Rivers
Content
Courses of Rivers: Upper Course Characteristics: Steep V-shaped Valley, Narrow/Shallow Channel, High Bedload Type of Erosion: Vertical Erosion, Weathering Features: V-shaped valley, Interlocking Spurs, Waterfalls, Gorges Middle Course Characteristics: Open/Gentle Sloping Valley with floodplain, Wider/deeper channel, more suspended Sediments Type of Erosion: Lateral Erosion, Transportation Features: Meanders, River Cliffs, Slip off Slopes Lower Course Characteristics: Open/Gentle Sloping Valley with floodplain, Flat and wide floodplain, wide & open valley, Very wide & deep Channel Type of erosion: Transportation, Deposition, Friction is reduced, so greater velocity Features: Ox-bow Lakes, Floodplains, Levees
SABINE NG SHUFEN
3E2
Factors affecting River Speed: Factor Gradient Roughness of river channel Wetted Perimeter Definition How steep a river is How rough a river channel is ( affected by obstacles in river ) Area in contact with the river, shape of the river channel affects the wetted perimeter ( Length and breadth in contact with the river ) High Speed River is steeper ( Usually at upper course ) Fewer obstacles Smoother Channel Faster Smaller Wetted perimeter Less friction Faster Low speed River is gentler ( Usually at lower course) More Obstacles Rougher Channel Slower Larger wetted perimeter More friction Faster
Factors affecting River Volume: Factor Permeability of Rocks Presence of vegetation cover Climate Definition Permeability is the ability of rocks being able to hold water Vegetation: Plants placed by the side of the river How rainfall / temperature affects the river Wet/dry weather water level fluctuates Drainage basin: An area which collects water ( Comprises of rivers ) High Volume Less Permeable rocksHolds less water High volume of water Sparse vegetation less rainwater infiltrates the ground More surface runoff High volume Hot/wet ( High evaporation ) High volume of water Bigger drainage Basin Larger surface area Higher Volume of water Low Volume Permeable rocksholds more water Lesser surface run-off Low volume of water Dense vegetation More rainwater infiltrates the ground Lesser surface run-off Low volume Dry low volume of water throughout the river Smaller Drainage basin Smaller surface area Small volume of water
Size of drainage basin
Important: High Speed + High Volume = HIGH ENERGY.
SABINE NG SHUFEN
3E2
River Processes
EROSION ( CASH ):
Corrasion: Also Known as Abrasion Scrapping, grinding & action of the river’s load Occurs when the river picks up the materials and rubs it along its bed and banks Most effective during load. Major method by which river erodes both vertically and laterally For example: Turbulent eddies in the current can swirl pebbles that are hollow in the river bed and pebble are likely to become trapped.
Attrition: - As bedload moves downstream, Boulders collide with each other. - Impact may break the rocks into smaller pieces - In time, Angular rocks become increasingly rounded Solution: Also known as corrosion This process not dependent on the river’s discharge or velocity When rock minerals dissolve in the water due to chemical reactions Common in Limestone areas Carbonation o Carbon dioxide dissolves in rainwater to form carbonic acid - Solution o Carbonic acid reacts with calcium carbonate (limestone) to form calcium bicarbonate, which is soluble in water. Hydraulic Action: - The action of flowing water hitting against the river banks and beds - Water surges into the cracks and joint to break and loosen pieces of rock and soil - Cavitation: A form of hydraulic action caused by bubbles of air collapsing
SABINE NG SHUFEN
3E2
TRANSPORTATION ( TSSS ):
Traction: - When the large Boulders and cobbles roll or slide along the river bed. - Examples: Rocks and Boulders Saltation: - When Pebbles, sand and gravel (bedload) are lifted up by the current and bounced along the bed in a hopping motion. - Examples: Stones, pebbles, sand and gravel Suspension: - When very fine particles such as clay and slit (suspended load) are dislodged and carried by turbulence in a fast flowing river. - Examples: Clay & Slit Solution: - When dissolved river materials such as calcium bicarbonate are along the riverbed. - Example: Calcium Bicarbonate
DEPOSITION:
A river may deposit its load along the course when: 1. Volume of flow is reduced and/or 2. Speed of flow is decreased 3. It is commonly found in the lower course of the river where the gradient is gentler and the load carries is at its maximum. 4. The bigger and heavier load will be deposited first whereas the finer particles will be transported into the sea or lake.
SABINE NG SHUFEN
3E2
River Management strategies:
Re-alignment: Re-alignment is the straightening of the river by removing meanders and reducing the length of river. This strategy has been implanted in the Mississippi River. This is effective as the shortened length of the river will lead to the increased speed of the river which will wash away the sediments faster. As the sediments get washed away faster, the channel would be deeper and will be able to contain a larger volume of water thereby decreasing the chances of flood. However, by shortening the length of the river, it will cause the wetland to shrink causing the marine organisms to perish. This Strategy is a short term measure as it would require long term maintenance. Rivers will still develop meanders over time. This will make the flow of the river slower and the area around the river will be prone to flooding. Also, Flooding problems may persist downstream as the increased speed of the river will lead to increased sedimentation downstream, making the river shallower. Lastly, it is a very expensive method as resources such as manpower and funds are needed to straighten the river. Vegetation Planting: Vegetation planting is about the use of vegetation such as mangroves to protect the bank against erosion. This strategy has been implanted at Pasir Ris Park and Sungei Api Api. This is an effective method as it reduces the rate of surface run off due to infiltration. At the same time, the root of the plant anchors the soil tightly together hence stabilizing the banks. Vegetation intercepts rainfall thus allowing more time for infiltration preventing large amounts of water from hitting the soil at any one time thereby reducing the amount of eroded sediments from being transported into the river reducing the chances of flood. However, the weight of the vegetation may lead to bank stress which results in the collapse of the banks. This will add debris to the river and might block the passage of water flow resulting in floods. Lastly, the planting of vegetation may result in the loss of aquatic life as excessive shade prevents sunlight from reaching the river-bed and this kills the aquatic plants and affects the aquatic food chains. Re-sectioning: Re-sectioning is the widening and the deepening of the river channel to increase its capability to hold water. This strategy has been implemented at the Singapore River and Kallang River. This strategy is effective as it makes the river channel deeper and increases the channel’s ability to hold water, This strategy also smoothens the river channel thereby reducing the friction and increasing the speed of the water flow allowing sediments to get washed away faster. However, it is not very effective as it may increase sedimentation downstream, making the downstream section of the river shallower and make it more prone to flooding. Lastly, it is a very expensive method as it requires long time maintenance such as dredging.
SABINE NG SHUFEN
3E2
Bank Protection: Bank protection involves the manipulating of the banks to increase river depth and to protect the banks from being eroded. This strategy has been implemented at the yellow river in China. Artificial levees/dykes can be built to increase the river depth while Gabions and revetments can be built to reduce bank erosion. Gabions and revetments reduce the amount of sediments in the river hence preventing the river from becoming shallower. This strategy is effective as a deeper river channel with less sediment increases the river’s capability to hold water. This prevents water from overflowing its banks during heavy rain, hence reducing the chances of flooding. Dykes have not been very successful because of the continual buildup of sediments on the riverbed, making the river shallower. This has caused water levels to rise higher than the dyke walls over the years. Higher dykes have to be built regularly or the sediments have to be dug up and removed from the riverbed. It is an expensive method as it incurs high maintenance costs. Dykes may not be effective in cases of heavy rainfall when the volume of the water in the channel may overwhelm effectiveness of the dykes retaining water in the channel. An example is the dykes along the Mississippi River that burst during the hurricane Katrina in 2005.
SABINE NG SHUFEN
3E2
SABINE NG SHUFEN
3E2
Factors affecting River Speed: Factor Gradient Roughness of river channel Wetted Perimeter Definition How steep a river is How rough a river channel is ( affected by obstacles in river ) Area in contact with the river, shape of the river channel affects the wetted perimeter ( Length and breadth in contact with the river ) High Speed River is steeper ( Usually at upper course ) Fewer obstacles Smoother Channel Faster Smaller Wetted perimeter Less friction Faster Low speed River is gentler ( Usually at lower course) More Obstacles Rougher Channel Slower Larger wetted perimeter More friction Faster
Factors affecting River Volume: Factor Permeability of Rocks Presence of vegetation cover Climate Definition Permeability is the ability of rocks being able to hold water Vegetation: Plants placed by the side of the river How rainfall / temperature affects the river Wet/dry weather water level fluctuates Drainage basin: An area which collects water ( Comprises of rivers ) High Volume Less Permeable rocksHolds less water High volume of water Sparse vegetation less rainwater infiltrates the ground More surface runoff High volume Hot/wet ( High evaporation ) High volume of water Bigger drainage Basin Larger surface area Higher Volume of water Low Volume Permeable rocksholds more water Lesser surface run-off Low volume of water Dense vegetation More rainwater infiltrates the ground Lesser surface run-off Low volume Dry low volume of water throughout the river Smaller Drainage basin Smaller surface area Small volume of water
Size of drainage basin
Important: High Speed + High Volume = HIGH ENERGY.
SABINE NG SHUFEN
3E2
River Processes
EROSION ( CASH ):
Corrasion: Also Known as Abrasion Scrapping, grinding & action of the river’s load Occurs when the river picks up the materials and rubs it along its bed and banks Most effective during load. Major method by which river erodes both vertically and laterally For example: Turbulent eddies in the current can swirl pebbles that are hollow in the river bed and pebble are likely to become trapped.
Attrition: - As bedload moves downstream, Boulders collide with each other. - Impact may break the rocks into smaller pieces - In time, Angular rocks become increasingly rounded Solution: Also known as corrosion This process not dependent on the river’s discharge or velocity When rock minerals dissolve in the water due to chemical reactions Common in Limestone areas Carbonation o Carbon dioxide dissolves in rainwater to form carbonic acid - Solution o Carbonic acid reacts with calcium carbonate (limestone) to form calcium bicarbonate, which is soluble in water. Hydraulic Action: - The action of flowing water hitting against the river banks and beds - Water surges into the cracks and joint to break and loosen pieces of rock and soil - Cavitation: A form of hydraulic action caused by bubbles of air collapsing
SABINE NG SHUFEN
3E2
TRANSPORTATION ( TSSS ):
Traction: - When the large Boulders and cobbles roll or slide along the river bed. - Examples: Rocks and Boulders Saltation: - When Pebbles, sand and gravel (bedload) are lifted up by the current and bounced along the bed in a hopping motion. - Examples: Stones, pebbles, sand and gravel Suspension: - When very fine particles such as clay and slit (suspended load) are dislodged and carried by turbulence in a fast flowing river. - Examples: Clay & Slit Solution: - When dissolved river materials such as calcium bicarbonate are along the riverbed. - Example: Calcium Bicarbonate
DEPOSITION:
A river may deposit its load along the course when: 1. Volume of flow is reduced and/or 2. Speed of flow is decreased 3. It is commonly found in the lower course of the river where the gradient is gentler and the load carries is at its maximum. 4. The bigger and heavier load will be deposited first whereas the finer particles will be transported into the sea or lake.
SABINE NG SHUFEN
3E2
River Management strategies:
Re-alignment: Re-alignment is the straightening of the river by removing meanders and reducing the length of river. This strategy has been implanted in the Mississippi River. This is effective as the shortened length of the river will lead to the increased speed of the river which will wash away the sediments faster. As the sediments get washed away faster, the channel would be deeper and will be able to contain a larger volume of water thereby decreasing the chances of flood. However, by shortening the length of the river, it will cause the wetland to shrink causing the marine organisms to perish. This Strategy is a short term measure as it would require long term maintenance. Rivers will still develop meanders over time. This will make the flow of the river slower and the area around the river will be prone to flooding. Also, Flooding problems may persist downstream as the increased speed of the river will lead to increased sedimentation downstream, making the river shallower. Lastly, it is a very expensive method as resources such as manpower and funds are needed to straighten the river. Vegetation Planting: Vegetation planting is about the use of vegetation such as mangroves to protect the bank against erosion. This strategy has been implanted at Pasir Ris Park and Sungei Api Api. This is an effective method as it reduces the rate of surface run off due to infiltration. At the same time, the root of the plant anchors the soil tightly together hence stabilizing the banks. Vegetation intercepts rainfall thus allowing more time for infiltration preventing large amounts of water from hitting the soil at any one time thereby reducing the amount of eroded sediments from being transported into the river reducing the chances of flood. However, the weight of the vegetation may lead to bank stress which results in the collapse of the banks. This will add debris to the river and might block the passage of water flow resulting in floods. Lastly, the planting of vegetation may result in the loss of aquatic life as excessive shade prevents sunlight from reaching the river-bed and this kills the aquatic plants and affects the aquatic food chains. Re-sectioning: Re-sectioning is the widening and the deepening of the river channel to increase its capability to hold water. This strategy has been implemented at the Singapore River and Kallang River. This strategy is effective as it makes the river channel deeper and increases the channel’s ability to hold water, This strategy also smoothens the river channel thereby reducing the friction and increasing the speed of the water flow allowing sediments to get washed away faster. However, it is not very effective as it may increase sedimentation downstream, making the downstream section of the river shallower and make it more prone to flooding. Lastly, it is a very expensive method as it requires long time maintenance such as dredging.
SABINE NG SHUFEN
3E2
Bank Protection: Bank protection involves the manipulating of the banks to increase river depth and to protect the banks from being eroded. This strategy has been implemented at the yellow river in China. Artificial levees/dykes can be built to increase the river depth while Gabions and revetments can be built to reduce bank erosion. Gabions and revetments reduce the amount of sediments in the river hence preventing the river from becoming shallower. This strategy is effective as a deeper river channel with less sediment increases the river’s capability to hold water. This prevents water from overflowing its banks during heavy rain, hence reducing the chances of flooding. Dykes have not been very successful because of the continual buildup of sediments on the riverbed, making the river shallower. This has caused water levels to rise higher than the dyke walls over the years. Higher dykes have to be built regularly or the sediments have to be dug up and removed from the riverbed. It is an expensive method as it incurs high maintenance costs. Dykes may not be effective in cases of heavy rainfall when the volume of the water in the channel may overwhelm effectiveness of the dykes retaining water in the channel. An example is the dykes along the Mississippi River that burst during the hurricane Katrina in 2005.
SABINE NG SHUFEN
3E2
