- Created by: oliviamaialucey
- Created on: 10-09-18 10:56
Patterns of Flow
Water flowing downwards over a smooth surface can flow in a simple sheet, with no eddies or meanders. This is known as laminar flow. Laminar flow can be observed on a smooth road surface or a paved area during heavy rainfall, but is very rare in nature because most surfaces exert enough friction for turbulence to disrupt the flowing sheet.
Water flowing in a river channel is subject to friction, both with the river bed and banks. This friction slows the water closest to the bed and banks and the water nearer the centre of the river overtakes the slow water. Because water is a liquid, this results in turbulence. Water at the sides of the river begins to eddy downwards. Both types of eddy operate at the same time and this leads to chaotic, turbulent flow.
Patterns of Flow Ctn.
Because the earth rotates, it sets up a force called the Coriolis force. This makes water flow in a meandering pattern. The line of fastest flow of (thalweg) follows a corkscrew or spiralling path as the river moves downstream around a meander bend. Water moves across the surface towards the outside of the meander bend. It then flows down the river cliff bank and then returns to the inside path of the bend by flowing close to the river bed. Water or small sediment follows this spiral path but may not always complete it.
Water flows fastest on the outer bend of the river where the channel is deeper and there is less friction. This faster flow causes lateral erosion on the outside bank which results in the undercutting of the river bank and the formation of a steep sided river cliff.
On the inner bend water is slow flowing. As a result, deposition occurs resulting in the inside of the meander becoming a shallower channel. This increases the amount of friction acting on the river flow, which further reduces the speed/velocity and further reduces the river’s energy which encourages further deposition. Over time, a small crescent shaped area of material builds up on the inner bend; this is called a point bar or a slip-off slope.
Riffle and Pool Sequence
River channels often have alternating shallow (riffle) and deep (pool) sections.
A riffle is an accumulation of fluvial sediment – which forms a bar. It is caused by the deposition of sand and gravel on the bed of a river channel causing the bed to be shallow and rough over which water flows in a turbulent flow. The space between the riffles is about 6 times the channel width.
A pool is a deeper area within a river channel in which water flows in a calmer, smoother, laminar flow.
Riffles and pools cause the river current to swing from side to side in its channel and therefore encourage the development of a meandering river channel. The deeper pools tend to be found where erosion is greatest, such as on the outside of developing meander bends, while the shallower of riffles are regularly spaced along the straighter parts of the channel and are separated by the deeper pools.
Waterfalls and Gorges
Waterfalls occur when there is a sudden change in the gradient of a river as it flows downstream. They form when a band of hard, resistant rock meets softer, less resistant rock. The softer rock erodes much faster than the harder rock, causing a step in the river bed. The step causes the water to speed up due to a lack of friction, giving it greater erosive power. The softer rock is eroded further by hydraulic action and cavitation and the harder rock experiences undercutting. The hard rock can then collapse. A plunge pool at the base of the waterfall is created by the abrasion of rock fragments which have collapsed from above. Eventually, more undercutting causes more collapse making the waterfall retreat leaving behind a steep-sided gorge.
When a waterfall retreats the waterfall often becomes smaller in size until it almost disappears producing a steeper, more turbulent section of river profile, often with a rough bed where outcrops of resistant rock are found. They are made with a combination of turbulent water and steep sections of hard rock. The water flows over the rock, eventually causing the rapids to become steeper and steeper as softer rock erodes away.
Potholes can form when water meets bedload, and it’s forced to flow over it. The water then down cuts behind the bedload, and water turbulence moves in a circular motion called eddy currents, which erodes the river bed and created cylindrical holes. Pebbles can become trapped in these depressions which carry out further abrasion. As a result, the circular hollows become deeper and wider.
Floodplains are large, flat areas of land located on either side of the river where the river floods when a high discharge is experienced. When a river floods an increase in friction causes the efficiency to decrease, so any load being carried is deposited.Regular flooding will result in the building up of layers of nutrient rich alluvium (silt) which forms a flat and fertile floodplain.
They are usually located in the lower parts of the river valley profile and are characterised by having an almost flat, low relief with very gentle gradients. Floodplains are edged by bluffs where they meet the valley sides. The junction where the valley slopes meet the edge of the floodplain is known as the bluff line.
Levees are natural, raised embankments formed as a river overflows its banks. The increase in friction and reduction in efficiency means that sediment is deposited. The largest and heaviest load is dropped first and finer material is deposited further away from the banks. Levees gradually increase the height of a river, decreasing the likelihood of flooding. Repeated flooding will cause layers of coarse alluvium to be built up onto into levees.
A delta is a fan-shaped feature that will form when vast quantities of sediment are deposited at the mouth of the river when it reaches the sea or a lake. The velocity and sediment carrying capacity of the river decreases upon entering the sea or lake and bedload is deposited. Deltas only form when the rate of deposition exceeds the rate of sediment removal. Flocculation occurs when fresh water mixes with sea water and clay particles coagulate due to the chemical reaction. These clay particles sink due to their increased weight. Deltas have three beds.The bottom set bed is composed mainly of clay, by flocculation and some other fine grain sediments. This bed reaches a far distance from the river mouth, since its fine sediment can be transported a reasonable distance. The foreset bed is composed of coarser, medium sized sediment that does not travel very far into the stationary body of water. Lastly, the top set bed is composed of larger, heavier particles.
The river channel often splits into several smaller, divergent channels on a delta and these are called distributaries.
The river channel often splits into several smaller, divergent channels on a delta and these are called distributaries. This may produce a range of different shaped deltas, the three most common being:
1. Arcuate – these have many distributaries, they are fan shaped and build out radially in a uniform shape.
2. Digital (or bird’s foot) – these are found where the river has vast amounts of load and splits into two or more channels which deposit their load along their sides and extend out into the sea or lake.
3. Cuspate – these form a pointed delta where there is a dominant channel shaped by tidal and longshore currents.
V-Shaped Valleys and Interlocking Spurs
The river is cut down by vertical erosion which has potential energy of height that can be converted to kinetic energy – to erode 10% of the river valley.
Hillslope process (weathering, mass movement, erosion and transportation) occur in the valley moving material downslope by either gravity (mass movement) or water wash. This accounts for 90% of material eroded and transported of a v-shaped valley.
Pieces of highland found on the inside of meander bends which join together interlock like a jigsaw puzzle. The river erodes vertically downwards because it has the potential energy of height which is changes to kinetic energy of water movement. Meandering occurs because of the Coriolis force which makes fluids move in a sinuous path.
The sequence starts with a pool-riffle sequence. First, even in straight channels the river will deposit sediment in alternating bars that will eventually create riffles. This will occur when there are low flows in the river, and the low hydraulic radius is enough to encourage deposition. Once created, the river will continue to lower the hydraulic radius for the area and water is seen to flow even more insufficiently over it. Secondly, water needs to find a way around these areas of higher frictional contact so it flows around them. This creates the variations in flow and introduces a side to side motion to the water, the start of meander development. A second theory for meandering is linked to the Coriolis force, this makes water move in a meandering pattern. Between those shallow riffle areas deeper areas called pools are eroded, mainly at times of higher discharge, so a series of pools and riffles develop over time.
As the water enters the meander, it is faster on the outside edge because it is deeper so there is less friction acting upon the water. On the inside bend of the meander it is shallower so more friction from the rivers bed and bedload act upon the water, slowing it down. As the velocity slows on the inside bend due to shallower water creating more friction, the critical deposition velocity is often reached and deposition occurs creating a point bar. The helicoidal flow of the river causes the river to move more laterally across the flood plain, forming a meander.
Oxbow lakes are horseshoe shaped lakes, separated from an adjacent river. They are the evolution of meanders that undergo extensive deposition and erosion. With meanders, strong erosion takes place on the outside bend and deposition takes place on the inside. Eventually, the neck of the meander begins to narrow. In times of high discharge, such as a flood, it’s more efficient for a river to flow through the neck of a meander than around it and when discharge levels resume to normal, the water continues to follow this course. The connected meander is now a cut-off. Deposition eventually separates the cut-off from the main channel, leaving behind an ox-bow lake. The water becomes stagnant, and in time the lake gradually splits up, becoming a crescent shaped stretch of marsh, called a meander scar.
Braided channels consist of two or more channels in an interlocking pattern. Braiding occurs when rivers are carrying a large amount of eroded sediment, such as sand and gravel. If a river has varying discharges, which are sometimes called ‘’unstable flow regimes’’, or the velocity drops, then the sediment load is far too heavy for the river to carry and the sediment is deposited. The river is then divided into small channels, separated by small islands called eyots. Vegetation may quickly colonise these islands in the braided channels which will help trap more sediment and increase the height of the eyot. This vegetation may become quite permanent and well established with trees and only then the eyot will only be covered briefly by water during very high discharge.The channels eventually re-join to form a single channel.