Geography AQA - Rivers, Floods and Management Notes
Here are a complete set of river notes:
Geography – Rivers, Floods and Management
1) The Drainage Basin Hydrological Cycle
A drainage basin is defined as the area of land drained by a river and its tributaries. It is normally surrounded by a ridge of highland known as a watershed. The water balance shows the state of equilibrium between inputs, outputs and changes in groundwater storage in a drainage basin.
Feedback loops are when changes in inputs and outputs move the system away from its state of equilibrium. There are two types of feedback loops: negative and positive. Movements of water in a drainage basin can be explained with the use of the hydrological cycle.
The hydrological cycle refers to the movement of water between atmosphere, lithosphere and biosphere. At a global scale it is seen as a closed system with no losses or gains. At a local scale (drainage basin) the cycle has a single input precipitation and two major losses evapotranspiration and runoff.
2) Factors Affecting River Discharge
Discharge is the quantity of water that passes a given point on the bank of a river within a given interval of time measured in Cumecs. A hydrograph is a graph showing changes in the discharge of a river over a period of time usually a year, this type of graphs are also known as river regimes.
A storm hydrograph shows the response of a river in terms of discharge, to a particular rainfall event. Key terms in a storm hydrograph: Baseflow is the normal level of the river which is fed by groundwater, Lag time is the time delay between peak rainfall and peak discharge, Rising limb shows how discharge rises and Recession limb shows how discharge falls.
Drainage density of a basin is defined as the total length of channels divided by the basin area. The higher the drainage density the higher the risk of flood since water reaches faster the river channel.
Factors affecting both river discharge and hydrographs are: the amount and nature of precipitation, the local rocks especially their porosity and permeability, the shape or morphology of the drainage basin, the amount and type of vegetation cover and the amount and type of soil cover. However, the most important factor affecting discharge on an annual basis is climate since this has an effect on amount of evaporation, rate of transpiration or plant growth.
3) The Long Profile
The long profile of a river is the gradient of the channel bed from source to mouth.
Water in a river channel has a given amount of energy. There are two types of energy kinetic (energy produced by movement of water) and potential (energy due to the weight of water). The energy is first used up in overcoming friction with bed and banks. Having overcome friction the energy is used up for transportation. Any surplus of energy left is used for erosion and when energy is reduced deposition occurs.
The ability of transportation of a river can be seen from competence (size of largest particle a river can carry) and capacity (largest amount of material a river can carry). The material carried by a river is called load and there are three types. Dissolved load are the chemicals dissolved in water which are invisibly transported. Suspended load is the sediment that has been whisked up by the water and then is carried by the flow, it is the main form of sediment transfer and explains why most rivers look muddy. Bedload are material which are too heavy to carry and may be rolled (traction) or bounced (saltation) along a river bed.
Erosion is the picking up and removal of material. There are four processes:
Corrasion is where particles of rock carried by the river grind away at the bed and banks; Hydraulic action is the sheer weight and impact of water against the river bed and banks; Solution is the dissolving of material;
Attrition is when particles carried by the river crash with one another
wearing away.
Deposition occurs when there is a reduction of river energy, this can happen when: river enters sea or lake, river floods, shallow water, smoother gradient, sudden increase in river load or decrease in discharge by reduced precipitation.
The main factor that controls transportation, erosion and deposition is velocity. This can be seen from the Hjulstrom curve which is a graph illustrating the relationship between velocity and competence. The mean or critical erosion velocity shows the velocity required to pick up (entrainment) and transport load. The mean fall or setting velocity curve shows the velocity at which each calibre of sediment is deposited.
4) The Valley Profile
The valley profile is the view of the valley from one side to the other. In the upper course the river flows in a narrow, steep-sided valley where it occupies the entire floor as a result of dominant vertical erosion. In the middle course there is a wider valley and flat floodplain as a result of lateral erosion. In the lower course there is a wide flat floodplain with valley sides difficult to locate and lack of erosion.
The graded profile is a state of balance where slope, width and other channel characteristics have adjusted to the volume of water and load carried under the prevailing conditions of a river. In this state all factors are in balance, all the energy is used for transportation with no excess erosion or deficit for deposition.
5) Changing channel characteristics
The hydraulic radius of a river is a measure of its efficiency and it is calculated by dividing cross sectional area into wetted perimeter. The hydraulic radius expresses the losses of energy in overcoming friction with a streams bed and banks. Higher values indicate higher efficiency.
Bradshaw’s model shows changes in channel characteristics.
6) Landforms of Fluvial Erosion and Deposition
A pothole is a circular depression on the river bed carved out of solid rock. It is formed by a kind of drilling action as pebbles are caught in eddy currents and whisked around within a small natural crack or hollow. As time passes, the drilling action enlarges the hollow to form a pothole. Potholes are commonly found below waterfalls or rapids where hydraulic action is a significant process.
A waterfall is a sudden step in a rivers long profile. It is often the result of a tougher more resistant band of rock cutting across the valley. Unable to erode the rock at the same rate as the neighbouring rocks, a step is formed and a waterfall results. Over time, the river cuts backwards into the resistant rock causing the waterfall to retreat backwards forming a narrow, steep-sided gorge.
Rapids are a series of small waterfalls associated with very disturbed turbulent water. Rapids are formed when rocks of varying resistances cut across a valley and erosive processes create a series of steps.
Menders are sudden bends in the course of a river. Meanders form when areas of alternating pools (deep water) and riffles (shallow water) develop at equally spaced intervals along a stretch of river. The distance between pools is usually 5-6 times the width of the river bed. Because water is deeper in pools, the river is more efficient when passing over them. The energy and erosive power is therefore increased when passing over these areas. On the other hand, the river is less efficient when passing over riffles as there is more friction causing the river to lose energy. This combination of the river gaining and losing efficiency at different intervals causes the river’s flow to become uneven, and maximum flow is concentrated on one side of the river. As the water speeds up, turbulence increases in and around pools. This leads to corkscrew-like currents in the river called helicoidal flow. These helicoidal currents spiral from bank to bank, causing more lateral erosion (abrasion and hydraulic action) and deepening of the pools – river cliff. This leads to the increased amount of eroded material being deposited on the inside of the next bend where the river loses energy – slip off slope. Combination of erosion and deposition exaggerates bend until large meanders are formed. These combined processes also give the meander’s asymmetric cross-section.
Oxbow lakes form because in meanders erosion is concentrated on the outer bend while deposition is predominant on the inner bend leading to the formation of point bars. The alternating zones of erosion and deposition cause meanders to migrate both across and down the valley. In addition meanders become more exaggerated and sinous. As opposite bends erode towards each other, the neck of a meander will get progressively narrower until, during a period of high discharge, the river will cut through forming an oxbow lake.
Braiding is when a river subdivides into smaller streams. In between these channels small islands of deposited sediment will form. Braiding occurs where there is a sudden decrease in energy causing deposition of large amounts of sediments making the main channel to subdivide.
A floodplain is a flat area of land bordering a river that is subjected to periodic flooding. It is made up of silts and sands which have been deposited over many years. As a river floods its velocity is dramatically decreased causing energy to be reduced and a bulk of sediment to be floating in a thin sheet of water on the floodplain. Once the water has evaporated a fresh layer of alluvium is left behind.
Levees are natural embankments of sediment formed when the river floods.
A delta is formed when a river enters a sea or lake loosing velocity and energy causing a large amount of sediment to be deposited as a delta. Rivers flowing over deltas tend to break into many smaller channels called distributaries. A type of delta is the bird’s foot.
7) Process and Impact of Rejuvenation
Rejuvenation causes the river to increase its downcutting activity.
Rejuvenation is caused by tectonic activity causing dynamic rejuvenation, an increase in the volume of water in the drainage system, changes in base level either sea level falls or land rises due to eustatic fall in sea level (global fall in the level of the sea) or an isostatic change in the level of the land (local uplift of land).
Knick points are breaks in gradient along the profile of a river usually marked by rapids or waterfalls.
River terraces are the remmants of the former floodplain prior to rejuvenation. Terraces create steps in the valley cross profile.
Incised meanders
8) Physical and Human Causes of Flooding
Physical causes of flooding are: intense precipitation events, higher than average precipitation over a short period of time, already saturated ground when precipitation event takes place, rapid snowmelt, sea level rise, storm surges in coastal areas.
Human causes of flooding are: deforestation, urbanisation, global warning, and hard engineering.
9) Flood Management Strategies
Hard-engineering solutions:
Flood embankments: artificial raise or straightening of channels allowing a greater capacity for water
Channel enlargement: this involves dredging and the removal of large boulders from the river bed. It increases channel efficiency and reduces roughness.
Flood relief channels: they take excess water around a settlement
Dams: allows water to be stored temporarily in a reservoir and regulates the rate at which water passes down the river
Removal of settlements
Soft-engineering solutions:
• Reforestation
• Forecasts and Warnings
• Land use zoning
• River restoration schemes
• Wetland and river bank conservation
9) Impacts of Floodings
Carlisle Floods, January 2005
Introduction:
Is a city in Cumbria in North West England
It is located at the confluence of the rivers Eden, Caldew and Petteril, 10 miles (16 km) south of the Scottish border
It is the largest settlement in the county of Cumbria with a population of 71,773 people (2001 census)
Short term impacts:
3 people died
120 flood-related injuries
Communications were affected
Roads became impassable therefore people were stranded
Long term impacts:
1925 homes and businesses flooded up to 2 meters
More than 3,000 people become homeless and some for more than 3 months40,000 addresses were left without power
3,000 jobs were put at risk
Damage to public perception of Carlisle
Emotional impacts on those involved in the floods
Extremely high financial costs
Flood Management Strategies:
•
The Lower Eden Strategic and Planning Appraisal Report (SPAR) came up with three main options for flood management
1) Do minimum – this meant sustaining the present level of flood defence
2) Upstream storage - this option would involve the creation of large-scale upstream storage reservoirs, for example through the construction of a large dam across the river Eden and excavation at the M6
3) Upstream managed realignment – this option would involve moving the existing line of flood defence back from the river to provide a larger area of natural floodplain storage and to raise existing flood defences
Finally, option 3 was selected since:
1) Option 1 was not acceptable owing to the risk to property and lives
2) Option 2 was rejected as it involved unsupportable economic and environmental costs
3) Option 3 was selected since it was identified as environmentally acceptable, with the latter option providing more opportunity for environmental enhancement
This option would provide flood protection to an estimated 1 in 200-year standard
Bangladesh Floods, July-September1998
Introduction:
Bangladesh is a country in South Asia
Bangladesh is the seventh most populous country in the world and is among the most densely populated countries in the world with a high poverty rate
Bangladesh has three of the most powerful rivers passing through it, the Ganges, the Brahmaputra and the Meghna
Bangladesh is subject to annual monsoon floods and cyclones every year
There are some facts that encourage flooding in this area:
• 1) Most of the country consists of a huge flood plain and delta
• 2) 70% of the total area is less than 1 meter above sea level
• 3) 10% of the land area is made up of Lakes and Rivers
• 4) Snowmelt from the Himalayas takes place in late spring & summer
• 5) Bangladesh experiences heavy monsoon rains, especially over the highlands
• Between July-September 1998, Bangladesh suffered one of its worse ever floods
Causes:
The flooding in July and August was caused by heavy intense monsoon rainfall accompanied by snowmelt, closely followed by heavy rains over Bangladesh in September which raised the already high discharge in the three main rivers even further
The depth of the Ganges in July was 13m, just below the flood level of 14m but rose to 15m in August
The Brahmaputra exceeded its flood level of 15m in July and again in August and September
The Meghna was above its flood level of 6m for the whole of July, August and September
The level of the sea in the Bay of Bengal was also high (a record 5.5m above average sea level on 10th September) and this slowed down the normal flow of water which resulted in higher depths in the rivers
The extraction of groundwater for irrigation had lowered the water table and caused the land to subside by about 2.5m
The use of water for irrigation upstream has reduced the amount of silt deposited so the level of the land has not been built up
Deforestation in the Himalayas has increased run off
Increased amounts of urbanisation has led to higher peak flow on the rivers with much shorter lag times and a greater frequency of floods
Short term impacts:
• 75% of the country was affected
• Over 1 million km² of area was flooded
• 31 million people were affected
• 1050 people died
• 30 million people were left homeless
Long term impacts:
• Floods destroyed a total of 668,529ha of crops
• Bangladesh export industries decreased a 20%
• 14,000 schools were flooded
• 4528km of flood embankments were damaged
• Communications were nearly destroyed
• 26,500 pieces of livestock were lost
• 980,000 houses were destroyed or partially damaged
• 300,000 rural hand tube wells (for irrigation) were flooded
Short term flood relief measures:
volunteers / aid workers worked to try and repair flood damage
international food aid programmes provided support
the distribution of free seed to farmers by the Bangladesh government to try and reduce the impact of food shortages
rapid creation of shelters for all the people who had been left homeless
Long term flood relief measures:
the creation of embankments (artificial levees) along the river to increase channel capacity and restrict flood waters
constructing flood protection shelters (large buildings raised above the ground) to shelter both people and animals
building flood proof storage sheds for grain and other food supplies
emergency flood warning systems and plans made for organising rescue and relief services
providing emergency medical stores in villages
dam construction upstream and major embankments around Dhaka have been suggested however lack of money has meant that these suggestions have not been taken further.
Geography – Rivers, Floods and Management
1) The Drainage Basin Hydrological Cycle
A drainage basin is defined as the area of land drained by a river and its tributaries. It is normally surrounded by a ridge of highland known as a watershed. The water balance shows the state of equilibrium between inputs, outputs and changes in groundwater storage in a drainage basin.
Feedback loops are when changes in inputs and outputs move the system away from its state of equilibrium. There are two types of feedback loops: negative and positive. Movements of water in a drainage basin can be explained with the use of the hydrological cycle.
The hydrological cycle refers to the movement of water between atmosphere, lithosphere and biosphere. At a global scale it is seen as a closed system with no losses or gains. At a local scale (drainage basin) the cycle has a single input precipitation and two major losses evapotranspiration and runoff.
2) Factors Affecting River Discharge
Discharge is the quantity of water that passes a given point on the bank of a river within a given interval of time measured in Cumecs. A hydrograph is a graph showing changes in the discharge of a river over a period of time usually a year, this type of graphs are also known as river regimes.
A storm hydrograph shows the response of a river in terms of discharge, to a particular rainfall event. Key terms in a storm hydrograph: Baseflow is the normal level of the river which is fed by groundwater, Lag time is the time delay between peak rainfall and peak discharge, Rising limb shows how discharge rises and Recession limb shows how discharge falls.
Drainage density of a basin is defined as the total length of channels divided by the basin area. The higher the drainage density the higher the risk of flood since water reaches faster the river channel.
Factors affecting both river discharge and hydrographs are: the amount and nature of precipitation, the local rocks especially their porosity and permeability, the shape or morphology of the drainage basin, the amount and type of vegetation cover and the amount and type of soil cover. However, the most important factor affecting discharge on an annual basis is climate since this has an effect on amount of evaporation, rate of transpiration or plant growth.
3) The Long Profile
The long profile of a river is the gradient of the channel bed from source to mouth.
Water in a river channel has a given amount of energy. There are two types of energy kinetic (energy produced by movement of water) and potential (energy due to the weight of water). The energy is first used up in overcoming friction with bed and banks. Having overcome friction the energy is used up for transportation. Any surplus of energy left is used for erosion and when energy is reduced deposition occurs.
The ability of transportation of a river can be seen from competence (size of largest particle a river can carry) and capacity (largest amount of material a river can carry). The material carried by a river is called load and there are three types. Dissolved load are the chemicals dissolved in water which are invisibly transported. Suspended load is the sediment that has been whisked up by the water and then is carried by the flow, it is the main form of sediment transfer and explains why most rivers look muddy. Bedload are material which are too heavy to carry and may be rolled (traction) or bounced (saltation) along a river bed.
Erosion is the picking up and removal of material. There are four processes:
Corrasion is where particles of rock carried by the river grind away at the bed and banks; Hydraulic action is the sheer weight and impact of water against the river bed and banks; Solution is the dissolving of material;
Attrition is when particles carried by the river crash with one another
wearing away.
Deposition occurs when there is a reduction of river energy, this can happen when: river enters sea or lake, river floods, shallow water, smoother gradient, sudden increase in river load or decrease in discharge by reduced precipitation.
The main factor that controls transportation, erosion and deposition is velocity. This can be seen from the Hjulstrom curve which is a graph illustrating the relationship between velocity and competence. The mean or critical erosion velocity shows the velocity required to pick up (entrainment) and transport load. The mean fall or setting velocity curve shows the velocity at which each calibre of sediment is deposited.
4) The Valley Profile
The valley profile is the view of the valley from one side to the other. In the upper course the river flows in a narrow, steep-sided valley where it occupies the entire floor as a result of dominant vertical erosion. In the middle course there is a wider valley and flat floodplain as a result of lateral erosion. In the lower course there is a wide flat floodplain with valley sides difficult to locate and lack of erosion.
The graded profile is a state of balance where slope, width and other channel characteristics have adjusted to the volume of water and load carried under the prevailing conditions of a river. In this state all factors are in balance, all the energy is used for transportation with no excess erosion or deficit for deposition.
5) Changing channel characteristics
The hydraulic radius of a river is a measure of its efficiency and it is calculated by dividing cross sectional area into wetted perimeter. The hydraulic radius expresses the losses of energy in overcoming friction with a streams bed and banks. Higher values indicate higher efficiency.
Bradshaw’s model shows changes in channel characteristics.
6) Landforms of Fluvial Erosion and Deposition
A pothole is a circular depression on the river bed carved out of solid rock. It is formed by a kind of drilling action as pebbles are caught in eddy currents and whisked around within a small natural crack or hollow. As time passes, the drilling action enlarges the hollow to form a pothole. Potholes are commonly found below waterfalls or rapids where hydraulic action is a significant process.
A waterfall is a sudden step in a rivers long profile. It is often the result of a tougher more resistant band of rock cutting across the valley. Unable to erode the rock at the same rate as the neighbouring rocks, a step is formed and a waterfall results. Over time, the river cuts backwards into the resistant rock causing the waterfall to retreat backwards forming a narrow, steep-sided gorge.
Rapids are a series of small waterfalls associated with very disturbed turbulent water. Rapids are formed when rocks of varying resistances cut across a valley and erosive processes create a series of steps.
Menders are sudden bends in the course of a river. Meanders form when areas of alternating pools (deep water) and riffles (shallow water) develop at equally spaced intervals along a stretch of river. The distance between pools is usually 5-6 times the width of the river bed. Because water is deeper in pools, the river is more efficient when passing over them. The energy and erosive power is therefore increased when passing over these areas. On the other hand, the river is less efficient when passing over riffles as there is more friction causing the river to lose energy. This combination of the river gaining and losing efficiency at different intervals causes the river’s flow to become uneven, and maximum flow is concentrated on one side of the river. As the water speeds up, turbulence increases in and around pools. This leads to corkscrew-like currents in the river called helicoidal flow. These helicoidal currents spiral from bank to bank, causing more lateral erosion (abrasion and hydraulic action) and deepening of the pools – river cliff. This leads to the increased amount of eroded material being deposited on the inside of the next bend where the river loses energy – slip off slope. Combination of erosion and deposition exaggerates bend until large meanders are formed. These combined processes also give the meander’s asymmetric cross-section.
Oxbow lakes form because in meanders erosion is concentrated on the outer bend while deposition is predominant on the inner bend leading to the formation of point bars. The alternating zones of erosion and deposition cause meanders to migrate both across and down the valley. In addition meanders become more exaggerated and sinous. As opposite bends erode towards each other, the neck of a meander will get progressively narrower until, during a period of high discharge, the river will cut through forming an oxbow lake.
Braiding is when a river subdivides into smaller streams. In between these channels small islands of deposited sediment will form. Braiding occurs where there is a sudden decrease in energy causing deposition of large amounts of sediments making the main channel to subdivide.
A floodplain is a flat area of land bordering a river that is subjected to periodic flooding. It is made up of silts and sands which have been deposited over many years. As a river floods its velocity is dramatically decreased causing energy to be reduced and a bulk of sediment to be floating in a thin sheet of water on the floodplain. Once the water has evaporated a fresh layer of alluvium is left behind.
Levees are natural embankments of sediment formed when the river floods.
A delta is formed when a river enters a sea or lake loosing velocity and energy causing a large amount of sediment to be deposited as a delta. Rivers flowing over deltas tend to break into many smaller channels called distributaries. A type of delta is the bird’s foot.
7) Process and Impact of Rejuvenation
Rejuvenation causes the river to increase its downcutting activity.
Rejuvenation is caused by tectonic activity causing dynamic rejuvenation, an increase in the volume of water in the drainage system, changes in base level either sea level falls or land rises due to eustatic fall in sea level (global fall in the level of the sea) or an isostatic change in the level of the land (local uplift of land).
Knick points are breaks in gradient along the profile of a river usually marked by rapids or waterfalls.
River terraces are the remmants of the former floodplain prior to rejuvenation. Terraces create steps in the valley cross profile.
Incised meanders
8) Physical and Human Causes of Flooding
Physical causes of flooding are: intense precipitation events, higher than average precipitation over a short period of time, already saturated ground when precipitation event takes place, rapid snowmelt, sea level rise, storm surges in coastal areas.
Human causes of flooding are: deforestation, urbanisation, global warning, and hard engineering.
9) Flood Management Strategies
Hard-engineering solutions:
Flood embankments: artificial raise or straightening of channels allowing a greater capacity for water
Channel enlargement: this involves dredging and the removal of large boulders from the river bed. It increases channel efficiency and reduces roughness.
Flood relief channels: they take excess water around a settlement
Dams: allows water to be stored temporarily in a reservoir and regulates the rate at which water passes down the river
Removal of settlements
Soft-engineering solutions:
• Reforestation
• Forecasts and Warnings
• Land use zoning
• River restoration schemes
• Wetland and river bank conservation
9) Impacts of Floodings
Carlisle Floods, January 2005
Introduction:
Is a city in Cumbria in North West England
It is located at the confluence of the rivers Eden, Caldew and Petteril, 10 miles (16 km) south of the Scottish border
It is the largest settlement in the county of Cumbria with a population of 71,773 people (2001 census)
Short term impacts:
3 people died
120 flood-related injuries
Communications were affected
Roads became impassable therefore people were stranded
Long term impacts:
1925 homes and businesses flooded up to 2 meters
More than 3,000 people become homeless and some for more than 3 months40,000 addresses were left without power
3,000 jobs were put at risk
Damage to public perception of Carlisle
Emotional impacts on those involved in the floods
Extremely high financial costs
Flood Management Strategies:
•
The Lower Eden Strategic and Planning Appraisal Report (SPAR) came up with three main options for flood management
1) Do minimum – this meant sustaining the present level of flood defence
2) Upstream storage - this option would involve the creation of large-scale upstream storage reservoirs, for example through the construction of a large dam across the river Eden and excavation at the M6
3) Upstream managed realignment – this option would involve moving the existing line of flood defence back from the river to provide a larger area of natural floodplain storage and to raise existing flood defences
Finally, option 3 was selected since:
1) Option 1 was not acceptable owing to the risk to property and lives
2) Option 2 was rejected as it involved unsupportable economic and environmental costs
3) Option 3 was selected since it was identified as environmentally acceptable, with the latter option providing more opportunity for environmental enhancement
This option would provide flood protection to an estimated 1 in 200-year standard
Bangladesh Floods, July-September1998
Introduction:
Bangladesh is a country in South Asia
Bangladesh is the seventh most populous country in the world and is among the most densely populated countries in the world with a high poverty rate
Bangladesh has three of the most powerful rivers passing through it, the Ganges, the Brahmaputra and the Meghna
Bangladesh is subject to annual monsoon floods and cyclones every year
There are some facts that encourage flooding in this area:
• 1) Most of the country consists of a huge flood plain and delta
• 2) 70% of the total area is less than 1 meter above sea level
• 3) 10% of the land area is made up of Lakes and Rivers
• 4) Snowmelt from the Himalayas takes place in late spring & summer
• 5) Bangladesh experiences heavy monsoon rains, especially over the highlands
• Between July-September 1998, Bangladesh suffered one of its worse ever floods
Causes:
The flooding in July and August was caused by heavy intense monsoon rainfall accompanied by snowmelt, closely followed by heavy rains over Bangladesh in September which raised the already high discharge in the three main rivers even further
The depth of the Ganges in July was 13m, just below the flood level of 14m but rose to 15m in August
The Brahmaputra exceeded its flood level of 15m in July and again in August and September
The Meghna was above its flood level of 6m for the whole of July, August and September
The level of the sea in the Bay of Bengal was also high (a record 5.5m above average sea level on 10th September) and this slowed down the normal flow of water which resulted in higher depths in the rivers
The extraction of groundwater for irrigation had lowered the water table and caused the land to subside by about 2.5m
The use of water for irrigation upstream has reduced the amount of silt deposited so the level of the land has not been built up
Deforestation in the Himalayas has increased run off
Increased amounts of urbanisation has led to higher peak flow on the rivers with much shorter lag times and a greater frequency of floods
Short term impacts:
• 75% of the country was affected
• Over 1 million km² of area was flooded
• 31 million people were affected
• 1050 people died
• 30 million people were left homeless
Long term impacts:
• Floods destroyed a total of 668,529ha of crops
• Bangladesh export industries decreased a 20%
• 14,000 schools were flooded
• 4528km of flood embankments were damaged
• Communications were nearly destroyed
• 26,500 pieces of livestock were lost
• 980,000 houses were destroyed or partially damaged
• 300,000 rural hand tube wells (for irrigation) were flooded
Short term flood relief measures:
volunteers / aid workers worked to try and repair flood damage
international food aid programmes provided support
the distribution of free seed to farmers by the Bangladesh government to try and reduce the impact of food shortages
rapid creation of shelters for all the people who had been left homeless
Long term flood relief measures:
the creation of embankments (artificial levees) along the river to increase channel capacity and restrict flood waters
constructing flood protection shelters (large buildings raised above the ground) to shelter both people and animals
building flood proof storage sheds for grain and other food supplies
emergency flood warning systems and plans made for organising rescue and relief services
providing emergency medical stores in villages
dam construction upstream and major embankments around Dhaka have been suggested however lack of money has meant that these suggestions have not been taken further.
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GEOGRAPHY AS LEVEL REVISION NOTES:
RIVERS, FLOODS AND MANAGEMENT
All Definitions:
• Discharge: the volume of water flowing in a river per second, measured in cumecs.
• Precipitation: all forms of moisture that reach the Earth’s surface.
• Evaporation: the transformation of water droplets into water vapour by heating.
• Evapotranspiration: the loss of water from a drainage basin into the atmosphere from the leaves of plants + loss from evaporation.
• Surface Storage: the total volume of water held on the Earth’s surface in lakes, ponds and puddles.
• Groundwater Storage: the storage of water underground in permeable rock strata.
• Infiltration: the downward movement of water into the soil surface.
• Percolation: the gravity flow of water within soil.
• Overland Flow: the movement of water over the surface of the land, usually when the ground is saturated or frozen or when precipitation is too intense from infiltration to occur.
• Throughflow: the movement of water downslope within the soil layer.
• Groundwater Flow: the deeper movement of water through underlying rock strata.
• Dynamic Equilibrium: rivers are constantly changing over time to reach a state of balance with the processes that determine their form. As the flows of energy and materials passing through a river system vary, the river changes to move towards this equilibrium.
• Velocity: the speed and direction at which a body of water moves.
• Erosion: the wearing away of the surface of the land. It includes the breakdown of rock and its removal by water, wind or ice.
• Interception: the prevention of rain reaching the Earth’s surface by trees and plants.
• Condensation: the process by which water vapour is converted into water.
• Channel Flow: the movement of water within the river channel.
• Soil Moisture: the total amount of water, including vapour, in an unsaturated soil.
• Hydrograph: a graph showing for a given point on a stream the discharge, depth, velocity, or other property of water with respect to time; a graphical representation of stream discharge during a storm or flood event.
• Flood: a temporary excess of water which spills over onto the land.
• Baseflow: water that reaches the channel largely through slow throughflow and from permeable rock below the water table.
• Storm Flow: water that reaches the channel largely through runoff. This may be a combination of overland flow and rapid throughflow.
• Hydraulic Radius: the ratio of the cross-sectional area of the channel and the length of its wetted perimeter.
• Wetted Perimeter: that portion of the perimeter of a stream channel cross-section that is in contact with water.
• Cross-Sectional Area: the total length of the bed and the bank sides in contact with the water in the channel.
• Attrition: the reduction and rounding of particles of sediment carried in water by repeated collision with each other and banks/the bed.
• Abrasion/Corrasion: the wearing away of the banks or bed by sediment carried by the river.
• Corrosion: includes the dissolving of carbonate rocks in water which help certain rocks disintegrate (chemical erosion).
• Topography: the arrangement of the natural and artificial physical features of an area.
• Sinuosity: the curving nature of a meander described as actual channel length/straight-line distance
• Isostatic: changes in sea level resulting from the rise and fall of land masses.
• Eustatic: changes in sea level induced by variations in the amount of water in the oceans.
• Recurrence Interval: the interval at which particular levels of flooding will occur.
• Urbanisation: an increase in the proportion of a country’s population living in urban areas. It is sometimes used to mean the process of moving from rural to urban areas.
• Frequency: how often floods occur.
• Magnitude: the size of the flood.
The Drainage basin Hydrological Cycle: The Water Balance:
• A Drainage Basin is an area of land drained by a river and its tributaries. They are separated from each other by high areas of land called Watersheds.
• Inputs – precipitation, solar energy for evaporation.
• Outputs – evaporation and transpiration, runoff into the sea, percolation to underlying rock strata.
• Stores – puddles, rivers, lakes, soil storage, on vegetation etc.
• Transfers or Flows – infiltration, percolation, overland flow throughflow and groundwater flow.
Systems Theory:
• Isolated – no input or output of energy or matter (universe)
• Closed – input, transfer and output of energy but not of matter or mass (earth)
• Open – inputs, outputs both of energy and matter (most environmental systems)
• When inputs and outputs are balanced the system is said to be in a state of Dynamic Equilibrium. Very rarely any balance.
• The balance between water inputs and outputs of a drainage basin may be shown as a water budget graph.
• Precipitation = (Q)Runoff + Evapotranspiration +/- Change in Storage.
• Soil Moisture Surplus – often November to April. Rise in river levels + runoff.
• Soil Moisture Utilisation – Summer. Water Surplus is used.
• Soil Moisture Recharge – Autumn. Soil reaches capacity again.
The Storm Hydrograph:
• A hydrograph indicates the pattern of flow in a certain period of time.
• It allows us to examine the relationship between a rainfall event and discharge.
Flood Case Study: Keswick, December of 1985:
Environmental Causes:
• Surrounded by Steep Slopes.
• Huge drainage basin. All rain passes through small area (Keswick). Via Greta and its three tributaries Glenderaterra Beck, Glendera Mackin, Saint John’s Beck.
• Greta is therefore very flashy (short lag time).
• One of the wettest areas in U.K.
• A large rainfall is 30mm in 24h. This time it was 175mm in 24h.
Human Causes:
• No adequate flood defences built, buildings placed in vulnerable areas.
• Land is generally impermeable as Keswick is built up therefore more overland flow.
• Drains, gutters (and maybe sewage) move water into river very quickly.
• Much water released into Saint John’s Beck through a reservoir at the time.
• People were not as well educated about what to do in case of floods.
Impacts:
• One casualty. Stroke through shock near to care home.
• Many homes, buildings and cars damaged.
• Shock to people and lasting psychological impacts.
• Croswaite road and High hill worst affected – boat at garage.
• Communication, transport, telephone lines and some electricity temporarily lost.
• Many people forced to evacuate.
Flood Defences:
• £1.5M spent on flood defence.
• Fitz park area massively changed;
o Drains and Embankments built near bridges.
o Weir placed to slow water reaching a meander preventing lateral erosion.
o Soft Engineering and boulder placement throughout Fitz park.
• Elsewhere, course was straightened to increase velocity.
• Levees and Culverts built where possible.
• Buffs built between bank and care home.
• Regular Dredging, particularly near bridges.
• Central support of bridges removed to prevent jamming of debris.
• ‘200-year-flood’ built near High hill. Almost breached less than 20 years later.
Changing Channel Characteristics:
• Velocity is influenced by three main factors:
o Channel shape in cross-section.
o Roughness of the channel’s bed and banks.
o Channel slope.
• Channel shape is measured with hydraulic radius = cross-sectional area/wetted perimeter
Erosion:
• Hydraulic Action; is the movement of sediment by the frictional drag of moving water. Where velocity is high, such as on meander outer beds, hydraulic action can remove material from the banks, which may lead to undercutting or eventual collapse. At waterfalls or rapids it may work on lines of weakness such as joints and bedding plains.
• Abrasion (Corrasion); is the rubbing or scouring of the bed and banks by the sedimentary material carried along by the river. This load ranges from finer particles to heavy boulders being slowly rolled along the bed. Where depressions exist in the floor, the turbulent flow of the river can cause pebbled to swirl around and enlarge such depressions into potholes.
• Attrition; refers to the reduction in size of the sediment particles as they collide with each other, the bed, and the banks.
• Corrosion; occurs where rocks dissolve into the water and are carried away. This process is most common where carbonate rocks are exposed in the channel.
Rivers may erode vertically and horizontally:
Vertical dominant when: Horizontal dominant when:
Faster rivers therefore sufficient energy Large floodplain river will meander
Larger, more angular bedload Evident particularly when floodplain is covered in fine alluvial sediment.
Steep valleys generated
UPPER COURSE LOWER COURSE
Transportation:
• Bedload: larger materials which are too heavy for the current to pick up pay roll of slide along the valley floor (traction). Materials ranging from pebbles to sand may be temporarily lifted and bounced along the floor (saltation).
• Suspended Load: usually forms the bulk of the transported sediment. Comprises fine muds, clays, sands etc. It accounts for the muddy colour of some river waster.
• Dissolved/Solution Load: most common where rivers run through areas with carbonate rocks. Weak acids may act on more soluble rocks and gradually remove material in solution.
Competence:
• Competence is the maximum size (calibre) of load a river is capable of transporting.
• Capacity is the total volume of sediment a river can transport.
• Both increase with higher velocity or during flood.
• The Hjulstrøm curve illustrates relationship between velocity and competence. It shows velocities at which particles will be eroded, transported or deposited.
o Very fine particles may require high velocities to be eroded as they coagulate
o Velocity to keep them in motion is low as they are so small when single.
Deposition:
• A river deposits when no longer competent, or no longer has the capacity to carry the load.
• The sudden change in velocity when the river meets the sea causes significant deposition of sand, silt and muds which may lead to the formation of deltas.
• Deposition frequently occurs when:
o There is a sudden reduction in gradient
o The river enters a lake or the sea
o Discharge has been reduced following a period of rainfall
o Where there is shallower water i.e. inside meander bends
o There is a sudden increase in calibre or volume of sediment available, such as at the confluence or where a landslide has occurred.
Long Profile:
Fluvial Landforms: Erosional:
V-Shaped Valleys and Interlocking Spurs:
• In the upper course rivers characteristically carry large loads. Such sediment is only transported when discharge has risen. At such times the bouncing and rolling of boulders often causes extensive vertical erosion. This produces a relatively steep V-shaped valley. Its exact shape is dependent on:
o Climate – sufficient water is needed for high discharge levels and so the wetter it is the steeper the valley.
o Geology – the type of rock and its structure often vary. Softer rocks lead to steeper sides i.e. carboniferous limestone.
o Vegetation – more vegetated slopes tend to bind the soil better and may lead to more stable valley sides.
• Interlocking spurs are also characteristic of upper course rivers and are formed when the river winds around protrusions, hills or ridges of a valley.
Rapids:
• Found where there is a sudden increase in the slope of the channel or where the river flows over a series of gently dipping harder bands of rock.
• As the water becomes more turbulent its erosive power increases.
Waterfalls:
• Most commonly found where there are marked changes in geology in the river valley.
• Where resistant rocks are underlain by less resistant beds, the plunge pool at the foot of the falls experiences the force of the swirling water around the rocks, leading to more erosion.
• This undercuts the beds above leaving them overhanging and prone to collapse.
• The waterfall therefore retreats upstream.
Fluvial Landforms: Depositional:
Floodplains:
• Formed during meander migration; over time, meanders move downstream and therefore a floodplain is eroded in their wake.
• When rivers are at bankfull level they may spill over onto the flat land – this is a floodplain.
• Fine sediment is deposited on this floodplain (often alluvial) as velocity in this shallower water is much lower and capacity drops.
• Pointbars may be left on the inside of meander bends by migrating rivers adding to the extent of the floodplain.
Levees:
• In some rivers, the dropping of coarser material closer to the river channel during a flood has led to the development of levees. They are parallel banks of sediment formed as the heavier sediment carried by the floodwater settles first, close to the channel, while the finer materials travel further over the floodplain. They are often made artificially as flood defences or natural ones are strengthened and heightened i.e. Mississippi.
• Over time, the levee may become so high that the river bed becomes higher than the original bank of the river.
Braiding:
• Occurs in areas where climate, geology and topography combine to generate periodic high sediment loads.
• As water levels fall and energy decreases, there is rapid deposition. The largest and coarsest load begins to block the main channel.
• If the main channel becomes less competent it may subdivide into a series of smaller diverging and converging channels to find the easiest route past the obstruction.
Deltas:
• Deltas are areas of sediment deposited at the mouth of a river when it enters a slow-moving body of water such as a sea or lake.
• Four types:
o Arcuate – rounded convex outer margin (the Nile)
o Cuspate – material evenly spread on either side of estuary (the Tiber)
o Bird’s Foot – many distributary channels in a fan shape (the Mississippi)
• They provide fertile soil and are associated with good fishing grounds and oil/gas deposits.
• They are composed of unconsolidated sediments and are subject to channel migration as well as to subsidence and incursion by the sea.
Meanders:
• All rivers are heading downslope toward the sea and will take the path of least resistance to them (like water trickling down a window).
• All channels will at times deposit sediment in alternating bars (riffles) when experiencing low flow conditions. The hydraulic radius is decreased at this area and so water flows inefficiently over it.
• In an attempt to flow preferentially around these areas of higher frictional contact, water flows around these riffles. At times of faster flow the water, as it flows around, is propelled by centripetal force toward one of the banks resulting in eroding it by undercutting.
• An outer concave bank is created while slower flow on the inside bend leads to deposition on the inside bend and a convex bank.
• The helicoidal flow of water allows material eroded from the outer bank to be deposited in part of the inner bank of the next meander bend.
GEOGRAPHY AS LEVEL REVISION NOTES:
RIVERS, FLOODS AND MANAGEMENT
All Definitions:
• Discharge: the volume of water flowing in a river per second, measured in cumecs.
• Precipitation: all forms of moisture that reach the Earth’s surface.
• Evaporation: the transformation of water droplets into water vapour by heating.
• Evapotranspiration: the loss of water from a drainage basin into the atmosphere from the leaves of plants + loss from evaporation.
• Surface Storage: the total volume of water held on the Earth’s surface in lakes, ponds and puddles.
• Groundwater Storage: the storage of water underground in permeable rock strata.
• Infiltration: the downward movement of water into the soil surface.
• Percolation: the gravity flow of water within soil.
• Overland Flow: the movement of water over the surface of the land, usually when the ground is saturated or frozen or when precipitation is too intense from infiltration to occur.
• Throughflow: the movement of water downslope within the soil layer.
• Groundwater Flow: the deeper movement of water through underlying rock strata.
• Dynamic Equilibrium: rivers are constantly changing over time to reach a state of balance with the processes that determine their form. As the flows of energy and materials passing through a river system vary, the river changes to move towards this equilibrium.
• Velocity: the speed and direction at which a body of water moves.
• Erosion: the wearing away of the surface of the land. It includes the breakdown of rock and its removal by water, wind or ice.
• Interception: the prevention of rain reaching the Earth’s surface by trees and plants.
• Condensation: the process by which water vapour is converted into water.
• Channel Flow: the movement of water within the river channel.
• Soil Moisture: the total amount of water, including vapour, in an unsaturated soil.
• Hydrograph: a graph showing for a given point on a stream the discharge, depth, velocity, or other property of water with respect to time; a graphical representation of stream discharge during a storm or flood event.
• Flood: a temporary excess of water which spills over onto the land.
• Baseflow: water that reaches the channel largely through slow throughflow and from permeable rock below the water table.
• Storm Flow: water that reaches the channel largely through runoff. This may be a combination of overland flow and rapid throughflow.
• Hydraulic Radius: the ratio of the cross-sectional area of the channel and the length of its wetted perimeter.
• Wetted Perimeter: that portion of the perimeter of a stream channel cross-section that is in contact with water.
• Cross-Sectional Area: the total length of the bed and the bank sides in contact with the water in the channel.
• Attrition: the reduction and rounding of particles of sediment carried in water by repeated collision with each other and banks/the bed.
• Abrasion/Corrasion: the wearing away of the banks or bed by sediment carried by the river.
• Corrosion: includes the dissolving of carbonate rocks in water which help certain rocks disintegrate (chemical erosion).
• Topography: the arrangement of the natural and artificial physical features of an area.
• Sinuosity: the curving nature of a meander described as actual channel length/straight-line distance
• Isostatic: changes in sea level resulting from the rise and fall of land masses.
• Eustatic: changes in sea level induced by variations in the amount of water in the oceans.
• Recurrence Interval: the interval at which particular levels of flooding will occur.
• Urbanisation: an increase in the proportion of a country’s population living in urban areas. It is sometimes used to mean the process of moving from rural to urban areas.
• Frequency: how often floods occur.
• Magnitude: the size of the flood.
The Drainage basin Hydrological Cycle: The Water Balance:
• A Drainage Basin is an area of land drained by a river and its tributaries. They are separated from each other by high areas of land called Watersheds.
• Inputs – precipitation, solar energy for evaporation.
• Outputs – evaporation and transpiration, runoff into the sea, percolation to underlying rock strata.
• Stores – puddles, rivers, lakes, soil storage, on vegetation etc.
• Transfers or Flows – infiltration, percolation, overland flow throughflow and groundwater flow.
Systems Theory:
• Isolated – no input or output of energy or matter (universe)
• Closed – input, transfer and output of energy but not of matter or mass (earth)
• Open – inputs, outputs both of energy and matter (most environmental systems)
• When inputs and outputs are balanced the system is said to be in a state of Dynamic Equilibrium. Very rarely any balance.
• The balance between water inputs and outputs of a drainage basin may be shown as a water budget graph.
• Precipitation = (Q)Runoff + Evapotranspiration +/- Change in Storage.
• Soil Moisture Surplus – often November to April. Rise in river levels + runoff.
• Soil Moisture Utilisation – Summer. Water Surplus is used.
• Soil Moisture Recharge – Autumn. Soil reaches capacity again.
The Storm Hydrograph:
• A hydrograph indicates the pattern of flow in a certain period of time.
• It allows us to examine the relationship between a rainfall event and discharge.
Flood Case Study: Keswick, December of 1985:
Environmental Causes:
• Surrounded by Steep Slopes.
• Huge drainage basin. All rain passes through small area (Keswick). Via Greta and its three tributaries Glenderaterra Beck, Glendera Mackin, Saint John’s Beck.
• Greta is therefore very flashy (short lag time).
• One of the wettest areas in U.K.
• A large rainfall is 30mm in 24h. This time it was 175mm in 24h.
Human Causes:
• No adequate flood defences built, buildings placed in vulnerable areas.
• Land is generally impermeable as Keswick is built up therefore more overland flow.
• Drains, gutters (and maybe sewage) move water into river very quickly.
• Much water released into Saint John’s Beck through a reservoir at the time.
• People were not as well educated about what to do in case of floods.
Impacts:
• One casualty. Stroke through shock near to care home.
• Many homes, buildings and cars damaged.
• Shock to people and lasting psychological impacts.
• Croswaite road and High hill worst affected – boat at garage.
• Communication, transport, telephone lines and some electricity temporarily lost.
• Many people forced to evacuate.
Flood Defences:
• £1.5M spent on flood defence.
• Fitz park area massively changed;
o Drains and Embankments built near bridges.
o Weir placed to slow water reaching a meander preventing lateral erosion.
o Soft Engineering and boulder placement throughout Fitz park.
• Elsewhere, course was straightened to increase velocity.
• Levees and Culverts built where possible.
• Buffs built between bank and care home.
• Regular Dredging, particularly near bridges.
• Central support of bridges removed to prevent jamming of debris.
• ‘200-year-flood’ built near High hill. Almost breached less than 20 years later.
Changing Channel Characteristics:
• Velocity is influenced by three main factors:
o Channel shape in cross-section.
o Roughness of the channel’s bed and banks.
o Channel slope.
• Channel shape is measured with hydraulic radius = cross-sectional area/wetted perimeter
Erosion:
• Hydraulic Action; is the movement of sediment by the frictional drag of moving water. Where velocity is high, such as on meander outer beds, hydraulic action can remove material from the banks, which may lead to undercutting or eventual collapse. At waterfalls or rapids it may work on lines of weakness such as joints and bedding plains.
• Abrasion (Corrasion); is the rubbing or scouring of the bed and banks by the sedimentary material carried along by the river. This load ranges from finer particles to heavy boulders being slowly rolled along the bed. Where depressions exist in the floor, the turbulent flow of the river can cause pebbled to swirl around and enlarge such depressions into potholes.
• Attrition; refers to the reduction in size of the sediment particles as they collide with each other, the bed, and the banks.
• Corrosion; occurs where rocks dissolve into the water and are carried away. This process is most common where carbonate rocks are exposed in the channel.
Rivers may erode vertically and horizontally:
Vertical dominant when: Horizontal dominant when:
Faster rivers therefore sufficient energy Large floodplain river will meander
Larger, more angular bedload Evident particularly when floodplain is covered in fine alluvial sediment.
Steep valleys generated
UPPER COURSE LOWER COURSE
Transportation:
• Bedload: larger materials which are too heavy for the current to pick up pay roll of slide along the valley floor (traction). Materials ranging from pebbles to sand may be temporarily lifted and bounced along the floor (saltation).
• Suspended Load: usually forms the bulk of the transported sediment. Comprises fine muds, clays, sands etc. It accounts for the muddy colour of some river waster.
• Dissolved/Solution Load: most common where rivers run through areas with carbonate rocks. Weak acids may act on more soluble rocks and gradually remove material in solution.
Competence:
• Competence is the maximum size (calibre) of load a river is capable of transporting.
• Capacity is the total volume of sediment a river can transport.
• Both increase with higher velocity or during flood.
• The Hjulstrøm curve illustrates relationship between velocity and competence. It shows velocities at which particles will be eroded, transported or deposited.
o Very fine particles may require high velocities to be eroded as they coagulate
o Velocity to keep them in motion is low as they are so small when single.
Deposition:
• A river deposits when no longer competent, or no longer has the capacity to carry the load.
• The sudden change in velocity when the river meets the sea causes significant deposition of sand, silt and muds which may lead to the formation of deltas.
• Deposition frequently occurs when:
o There is a sudden reduction in gradient
o The river enters a lake or the sea
o Discharge has been reduced following a period of rainfall
o Where there is shallower water i.e. inside meander bends
o There is a sudden increase in calibre or volume of sediment available, such as at the confluence or where a landslide has occurred.
Long Profile:
Fluvial Landforms: Erosional:
V-Shaped Valleys and Interlocking Spurs:
• In the upper course rivers characteristically carry large loads. Such sediment is only transported when discharge has risen. At such times the bouncing and rolling of boulders often causes extensive vertical erosion. This produces a relatively steep V-shaped valley. Its exact shape is dependent on:
o Climate – sufficient water is needed for high discharge levels and so the wetter it is the steeper the valley.
o Geology – the type of rock and its structure often vary. Softer rocks lead to steeper sides i.e. carboniferous limestone.
o Vegetation – more vegetated slopes tend to bind the soil better and may lead to more stable valley sides.
• Interlocking spurs are also characteristic of upper course rivers and are formed when the river winds around protrusions, hills or ridges of a valley.
Rapids:
• Found where there is a sudden increase in the slope of the channel or where the river flows over a series of gently dipping harder bands of rock.
• As the water becomes more turbulent its erosive power increases.
Waterfalls:
• Most commonly found where there are marked changes in geology in the river valley.
• Where resistant rocks are underlain by less resistant beds, the plunge pool at the foot of the falls experiences the force of the swirling water around the rocks, leading to more erosion.
• This undercuts the beds above leaving them overhanging and prone to collapse.
• The waterfall therefore retreats upstream.
Fluvial Landforms: Depositional:
Floodplains:
• Formed during meander migration; over time, meanders move downstream and therefore a floodplain is eroded in their wake.
• When rivers are at bankfull level they may spill over onto the flat land – this is a floodplain.
• Fine sediment is deposited on this floodplain (often alluvial) as velocity in this shallower water is much lower and capacity drops.
• Pointbars may be left on the inside of meander bends by migrating rivers adding to the extent of the floodplain.
Levees:
• In some rivers, the dropping of coarser material closer to the river channel during a flood has led to the development of levees. They are parallel banks of sediment formed as the heavier sediment carried by the floodwater settles first, close to the channel, while the finer materials travel further over the floodplain. They are often made artificially as flood defences or natural ones are strengthened and heightened i.e. Mississippi.
• Over time, the levee may become so high that the river bed becomes higher than the original bank of the river.
Braiding:
• Occurs in areas where climate, geology and topography combine to generate periodic high sediment loads.
• As water levels fall and energy decreases, there is rapid deposition. The largest and coarsest load begins to block the main channel.
• If the main channel becomes less competent it may subdivide into a series of smaller diverging and converging channels to find the easiest route past the obstruction.
Deltas:
• Deltas are areas of sediment deposited at the mouth of a river when it enters a slow-moving body of water such as a sea or lake.
• Four types:
o Arcuate – rounded convex outer margin (the Nile)
o Cuspate – material evenly spread on either side of estuary (the Tiber)
o Bird’s Foot – many distributary channels in a fan shape (the Mississippi)
• They provide fertile soil and are associated with good fishing grounds and oil/gas deposits.
• They are composed of unconsolidated sediments and are subject to channel migration as well as to subsidence and incursion by the sea.
Meanders:
• All rivers are heading downslope toward the sea and will take the path of least resistance to them (like water trickling down a window).
• All channels will at times deposit sediment in alternating bars (riffles) when experiencing low flow conditions. The hydraulic radius is decreased at this area and so water flows inefficiently over it.
• In an attempt to flow preferentially around these areas of higher frictional contact, water flows around these riffles. At times of faster flow the water, as it flows around, is propelled by centripetal force toward one of the banks resulting in eroding it by undercutting.
• An outer concave bank is created while slower flow on the inside bend leads to deposition on the inside bend and a convex bank.
• The helicoidal flow of water allows material eroded from the outer bank to be deposited in part of the inner bank of the next meander bend.
Pablin888
these are for the new specification and i will also be adding tomorrow complete notes for cold environments, food supply issues and population
Ah they wont be very useful for me then (Im old spec, however some of it is similar to my spec)
Anyway, well done for posting them - but go post them on the Revision Section so people will more likely come across them.
http://www.thestudentroom.co.uk/wiki/Revision_Notes
continued:
Oxbow Lakes:
• As meanders develop, erosion of the outside bend tends to move them slowly downstream and downslope.
• The sinuosity of the meander may become more pronounced, with erosion of the outer bank and deposition on the inner bank decreasing the width of the neck of land between the start and end of meander.
• At times of flood, this neck can be eroded away, giving the river a shorter and straighter route downstream.
• Initially the truncated meander loop forms a curved lake (hence the name oxbow), cut off from the main channel by deposition.
• Over time it may get infilled with sediment and vegetation; these are known as meander scars.
Rejuvenation:
• The long profile of a river (graded getting gradually less steep) reflects the fact that water does not have as far to fall as it nears the sea and so has less erosive power.
• However, over time, if the relative heights of the land and the sea alter, this situation may change.
• Such change is either Isostatic (land rising in relation to sea level such as when the weight of ice caps is removed) or Eustatic (sea level change often due to ice melt or water freezing).
• If sea level drops or land rises, existing valley floors may be cut into as the river attempts to regrade itself in keeping with the new energy levels exhibited by the river. This begins in the channel nearest the sea and then migrates back upstream.
• The current limit of the regarding is marked by a knick point.
Incised Meanders:
• An incised meander is one which lies at the bottom of a steep-walled canyon.
• This most often occurs at an existing meander after the rejuvenation of a river – there is then severe downwards erosion creating a steep-walled canyon.
• There are two types of ingrown meander:
o Entrenched meanders have a symmetrical cross-section resulting from very rapid incision by the river of valley sides being made of hard, resistant rock. The River Wye at Durham is an example.
o Ingrown meanders are formed when the incision or uplift is less rapid and the river may ‘shift’ laterally thus producing an asymmetrical cross section shape. The river Wye at Tintern Abbey is an example.
Flooding and Flood Management:
(Page 4 for Keswick case study)
• Floods occur when large volumes of water enter a river system quickly. Discharge increases to the point where it cannot be contained in the channel and water spills out onto the floodplain.
• Natural causes of flooding may be classified as follows:
o Primary causes are usually the result of climatic factors.
o Secondary causes tend to be drainage-basin specific (e.g. dependent on geology, soil, topography and vegetation).
• In addition to natural causes, human influences on the drainage basin increase the risk of flooding as development of once natural landscapes for residential, industrial and agricultural purposes tends to reduce infiltration and increase runoff.
• Urbanisation helps to increase the frequency and magnitude of flooding in several ways;
o Creating impermeable surfaces, e.g. car parks, roofs, roads and pavements.
o Speeding up the drainage of water in built-up areas via artificial conduits, e.g. sewers and drains.
o Impeding channel flow by building alongside or in the river, e.g. bridge supports.
o Straightening of channels to increase speed of flow which results in flooding downstream.
o Changing land use associated with development, e.g. deforestation, ploughing and overgrazing, which results in increased risk of flooding through increased runoff and increased levels of sediment washed into streams blocking channels.
Flood Management Strategies:
• Flood management strategies seek to reduce the effects of flooding on the human environment.
• The main strategies can be grouped as follows:
o Structural methods – offering protection through engineering
o River basin management – seeking to reduce the likelihood of flooding by managing land use
o Modifying the burden of loss – by insurance schemes
o Bearing the cost of flood damage – a ‘do nothing’ approach that only deals with the issues when they arise
• Unsurprisingly, developing countries tend to avoid expensive engineering solutions and in more developed countries; a more preventative attitude may prevail.
• Preventing floods is increasingly seen as impossible and river management concentrates more on reducing losses due to flooding.
Structural Methods:
Flood walls, Embankments and Levees:
• Flood walls are designed to increase the height of the channel to stop water spilling out onto the floodplain. Most commonly used in towns, they restrict access to the riverside and offer little in the way of floodwater storage capacity.
• Embankments are often made of earth with rubble fill and are more common outside the town centre where there is more room. If set back from the channel, they can provide storage for excess floodwaters while inhabited areas remain unaffected.
• Levees may be artificially enhanced or introduced to raise the level of the river banks.
• Each of these methods may reduce flooding at the expense of speeding water downstream to create problems elsewhere.
Channel Improvements:
• Attempt to restrict floods either by creating a smoother channel for faster flow to get water out of the area as soon as possible or by deepening/widening the channel.
• Channels may be smoothened by lining the channel with concrete.
• Deepening and widening may be achieved by regular dredging.
• Both may increase flood risk further downstream and both need regular maintenance as deposition and erosion revert the channel to its more natural form.
Relief Channels:
• Are constructed to redirect excess water upstream of a settlement via an alternative route. Water is able to re-enter the main channel further downstream, thereby reducing flood risk.
• By creating a bypass that can only be accessed at high discharge levels, the peak flow in the main channel is reduced and the relief channel may often remain dry until needed.
Flood Storage Reservoirs:
• Aim to store excess water in the upper reaches of the catchment area.
• They are expensive to construct and require huge amounts of land. In the U.K. no reservoirs are built with the sole purpose of preventing flood.
Flood Interception Schemes:
• May include re-routing a river to effect a bypass, using new channels to store excess water and flood embankments to contain flooding well away from settlements under threat.
• May also include flood retention basins, washland areas and polders. These are areas of land deliberately flooded upstream of towns and cities. They are low-value and provide temporary storage of floodwater. They may also be wildlife refuges or have amenity value.
River Basic Management:
• Seeks to reduce the harm done by flooding when it does occur.
Flood Abatement:
• Abatements measures aim to reduce the possibility of flooding by managing land use upstream. Includes;
o Afforestation to increase interception storage and evapotranspiration to help reduce runoff as well as holding the soil together to reduce silting up of river channels.
o Farming practices like contour ploughing and reducing the amount of bare earth to avoid excessive runoff problems.
Flood Proofing:
• May be temporary or permanent. Buildings can be constructed with flood-proof ground floor walls or have temporary gates ready to be installed at times of high risk.
• Potential damage of floods can be reduced by placing car parks etc on the ground-level sites.
Floodplain Zoning:
• Zones of relative risk can be mapped;
o Zone A: Prohibitive Zones – areas nearer to the channel with a relatively high risk of flooding. Essential waterfront development may be permitted by development is unlikely to be allowed here.
o Zone B: Restrictive Zones – little development is allowed and that which is should be flood-proofed. They are best suited to low-intensity or low-value land uses such as pasture, playing fields or car parks.
o Zone C: Warming Zones – situated on higher land and further away. There is more development here but inhabitants are made aware of imminent flood damage and are instructed how to react when floods do occur.
Flood Prediction and Warning:
• Records of river discharge and flooding are kept to help predict future flood events.
• The main methods of collecting data to aid flood forecasting are weather radar and the information from automatic rainfall and river gauges.
• Flood prediction software helps to model likely outcomes, and warning may be issued in terms of the potential severity of the flood risk and the areas that could be affected.
• A method called Risk Assessment for Strategic Planning (RASP) is used to help place areas into three distinct categories:
o Low – the chance of flooding each year is below 0.5%.
o Moderate – the chance of flooding each year is between 1.3% (1 in 75) and 0.5%.
o Significant – the chance of flooding is above 1.3%.
• In flood prone areas like York, automatic phone warnings are issues to alert inhabitants in potential flooding zones, and visits by flood wardens ensure that temporary defences are in place and/or evacuations are carried out when necessary.
Channelisation or Neutralisation:
• Channelisation is an attempt to alter the natural geometry of a watercourse.
• It can help to prevent flooding by increasing channel capacity and preventing bank erosion, both of which reduce likelihood of a river breaking out of its channel at times of high flow.
• The dual benefits of flood prevention and land extension mean that such hard engineering solutions have massively increased in popularity.
• Resectioning a river involves widening and deepening a channel to improve its hydraulic efficiency. This increase capacity and moves water out of an area much more quickly. Dredging is one way of removing surplus sediment from the river bed.
• Realignment (straightening) involves shortening the river course by removal of meanders. The increase in gradient moves floodwaters away more quickly while improving navigation.
• Revetments made of concrete blocks, steel or gabions (wire mesh cubes filled with boulders) are used to strengthen banks.
• Wing Dykes or training walls which jut out from the sides of the channel may be employed to focus the main river current in the centre of the channel and away from the banks.
• In urban rivers the entire channel may be lines with concrete to decrease friction and increase flow velocity.
• In cities, rivers may be covered over and confined to concrete culverts to reduce the inconvenience to development and to help remove the increased amount of runoff from impermeable surfaces.
• All the above measures are expensive and offer relatively short-term advantages with high maintenance costs. In the long term, disadvantages include the effects on upstream sections (downcutting) and downstream sections (more deposition) which could potentially lead to catastrophic flooding.
Wetland and River Bank Conservation and River Restoration:
• River restoration can include a variety of strategies to reclaim rivers, for example re-routing the river from its straightened course into new meandering channels.
• Wetlands are areas that are deliberately allowed to flood at times of high discharge. They are also valuable as wildlife habitats.
• E.g. the Nene washes upstream from Peterborough for the River Nene.
• Many feel that restoration of peat bogs in northern uplands would slow water reaching lowland streams and rivers, reducing the threat to Sheffield, Ripon and Hull – all of which are particularly prone to flooding.
• On a larger scale, the frequent flooding of the river Rhine, culminating in the 1995 floods, led to a rethink on how to manage flood problems. A ‘Room for the River’ programme is currently being translated into land-use change and relocation of inhabitants on floodplains.
o Arable land is being converted to forest, marsh or wet grazing meadows.
o Inhabitants are being relocated, with compensation, to higher elevations, and the entire floodplain cross-section can accommodate a much larger volume of water.
• Measures taken include:
o An increase in ‘water meadows’ which can be allowed to flood when necessary.
o A reduction in the use of tarmac or concrete in vulnerable areas to slow water runoff into the rivers.
o Increased ground coverage of vegetation with woodlands and grasslands.
o Restrictions on the use of soil fertilisers which affect the soil structure, reducing its ability to retain water.
o Metres of silt accumulated over many years have been stripped and deep trenches constructed to allow more storage space for water in the event of flooding, and more room for trees which stabilise the soil and improve the ecological balance and help to evapotranspirate moisture away from saturated soils.
Case Study:
Flooding In Bangladesh:
• Major flooding in Bangladesh occurs frequently, regularly inundating between 20% and 30% of the country and leading to enormous loss of life.
• Flash Flooding – extremely heavy rainfall occurs on surrounding upland areas. Not all of it can be infiltrated into the soil and excess water forms runoff which leads to rapid filling of river channels. Where this spills onto the floodplain much sediment can be deposited, damaging crops.
• River Floods – mainly caused by meltwaters from the Himalayan mountains and heavy monsoon rains. Where the Brahmaputra and the Ganges meet, heavy levels of discharge breach embankments and flooding often ensues. This is particularly common along the Brahmaputra and Meghna rivers in early June. Widespread flooding can threaten settlements and heavy silt deposits may bury crops.
• Rainwater Floods – heavy prolonged rainfall within Bangladesh causes runoff to accumulate in surface depressions, trapped by rising river levels. This may occur before the monsoon and lead to topsoil being washed off farmland and into adjoining depressions.
• Storm Surges – these mainly affect the southern coastal fringe of the country, where cyclones moving up the Bay of Bengal create storm surges which inundate the low-lying coastal strip. Significant losses of life may ensue in the few hours of the storm.
Geography of Bangladesh Physical flood causes Human flood causes
Population 125million Most the country is a floodplain and delta Urbanisation of the flood plain increased magnitude and frequency of floods
70% of total area is under 1m above sea level Global warming blamed for sea level rise, increased snow melt and rainfall Global warming blamed for sea level rise, increased snow melt and rainfall
Experiences floods and tropical rainstorms annually Experiences heavy monsoon rains esp. over the highland Deforestation in Nepal and the Himalayas increases run off and adds to deposition and flooding downstream
One of the world’s poorest countries – GNP $200 10% of land is lakes and rivers
One of the world’s most densely populated countries Tropical storms bring heavy rains and coastal flooding The building of dams in India has increased the probability of sedimentation in Bangladesh
The entire country is a delta – contains virtually no raw materials or rock The Ganges, Meghna and Brahmaputra all pass through it
Embankments are poorly maintained due to the poverty and therefore leak and collapse during high discharge
3 of the world’s most powerful rivers flow through it all three reached peak flow at the same time
• Impacts of the 1988 and 1998 Bangladesh Floods:
1988 1998
Duration of Floods 21 days 65 days
Percentage of Country Affected 60% 75%
Percentage of Capital City covered by flooding 67% 50%
Area flooded 2,282,000km2 Over 1million km2
People Affected 45 million 31 million
Houses totally or partially damaged 7.2 million 980,000
Human lives lost 2379 1050
Livestock lost (cattle and goats) 172,000 26,500
Rice production lost 2 million tonnes 2.2 million tonnes
Trunk roads damaged 3000km 15,900km
Flood Embankments damaged 1990km 4528km
Industrial units flooded Over 1000 Over 5000
Schools flooded 19,000 14,000
Rural irrigation tubewells flooded 240,000 300,000
Flood Defences:
• After the 1988 floods, which affected 45 million and killed over 2000, a Flood Action Plan (FAP) was devised. The overarching aim for the plan is to create flood protection for Bangladesh.
• One key part is to construct new embankments alongside the Brahmaputra and Ganges in Bangladesh, starting with the upstream areas. The aim is not to completely stop the floods, but to keep them at a manageable level. Behind the embankment compartments of land are created by building internal walls to link up with the embankments.
• A flood forecasting system is planned to alert local inhabitants of impending floods.
• Preparation to deal with the consequences of flooding will include the provision of boats so that people can escape to shelters on higher land.
• The Jamalpur Priority Project Study illustrates four issues surrounding the potential impacts of embankment construction to help decide on optimum solutions;
o Flood proofing and drainage improvement
o Controlled flooding of the entire area with some compartmentalisation
o Controlled flooding of about half the area
o All areas compartmentalised – all river flooding excluded
• Economic and social impacts of the latter two meant they were soon rejected.
• The first offer seemed more beneficial to the fishing, non-farming and landless population whereas the second benefitted farming households and land owners, with the promise of greater economic growth for the area as a whole.
• The threat of sabotage and concern that areas outside of this scheme would suffer worse floods as a consequence made decision making much harder.
Embankment Issues:
Positioning:
• Many people in Bangladesh wanted embankments close to the channels to protect as many as possible and to maximise farmland.
• However, building close to the channel increases river depth and velocity at times of high flow in unstable, braided and meandering channels.
• Studies show there is a far greater risk of erosion and collapse when embankments are closer to channels.
• The more distant embankment option would cost half as much to build and maintain (up to 5km from channel) but an additional 5 million people would be in a flood zone.
Longer-term Impacts:
• From the famous Mississippi breach, we see that over time the river bed will rise due to deposition and eventually will exceed the former bank full level and require much larger embankments. Any breaches at this stage can be catastrophic.
• Faster and deeper flow regimes in upstream channel sections controlled by embankments inevitably lead to increased erosion producing greater sedimentation downstream as the river slows down. This may result in channel obstruction and an increased likelihood of flooding.
• Although compartmentalisation controls floodwaters when they occur the retention of large amounts of river water in smaller areas has implications for human health, crop production and fishing.
Oxbow Lakes:
• As meanders develop, erosion of the outside bend tends to move them slowly downstream and downslope.
• The sinuosity of the meander may become more pronounced, with erosion of the outer bank and deposition on the inner bank decreasing the width of the neck of land between the start and end of meander.
• At times of flood, this neck can be eroded away, giving the river a shorter and straighter route downstream.
• Initially the truncated meander loop forms a curved lake (hence the name oxbow), cut off from the main channel by deposition.
• Over time it may get infilled with sediment and vegetation; these are known as meander scars.
Rejuvenation:
• The long profile of a river (graded getting gradually less steep) reflects the fact that water does not have as far to fall as it nears the sea and so has less erosive power.
• However, over time, if the relative heights of the land and the sea alter, this situation may change.
• Such change is either Isostatic (land rising in relation to sea level such as when the weight of ice caps is removed) or Eustatic (sea level change often due to ice melt or water freezing).
• If sea level drops or land rises, existing valley floors may be cut into as the river attempts to regrade itself in keeping with the new energy levels exhibited by the river. This begins in the channel nearest the sea and then migrates back upstream.
• The current limit of the regarding is marked by a knick point.
Incised Meanders:
• An incised meander is one which lies at the bottom of a steep-walled canyon.
• This most often occurs at an existing meander after the rejuvenation of a river – there is then severe downwards erosion creating a steep-walled canyon.
• There are two types of ingrown meander:
o Entrenched meanders have a symmetrical cross-section resulting from very rapid incision by the river of valley sides being made of hard, resistant rock. The River Wye at Durham is an example.
o Ingrown meanders are formed when the incision or uplift is less rapid and the river may ‘shift’ laterally thus producing an asymmetrical cross section shape. The river Wye at Tintern Abbey is an example.
Flooding and Flood Management:
(Page 4 for Keswick case study)
• Floods occur when large volumes of water enter a river system quickly. Discharge increases to the point where it cannot be contained in the channel and water spills out onto the floodplain.
• Natural causes of flooding may be classified as follows:
o Primary causes are usually the result of climatic factors.
o Secondary causes tend to be drainage-basin specific (e.g. dependent on geology, soil, topography and vegetation).
• In addition to natural causes, human influences on the drainage basin increase the risk of flooding as development of once natural landscapes for residential, industrial and agricultural purposes tends to reduce infiltration and increase runoff.
• Urbanisation helps to increase the frequency and magnitude of flooding in several ways;
o Creating impermeable surfaces, e.g. car parks, roofs, roads and pavements.
o Speeding up the drainage of water in built-up areas via artificial conduits, e.g. sewers and drains.
o Impeding channel flow by building alongside or in the river, e.g. bridge supports.
o Straightening of channels to increase speed of flow which results in flooding downstream.
o Changing land use associated with development, e.g. deforestation, ploughing and overgrazing, which results in increased risk of flooding through increased runoff and increased levels of sediment washed into streams blocking channels.
Flood Management Strategies:
• Flood management strategies seek to reduce the effects of flooding on the human environment.
• The main strategies can be grouped as follows:
o Structural methods – offering protection through engineering
o River basin management – seeking to reduce the likelihood of flooding by managing land use
o Modifying the burden of loss – by insurance schemes
o Bearing the cost of flood damage – a ‘do nothing’ approach that only deals with the issues when they arise
• Unsurprisingly, developing countries tend to avoid expensive engineering solutions and in more developed countries; a more preventative attitude may prevail.
• Preventing floods is increasingly seen as impossible and river management concentrates more on reducing losses due to flooding.
Structural Methods:
Flood walls, Embankments and Levees:
• Flood walls are designed to increase the height of the channel to stop water spilling out onto the floodplain. Most commonly used in towns, they restrict access to the riverside and offer little in the way of floodwater storage capacity.
• Embankments are often made of earth with rubble fill and are more common outside the town centre where there is more room. If set back from the channel, they can provide storage for excess floodwaters while inhabited areas remain unaffected.
• Levees may be artificially enhanced or introduced to raise the level of the river banks.
• Each of these methods may reduce flooding at the expense of speeding water downstream to create problems elsewhere.
Channel Improvements:
• Attempt to restrict floods either by creating a smoother channel for faster flow to get water out of the area as soon as possible or by deepening/widening the channel.
• Channels may be smoothened by lining the channel with concrete.
• Deepening and widening may be achieved by regular dredging.
• Both may increase flood risk further downstream and both need regular maintenance as deposition and erosion revert the channel to its more natural form.
Relief Channels:
• Are constructed to redirect excess water upstream of a settlement via an alternative route. Water is able to re-enter the main channel further downstream, thereby reducing flood risk.
• By creating a bypass that can only be accessed at high discharge levels, the peak flow in the main channel is reduced and the relief channel may often remain dry until needed.
Flood Storage Reservoirs:
• Aim to store excess water in the upper reaches of the catchment area.
• They are expensive to construct and require huge amounts of land. In the U.K. no reservoirs are built with the sole purpose of preventing flood.
Flood Interception Schemes:
• May include re-routing a river to effect a bypass, using new channels to store excess water and flood embankments to contain flooding well away from settlements under threat.
• May also include flood retention basins, washland areas and polders. These are areas of land deliberately flooded upstream of towns and cities. They are low-value and provide temporary storage of floodwater. They may also be wildlife refuges or have amenity value.
River Basic Management:
• Seeks to reduce the harm done by flooding when it does occur.
Flood Abatement:
• Abatements measures aim to reduce the possibility of flooding by managing land use upstream. Includes;
o Afforestation to increase interception storage and evapotranspiration to help reduce runoff as well as holding the soil together to reduce silting up of river channels.
o Farming practices like contour ploughing and reducing the amount of bare earth to avoid excessive runoff problems.
Flood Proofing:
• May be temporary or permanent. Buildings can be constructed with flood-proof ground floor walls or have temporary gates ready to be installed at times of high risk.
• Potential damage of floods can be reduced by placing car parks etc on the ground-level sites.
Floodplain Zoning:
• Zones of relative risk can be mapped;
o Zone A: Prohibitive Zones – areas nearer to the channel with a relatively high risk of flooding. Essential waterfront development may be permitted by development is unlikely to be allowed here.
o Zone B: Restrictive Zones – little development is allowed and that which is should be flood-proofed. They are best suited to low-intensity or low-value land uses such as pasture, playing fields or car parks.
o Zone C: Warming Zones – situated on higher land and further away. There is more development here but inhabitants are made aware of imminent flood damage and are instructed how to react when floods do occur.
Flood Prediction and Warning:
• Records of river discharge and flooding are kept to help predict future flood events.
• The main methods of collecting data to aid flood forecasting are weather radar and the information from automatic rainfall and river gauges.
• Flood prediction software helps to model likely outcomes, and warning may be issued in terms of the potential severity of the flood risk and the areas that could be affected.
• A method called Risk Assessment for Strategic Planning (RASP) is used to help place areas into three distinct categories:
o Low – the chance of flooding each year is below 0.5%.
o Moderate – the chance of flooding each year is between 1.3% (1 in 75) and 0.5%.
o Significant – the chance of flooding is above 1.3%.
• In flood prone areas like York, automatic phone warnings are issues to alert inhabitants in potential flooding zones, and visits by flood wardens ensure that temporary defences are in place and/or evacuations are carried out when necessary.
Channelisation or Neutralisation:
• Channelisation is an attempt to alter the natural geometry of a watercourse.
• It can help to prevent flooding by increasing channel capacity and preventing bank erosion, both of which reduce likelihood of a river breaking out of its channel at times of high flow.
• The dual benefits of flood prevention and land extension mean that such hard engineering solutions have massively increased in popularity.
• Resectioning a river involves widening and deepening a channel to improve its hydraulic efficiency. This increase capacity and moves water out of an area much more quickly. Dredging is one way of removing surplus sediment from the river bed.
• Realignment (straightening) involves shortening the river course by removal of meanders. The increase in gradient moves floodwaters away more quickly while improving navigation.
• Revetments made of concrete blocks, steel or gabions (wire mesh cubes filled with boulders) are used to strengthen banks.
• Wing Dykes or training walls which jut out from the sides of the channel may be employed to focus the main river current in the centre of the channel and away from the banks.
• In urban rivers the entire channel may be lines with concrete to decrease friction and increase flow velocity.
• In cities, rivers may be covered over and confined to concrete culverts to reduce the inconvenience to development and to help remove the increased amount of runoff from impermeable surfaces.
• All the above measures are expensive and offer relatively short-term advantages with high maintenance costs. In the long term, disadvantages include the effects on upstream sections (downcutting) and downstream sections (more deposition) which could potentially lead to catastrophic flooding.
Wetland and River Bank Conservation and River Restoration:
• River restoration can include a variety of strategies to reclaim rivers, for example re-routing the river from its straightened course into new meandering channels.
• Wetlands are areas that are deliberately allowed to flood at times of high discharge. They are also valuable as wildlife habitats.
• E.g. the Nene washes upstream from Peterborough for the River Nene.
• Many feel that restoration of peat bogs in northern uplands would slow water reaching lowland streams and rivers, reducing the threat to Sheffield, Ripon and Hull – all of which are particularly prone to flooding.
• On a larger scale, the frequent flooding of the river Rhine, culminating in the 1995 floods, led to a rethink on how to manage flood problems. A ‘Room for the River’ programme is currently being translated into land-use change and relocation of inhabitants on floodplains.
o Arable land is being converted to forest, marsh or wet grazing meadows.
o Inhabitants are being relocated, with compensation, to higher elevations, and the entire floodplain cross-section can accommodate a much larger volume of water.
• Measures taken include:
o An increase in ‘water meadows’ which can be allowed to flood when necessary.
o A reduction in the use of tarmac or concrete in vulnerable areas to slow water runoff into the rivers.
o Increased ground coverage of vegetation with woodlands and grasslands.
o Restrictions on the use of soil fertilisers which affect the soil structure, reducing its ability to retain water.
o Metres of silt accumulated over many years have been stripped and deep trenches constructed to allow more storage space for water in the event of flooding, and more room for trees which stabilise the soil and improve the ecological balance and help to evapotranspirate moisture away from saturated soils.
Case Study:
Flooding In Bangladesh:
• Major flooding in Bangladesh occurs frequently, regularly inundating between 20% and 30% of the country and leading to enormous loss of life.
• Flash Flooding – extremely heavy rainfall occurs on surrounding upland areas. Not all of it can be infiltrated into the soil and excess water forms runoff which leads to rapid filling of river channels. Where this spills onto the floodplain much sediment can be deposited, damaging crops.
• River Floods – mainly caused by meltwaters from the Himalayan mountains and heavy monsoon rains. Where the Brahmaputra and the Ganges meet, heavy levels of discharge breach embankments and flooding often ensues. This is particularly common along the Brahmaputra and Meghna rivers in early June. Widespread flooding can threaten settlements and heavy silt deposits may bury crops.
• Rainwater Floods – heavy prolonged rainfall within Bangladesh causes runoff to accumulate in surface depressions, trapped by rising river levels. This may occur before the monsoon and lead to topsoil being washed off farmland and into adjoining depressions.
• Storm Surges – these mainly affect the southern coastal fringe of the country, where cyclones moving up the Bay of Bengal create storm surges which inundate the low-lying coastal strip. Significant losses of life may ensue in the few hours of the storm.
Geography of Bangladesh Physical flood causes Human flood causes
Population 125million Most the country is a floodplain and delta Urbanisation of the flood plain increased magnitude and frequency of floods
70% of total area is under 1m above sea level Global warming blamed for sea level rise, increased snow melt and rainfall Global warming blamed for sea level rise, increased snow melt and rainfall
Experiences floods and tropical rainstorms annually Experiences heavy monsoon rains esp. over the highland Deforestation in Nepal and the Himalayas increases run off and adds to deposition and flooding downstream
One of the world’s poorest countries – GNP $200 10% of land is lakes and rivers
One of the world’s most densely populated countries Tropical storms bring heavy rains and coastal flooding The building of dams in India has increased the probability of sedimentation in Bangladesh
The entire country is a delta – contains virtually no raw materials or rock The Ganges, Meghna and Brahmaputra all pass through it
Embankments are poorly maintained due to the poverty and therefore leak and collapse during high discharge
3 of the world’s most powerful rivers flow through it all three reached peak flow at the same time
• Impacts of the 1988 and 1998 Bangladesh Floods:
1988 1998
Duration of Floods 21 days 65 days
Percentage of Country Affected 60% 75%
Percentage of Capital City covered by flooding 67% 50%
Area flooded 2,282,000km2 Over 1million km2
People Affected 45 million 31 million
Houses totally or partially damaged 7.2 million 980,000
Human lives lost 2379 1050
Livestock lost (cattle and goats) 172,000 26,500
Rice production lost 2 million tonnes 2.2 million tonnes
Trunk roads damaged 3000km 15,900km
Flood Embankments damaged 1990km 4528km
Industrial units flooded Over 1000 Over 5000
Schools flooded 19,000 14,000
Rural irrigation tubewells flooded 240,000 300,000
Flood Defences:
• After the 1988 floods, which affected 45 million and killed over 2000, a Flood Action Plan (FAP) was devised. The overarching aim for the plan is to create flood protection for Bangladesh.
• One key part is to construct new embankments alongside the Brahmaputra and Ganges in Bangladesh, starting with the upstream areas. The aim is not to completely stop the floods, but to keep them at a manageable level. Behind the embankment compartments of land are created by building internal walls to link up with the embankments.
• A flood forecasting system is planned to alert local inhabitants of impending floods.
• Preparation to deal with the consequences of flooding will include the provision of boats so that people can escape to shelters on higher land.
• The Jamalpur Priority Project Study illustrates four issues surrounding the potential impacts of embankment construction to help decide on optimum solutions;
o Flood proofing and drainage improvement
o Controlled flooding of the entire area with some compartmentalisation
o Controlled flooding of about half the area
o All areas compartmentalised – all river flooding excluded
• Economic and social impacts of the latter two meant they were soon rejected.
• The first offer seemed more beneficial to the fishing, non-farming and landless population whereas the second benefitted farming households and land owners, with the promise of greater economic growth for the area as a whole.
• The threat of sabotage and concern that areas outside of this scheme would suffer worse floods as a consequence made decision making much harder.
Embankment Issues:
Positioning:
• Many people in Bangladesh wanted embankments close to the channels to protect as many as possible and to maximise farmland.
• However, building close to the channel increases river depth and velocity at times of high flow in unstable, braided and meandering channels.
• Studies show there is a far greater risk of erosion and collapse when embankments are closer to channels.
• The more distant embankment option would cost half as much to build and maintain (up to 5km from channel) but an additional 5 million people would be in a flood zone.
Longer-term Impacts:
• From the famous Mississippi breach, we see that over time the river bed will rise due to deposition and eventually will exceed the former bank full level and require much larger embankments. Any breaches at this stage can be catastrophic.
• Faster and deeper flow regimes in upstream channel sections controlled by embankments inevitably lead to increased erosion producing greater sedimentation downstream as the river slows down. This may result in channel obstruction and an increased likelihood of flooding.
• Although compartmentalisation controls floodwaters when they occur the retention of large amounts of river water in smaller areas has implications for human health, crop production and fishing.
continued:
Oxbow Lakes:
• As meanders develop, erosion of the outside bend tends to move them slowly downstream and downslope.
• The sinuosity of the meander may become more pronounced, with erosion of the outer bank and deposition on the inner bank decreasing the width of the neck of land between the start and end of meander.
• At times of flood, this neck can be eroded away, giving the river a shorter and straighter route downstream.
• Initially the truncated meander loop forms a curved lake (hence the name oxbow), cut off from the main channel by deposition.
• Over time it may get infilled with sediment and vegetation; these are known as meander scars.
Rejuvenation:
• The long profile of a river (graded getting gradually less steep) reflects the fact that water does not have as far to fall as it nears the sea and so has less erosive power.
• However, over time, if the relative heights of the land and the sea alter, this situation may change.
• Such change is either Isostatic (land rising in relation to sea level such as when the weight of ice caps is removed) or Eustatic (sea level change often due to ice melt or water freezing).
• If sea level drops or land rises, existing valley floors may be cut into as the river attempts to regrade itself in keeping with the new energy levels exhibited by the river. This begins in the channel nearest the sea and then migrates back upstream.
• The current limit of the regarding is marked by a knick point.
Incised Meanders:
• An incised meander is one which lies at the bottom of a steep-walled canyon.
• This most often occurs at an existing meander after the rejuvenation of a river – there is then severe downwards erosion creating a steep-walled canyon.
• There are two types of ingrown meander:
o Entrenched meanders have a symmetrical cross-section resulting from very rapid incision by the river of valley sides being made of hard, resistant rock. The River Wye at Durham is an example.
o Ingrown meanders are formed when the incision or uplift is less rapid and the river may ‘shift’ laterally thus producing an asymmetrical cross section shape. The river Wye at Tintern Abbey is an example.
Flooding and Flood Management:
(Page 4 for Keswick case study)
• Floods occur when large volumes of water enter a river system quickly. Discharge increases to the point where it cannot be contained in the channel and water spills out onto the floodplain.
• Natural causes of flooding may be classified as follows:
o Primary causes are usually the result of climatic factors.
o Secondary causes tend to be drainage-basin specific (e.g. dependent on geology, soil, topography and vegetation).
• In addition to natural causes, human influences on the drainage basin increase the risk of flooding as development of once natural landscapes for residential, industrial and agricultural purposes tends to reduce infiltration and increase runoff.
• Urbanisation helps to increase the frequency and magnitude of flooding in several ways;
o Creating impermeable surfaces, e.g. car parks, roofs, roads and pavements.
o Speeding up the drainage of water in built-up areas via artificial conduits, e.g. sewers and drains.
o Impeding channel flow by building alongside or in the river, e.g. bridge supports.
o Straightening of channels to increase speed of flow which results in flooding downstream.
o Changing land use associated with development, e.g. deforestation, ploughing and overgrazing, which results in increased risk of flooding through increased runoff and increased levels of sediment washed into streams blocking channels.
Flood Management Strategies:
• Flood management strategies seek to reduce the effects of flooding on the human environment.
• The main strategies can be grouped as follows:
o Structural methods – offering protection through engineering
o River basin management – seeking to reduce the likelihood of flooding by managing land use
o Modifying the burden of loss – by insurance schemes
o Bearing the cost of flood damage – a ‘do nothing’ approach that only deals with the issues when they arise
• Unsurprisingly, developing countries tend to avoid expensive engineering solutions and in more developed countries; a more preventative attitude may prevail.
• Preventing floods is increasingly seen as impossible and river management concentrates more on reducing losses due to flooding.
Structural Methods:
Flood walls, Embankments and Levees:
• Flood walls are designed to increase the height of the channel to stop water spilling out onto the floodplain. Most commonly used in towns, they restrict access to the riverside and offer little in the way of floodwater storage capacity.
• Embankments are often made of earth with rubble fill and are more common outside the town centre where there is more room. If set back from the channel, they can provide storage for excess floodwaters while inhabited areas remain unaffected.
• Levees may be artificially enhanced or introduced to raise the level of the river banks.
• Each of these methods may reduce flooding at the expense of speeding water downstream to create problems elsewhere.
Channel Improvements:
• Attempt to restrict floods either by creating a smoother channel for faster flow to get water out of the area as soon as possible or by deepening/widening the channel.
• Channels may be smoothened by lining the channel with concrete.
• Deepening and widening may be achieved by regular dredging.
• Both may increase flood risk further downstream and both need regular maintenance as deposition and erosion revert the channel to its more natural form.
Relief Channels:
• Are constructed to redirect excess water upstream of a settlement via an alternative route. Water is able to re-enter the main channel further downstream, thereby reducing flood risk.
• By creating a bypass that can only be accessed at high discharge levels, the peak flow in the main channel is reduced and the relief channel may often remain dry until needed.
Flood Storage Reservoirs:
• Aim to store excess water in the upper reaches of the catchment area.
• They are expensive to construct and require huge amounts of land. In the U.K. no reservoirs are built with the sole purpose of preventing flood.
Flood Interception Schemes:
• May include re-routing a river to effect a bypass, using new channels to store excess water and flood embankments to contain flooding well away from settlements under threat.
• May also include flood retention basins, washland areas and polders. These are areas of land deliberately flooded upstream of towns and cities. They are low-value and provide temporary storage of floodwater. They may also be wildlife refuges or have amenity value.
River Basic Management:
• Seeks to reduce the harm done by flooding when it does occur.
Flood Abatement:
• Abatements measures aim to reduce the possibility of flooding by managing land use upstream. Includes;
o Afforestation to increase interception storage and evapotranspiration to help reduce runoff as well as holding the soil together to reduce silting up of river channels.
o Farming practices like contour ploughing and reducing the amount of bare earth to avoid excessive runoff problems.
Flood Proofing:
• May be temporary or permanent. Buildings can be constructed with flood-proof ground floor walls or have temporary gates ready to be installed at times of high risk.
• Potential damage of floods can be reduced by placing car parks etc on the ground-level sites.
Floodplain Zoning:
• Zones of relative risk can be mapped;
o Zone A: Prohibitive Zones – areas nearer to the channel with a relatively high risk of flooding. Essential waterfront development may be permitted by development is unlikely to be allowed here.
o Zone B: Restrictive Zones – little development is allowed and that which is should be flood-proofed. They are best suited to low-intensity or low-value land uses such as pasture, playing fields or car parks.
o Zone C: Warming Zones – situated on higher land and further away. There is more development here but inhabitants are made aware of imminent flood damage and are instructed how to react when floods do occur.
Flood Prediction and Warning:
• Records of river discharge and flooding are kept to help predict future flood events.
• The main methods of collecting data to aid flood forecasting are weather radar and the information from automatic rainfall and river gauges.
• Flood prediction software helps to model likely outcomes, and warning may be issued in terms of the potential severity of the flood risk and the areas that could be affected.
• A method called Risk Assessment for Strategic Planning (RASP) is used to help place areas into three distinct categories:
o Low – the chance of flooding each year is below 0.5%.
o Moderate – the chance of flooding each year is between 1.3% (1 in 75) and 0.5%.
o Significant – the chance of flooding is above 1.3%.
• In flood prone areas like York, automatic phone warnings are issues to alert inhabitants in potential flooding zones, and visits by flood wardens ensure that temporary defences are in place and/or evacuations are carried out when necessary.
Channelisation or Neutralisation:
• Channelisation is an attempt to alter the natural geometry of a watercourse.
• It can help to prevent flooding by increasing channel capacity and preventing bank erosion, both of which reduce likelihood of a river breaking out of its channel at times of high flow.
• The dual benefits of flood prevention and land extension mean that such hard engineering solutions have massively increased in popularity.
• Resectioning a river involves widening and deepening a channel to improve its hydraulic efficiency. This increase capacity and moves water out of an area much more quickly. Dredging is one way of removing surplus sediment from the river bed.
• Realignment (straightening) involves shortening the river course by removal of meanders. The increase in gradient moves floodwaters away more quickly while improving navigation.
• Revetments made of concrete blocks, steel or gabions (wire mesh cubes filled with boulders) are used to strengthen banks.
• Wing Dykes or training walls which jut out from the sides of the channel may be employed to focus the main river current in the centre of the channel and away from the banks.
• In urban rivers the entire channel may be lines with concrete to decrease friction and increase flow velocity.
• In cities, rivers may be covered over and confined to concrete culverts to reduce the inconvenience to development and to help remove the increased amount of runoff from impermeable surfaces.
• All the above measures are expensive and offer relatively short-term advantages with high maintenance costs. In the long term, disadvantages include the effects on upstream sections (downcutting) and downstream sections (more deposition) which could potentially lead to catastrophic flooding.
Wetland and River Bank Conservation and River Restoration:
• River restoration can include a variety of strategies to reclaim rivers, for example re-routing the river from its straightened course into new meandering channels.
• Wetlands are areas that are deliberately allowed to flood at times of high discharge. They are also valuable as wildlife habitats.
• E.g. the Nene washes upstream from Peterborough for the River Nene.
• Many feel that restoration of peat bogs in northern uplands would slow water reaching lowland streams and rivers, reducing the threat to Sheffield, Ripon and Hull – all of which are particularly prone to flooding.
• On a larger scale, the frequent flooding of the river Rhine, culminating in the 1995 floods, led to a rethink on how to manage flood problems. A ‘Room for the River’ programme is currently being translated into land-use change and relocation of inhabitants on floodplains.
o Arable land is being converted to forest, marsh or wet grazing meadows.
o Inhabitants are being relocated, with compensation, to higher elevations, and the entire floodplain cross-section can accommodate a much larger volume of water.
• Measures taken include:
o An increase in ‘water meadows’ which can be allowed to flood when necessary.
o A reduction in the use of tarmac or concrete in vulnerable areas to slow water runoff into the rivers.
o Increased ground coverage of vegetation with woodlands and grasslands.
o Restrictions on the use of soil fertilisers which affect the soil structure, reducing its ability to retain water.
o Metres of silt accumulated over many years have been stripped and deep trenches constructed to allow more storage space for water in the event of flooding, and more room for trees which stabilise the soil and improve the ecological balance and help to evapotranspirate moisture away from saturated soils.
Case Study:
Flooding In Bangladesh:
• Major flooding in Bangladesh occurs frequently, regularly inundating between 20% and 30% of the country and leading to enormous loss of life.
• Flash Flooding – extremely heavy rainfall occurs on surrounding upland areas. Not all of it can be infiltrated into the soil and excess water forms runoff which leads to rapid filling of river channels. Where this spills onto the floodplain much sediment can be deposited, damaging crops.
• River Floods – mainly caused by meltwaters from the Himalayan mountains and heavy monsoon rains. Where the Brahmaputra and the Ganges meet, heavy levels of discharge breach embankments and flooding often ensues. This is particularly common along the Brahmaputra and Meghna rivers in early June. Widespread flooding can threaten settlements and heavy silt deposits may bury crops.
• Rainwater Floods – heavy prolonged rainfall within Bangladesh causes runoff to accumulate in surface depressions, trapped by rising river levels. This may occur before the monsoon and lead to topsoil being washed off farmland and into adjoining depressions.
• Storm Surges – these mainly affect the southern coastal fringe of the country, where cyclones moving up the Bay of Bengal create storm surges which inundate the low-lying coastal strip. Significant losses of life may ensue in the few hours of the storm.
Geography of Bangladesh Physical flood causes Human flood causes
Population 125million Most the country is a floodplain and delta Urbanisation of the flood plain increased magnitude and frequency of floods
70% of total area is under 1m above sea level Global warming blamed for sea level rise, increased snow melt and rainfall Global warming blamed for sea level rise, increased snow melt and rainfall
Experiences floods and tropical rainstorms annually Experiences heavy monsoon rains esp. over the highland Deforestation in Nepal and the Himalayas increases run off and adds to deposition and flooding downstream
One of the world’s poorest countries – GNP $200 10% of land is lakes and rivers
One of the world’s most densely populated countries Tropical storms bring heavy rains and coastal flooding The building of dams in India has increased the probability of sedimentation in Bangladesh
The entire country is a delta – contains virtually no raw materials or rock The Ganges, Meghna and Brahmaputra all pass through it
Embankments are poorly maintained due to the poverty and therefore leak and collapse during high discharge
3 of the world’s most powerful rivers flow through it all three reached peak flow at the same time
• Impacts of the 1988 and 1998 Bangladesh Floods:
1988 1998
Duration of Floods 21 days 65 days
Percentage of Country Affected 60% 75%
Percentage of Capital City covered by flooding 67% 50%
Area flooded 2,282,000km2 Over 1million km2
People Affected 45 million 31 million
Houses totally or partially damaged 7.2 million 980,000
Human lives lost 2379 1050
Livestock lost (cattle and goats) 172,000 26,500
Rice production lost 2 million tonnes 2.2 million tonnes
Trunk roads damaged 3000km 15,900km
Flood Embankments damaged 1990km 4528km
Industrial units flooded Over 1000 Over 5000
Schools flooded 19,000 14,000
Rural irrigation tubewells flooded 240,000 300,000
Flood Defences:
• After the 1988 floods, which affected 45 million and killed over 2000, a Flood Action Plan (FAP) was devised. The overarching aim for the plan is to create flood protection for Bangladesh.
• One key part is to construct new embankments alongside the Brahmaputra and Ganges in Bangladesh, starting with the upstream areas. The aim is not to completely stop the floods, but to keep them at a manageable level. Behind the embankment compartments of land are created by building internal walls to link up with the embankments.
• A flood forecasting system is planned to alert local inhabitants of impending floods.
• Preparation to deal with the consequences of flooding will include the provision of boats so that people can escape to shelters on higher land.
• The Jamalpur Priority Project Study illustrates four issues surrounding the potential impacts of embankment construction to help decide on optimum solutions;
o Flood proofing and drainage improvement
o Controlled flooding of the entire area with some compartmentalisation
o Controlled flooding of about half the area
o All areas compartmentalised – all river flooding excluded
• Economic and social impacts of the latter two meant they were soon rejected.
• The first offer seemed more beneficial to the fishing, non-farming and landless population whereas the second benefitted farming households and land owners, with the promise of greater economic growth for the area as a whole.
• The threat of sabotage and concern that areas outside of this scheme would suffer worse floods as a consequence made decision making much harder.
Embankment Issues:
Positioning:
• Many people in Bangladesh wanted embankments close to the channels to protect as many as possible and to maximise farmland.
• However, building close to the channel increases river depth and velocity at times of high flow in unstable, braided and meandering channels.
• Studies show there is a far greater risk of erosion and collapse when embankments are closer to channels.
• The more distant embankment option would cost half as much to build and maintain (up to 5km from channel) but an additional 5 million people would be in a flood zone.
Longer-term Impacts:
• From the famous Mississippi breach, we see that over time the river bed will rise due to deposition and eventually will exceed the former bank full level and require much larger embankments. Any breaches at this stage can be catastrophic.
• Faster and deeper flow regimes in upstream channel sections controlled by embankments inevitably lead to increased erosion producing greater sedimentation downstream as the river slows down. This may result in channel obstruction and an increased likelihood of flooding.
• Although compartmentalisation controls floodwaters when they occur the retention of large amounts of river water in smaller areas has implications for human health, crop production and fishing.
Oxbow Lakes:
• As meanders develop, erosion of the outside bend tends to move them slowly downstream and downslope.
• The sinuosity of the meander may become more pronounced, with erosion of the outer bank and deposition on the inner bank decreasing the width of the neck of land between the start and end of meander.
• At times of flood, this neck can be eroded away, giving the river a shorter and straighter route downstream.
• Initially the truncated meander loop forms a curved lake (hence the name oxbow), cut off from the main channel by deposition.
• Over time it may get infilled with sediment and vegetation; these are known as meander scars.
Rejuvenation:
• The long profile of a river (graded getting gradually less steep) reflects the fact that water does not have as far to fall as it nears the sea and so has less erosive power.
• However, over time, if the relative heights of the land and the sea alter, this situation may change.
• Such change is either Isostatic (land rising in relation to sea level such as when the weight of ice caps is removed) or Eustatic (sea level change often due to ice melt or water freezing).
• If sea level drops or land rises, existing valley floors may be cut into as the river attempts to regrade itself in keeping with the new energy levels exhibited by the river. This begins in the channel nearest the sea and then migrates back upstream.
• The current limit of the regarding is marked by a knick point.
Incised Meanders:
• An incised meander is one which lies at the bottom of a steep-walled canyon.
• This most often occurs at an existing meander after the rejuvenation of a river – there is then severe downwards erosion creating a steep-walled canyon.
• There are two types of ingrown meander:
o Entrenched meanders have a symmetrical cross-section resulting from very rapid incision by the river of valley sides being made of hard, resistant rock. The River Wye at Durham is an example.
o Ingrown meanders are formed when the incision or uplift is less rapid and the river may ‘shift’ laterally thus producing an asymmetrical cross section shape. The river Wye at Tintern Abbey is an example.
Flooding and Flood Management:
(Page 4 for Keswick case study)
• Floods occur when large volumes of water enter a river system quickly. Discharge increases to the point where it cannot be contained in the channel and water spills out onto the floodplain.
• Natural causes of flooding may be classified as follows:
o Primary causes are usually the result of climatic factors.
o Secondary causes tend to be drainage-basin specific (e.g. dependent on geology, soil, topography and vegetation).
• In addition to natural causes, human influences on the drainage basin increase the risk of flooding as development of once natural landscapes for residential, industrial and agricultural purposes tends to reduce infiltration and increase runoff.
• Urbanisation helps to increase the frequency and magnitude of flooding in several ways;
o Creating impermeable surfaces, e.g. car parks, roofs, roads and pavements.
o Speeding up the drainage of water in built-up areas via artificial conduits, e.g. sewers and drains.
o Impeding channel flow by building alongside or in the river, e.g. bridge supports.
o Straightening of channels to increase speed of flow which results in flooding downstream.
o Changing land use associated with development, e.g. deforestation, ploughing and overgrazing, which results in increased risk of flooding through increased runoff and increased levels of sediment washed into streams blocking channels.
Flood Management Strategies:
• Flood management strategies seek to reduce the effects of flooding on the human environment.
• The main strategies can be grouped as follows:
o Structural methods – offering protection through engineering
o River basin management – seeking to reduce the likelihood of flooding by managing land use
o Modifying the burden of loss – by insurance schemes
o Bearing the cost of flood damage – a ‘do nothing’ approach that only deals with the issues when they arise
• Unsurprisingly, developing countries tend to avoid expensive engineering solutions and in more developed countries; a more preventative attitude may prevail.
• Preventing floods is increasingly seen as impossible and river management concentrates more on reducing losses due to flooding.
Structural Methods:
Flood walls, Embankments and Levees:
• Flood walls are designed to increase the height of the channel to stop water spilling out onto the floodplain. Most commonly used in towns, they restrict access to the riverside and offer little in the way of floodwater storage capacity.
• Embankments are often made of earth with rubble fill and are more common outside the town centre where there is more room. If set back from the channel, they can provide storage for excess floodwaters while inhabited areas remain unaffected.
• Levees may be artificially enhanced or introduced to raise the level of the river banks.
• Each of these methods may reduce flooding at the expense of speeding water downstream to create problems elsewhere.
Channel Improvements:
• Attempt to restrict floods either by creating a smoother channel for faster flow to get water out of the area as soon as possible or by deepening/widening the channel.
• Channels may be smoothened by lining the channel with concrete.
• Deepening and widening may be achieved by regular dredging.
• Both may increase flood risk further downstream and both need regular maintenance as deposition and erosion revert the channel to its more natural form.
Relief Channels:
• Are constructed to redirect excess water upstream of a settlement via an alternative route. Water is able to re-enter the main channel further downstream, thereby reducing flood risk.
• By creating a bypass that can only be accessed at high discharge levels, the peak flow in the main channel is reduced and the relief channel may often remain dry until needed.
Flood Storage Reservoirs:
• Aim to store excess water in the upper reaches of the catchment area.
• They are expensive to construct and require huge amounts of land. In the U.K. no reservoirs are built with the sole purpose of preventing flood.
Flood Interception Schemes:
• May include re-routing a river to effect a bypass, using new channels to store excess water and flood embankments to contain flooding well away from settlements under threat.
• May also include flood retention basins, washland areas and polders. These are areas of land deliberately flooded upstream of towns and cities. They are low-value and provide temporary storage of floodwater. They may also be wildlife refuges or have amenity value.
River Basic Management:
• Seeks to reduce the harm done by flooding when it does occur.
Flood Abatement:
• Abatements measures aim to reduce the possibility of flooding by managing land use upstream. Includes;
o Afforestation to increase interception storage and evapotranspiration to help reduce runoff as well as holding the soil together to reduce silting up of river channels.
o Farming practices like contour ploughing and reducing the amount of bare earth to avoid excessive runoff problems.
Flood Proofing:
• May be temporary or permanent. Buildings can be constructed with flood-proof ground floor walls or have temporary gates ready to be installed at times of high risk.
• Potential damage of floods can be reduced by placing car parks etc on the ground-level sites.
Floodplain Zoning:
• Zones of relative risk can be mapped;
o Zone A: Prohibitive Zones – areas nearer to the channel with a relatively high risk of flooding. Essential waterfront development may be permitted by development is unlikely to be allowed here.
o Zone B: Restrictive Zones – little development is allowed and that which is should be flood-proofed. They are best suited to low-intensity or low-value land uses such as pasture, playing fields or car parks.
o Zone C: Warming Zones – situated on higher land and further away. There is more development here but inhabitants are made aware of imminent flood damage and are instructed how to react when floods do occur.
Flood Prediction and Warning:
• Records of river discharge and flooding are kept to help predict future flood events.
• The main methods of collecting data to aid flood forecasting are weather radar and the information from automatic rainfall and river gauges.
• Flood prediction software helps to model likely outcomes, and warning may be issued in terms of the potential severity of the flood risk and the areas that could be affected.
• A method called Risk Assessment for Strategic Planning (RASP) is used to help place areas into three distinct categories:
o Low – the chance of flooding each year is below 0.5%.
o Moderate – the chance of flooding each year is between 1.3% (1 in 75) and 0.5%.
o Significant – the chance of flooding is above 1.3%.
• In flood prone areas like York, automatic phone warnings are issues to alert inhabitants in potential flooding zones, and visits by flood wardens ensure that temporary defences are in place and/or evacuations are carried out when necessary.
Channelisation or Neutralisation:
• Channelisation is an attempt to alter the natural geometry of a watercourse.
• It can help to prevent flooding by increasing channel capacity and preventing bank erosion, both of which reduce likelihood of a river breaking out of its channel at times of high flow.
• The dual benefits of flood prevention and land extension mean that such hard engineering solutions have massively increased in popularity.
• Resectioning a river involves widening and deepening a channel to improve its hydraulic efficiency. This increase capacity and moves water out of an area much more quickly. Dredging is one way of removing surplus sediment from the river bed.
• Realignment (straightening) involves shortening the river course by removal of meanders. The increase in gradient moves floodwaters away more quickly while improving navigation.
• Revetments made of concrete blocks, steel or gabions (wire mesh cubes filled with boulders) are used to strengthen banks.
• Wing Dykes or training walls which jut out from the sides of the channel may be employed to focus the main river current in the centre of the channel and away from the banks.
• In urban rivers the entire channel may be lines with concrete to decrease friction and increase flow velocity.
• In cities, rivers may be covered over and confined to concrete culverts to reduce the inconvenience to development and to help remove the increased amount of runoff from impermeable surfaces.
• All the above measures are expensive and offer relatively short-term advantages with high maintenance costs. In the long term, disadvantages include the effects on upstream sections (downcutting) and downstream sections (more deposition) which could potentially lead to catastrophic flooding.
Wetland and River Bank Conservation and River Restoration:
• River restoration can include a variety of strategies to reclaim rivers, for example re-routing the river from its straightened course into new meandering channels.
• Wetlands are areas that are deliberately allowed to flood at times of high discharge. They are also valuable as wildlife habitats.
• E.g. the Nene washes upstream from Peterborough for the River Nene.
• Many feel that restoration of peat bogs in northern uplands would slow water reaching lowland streams and rivers, reducing the threat to Sheffield, Ripon and Hull – all of which are particularly prone to flooding.
• On a larger scale, the frequent flooding of the river Rhine, culminating in the 1995 floods, led to a rethink on how to manage flood problems. A ‘Room for the River’ programme is currently being translated into land-use change and relocation of inhabitants on floodplains.
o Arable land is being converted to forest, marsh or wet grazing meadows.
o Inhabitants are being relocated, with compensation, to higher elevations, and the entire floodplain cross-section can accommodate a much larger volume of water.
• Measures taken include:
o An increase in ‘water meadows’ which can be allowed to flood when necessary.
o A reduction in the use of tarmac or concrete in vulnerable areas to slow water runoff into the rivers.
o Increased ground coverage of vegetation with woodlands and grasslands.
o Restrictions on the use of soil fertilisers which affect the soil structure, reducing its ability to retain water.
o Metres of silt accumulated over many years have been stripped and deep trenches constructed to allow more storage space for water in the event of flooding, and more room for trees which stabilise the soil and improve the ecological balance and help to evapotranspirate moisture away from saturated soils.
Case Study:
Flooding In Bangladesh:
• Major flooding in Bangladesh occurs frequently, regularly inundating between 20% and 30% of the country and leading to enormous loss of life.
• Flash Flooding – extremely heavy rainfall occurs on surrounding upland areas. Not all of it can be infiltrated into the soil and excess water forms runoff which leads to rapid filling of river channels. Where this spills onto the floodplain much sediment can be deposited, damaging crops.
• River Floods – mainly caused by meltwaters from the Himalayan mountains and heavy monsoon rains. Where the Brahmaputra and the Ganges meet, heavy levels of discharge breach embankments and flooding often ensues. This is particularly common along the Brahmaputra and Meghna rivers in early June. Widespread flooding can threaten settlements and heavy silt deposits may bury crops.
• Rainwater Floods – heavy prolonged rainfall within Bangladesh causes runoff to accumulate in surface depressions, trapped by rising river levels. This may occur before the monsoon and lead to topsoil being washed off farmland and into adjoining depressions.
• Storm Surges – these mainly affect the southern coastal fringe of the country, where cyclones moving up the Bay of Bengal create storm surges which inundate the low-lying coastal strip. Significant losses of life may ensue in the few hours of the storm.
Geography of Bangladesh Physical flood causes Human flood causes
Population 125million Most the country is a floodplain and delta Urbanisation of the flood plain increased magnitude and frequency of floods
70% of total area is under 1m above sea level Global warming blamed for sea level rise, increased snow melt and rainfall Global warming blamed for sea level rise, increased snow melt and rainfall
Experiences floods and tropical rainstorms annually Experiences heavy monsoon rains esp. over the highland Deforestation in Nepal and the Himalayas increases run off and adds to deposition and flooding downstream
One of the world’s poorest countries – GNP $200 10% of land is lakes and rivers
One of the world’s most densely populated countries Tropical storms bring heavy rains and coastal flooding The building of dams in India has increased the probability of sedimentation in Bangladesh
The entire country is a delta – contains virtually no raw materials or rock The Ganges, Meghna and Brahmaputra all pass through it
Embankments are poorly maintained due to the poverty and therefore leak and collapse during high discharge
3 of the world’s most powerful rivers flow through it all three reached peak flow at the same time
• Impacts of the 1988 and 1998 Bangladesh Floods:
1988 1998
Duration of Floods 21 days 65 days
Percentage of Country Affected 60% 75%
Percentage of Capital City covered by flooding 67% 50%
Area flooded 2,282,000km2 Over 1million km2
People Affected 45 million 31 million
Houses totally or partially damaged 7.2 million 980,000
Human lives lost 2379 1050
Livestock lost (cattle and goats) 172,000 26,500
Rice production lost 2 million tonnes 2.2 million tonnes
Trunk roads damaged 3000km 15,900km
Flood Embankments damaged 1990km 4528km
Industrial units flooded Over 1000 Over 5000
Schools flooded 19,000 14,000
Rural irrigation tubewells flooded 240,000 300,000
Flood Defences:
• After the 1988 floods, which affected 45 million and killed over 2000, a Flood Action Plan (FAP) was devised. The overarching aim for the plan is to create flood protection for Bangladesh.
• One key part is to construct new embankments alongside the Brahmaputra and Ganges in Bangladesh, starting with the upstream areas. The aim is not to completely stop the floods, but to keep them at a manageable level. Behind the embankment compartments of land are created by building internal walls to link up with the embankments.
• A flood forecasting system is planned to alert local inhabitants of impending floods.
• Preparation to deal with the consequences of flooding will include the provision of boats so that people can escape to shelters on higher land.
• The Jamalpur Priority Project Study illustrates four issues surrounding the potential impacts of embankment construction to help decide on optimum solutions;
o Flood proofing and drainage improvement
o Controlled flooding of the entire area with some compartmentalisation
o Controlled flooding of about half the area
o All areas compartmentalised – all river flooding excluded
• Economic and social impacts of the latter two meant they were soon rejected.
• The first offer seemed more beneficial to the fishing, non-farming and landless population whereas the second benefitted farming households and land owners, with the promise of greater economic growth for the area as a whole.
• The threat of sabotage and concern that areas outside of this scheme would suffer worse floods as a consequence made decision making much harder.
Embankment Issues:
Positioning:
• Many people in Bangladesh wanted embankments close to the channels to protect as many as possible and to maximise farmland.
• However, building close to the channel increases river depth and velocity at times of high flow in unstable, braided and meandering channels.
• Studies show there is a far greater risk of erosion and collapse when embankments are closer to channels.
• The more distant embankment option would cost half as much to build and maintain (up to 5km from channel) but an additional 5 million people would be in a flood zone.
Longer-term Impacts:
• From the famous Mississippi breach, we see that over time the river bed will rise due to deposition and eventually will exceed the former bank full level and require much larger embankments. Any breaches at this stage can be catastrophic.
• Faster and deeper flow regimes in upstream channel sections controlled by embankments inevitably lead to increased erosion producing greater sedimentation downstream as the river slows down. This may result in channel obstruction and an increased likelihood of flooding.
• Although compartmentalisation controls floodwaters when they occur the retention of large amounts of river water in smaller areas has implications for human health, crop production and fishing.
holy moly!
that is the most perfect thing that i needed right now
thanks sooo much!!
do you have the same for:
Cold Environments?
Human Core Population
or Health?
Thanks sooo much!! if my rep was posotive i would rep you definitly!
by the way, we're discussing this exam here --> http://www.thestudentroom.co.uk/showthread.php?p=18682772
that is the most perfect thing that i needed right now
thanks sooo much!!
do you have the same for:
Cold Environments?
Human Core Population
or Health?
Thanks sooo much!! if my rep was posotive i would rep you definitly!
by the way, we're discussing this exam here --> http://www.thestudentroom.co.uk/showthread.php?p=18682772
neomilan
holy moly!
that is the most perfect thing that i needed right now
thanks sooo much!!
do you have the same for:
Cold Environments?
Human Core Population
or Health?
Thanks sooo much!! if my rep was posotive i would rep you definitly!
by the way, we're discussing this exam here --> http://www.thestudentroom.co.uk/showthread.php?p=18682772
that is the most perfect thing that i needed right now
thanks sooo much!!
do you have the same for:
Cold Environments?
Human Core Population
or Health?
Thanks sooo much!! if my rep was posotive i would rep you definitly!
by the way, we're discussing this exam here --> http://www.thestudentroom.co.uk/showthread.php?p=18682772
yes i have for cold environments but i dont have case studies for tundra or the antarctica, i will be posting my notes tomorrow
Pablin888
Dont you have case studies for tundra areas and the antarctica
And notes on food supply issues?
And notes on food supply issues?
Sort of. Since I do the old spec, the topics are split up differently.
And we havent studied food supply issues - we studied "Changes in Population over the last 30years and resources available", but I presume the stuff is still pretty similar.
The stuff about flood management was in our second module whilst the stuff about rivers was in the first.
Pablin888
yes i have notes on cold environments which i will be uploading tomorrow but i dont have case studies for tundra areas or the antarctica
I wish I could help you but we havent studied antarctica for our exam.
We did a bit on African Savanas but I think that the content would be too different to your 'tundra areas' topic
Pablin888
yes i have notes on cold environments which i will be uploading tomorrow but i dont have case studies for tundra areas or the antarctica
i might be able to get antartica stuff
it is very similar to arctic case study details
thanks sooo very much for these notes you are a life saver! i owe you one
I've put all those notes into a nice word document ready to print or view:
I have cold environments, energy and population if anyone is interested but it exceeds the upload limit by a bit so would have to email it.
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