On the earth's surface, there are numerous types of water. Rivers, lakes, seas, oceans, streams, and dams are examples. Depending on how they are generated, these water shapes might be either natural or manufactured. Water covers approximately 75% of the earth's surface. The majority of the water is salty, particularly that found in oceans and lakes. The primary topic of this talk will be rivers, specifically river bed and shapes. Initially, we shall define rivers and briefly detail the numerous types of rivers found on Earth's surface. Later on, we shall talk about bed shapes in connection to rivers and the numerous sorts that occur. Also, we will look at the function of each of these forms.
A river is a stream or channel of water flowing from a particular point, usually called the sources to an end point, usually called the delta. The sources of a river may include a mountain, forest or lake. The delta of the river is usually a water body such as a lake, ocean or sea. Generally, rivers contain fresh water as their sources mostly contain pure water. The way and nature of the flow of a river affect the topography of the land. The river beds of different rivers vary in form due to the force of water, nature of the river bed and even the type of soil underneath. All these factors result in an average effect known as a bed configuration. Bed configuration refers to the nature in which the river bed is aligned in relation to the ground, its source, and delta. The various forms of bed configuration give rise to the different bed forms. Bed forms have different characteristics and behavior depending on the way the river flows. For a long time now, geologists and scientists have launched various researches to look into the different reasons behind the various river bed forms. The knowledge of various specialists is of great importance in considering the various reasons behind these forms.
Bed forms have a great impact on the land beneath the river. They have been noted to change the course of the river and also the rate of flow of the volume of water of the river. For example, rough bed forms increase the speed of water flow as water attempts to use force to overcome the coarse nature of the land. Smooth bed form, on the other hand, decreases the rate of flow of water as water meanders smoothly through the land.
To get a better understanding, we will take a look at various definitions. Flow regime refers to the rate of flow of water in the river bed. The flow regime may be high or low depending on the volume of water. The type of bed forms influences the flow regime of a river. It can either be high or low depending on the nature of the river bed. There are two main types of flow regime, low and high. Low flow regime is characterized by low rate of sedimentation, contact load and a bed form that is usually out of phase. Upper flow regime on the other hand is characterized by high load suspension and high rate of transportation of sediments. The bed forms are usually in phase with the water flow.
Formation of a Bed Form
A bed form is created when an interaction between the flow of water and cohesion of sediments occurs on a river bed. Some examples of bed forms are sand dunes and ripples in the sand in a flowing stream. Sand dunes are formed when there is a flow of water followed by air flow. The ripples and dunes usually form patterns. These patterns result from the action of the flow of the fluids followed by the various formations of bed forms thereby creating specific layers and structures within the strata. The sedimentary structures that are generated from these bed forms give specific details on the current. Such details include its strength, depth of flow, and direction of flow of the sediments.
A fluid that is flowing over a given area can be divided into three sections, these are a free stream, boundary layer, and the viscous sub layer. Free stream is the area of the fluid that is not affected by any boundary effect. The boundary layer refers to the area which the velocity begins to decrease as a result of friction with the river bed. Finally, the viscous sublayer refers to the region where there is reduced turbulence. The region is usually very shallow, about a meter deep. This region is independent in flow depth. However, the rate of flow increases with increasing velocity.
There is a significant relation between the thickness of the viscous sublayer and size of the particles on the bed. These two play a huge role in defining the property of flow. The flow of particles can either give rise to a hydraulically smooth surface or hydraulically rough surface. The surface is said to be hydraulically smooth when the particles are found within the sub layer. When these particles project upwards, then they are said to be hydraulically rough.
There are different types of bed forms, depending on the flow of water and wind and also the type of water body. It is important to note that the properties of fluid dynamics have a great impact on the type of features that are formed. Below, we will take a look at the various types of bed forms in existence.
Current Ripples
Materials within the viscous sublayer are exposed to turbulent sweeps. These sweeps move the grains of the substances by rolling or saltation thereby creating a group of grains. The cluster begins to form with very few grains. As they develop, they form steps which influence the flow of other grains along the surface. When one looks at the flow, it appears to be streamlined in relation to the fluid. There are imaginary lines created which indicate the direction of flow. These imaginary lines lie parallel to the bed or at the sides of the cylindrical pipe. However, where there seems to be an irregularity, there must have been an accumulation of the grains. Here, these lines seem to converge thereby increasing the transport rate of the materials.
At the top of the step, there is usually a streamline that tends to separate the bed surface and the boundary layer. Such steps create a flow of separation point and attachment point. Just below this streamline, there is a section referred to as the separation zone (bubble). Whenever there is an expansion above the step, there is a consequent increase in pressure. The rate of sedimentation is therefore reduced resulting in an increase in a deposition on the leeward side.
The current ripples are therefore tiny bed forms that form when there is a boundary separation on the bed of sand. These small clusters of grains will develop from the crest of a ripple. This causes separation to occur at this particular point. These grains of sand will roll up to the crest on the upper side of the ripple. This results in an avalanche effect on the leeward side of the ripple. These grains will come to a rest at the slope of the leeward side. This action results in the formation of a trough of the ripple.
Cross Lamination
As a ripple migrates downstream, sand accumulates on the crest and accretes on the leeward side. This action results in a movement of the crest, consequently, separation point downstream. The separation, in turn, moves the attachment point and trough downstream. These result in the supply of sand by the scour in the trough and also the base of the stoss side of the ripple. The sand moves upwards the slope leaving behind a train of troughs and crest advancing downstream. On the leeward side, there is sand resulting from the avalanche action. The sand forms a series of thin layers at the base of the slope at an angle known as cross-laminae. These, in turn, build up forming a structure known as cross lamination.
An aerial view of this scenery indicates a variety of current ripples formed. The ripples vary from straight to sinuous crests forming linguoid ripples. There is a close relationship between the above-named bed forms. Both have a relation in the rate of flow and velocity. Also, they have ripples that evolve into linguoid forms resulting from the influence of the velocity. There are two types of cross-laminations formed, planar cross-lamination and trough cross-lamination. A planar cross lamination results from a perfect straight ripple that generates these laminae in the same direction and plane. On the other hand, a trough cross-lamination refers to one that forms from curved cross-laminae between adjacent ripples.
Creating and Preserving Cross-Lamination
Current ripples move downstream by the removal of sand on the upstream side of the ripple. This results in the deposition of sand on the leeward side. When the amount of sand is fixed, then the ripple will migrate as a simple ripple, eroding the troughs thus matching additions to the crests. These ripples are then said to be starved. The starved ripples form as a result of blanketing of mud. If the current has a lot of sand than it is carrying away, then the sand is deposited on the leeward side resulting in an increase of materials on this side of the ripple.
Linguoid Ripple
An increase in the amount of sand results in a consequent increase in the growth of the ripple. No sand is removed from the stoss side. As such, each ripple migrates upwards as the ripples develop upfront. These results in features known as climbing ripples. When the sediments continue to accumulate further, there is an increase in the current, resulting in a consequent movement of the ripple forward. Climbing ripples indicate that there is great sedimentation occurring on the both sides of the ripple. The addition of sand results in either an equal or greater migration of the ripples downstream.
Constraint on Current Ripple Formation
For ripples to form efficiently, there must be a moderate flow of velocity over the hydrodynamic smooth bed. These only form in areas where sand is predominantly the main material. The grains need not be more than 0.6mm as this makes them coarser. Coarse sand results in great turbulent mixing which prevents the small scale flow separation that is required to pave way for ripple formation.
The process of ripple formation is mainly controlled by the viscous sublayer, and their formation is independent of the depth of water and direction of current of the ripples. In contrast to the other features, water depth here is dependent. The ratio of the wavelength to height is approximately 1:4. Evidence has shown a relationship between the grain size and wavelength of a ripple.
Starved Ripple
Rivers, estuaries, beaches and marine environments have a distinct formation of bed forms, which are typically larger than ripples. Ripples are small sized types of bed forms. Such large bed forms are referred to as dunes or mega-ripples. There is no clear evidence on the hydrodynamics behind the formation of a dune. However, it is believed that dunes are formed in the same way as ripples. The difference in the formation depends on the wavelength involved. Long wavelengths result in the formation of dunes while short wavelengths result in the formation of ripples. The formation of dunes is related to strong turbulences that have a great flow which is able to carry a lot of materials with it. The depth of water also has a great influence on the type of feature formed.
Dunes and Cross-Bedding
The structure and mode of formation of these types of the dune are similar to a ripple. However, there is a stoss side which emerges to a crest and avalanches down the leeward side. The direction of flow of water results in different features. When a subaqueous dune migrates, it leads to the construction of consecutive slopes of layers referred to as cross beds. The difference in the rate of flow results in the creation of a zone known as roller vortex. There are different types of cross-bedding that can be formed. These are planar, tubular and trough cross beddings.
Bar Forms
These are bed forms which form within the channels that are of a larger scale than dunes. They differ in width, height, and magnitude. They vary in the amount and the type of sand sediments. Other than sand, they may contain gravel and coarse grain sized materials. The bars can be classified differently depending on their shape and position in relations to the channels. They vary from lens-shaped to cross-stratification.
Plane Bedding and Planar laminar
When the sand is deposited horizontally, it results in the formation of a feature known as plane bedding. This bedding consequently produces a feature known as planar lamination in the sedimentary rocks. Ripples will only form when there are grains that are smaller than the thickness of the viscous sublayer. A rough bed form produces a small-scale flow separation. Such a flow is necessary for the formation of ripples. If the flow is not there, then the grains will roll downslope.
When the rate of flow is high, then a plane bedding is observed. Plane beds result in a properly defined planar lamination feature which is typically 5-20 mm thick referred to as primary current lineation. During the formation of this feature, there is a “sweep” within the viscous sublayer which pushes the grains sideways forming ridge-like structures.
Supercritical Flow
A flow is said to be either subcritical or supercritical depending on the terrain of the surface. A smooth terrain results in a subcritical flow while a rough terrain results in a supercritical flow. Using Froude number, a relationship between the velocity and depth of flow can be derived, using gravity as the acceleration. Froude number is said to be a ratio of the flow of velocity to the wave flow. A greater Froude number indicates a greater flow, thereby resulting in a supercritical flow. On the other hand, a small Froude number indicates slow flow, thereby resulting in the subcritical flow. From the supercritical flow, features known as anti-dunes can be obtained.
Types of Bed Forms
The type of bed form formed highly depends on the type of flow regime. There are two main types of flow regime, lower and upper. Lower flow regime results in the formation of features such as a lower plane bed, ripple marks, sand waves, and dunes or mega-ripples. Each of these bed forms has different ways of preserving materials. Sand waves preserve the least amount of materials. The rest preserve very high amounts of materials.
To lower plane bed is identified by the presence of flat laminae and an almost disappearing current. Ripple marks are usually tiny and are categorized by undulations. Sand waves are the rarest type of bed forms. They are classified by longer wavelengths compared to those of ripples. Dunes are the largest and classified by meter scale ripples.
Upper flow regime results in the formation of upper plane bed, antidunes and pool and chute. Upper plane bed has the highest tendency to retain materials while pool and chute have the lowest tendency to retain materials. The upper plane bed is recognized by flat laminae and aligned grains. Anti-dunes are categorized by low angle and subtle laminae. Pool and chute are mostly erosional features. This explains their tendency to retain the least amount of materials.
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