Importance of Bedload Transport in River Systems
The bedload of a river is defined as the sediment being actively moved by the current, typically composed of small stones, gravel, and other materials. It is the most important form of sediment transport in water bodies because the bedload controls the development of most morphological river features and thus influences sediment transport in the river reaches. The Mississippi River is well known for its large sediment transport, which mostly consists of the bedload. The mechanisms by which the river transports a bedload involve complex physical processes that depend on the microscopic interactions of grains within the sediment.
Bedload transport is related to several physical processes such as sediment storage, grain size, fluid shear stress and sediment concentration. Firstly, sediment storage is defined as the amount of particles that can be stored near or on the river bed and will represent the available material for transport. This material can come from suspension or dissolved material that is further transported downstream by the current. Secondly, grain size and density have the power to determine the hydrodynamic processes involved during the transport, such as turbulence and effect of gravity. The flow velocity and particle density will affect the threshold of sediment motion. Lastly, sediment concentration will determine the variability of bedload transport, due to the fact that in less sedimented reaches, the current will be able to move more material, compared to sedimented reaches.
The Mississippi River plays a special role in sediment transport process, forming part of its productive capacity. However, sediment flux from the river is decreasing due to several anthropogenic and natural causes. Its bedload transport is mainly dominated by variations in water discharge, the grain size and composition of sediment, and the composition of sediment types. Along with this, fluvial processes, such as differently activated sediment transport processes and sedimentation, also influence water quality.
Sediment Transport by Flow Processes
Sediment transport is the process by which a river carries its bedload. The Mississippi River has a large channel, with strong erosional capacity, which allows it to displace large loads of sand and gravel. The flow velocity of the Mississippi plays a crucial role in the transport of the particles. The faster the flow, the greater the rate of transport and the further the sediment can travel, while slower flows lead to precipitation of the material.
The behavior of the liquid is important in the transport of sediment. Flow process depends on the dominus law of fluid motion, which states that the product of the velocity and the density must remain constant. In other words, when the river water velocity increases, the density of the particles decreases and vice versa. This explains why sediment is more likely to be transported in areas with high velocity but low sediment concentration and vice versa.
Moreover, the flow over the riverbed sediments has to interact with the grains in order to mobilize the particles. This interaction occurs when the water velocity overcomes a certain strength, known as the threshold velocity. Smaller particles are easier to transport, because their threshold velocity is lower, compared to larger particles. As the diameter of the particles increases, the grain-to-grain interaction also increases, and hence it takes more energy to transport them.
Density Currents and Bedforms
Density currents are driven by the differences in density between the water and the sediment particles, and they play an important role in the sediment transport. Turbulence increases near the bed of the river and the sediment particles tend to settle down under the influence of gravity. This creates a dense layer of sediment particles on the bed, known as the bedload. The bedforms on the riverbed form when the top-most layer of sediment transported by the flow. Bedforms are characterised by their characteristic shapes, such as dunes, ripples, and antiripples.
Density currents contribute to sediment transport in several ways. They can be used to move the sediment particles down the channel, which is known as saltation. On other occasions, they can be suspended in the water, forming a steady flow of sediment. In addition, density currents lead to an increase of the suspended load due to their turbulent nature that enables the fluid to carry more particles in the water column. Finally, they also contribute to the deposition of sediment, due to their ability to reduce the velocity of the flow around the particles.
Bedload at the Mouth of the Mississippi
The bedload transported along the Mississippi decreases as the river approaches its mouth. As the water moves downstream, the majority of sediment is transported in suspension, due to the lower velocities and turbidity of the water near the coast. Nonetheless, the interaction of the waves and tides with the river sediments, can resuspend large amounts of sand and gravel, leading to sedimentation taking place near the delta.
In most cases, the river flow transports finer materials while the waves and tides contribute mostly to the transport of coarser material. The coarser material, because of its higher density, eventually ends up being deposited in the intertidal areas and the bay, leading to formation of sedimentary environments such as salt marshes, and estuaries.
The sediment deposition also depends on the topography of the riverbed near its mouth. In the case of the Mississippi mouth, the riverbed is relatively flat and the sediment deposition is rather slow. This is why it may take several years for the sediment to reach the shores of the Gulf of Mexico. On the other hand, sediment deposition is faster in estuaries due to their shallow depths and narrow widths. In this case, the sediment is able to reach the shore much faster, leading to the formation of beaches, dunes, and barriers.
Influence of Human Activity
Human activities, such as levee construction, can further affect the ability of the Mississippi to transport its bedload. The main goal of these structures is to protect the lands from floods and other river hazards, however, they can also prevent the morphological adjustments of the river, by trapping sediment. As a result, the river is able to carry a lower amount of sediment compared to what it would carry in the absence of levees.
In addition, the quality and amount of sediment in the river can also be affected by human activities. For example, in the upstream reaches of the river, deforestation is known to favour soil erosion and reduce sediment load. On the other hand, agricultural practices, such as tilling, have an effect of increasing the amount of sediment in the river, due to their ability to mobilize larger particles. All these activities, along with urbanization and industrial practices, have led to alteration of the river’s bedload transport capacity.
Impact of Bedload Transport on Water Quality
Bedload transport has a direct impact on the water quality of rivers. Sediment can trap and store contaminants and increase suspended sedimentation in the water column. This, in turn, can reduce light penetration and the available oxygen for aquatic organisms. In addition, sediment deposition can lead to the formation of reservoirs of pollutants near the river bed, causing further harm for aquatic ecosystems. Ultimately, the particle size and type of sediment, as well as the rate of sediment transport, determine the extent of water pollution in a river reach.
In the case of water nutrients, most of them are transported by the river sediment. It has been found that the labile phases of phosphorus and other pollutants are adsorbed by the suspended sediment particles, and transported downstream. This environment can lead to accumulation of these pollutants in water bodies, leading to water eutrophication.
Overall, the bedload transport plays a critical role in river morphological and ecological processes. In the case of the Mississippi, human activities have led to an alteration of the river’s sediment transport capacity, with severe effects on water quality. Despite this, the river continues to be responsible for the large-scale sediment transport, offering an important source of sediment to the sedimentary systems of the Gulf of Mexico.
Measures to Reduce Impact of Bedload Transport
Despite the importance of sediment transport processes in rivers, they are subjected to considerable modifications due to human activities. For example, agricultural practices and urbanization have been known to cause considerable changes to the bedload transport by increasing the rate of sediment accumulation, leading to water pollution and water quality degradation.
One of the main measures to limit the impact of bedload transport is to reduce the amount of topsoil loss from agricultural lands. This can be achieved by changing agricultural practices, by reducing the intensity of tillage, or by changing crop management plans. In addition, different types of erosion control practices, such as terrace construction and cover crop planting, can also be implemented. These practices are known to reduce the amount of sediment being transported by the river, leading to improvement in water quality.
For urban areas, it is important to limit the construction of impervious surfaces, such as roads and buildings, to avoid runoff and erosion, which can lead to increased sediment transport. In addition, best management practices, such as sediment retention basins and reforestation, can also reduce runoff, leading to improvement in water quality. Currently, efforts are being made to improve the management of sediment transported by the Mississippi, with the aim of reducing its impact on water quality.
Conclusion
The sediment in rivers plays a critical role in controlling the shape and movement of the water, as well as water quality. The Mississippi River is known for its large sediment transport capacity, which mostly consists of the bedload. The river transports a bedload by different flow processes that involve sediment storage, grain size, and fluid shear stress. The bedload at the mouth of the Mississippi is mainly transported by the flow and the waves, leading to the formation of sedimentary environments such as salt marshes. Human activities, such as deforestation and urbanization, have led to an alteration of the river’s bedload transport capacity, with severe effects on water quality. Nevertheless, the river continues to be a crucial source of sediment for the sedimentary systems of the Gulf of Mexico.
