Freshwater - flooding and flood mitigation Flashcards
Define hard engineering management strategies
Management strategies whereby predominately technology and built-up infrastructure are used to regulate the flow of the river, control floods and manage the floodplain and its uses.
What are the structural measures used to mitigate floods?
- Dams
- Afforestation
- Channel modification
- Levée strengthening
Define channelization
A deliberate attempt to alter the natural geometry of the channel.
Describe the ways in which channelization can mitigate flooding
- The river can be deepened and widened to increase the capacity of the channel. This increases its hydraulic efficiency and allows a larger discharge to be contained within the channel. This will help prevent flooding.
- The channel can be made straighter through the use of artificial cut-offs. The channel can also be realigned to increase the long profile gradient. These will speed up the flow and also aid navigation so flood water can be removed more quickly.
Channelization is often achieved through …. …. the banks and bed. This prevents bed and bank …. . Fill in the gaps.
- concrete lining
- erosion
Give place context for The Channelization of the River Kissimmee
- Flows from Lake Kissimmee to Lake Okeechobee in Florida
- Before being channelized the river meandered some 160 kilometres
- It frequently flooded its banks creating a vast wetland several kilometres wide
Outline the the Channelization of the River Kissimmee
- It was channelized by the Army Corps Engineers in the 1960s
- They straightened the river into a 90 kilometre concrete-lined canal
What are the advantages of channelization? (River Kissimmee context)
- Created a lot more space for urban development, agriculture and industry
- Protected a growing population from the risk of flooding
- Enabled use of the floodplain for important transport routes
What are the disadvantages of channelization? (River Kissimmee context)
- 35,000 acres of wetland were drained which led to the loss of a unique wetland ecosystem
- The natural filter of the wetland for storing and breaking down pollutants was lost. As a result, the canal became badly polluted by phosphates and nitrate run-off. This has led to the contamination and eutrophication of Lake Okeechobee and increased coral bleaching in coastal waters.
- Because of environmental impacts, close to $1 billion was spent to restore it back to its meandering course
Simply put, what are dams?
Major infrastructure projects that store water in large reservoirs held back by huge concrete walls. They can release water when it’s needed and often to produce hydroelectric power.
What are examples of the many services dams can provide?
- Flood defence
- Energy
- Internal transport routes
- Industry
- Irrigation
- Leisure opportunities
Place context for the Three Gorges Dam
Located along the Yangtze River in China
Advantages of the Three Gorges Dam
- It’s seen as a major feat of engineering which brings prestige to China
- Produces 95 TWh of clean energy
- Protects 10 million people from the risk of floods
- Provides a 600 kilometre internal reservoir that helps provide transport and trade in the interior of China
- Acted as a catalyst for reduced sewage and industrial effluence discharge and water processing
Disadvantages of the Three Gorges Dam
- Led to the displacement of more than 2 million people from many cities. Landslides along the reservoir banks led to further displacements.
- Led to increased pollution due to upstream sewage run-off and effluence
- Increased downstream erosion due to erratic discharge rates
- Impacted river ecology including the endangered Yangtze dolphin
What is ‘making room for the river’?
A land use management project that the Netherlands embarked on to make room for the river Ijssel to flood. The project moves away from hard engineered flood mitigation strategies, like containing channels behind dykes, towards a floodplain storage plan that targets over 30 different locations along the river.
What is an example of the floodplain management being done in the ‘making room for the river’ project?
Dykes that would normally have held back the River at a section of the River Ijssel have been broken and replaced with a new meandering section that will allow the floodplain to flood and store water.
What are the different engineering projects associated with the ‘make room for the river’ project on the River Ijssel?
- Lowering the floodplain
- Extending the floodplain
- Deepening channel beds
- High water channel
- Adapting groynes
How does lowering the floodplain make room for the river?
Centuries of flooding has built up the floodplain with sediments. By excavating them you lower the floodplain giving the river more space during periods of high flow.
How does extending the floodplain make room for the river?
By moving dykes further away from the channel, a larger floodplain with more storage is created. Therefore, during periods of high flow there is more room for the river to flood.
How does deepening channel beds make room for the river?
Additional capacity is created reducing the likelihood of floods.
How does engineering a high water channel make room for the river?
It provides an extra channel that can be used to divert extra flow during times of high flow.
What are disadvantages of the ‘make room for the river’ project?
- Can be time consuming and costly in terms of delays and land values. Affordable housing tends to be built on floodplains which is no longer advisable
- Costs for industry and vital services can be higher due to high land costs in more strategic locations
- Actual flooding is not always reduced and some places are not prioritized leaving to social and economic disruption
- The Nijmegen project alone cost over 300 million euros - expensive
What are advantages of the ‘make room for the river’ project?
- Relatively low cost of channel and floodplain management compared to harder engineered approaches
- Careful land use planning to reduce impacts on housing, industry and vital services
- A whole catchment approach that addresses the need to manage the drainage basin as a whole and not just small river sections
- Reduced regulation of the river allowing for a more natural approach
What is the main aim of floodplain restoration?
To bring back the floodplain as a natural store of flood water
What are the two main features of the floodplain restoration project on the downstream section of the River Dijle?
- A short diversion channel and a sluice gate, which when closed floods water upstream to cover a washland of roughly 2 km^2
- A small reservoir downstream from the sluice gate which slows down flow and deposits sediment, preventing build-up in Leuven. The reservoir is dredged every two years.
Describe the second project further upstream in the Dijle valley aimed at restoring the floodplain.
It is in a nature reserve called the Doode Bemde. The drainage ditches were dismantled allowing the water table to slowly rise. In addition, vegetation clearance of the river banks was stopped, which slowed down flow leading to increased deposition and a fall in channel capacity. This led to rapid meander migration, in some sections greater than 1m a year. The Doode Bemde now has the capacity to store 25% of the maximum flood for Leuven.
The …. Bemde together with the …. …. and washland have the potential to fully prevent Leuven’s 100 year flood event. Fill in the gaps.
- Doode
- sluice gate
Advantages of the floodplain restoration of the River Dijle
- Very low cost in comparison to heavy-engineered projects
- Significant prevention of floods through floodplain storage
- Significant cost saving - 395 million euros if the 1891 flood occurred today
- Good ecological benefits that replicate natural flooding and ecosystems
Disadvantages of the floodplain restoration of the River Dijle
- Local flooding impacts in the designated washlands
- Washlands need to be large in size in order to avoid high-depth floods that can be ecologically harmful
- Loss of agriculturally productive land
How can afforestation schemes mitigate floods?
Dense forest reduces surface run-off by as much as 40%. Therefore, increased forest cover across a drainage basin is likely to have a major influence on the frequency and magnitude of floods.
Through which 2 ways does vegetation reduce surface run-off?
- It intercepts precipitation preventing it from reaching the ground
- Precipitation reaches the ground slowly as throughfall and so infiltrates the soil
Place context for the project called ‘Slowing the Flow’
Pickering, UK
Describe the ‘Slowing the Flow’ project
It was a flood mitigation project based around the planting of 40,000 trees over 44 hectares in Pickering, UK. In addition, 167 leaky dams and 187 heather bale dams were placed within streams.
How do leaky and heather bale dams slow down flow rates?
Tree trunks or heather bales are placed across small streams. They slow water down, hold it back and regulate how fast water leaks through. When replicated in high numbers across a catchment area they are quite effective in slowing down flow rates.
What are the advantages of the ‘Slowing the Flow’ project / afforestation schemes?
- Reduced peak river flow by 20% at times of extreme heavy rainfall and saturated soils
- Only costed 500,000 pounds
- Ecologically sound and can enrich the wetland ecosystem
- Can be used in a wide variety of rural landscapes
What are the disadvantages of the ‘Slowing the Flow’ project / afforestation schemes?
- Requires a lot of land
- Can be resisted by farming communities due to how increased forest cover may isolate farmers further
- Not ideal for all environments, such as urban areas
Give an example of an adaptive strategy to help people cope and respond to flooding to minimise the disruption they cause.
The UK’s Environmental Agency carefully monitors rivers with smart flow systems that can give live feedback on the risk of flooding , and with good data, they can provide real-time flood alerts for warning people of when a flood is most likely. This live data is available online and provides individuals and businesses with the correct information at the right time, allowing measures to be put in place to protect properties and also to evacuate.
Another adaptive strategy to flood mitigation is to allow the water to flood in designated areas through …. …. . Fill in the gaps.
urban zoning
Describe urban zoning
Urban zoning is a flood management approach used to designate areas to allow water to flood in. This tends to be areas along the floodplain where the costs will be lowest and property is most protected, for example farm fields, parks and large car parks. It ensures dense housing areas and vital infrastructure is not built in areas at greatest risk of flooding.
Describe sustainable flood management
It involves increasing the resilience to flood risk by reducing the frequency and impacts of flooding, but also reducing the potential flood risks and provide protection and warning where appropriate.
Define flood recurrence intervals
The frequency of floods of a certain flood magnitude
The scale and frequency of floods can be calculated using the …. …. …. . Fill in the gaps.
flood recurrence interval
How is the recurrence interval of a flood calculated?
By dividing the number of recorded floods plus 1 by its ranks in terms of flood magnitude
What can be determined from flood recurrence intervals?
The largest magnitude floods recur the least frequently, whereas the low magnitude floods occur the most often
What do recurrence intervals give environment agencies?
Useful data to enable long-term planning linked to flood prediction
What hydrological factors do hydrologists measure?
- The water levels in stream, rivers and lakes
- The water content of snow
- Recent precipitation amounts
- How much more precipitation is expected
What can the data measured by hydrologists used for? (flood prediction)
The data can be sent to river forecast centres where computer models are used to predict river and stream levels in their areas of responsibility
Explain the FLASH project
FLASH is a USA based project that stands for Flooded Locations and Simulation Hydrographs. The project relies on minute-by-minute data feeding into the models and can help predict the timing and location of potential floods for each square kilometre.
What do the FLASH models rely on?
Multiple radar and multi-sensor systems that combine radar data on upper atmospheric conditions as well as surface storage and flow patters in rivers.
How is climate change impacting climate models?
Climate change is impacting weather patterns. Storm and rainfall events appear to be getting more severe and less predictable in many regions of the world. Predicting global climate and its impacts at the local scale is therefore very difficult and full of uncertainty.
Define hydrograph
A graph that visuals the response of river’s discharge to a rainfall event
Define peak rainfall
The point in time when rainfall reached its max
Define peak discharge
The point in time when discharge reached its max
Define lag time
The time between peak rainfall and peak discharge
Define rising limb
The upwards-sloping curve of the discharge line
Define falling limb
The downwards sloping curve of the discharge line
Define throughflow
The section of the discharge made up of water reaching the river via the soil layer
Define throughfall
Water flowing to the surface from leaves and the trunk of the tree
Define surface run-off
The section of the discharge made up of water reaching the river via overland flow
Define base flow
The amount of water that would always be in the river regardless of rainfall
Why is the falling limb a little less steep in comparison to the rising limb?
Because water reaching the river via throughflow takes more time to transfer to the river in comparison to run-off, hence why it takes longer to return to base flow
What drainage basin factors influence how rivers respond to rainfall?
- Size and shape
- Rock and soil type
- Density of vegetation
- How built up the drainage basin is
Define drainage density
The length of streams in relation to the area of the drainage basin
Define drainage frequency
The number of tributaries and streams in the drainage basin
Distinguish between a hydrograph for a large drainage basin versus one for a small drainage basin
Large drainage basins drain a larger surface area and so reach higher peak discharge. However, because water is required to flow a greater distance it has a larger time lag. In contrast, small drainage basins have a smaller time lag but because the area drained is smaller the peak is much lower.
Distinguish between a hydrograph for forest cover versus one for bare ground cover
Forest cover intercepts rainfall and slows it down. It encourages throughfall, infiltration and transpiration, so less water reaches the river and there’s a greater time lag. Therefore, peak discharge is lower but the falling limb is longer. In contrast, bare ground intercepts less and increases surface run-off. This creates flashy responses with a higher peak discharge and shorter time lags. The falling limb is also much shorter as very little water reaches the river via throughflow.
Distinguish between a hydrograph for a steep relief versus one for gentle relief
Steep valleys increase surface run-off whilst gentle sloping valleys and plateaus lead to infiltration and throughflow. As a result, steeper valleys have more flashy responses with higher peak discharge and shorter time lags whereas gentle valleys have lower peak discharge and longer time lags.
Distinguish between a hydrograph for impermeable surfaces and rocks versus one for permeable surfaces and rocks
Impermeable surfaces and rocks lead to run-off and quick transfers to the river, hence peak discharge tends to be high and has a short time lag. In contrast, permeable surfaces and rocks allow for infiltration and percolation and so surface run-off is reduced. Hence, the peak discharge is lower but the falling limb is longer.
Define river regime
The flow of water in a river over a year
River regimes vary based on …. changes in the …. …. . Fill in the gaps.
- seasonal
- water balance
What factors affect the rates of run-off?
- Inputs
- The balance between storage and evapotranspiration
The Lyons Saone flows from the …. and is influenced by a …. climate. Fill in the gaps.
- north
- temperate
How does the River Saone’s discharge vary through the year?
It varies in line with the seasons, increasing in the winter and falling in the summer.
The Lyons Rhone is more influenced by an …. climate. Fill in the gaps.
- Alpine
Where does the River Rhone flow from and when does it experience snow melt?
- It flows from the Alps
- It experiences snow melt in the spring and summer
Why is the River Rhone’s discharge lower during winter?
In winter, the temperatures are lower. Therefore, more water is held in storage as water and ice.
The Isere also has …. influences and …. in discharge due to snow melt. Fill in the gaps.
- alpine
- increases
Why does the River Isere not reach the same discharge levels as the River Rhone?
Because the River Isere is a smaller river.
Why is the discharge greatest at Beaucaire?
Because the drainage basin here it much larger as the Rhone has been fed by all its tributaries
Outline the River Rhone and all its tributaries
The source of the Rhone is located in the Alps. The Rhone flows down the alps, passes Lake Geneva and meets the River Saone at Lyons. Then it flows southward towards the Mediterranean sea, during which it is fed by the River Isere and other tributaries.
How does a greater vegetation density reduce flood magnitude and frequency?
Forest cover intercepts rainfall, stores it and releases it slowly to the river as well as through transpiration.
How do urbanised drainage basins increase the likelihood of flooding?
Because they are designed to quickly drain water through impermeable surfaces and infrastructure into the river
How can dams and reservoirs reduce flood magnitude and frequency?
They can hold water back
How could channelized river and diversion channels reduce flood frequency in some locations but increase flood magnitude in others?
They move water quickly downstream to avoid towns, reducing flood frequency there, but may have the negative consequence of increasing flood magnitude downstream.
Describe how softer approaches to managing rivers can reduce flood risks
Softer approaches aim to increase storage capacity across the drainage basin as well as zoning rivers by allowing them to flood in some locations where it is safe and preventing floods in areas of high population density.
