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The revision notes by Mdm Wendy:)

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  1. 1. GEOGRAPHY REVISION TOPIC: RIVERS (N2002 P2 Q3) 1a. Study Fig. 1.i. What is the general relationship shown by the graph? [2] The graph shows that as the speed of the river increase, the river is able to transport bigger materials. (Note: Very small particles of 0.01mm and less cannot be deposited at any river velocity because they are too small and light to settle on the river bed. Particles of size 0.01mm and above will remain deposited until river velocity exceeds that of the minimum velocity required to transport the particles for their various sizes.)ii. Describe how the river might transport different sized particles as shown in Fig 1. [3] Eroded materials such as mud, sand and boulders are carried or transported along by the river. These materials, called load, can be transported in many ways. One way is by suspension. Small particles such as clay (0.001 to 0.01mm) move in suspension and are carried away without touching the river bed. Another way is saltation, where larger particles such as silt, sand and gravel are too heavy to be suspended, are carried along in a series of bouncing motions along the river bed. Finally, traction is when large stones such as cobbles or boulders roll or slide along the river bed.
  2. 2. Fig.2 (N2003 P2 Q4)2a. Fig. 2 shows part of a river and its valley together with a cross section of the river’s channel.i. Describe how and explain why the river will change its course at X on Fig 2. [4] The meander at X may slowly grow into a pronounced loop. An ox bow lake will formed as a result of river deposition at both ends of the cut-off of a meander. With constant erosion of the outer/concave bank and deposition on the inner/convex bank, a meander becomes very pronounced, forming a loop and separated by a narrow neck of land (Fig. A). The neck becomes narrower with continued erosion and deposition and the two meanders eventually meet (Fig. B). Overtime, deposition begins to cut the river from the meander. The river then flows straight through the channel (Fig. C). Eventually, the sediments deposited will dam up the water, causing the loop to be cut-off, forming an oxbow lake. (Fig. D). As it is cut off from the main river, the oxbow lake will eventually dry up due to evaporation, encouraging the growth of vegetation. Note: Refer to Rivers Notes Pg 15 for annotated diagrams on formation of oxbow lakes.ii. At which location P or Q or R along the river was the cross section A-B drawn. Give a reason for your answer explaining also why you reject the other two locations. [4] Q. At Q, the river has an acute meander with erosion on the concave or outer bank of the river which will eventually become the steep-sided river cliff facing the deposited slip-off slope. The river does not meander at P and at R, the opposite cross-section should be shown.
  3. 3. iii. Explain the shape of the river channel shown in the cross section. [4] The river channel is at its middle course where lateral erosion is greater than vertical erosion. Water flow is fastest along the outer bank resulting in erosion. Water flow is slow in the inner bank resulting in deposition. The eroded materials from the concave bank are transported by eddies to the inner convex bank and deposited there. (Note: An eddy is a place in a river where the water is moving in a different direction or different speed than the main current.)(N2008 P1 Q3)3. Compare the effective of the different measures of river channel management [8]- Question ask to compare, so you will need to compare the different measures. Below is a suggested answer.- Some questions may ask on effectiveness without comparison, then answer without comparing the measures.- Note: “River channelization” = River Management Strategies = Measures = Ways[Intro]Channel management of rivers is important in the prevention of floods which may result isnegative impacts of the environment, human settlements and their properties. Methods usedinclude re-alignment, re-sectioning , bank protection and planting vegetation. However, theeffectiveness of these methods varies as the different methods result in different consequences.[Measure 1]Re-alignment of a river channel by straightening a meandering river is an efficient wayincreasing the speed and the volume of water flow. This method would shorten the river courseand effectively allows water and its sediment load to flow away from the area more quickly. Inthe case of Mississippi River, USA, shortening the river by 240km has helped reduce flowingand preventing economic and property losses.However, re-alignment of a section of the river course may result in flooding downstream whereno improvement was carried out. This is because of the sudden increase in the volume of waterdownstream. As more sediments are transported downstream, the rate of deposition willincrease and consequently reducing its capacity of flow. Excess sediments deposited at theriver mouth will also cause less sunlight to reach the seabed and affect the growth of marine lifesuch as sea grass and coral reefs.The above are also the consequences of re-sectioning of the river where the river bed is linedwith cement. However, the effects are more pronounced in the re-alignment method as it isusually done higher up in the river course.[Measure 2]Re-sectioning by widening and deepening sections of the river channel and lining with cementincreases the channel’s ability to hold water. The smoothened river bed decreases friction andincrease the speed of flow and this reduce flooding. This method is used at the Singapore Riverby dredging the river bed and its banks are reinforced with concrete to reduce erosion andfriction, allowing the river to flow efficiently. The amount of surface runoff flowing from thesurrounding areas is increased because more surface runoff can now enter the river withoutcausing it to flood.
  4. 4. However, this method may not be very effective as sedimentation may increase downstreammaking the river shallow and reducing its capacity to carry water, thereby causing floodingdownstream. In addition, this method could be costly as sediments accumulate on the river bedover time, and regular dredging of the river has to be carried out to maintain its effectiveness.[Measure 3]The bank protection method involved building of concrete structures such as dykes, gabionsand revetments, to prevent floods and reduce erosion along river banks. For example, dykessupported by stones have been built along the Yellow River in China to reduce the effects ofriver flooding into populated and agricultural areas. This measure is effective as a short termmeasure as dykes reduce the risk of bank collapse and erosion during heavy rains whilegabions and revetments prevent bank erosion by diverting the flow of water from banks to thecenter of the river, reducing sediment build-up, increasing water holding capacity and hencereduces chances of flooding.This method has not been very successful because deposition takes place continually along thedykes, gabions and revetments that are made. This build up of deposition causes the channel tobecome shallower thus decreasing its ability to carry large volume of water. Over the years, thewater level rise higher than the protection walls, causing flooding. For example, breaches indykes near the city of Xhengzhou in China have cause destruction and devastation to its people.[Measure 4]Planting of vegetation along the river can also protect its bank against soil erosion as the rootsof trees and plants are able to hold the soil firmly, thereby reducing the chance of soil erodinginto the river channel. The vegetation also provides interception and slows down the speed ofsurface water flow, thus preventing huge surface runoff from entering the river. This method hasproved to be an efficient way of controlling floods as unlike other methods, its purpose is not toincrease river capacity but to decrease the surface run off into rivers.However, a consequence of planting vegetation is that it prevents sunlight from reaching theriver bed due to its excessive shade, thus killing aquatic plants and affecting aquatic food chains.It should be noted that all the measures may lead to the destruction of ecosystems, causing lossof marine life and wetlands. This is especially so for re-sectioning of the river bed where it maybe lined with cement.[Suggested Conclusion]Each measure has its strengths and weaknesses. A careful study of each channelization projectis necessary to enhance the positive effects and minimize the negative effects onto theenvironment. Adopting a combination of measures may be the best strategy so as to achievethe dual objectives.Remarks: Please refer to River Notes pages 17-20 for details on River Channel Management Strategies.