River channel processes and landforms.pptx

A LEVEL GEOGRAPHY
HYDROLOGY AND FLUVIAL GEOMORPHOLOGY

1. River channel processes and landforms

2. River channels and their variables Rivers and streams are bodies of water that flow in open channels. Rivers get water and material as inputs from overland flow, precipitation and groundwater flow and erosion Rivers then transfer this water and material along to the sea (flow) Rivers give up their water and material to the sea/land as an output.

3. The river channel shape is constantly adjusting to changes in inputs of water and material, so that it can transfer the water and material downstream. This constantly adjusting balance is called Dynamic Equilibrium. There are Seven  variables  of the river channel that are involved in this constant adjustment. Each variable can change as you move along a river’s course , Each variable is linked to the others , so if one changes, the others can too.

4. answer the questions below 1 )How might the channel depth change if discharge increases? 2)How might velocity change if gradient changes? 3)If channel width and depth increase then what will happen to cross-sectional area?

5. 1)How might the channel depth change if discharge increases? The channel will get deeper as discharge increases (to a certain extent) 2) How might velocity change if gradient changes? If gradient is steeper = higher velocity If gradient is gentler = lower velocity 3) If channel width and depth increase then what will happen to cross-sectional area? If width and depth increase then so will cross-sectional area

6. Channel efficiency and a river’s energy A river’s energy is used to erode it’s beds and banks and transport material downstream The more energy it has = the more work it can do However, some of it’s energy has to be used to overcome  friction , so it is not always  efficient.

7. Two key ideas linked to river energy and efficiency…. Hydraulic radius Manning’s equation(channel roughness)

8. Hydraulic Radius The  efficiency o f a river channel = a measure of the ability of a river channel to move water and sediment. It depends on the friction the water comes up against which is shown by the  hydraulic radius  of the river This is the proportion of water in a channel that is in contact with the beds and banks

9. The answers The higher the hydraulic radius the more efficient the river and  the faster it will flow with more energy

10. Manning’s equation Manning (an engineer) worked out that water in channels 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 is on the river’s bed and banks His roughness coefficient is measured using Manning’s ‘n’, which shows the relationship between channel roughness and velocity.  The equation is as follows:

11. Velocity= Hydraulic radius x Gradient/  roughness (n) A high-value indicates a rough bed

12. Erosional processes

13. Cavitation Cavitation  is an interesting method of  erosion . Air bubbles trapped in the water get compressed into small spaces like cracks in the  river's  banks. These bubbles eventually implode creating a small shockwave that weakens the rocks. 

14. types of flow: Erosion is affected by  types of flow: Laminar flow =  horizontal movement of water parallel to the river bed (very rare) Turbulent flow =  a series of eddies (swirls) both vertical and horizontal in a downstream direction (most common)

15. Turbulent flow  causes the most erosion -Horizontal turbulent flow is known has  helicoidal flow   and is associated with hydraulic action as water swirls against the beds and banks which weakens them. This can widen the channel Vertical turbulence is associated with pot holes in the channel bed where vertical abrasion occurs as rocks grind into the bed. This can deepen the channel.

16. Transportational processes The load of a river is transported downstream in 4 ways. 1) Suspension (suspended load)  = Fine clay and sand particles are carried along within the water even at low discharges. 2) Saltation (saltated load)=  Sand, gravels and small stones are bounced along the river bed by the flow of water 3) Traction ( tracted load) =  Boulders and pebbles are rolled along the river bed at times of high discharge 4) Solution (dissolved load) =  Some minerals dissolve in water such as calcium carbonate. This requires very little energy.

17. Hjulstrom curve The  capacity  of a stream refers to the largest amount of debris that a stream can carry -The  competence  refers to the diameter of the largest particle that can be carried -The  critical erosion velocity  is the lowest velocity at which grains of a given size can be moved -The relationship between these variables is shown by means of the  Hjulstrom Curve

18. The Hjulstrom Curve- What is it all about? It shows what velocity is needed to pick up  (entrain)  and erode material, transport it, or deposit it. It shows this for -Clay, Silt, Sand, Pebbles and Boulders

19. Important things to note Most of the time, larger particles such as boulders, need a higher velocity for them to be picked up because of their large size However, the exception to this rule is clay and silt, as even though the particles are very small , the particles tend to stick together, making them hard to pick up. Higher velocities are needed for picking up  (entrainment)  than just for transporting. When velocity falls below a certain level  (settling velocity),  particles are deposited

20. Landforms and channel types found along a river’s course A river channel can be s traight , meandering or braided . *But what do these terms mean?

21. A  straight channel  is very rare, but is exactly what it sounds like! The river is straight along its course!

22. A  meandering channel  is common and occurs where the river moves from side to side When describing meandering or straight channels you can use the term  sinuosity,  which is an indication of how meandering a channel is. It is measured by looking at how long the channel is compared to the valley length through which it runs. For example –  a sinuosity figure higher than 1.5 means the channel meanders, whereas a sinuosity figure lower than 1.5 means it is fairly straight. Over 4.4. means a river has a high sinuosity, and meanders a lot.

23. A Braided Channel  is a channel which is divided by islands or bars . They occur when a river does not have the capacity to transport its load in a single channel.

24. Erosion, transportation and deposition are all processes that create the characteristic features of meanders.

25. There are several stages involved in the creation of meanders.  Stage 1 In low flow conditions straight river channels have bars of sediment on their beds. Flowing water weavers around these bars of sediment. This creates deeper pathways where most of the water flows called pools and shallow areas where less water flows called riffles. This causes the river flow to swing from side to side.

26. Stage 2 Where the river swings towards the bank erosion causes undercutting. On the opposite side of the channel where the velocity is lower material is deposited. Therefore the river does not get any wider.

27. Stage 3 Continued erosion along the outer bank, as the result of hydraulic action and abrasion, creates a river cliff or bluff. A point bar forms on the inner bank. This is a gently sloping deposit of sand, gravel and pebbles.

28. Stage 4 Meanders are perpetuated through a process called helicoidal flow. As the surface flow of water hits the outer bank it corkscrews, flows along the river bed then deposits eroded material on the inner bank.  

29. Step 5 Erosion is greatest beyond the middle of the bend in the meander. This causes the meander to migrate downstream over time. 

30. A braided channel is one that is divided into smaller channels by temporary islands called eyots . Braided channels tend to form ; in rivers that have a significant amount of sedimentary load, a steep profile and where discharge regularly fluctuates

31. When the river’s carrying capacity is exceeded the river deposits its load into the channel and eyots form..

32. The long profile shows how, in the upper stage of a river’s course, the river’s gradient is steep but it gradually flattens out as the river erodes towards its base level.

33. As a river travels along it’s course, the shape of the channel and the landforms which are created by a river change. Some landforms are caused by erosion, some by deposition, some a mixture of both!

34. Processes in the Upper Course In the upper course, the river has a lot of gravitational potential energy so it has a lot of energy to erode vertically. The bed of the river is eroded greatly while the banks aren’t eroded as much. The river mainly transports large pieces of angular rock and does so by traction because it doesn’t have enough kinetic energy to move the load in any other way. This increases erosion of the bed by corrasion as a result of the load being dragged along the bed of the river.

35. Vertical erosion is further increased by the rough nature of the channel in the upper course which increases the water’s turbulence and its ability to erode. Erosion and transportation only takes place in large quantities in the upper course when the river’s discharge is high after periods of heavy precipitation. When the river’s discharge falls the river stops transporting the large boulders its transporting and deposits them.

36. Processes in the Middle Course In the middle course, the river has less gravitational potential energy and more kinetic energy so erosion shifts from vertical to lateral erosion. Corrasion is still the main erosive process as large particles are transported by saltation. The average load size has decreased in the middle course, so more load is being transported in suspension. In the middle course, the river can flood and in doing so, it deposits gravel and sand sized particles onto its flood plain.

37. Processes in the Lower Course In the lower course, the river has next to no gravitational potential energy so erosion is almost exclusively lateral. There isn’t much erosion though because the channel is smoother resulting in less turbulent flow. The main place where erosion takes place is where the river  meanders . The average particle size is very small now, another reason for the reduction in erosion. The river’s load is mainly composed of silts and clays and it is transported in suspension or even solution. Like in the middle course, when the river floods it deposits its load but deposition now also takes place at the mouth of the river where the river meets the sea or a stationary body of water.

38. River Cross Profiles In the upper course, the valley and channel are narrow and deep as a result of the large amount of vertical erosion and little lateral erosion. The sides of a river’s valley in the upper course are very steep earning these valleys the nickname  “V-Shaped Valley”  since they look like a letter V. The river’s valley can be anything from a few meters to a few hundred metres in width depending on the lithology but the channel rarely more than 5m or 6m wide.

39. In the middle course, the valley has increased in width due to the increase in lateral erosion but its depth hasn’t changed significantly because vertical erosion has slowed down. Similarly, the channel’s width has increased but it’s still roughly the same depth. The land to either side of the channel in the valley is now the river’s floodplain and the valley’s sides are much more gentle.

40. In the lower course the valley is now very wide (often several kilometres ) and the floodplain has increased greatly in size. The channel is a little wider but not much deeper.

41. depositional landforms – as they are mainly created through the process of deposition. Deposition occurs where a river slows down and loses its energy so drops its load. In the upper course of a river deposition might occur behind large boulders in slack water In the middle course, deposition increases when the speed of the river is reduced –such as at the inner bend of a meander or when it enters a lake or behind a dam. In the lower course, deposition can occur as river’s enter a standing body of water such as the sea and slows down.

42. Depositional landforms are created anywhere that alluvium (river material) is deposited on land or at sea.

43. 1)Point bar This occurs on the inside bend of a meander when deposition accumulates on the slip of slope and it breaks through the surface of the water to form a bank of land.

44. 2) Floodplain These are formed when river’s burst their banks and flood on a regular basis. Water deposits finer material further away from the river creating sections of flat land around the river.

45. 3) Levees Coarse material is deposited near the channel during repeated floods, building up a bank on either side of a river. Finer material travels further onto the flood plain

46. 4) Alluvial fans Alluvium (material in a river) is dropped by the river when it loses momentum as it enters a wide, flat  valley after leaving a narrow mountain channel. This happens as water velocity, gradient and speed reduces as the water enters a wide unconfined channel, so it is deposited.

47. 5) Deltas These form where the river flows into a standing body of water such as a sea, and deposits it load. The river needs to be carrying a large volume of sediment. Deposition is increased if the water is salty because salt causes clay particles to stick together (flocculate).

48. TYPES OF DELTAS a)Arcuate Delta This shape of delta is made where deposition occurs into a fan shape , found in areas where long shore drift occurs to trim the edges of the landform

49. Arcuate Delta

50. b) Bird’s foot Delta This shape of delta is made where the river brings down a lot of fine silt which can be carried for a very long distance into the sea which makes it long and thin.

51. Bird’s foot Delta

52. c) Cuspate Delta This shape of delta is made where deposition occurs into a point like a tooth. It occurs as two opposing currents hit the deposits from either side.

53. Cuspate Delta

54. Human impact on – Interception, Infiltration and soil water

55. Urbanisation creates impermeable surfaces with compacted soil -This reduces infiltration, so overland flow increases, and floods can occur -Deforestation reduces interception and infiltration and increases overland flow -This is because trees normally channel water down their stems and roots

56. Grazing animals compacts soil, as reduces infiltration -Ploughing for crops increases infiltration as it loosens soil -If the water table is close to the surface, evaporation of water leaves salts behind and can form an impermeable crust -Dams can increase surface storage

57. Human impact on – Changing groundwater,  -Irrigation can reduce groundwater stores -This has occurred in the High Plains of Texas where groundwater has been used to supply centre -pivot irrigation schemes The aquifer has reduced by 50%

58. Cont … In areas where industrial activity has reduced less groundwater is taken out and groundwater levels rise This rise can have implications such as flooding, increase in river flows, pollution, flooding of basements, slope stability reduced, attacks on building foundations Ground water can be recharged by water spreading or pumping water into deep pits or wells (This is used in Israel)

59. Mini Case study of the Aral sea - Aral sea began to shrink in 1960s when water was taken from Syr Darya and Amu Darya rivers -By 1994 shoreline fallen by 16m, surface area declined by 50%, volume reduced by 75% -Salinity levels had increased by 300% -Increased salinity killed fishing industry -Soil fertility decreased -Dust storms occur -Drinking water polluted -Rise in respiratory and stomach disorders  

60. Human impact on – The effects of dams -More large dams being built- Aswan Dam was built in Egypt. Aswan dam has had many advantages and disadvantages Positives - It controls floods and droughts -Helps to irrigate land and grow crops -Produces HEP -Improves navigation and recreation and tourism

61. Negatives there are water losses from evaporation Salinisation has occurred Seepage from reservoir has increased ground water levels 100,000 Nubians displaced from their land Archaeological sites drowned Can create seismic stress Lake has deposition from sediments not carried on Nile delta is eroding Nutrients are lost to flood plain Fish catches are reduced

62. Flooding – CAUSES A flood  is a discharge great enough to cause a body of water to overflow its channel and submerge surrounding land. Bank full discharge  is when a river’s channel fills completely, so that any more water results overflowing the banks Over bank full discharge  is when the level of water in the channel over tops the banks and causes a flood.

63. Some key terms related to flooding The  recurrence interval  refers to the regularity of a flood of a given size Small floods  are expected to occur regularly Large floods  occur less often A  flash flood  is a flood in which the lag time is exceptionally short. A  slow rise flood  is a flood in which the lag time is longer

64. Natural causes of floods Types of precipitation : Climate: Rock type

65. Human causes/intensification of floods Urbanisation: Drainage systems :  Deforestation : Dams:

66. Flooding Solutions There are three major solutions to reduce the impacts of floods 1)Prediction of floods- forecasting and warning 2)Preparing people for floods 3)Prevention and amelioration of floods

67. 1) Prediction -Using weather satellites to predict high rainfall amounts -Estimating rainfall and snow pack amounts -Using river gauges to study river levels over time and map flood recurrence -Create computer flooding models including information on human infrastructure and what would be most at risk.

68. 2) Preparing people for floods Loss sharing adjustments (e.g. disaster aid and insurance) –Removal of settlements from flood plains

69. 3) Prevention and amelioration of floods There are two types of flood protection methods which act to prevent or ameliorate flooding. Hard engineering=   Defence schemes that halt a rivers natural processes. Soft engineering =  involves the use of the natural environment surrounding a river, and the schemes often work with the river’s natural processes.