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.
