R I V E R S

[Virtual Presenter] The presentation on Hydrology and Fluvial Geomorphology welcomes us to explore the captivating world of river channels and their processes. We will investigate the different types of rivers, streams, and channels, analyzing their features and roles. By examining erosion and transportation, deposition, and landform creation, we will discover the complex connections between these natural systems. Our journey begins through the world of hydrology and fluvial geomorphology..

[Audio] Rivers and streams are bodies of water that flow in open channels. The movement of water through a channel downstream is influenced by the gradient of the slope. Rivers obtain water from precipitation, surface runoff, and base flow - all of which is considered input. Alongside this water, rivers also bring in materials like soil, organic matter, and rocks through surface runoff and erosion. These materials are referred to as Load or Alluvium. As rivers transport this water and material, they eventually release it into the sea, marking the beginning of the flow process. Ultimately, rivers give up their water and material to the sea or land, serving as the output..

[Audio] Dynamic equilibrium is a state where the river channel shape is constantly adjusting to changes in inputs of water and material, allowing it to transfer the water and material downstream. This balance is achieved through adjustments in various river channel variables, such as channel depth, gradient, width, velocity, wetted perimeter, cross-sectional area, channel roughness, load size, and discharge. The Bradshaw model illustrates these variables and their interactions..

[Audio] The river channel's depth is the average distance between its bed and water level, giving us an idea of how deep the river is. The channel width, on the other hand, refers to the distance between one riverbank and the other, spanning across the river's channel. Notably, the width of the river changes as we move downstream, starting from a narrower point at the source and gradually increasing further downstream..

[Audio] As we can see, the gradient of a river plays a crucial role in determining its velocity. When the gradient is steep, the velocity is also higher, but if the gradient becomes gentler, the velocity reduces. However, this relationship between gradient and velocity is not absolute, as other factors such as channel roughness and hydraulic radius can influence the velocity. The formula used to calculate velocity takes into account the distance traveled and the time taken. This highlights the importance of understanding the complex interactions between these variables in order to fully comprehend the dynamics of a river's flow..

[Audio] Discharge is the volume of water that passes through a river at a given point. This volume is calculated using the formula Discharge equals Area times Velocity. The unit of measurement for discharge is cubic meters per second, abbreviated as cumecs. Wetted Perimeter refers to the portion of the river's bed and bank that comes into contact with water. Cross-sectional area is determined by multiplying the stream's width by its average water depth..

[Audio] As the hydraulic radius increases, the river becomes more efficient and flows faster with greater energy. This energy allows rivers to erode their beds and banks, transporting materials downstream. However, some of this energy is lost due to friction, making the river's efficiency dependent on its hydraulic radius..

[Audio] Water flowing through a channel has different energy and efficiency depending on how rough the channel is. Channel roughness causes friction, which slows down the velocity of the water. Friction is caused by what's on the river's bed and banks. Channel roughness is more pronounced near the mouth of the river, where it can reduce downstream velocities. According to the Manning's equation, velocity equals hydraulic radius times gradient, all multiplied by roughness. A higher value for roughness indicates a rougher bed..

[Audio] Hydraulic action and abrasion are two processes of river erosion. Hydraulic action occurs when the force of air and water within the cracks on the river bed and bank weakens them over time. This process can cause the river to deepen or widen. Abrasion, on the other hand, is the wearing away of the bed and bank by the grinding action of the river's load. The load acts like a sandpaper against the bed and bank, leading to undercutting and caving. The rate of erosion by abrasion depends on the hardness of the impacting rocks and the resistance of the rocks that form the banks and bed. As the velocity of the river increases, so does the rate of erosion by abrasion..

[Audio] Rivers undergo different processes that shape the landscape. Attrition occurs when the load carried by the river smashes against each other within the water flow. As a result, particles and rocks become smaller and rounder over time and downstream. Corrosion/solution involves the removal of soluble materials from the bed and bank of the river by water, typically occurring where a fast-flowing river passes over soluble rocks such as limestone. These processes play a crucial role in shaping the river's landscape, creating various landforms and features along its course..

[Audio] Water flowing through a river channel can exhibit two main types of flow: laminar and turbulent. Laminar flow is characterized by a smooth, parallel movement of water with no internal eddying or differences in velocity. In contrast, turbulent flow is marked by an irregular flow with a series of eddies both horizontally and vertically, resulting from bed roughness and high velocities. While laminar flow is theoretically capable of causing limited erosion, it is rarely observed in natural river channels due to its rarity. On the other hand, turbulent flow plays a significant role in shaping the river's landscape through erosion..

[Audio] Water flowing through a meandering channel exhibits a unique behavior called helicoidal flow. This phenomenon is characterized by a cork-screwing motion, where the water flows from the outside of one bend to the inside of the next downstream bend. This movement is caused by the presence of riffles and pools on the river bed, which allows the water to flow through pools while avoiding riffles, resulting in a zig-zag motion. As the water swirls against the bed and banks, it wears them off over time, contributing to the formation of meanders and channel widening. Additionally, helicoidal flow plays a crucial role in transporting materials from the outside of a meander bend to the inside of the next bend, depositing them along the way..

[Audio] Vertical turbulence plays a crucial role in shaping the river channel. One of its effects is the creation of potholes on the channel bed. These potholes are formed through vertical abrasion, where rocks grind into the bed, causing erosion. As this process continues, it can lead to deepening of the channel..

[Audio] The processes of river transport involve different mechanisms by which rivers move their loads. Saltation occurs where sand, gravel, and small stones are bounced along the river bed by the flow of water because this load is lightweight, water can carry it for a short distance before dropping it. Suspension takes place where fine clay and sand particles are carried along within the turbulent flow of water, even at low discharge and speeds, changing the color of the water. For instance, if the river is carrying clay soils, the water appears greyish in color. Solution occurs when some minerals dissolve in water, such as calcium carbonate from limestone rock, requiring very little energy. Traction involves the rolling of large boulders and pebbles along the river bed at times of high discharge, with the tracted load being heavy and capable of being dragged only during high discharge and speed. Any slight reduction in velocity will lead to deposition of this load..

[Audio] Fine material, such as sand and silt, is suspended in the water as it flows over the river bed. This process is known as suspension. The material is carried along by the river's current, often altering the color of the water as it does so. In this manner, the river is capable of transporting these fine materials over considerable distances..

[Audio] A river performs three main functions along its course. It erodes materials, transports its load, also known as alluvium, and deposits its load, also known as alluvium. These three functions depend on three variables: level of river discharge, velocity, and nature and size of the load. The relationship between these functions and variables can be explained with the help of the Hjulström curve..

[Audio] The Hjulstrom curve illustrates the connection between sediment size and the speed required to erode, transport, and deposit sediment. This relationship is demonstrated for various materials, including clay, silt, sand, pebbles, and boulders..

[Audio] Larger particles such as boulders require a higher velocity of traction to be picked up due to their large size and density, whereas gravel, being smaller, needs a velocity of suspension to be picked up. However, clay and silt are exceptions to this rule, as despite being very small, they tend to stick together strongly, making them difficult to pick up and requiring high velocity of entrainment to be entrained..

[Audio] Solution refers to the process where dissolved substances such as salts, minerals, and nutrients are carried by the flowing water from one location to another. In a river, this can occur when rainwater or melted snow flows over land and picks up these substances, carrying them downstream. Suspension, on the other hand, is the process where particles like silt, clay, and small rocks are transported by the flowing water. These particles remain suspended in the water column due to their size and density, allowing them to be carried along by the current. Saltation occurs when larger particles like gravel and pebbles are rolled or bounced along the riverbed by the flowing water. This process is different from suspension because the particles are not suspended in the water column but rather move along the surface of the riverbed. The Hjulstrom curve is a graphical representation of the relationship between the size of sediment particles and the velocity of the flowing water required to transport those particles. The curve shows that smaller particles require higher velocities to be transported, while larger particles require lower velocities. This means that rivers can transport a range of particle sizes depending on their velocity, and the Hjulstrom curve provides a useful tool for understanding this process..

Types of Rivers/Stream/Channel.

[Audio] A river channel can be one of three main types. These are straight, meandering, or braided. A straight channel is a river that flows straight along its course. It's quite rare, actually..

[Audio] A meandering channel is a type of river channel that winds back and forth across its floodplain. This occurs when the river's flow rate is relatively constant, allowing sediment to settle and build up over time. As the river continues to flow, it erodes the outer banks and deposits sediment on the inner curves, creating a sinuous shape. Meandering channels are common in areas where the river's gradient is low, such as in alluvial plains. They can also occur in areas where there is a change in the river's direction, such as where it flows around a bend or changes course..

[Audio] The degree of meandering or straightness of a river channel is crucial in understanding its behavior and evolution over time. Sinuosity is a valuable tool for measuring this degree. By comparing the length of the channel with the length of the valley in which it flows, we can determine whether the channel is meandering or relatively straight. A sinuosity figure above 1.5 indicates significant meandering, while a value below 1.5 suggests a relatively straight course. This concept is essential in hydrology and fluvial geomorphology, as it helps us comprehend the complex interactions between rivers and their environments..

[Audio] River channel processes and landforms are crucial aspects of hydrology and fluvial geomorphology. One type of channel that we will focus on is the braided channel, which is characterized by being divided by islands or bars. This occurs when a river does not have the capacity to transport its load in a single channel, instead depositing its load in the middle of the valley and separating into more streams. These streams may eventually rejoin downstream..