, or, in the coastal areas can be moved by the actions of the currents (i.e. the longshore current) and transported along the shoreline. However, the processes inducing the solid particles detachment can be very different depending on the considered case.

Figure 1.

Examples of eroding slopes and the natural erosion/sedimentation processes – Picture courtesy of Prof. Marcello Di Risio.

The physical mechanisms by which sediment is transported downstream of a river are as suspended sediments (i.e. in the case of fine particles) or as bedload sediments (coarse particles) e.g. [3, 4] However, the great part (namely about the 70%) of sediments reaching the coastal area is suspended e.g. [4, 5].

In all these cases, the process can be considered natural, and without external forcing or changes, it will reach an equilibrium if the forcing term does not change significantly.

However, especially in the last decades, the human activities induced considerable changes in erosion/sedimentation rate. Probably, the most striking examples of the effect of human activities on sediment transport processes are the changes in land use of large watershed areas, the deployment of artificial reservoir, and the roads or railways construction. Julien P.Y. [3] highlights that the erosion rate can be (in the worst situations) 100 times greater than the natural (i.e. geological) conditions producing the modification of the runoff characteristics of internal areas also modifying the total sediment budget of the rivers (see Figure 2).

Figure 2.

Example of an uncontaminated coastal areas with the presence of a coastal dune – Picture courtesy of Prof. Marcello Di Risio.

Moreover, dams or small deployments for electricity energy production, can result in sediment entanglement in reservoirs [6], that is, inducing the decrease of storage capacity (i.e. a decrease in energy production) e.g. [3, 4, 7]. The effect of this occurrence is a decrease of sediment input on the coastal environments, and, therefore, a generalized deficit in the sediment budget and a consequent possibility of coastal erosion. This tendency is less pronounced in coastal areas with a good dunal system that allow restoring a portion of the sediment budget. However, the urbanization growth in coastal or/and flat areas, the increase of human pressure, and the natural phenomena modified, in some case in irreversible way, the coastal dynamics e.g. [7].

Moreover, the sediment accumulation in the upstream area can induce bed erosion, change the morphology diversity and can modify the topology of the connection between different close areas e.g. [4].

By the way often, dredging activities e.g. [8, 9] are required to collect sediment finalized to “soft” techniques (see Figure 3, left panel) to restore beaches or to move the sand trapped in the harbor (clean or contaminated). Another possibility to counteract coastal dynamics, is to use “hard” techniques, that is, coastal protections (see Figure 3, right panel), that normally induced changes in hydrodynamics and morphodynamics inducing, in some cases, modification also in the sediment transport regime.

Figure 3.

Examples of “soft” (left panel) and “hard” (right panel) techniques used to act on shoreline dynamics [courtesy of M. Di Risio].

Therefore, it is clear that sediment transport and its fate can induce deep changes not only in the internal and in the watershed areas but can include coastal and also urban areas increasing, in some case, the vulnerability of a specific area and consequently the risk intensity.

As a seek of example, a proper maintenance of river mouths can promote regular water runoff and avoid sedimentation phenomena that can inhibit runoff. Figure 4 shows an example of sedimentation of a river mouth.

Figure 4.

Example of river mouth. In can be observed that the mouth is protected by lateral groins and it is prone to the sedimentation [courtesy of M. Di Risio].

Also, another human effect connected to the sediment transport is the water quality. Indeed, suspended sediments in water can indirectly transport both nutrients and pollution. On the other hand, it should be considered the effect of the water quality on the suspended sediments. It has to be underlined that, however, not only pollutants can be a issue for water quality, but also the presence of heavy metals and metal compounds, which in excessive amounts can be a problem, in general for the environment e.g. [10, 11, 12].

This problem can be directly related to bathing water. Indeed, the quality of the water of coastal area depends on the one hand from sea water pollution, and on the other hand from pollution and water quality coming from rivers and estuaries. Indeed, in the case of storm runoff, and in particular during extraordinary and uncontrolled storm events, there is the possibility of water discharges containing sewer overflows, high concentration of Escherichia coli, fecal or other bacteria that can easily modify the water quality levels e.g. [13, 14].

3. Sediment transport modeling and monitoring

In light of what has been discussed so far, it is clear the increase of attention in modeling and monitoring of sediment transport, not only by researchers but also by stakeholders and managers e.g. [15]. As underlined by [15] there are a lot of models that are able to reach this goal. However, due to the complexity of the problem, the amount of input data at the number of different spatial and temporal scales, the choice of the correct model is always a hard task.

From a general point of view the range of model types includes: (i) empirical models (ii) statistical models (iii) numerical models (iv) analytical models.

While some years ago numerical components are rare ad used only in some detailed cases, with the development of computing power at present they are a good alternative. However, there are no general indications or prescriptions in the use in the use of one or the other. It often depends on the available input data, on the dimension of the spatial scale (i.e. the dimension of the watershed), or on the physics to describe [15].

Empirical models are probably the simplest but, at the same time, the most criticized approach. The reason relies on the simplification in the assumptions, that is, the catchment is often considered homogeneous, the nonlinearities are ignored, etc. Nevertheless, they are frequently used when input parameters are limited, or in the early step of a study or a project when a fast and general overview is needed.

Statistical models, especially with the advent of artificial intelligence, are a good tool when a lot of parameters and information on the study area are available. As obvious they are powerful and allow to elaborate scenarios at different time scales. However, they often lack a physical basis and the larger the time scale (or the larger spatial scale) the greater the needed computational power.

Also, numerical models had (in particular in the last decades) a great impact on the study of sediment transport. Depending on the complexity of the problem, they can be one-, two- o three-dimensional and can be used to model sediment transport, sediment sources, river floods, eroded and deposition areas, etc. Also in this case (as for statistical models) the biggest issue is related to the computational costs. The most used, also in practical applications are probably 1D and 2D depending on the complexity of the problem. 3D models are often used to model very detailed situations requiring particular attention, that is, for environmental impact.

Finally, an important class of models is the analytical one. They are based on the solution of the equation of streamflow, hydrodynamics, advection, and diffusion, etc. It is intuitive that to model real situations, the hypothesis that has been made to write the equations can lead to an oversimplification of the problem. However, they allow modeling (accepting some simplification) of complex system with a physics-based approach. From the perspective of a modeling framework (i.e. the complexity of the model increases as the level of detail increases) approach, they can help to build a general overview of the problem, overcoming the uncertainties of empirical methods e.g. [8, 16].

An important component of sediment transport is, certain, the monitoring. In particular for long-term assessment, regular monitoring should be performed. Acquired data are fundamental in the case of numerical model validations, to calibrate empirical approaches and, in general, to check the reliability of all the described models e.g. [16].

The complexity of the study of sediment transport suggests that there is no better model to use. The choice is subjective and driven by the complexity of the case at hand.

4. Concluding remarks

Sediment transport is one of the most challenging topics in different fields (e.g. coastal, hydraulic and environmental engineering). Indeed, it appears in seas and oceans, lakes, rivers, harbors, and many other natural systems. Historically, all these aspects are related to specific research areas ranging from engineering, geology, geomorphology, biology, etc., but it is difficult to find a comprehensive overview of these topics. At present, behind the natural process inducing soil erosion, sediment transport, and deposition, human activities (i.e. dam, railways, bridges, etc.) have induced considerable changes in sediment transport rate. These changes have resulted in changes in the coastal sediment budget with consequences for shoreline dynamics, and of course, in the rivers’ morphodynamics and in the water quality. In this scenario, considering also the future possible impact of climate change, the aim of this book is intended to organize in a single volume, original works, and review concerning numerical and experimental investigation, theoretical works, methodological approaches, and any other technique that allow giving the actual state-of-the-art in the field of sediment transport.