Floods (Flood Hydrograph, Flood Risk, Flood Mitigation & Prediction) Flashcards
What is on the axis of a flood hydrograph?
Discharge & hours from the start of rainstorm
What is lag time?
Period of time between peak rainfall & peak discharge
Factors causing flood hydrographs to differ
- geology
- land use
- climate
- amount of vegetation
- relief
- area shape
- soil
- drainage density
When there is a long lag time, the graph will be…
Subdued
When there is a short lag time, the graph will be…
Flashy
What is a river regime?
The variability in a rivers discharge throughout the year
How can we define ‘distribution’?
- Magnitude
- Frequency
What is mitigation?
Reducing flood risk through hard & soft engineering schemes
What is prediction?
Forecasting when a flood will occur
Some advantages of dams & reservoirs are…
- water is held back by the dam and released gradually
- water is usually stored in a reservoir behind the dam
- water can be used for hydroelectric or for recreation purposes
Some disadvantages of dams & reservoirs are…
- expensive to build
- sediment is often trapped behind the wall = erosion downstream
- settlements & agricultural land may be lost
List some hard engineering techniques for rivers
- dams & reservoirs
- artificial levees & embankments
- channelisation
- containment
- dredging
What is channelisation?
A deliberate attempt to alter the natural geometry of a river
What are some advantages of channelisation?
- floodplain reclaimed
- speeds up flow & moves water fast downstream
- creates space on floodplains
- act as route ways
What are some disadvantages of channelisation?
- natural beauty & ecology lost
- natural filter & storage capacity of floodplain lost
- cannot prevent major flood events (and may worsen them)
- become heavily polluted
- high cost
What is containment?
An extreme form of channelisation, when the river is entirely contained underground within channels
What are some advantages of containment?
- prevents flooding within the contained area
- allows urban function
- upfront costs are high -> but reduced in the long term
What are some disadvantages of containment?
- can be difficult to maintain (access problems)
- loss of ecology & beauty
- high construction cost
- reduces the variety for recreational space within the city
What is dredging?
Removal of sediment from the channel in order to increase capacity and channel efficiency
What are the ways of dredging?
- Dredging machine
- Reservoir/ abandoned meander loops
What are some advantages of dredging?
- maintains the natural river channel
- limits upfront costs of big infrastructure projects
- sediments rich in nutrients = economic value
What are some disadvantages of dredging?
- ecologically disruptive
- high ongoing costs (labor intensive)
- can be an eyesore, noisy
What is afforestation?
Process of establishing a forest
Why is afforestation good?
- Increased interception and surface runoff
- protection of vegetation (wildfires, overgrazing and deforestation)
What is the main aim of wetland/ floodplain restoration?
Bring back floodplains as a natural source of flood waters
What is the case study for wetland restoration?
Kissimmee river, Florida
What are some advantages of wetland/ floodplain restoration?
- low cost
- significant prevention of floods
- cost savings
- good ecological benefits
What are some disadvantages of wetland/ floodplain restoration?
- wetlands need to be large
- local flooding impacts
- loss of agriculturally productive land may
What is floodplain zoning?
A systems for setting the level of flooding risk for different areas of a country
What are some advantages of riverbank conservation?
- low cost
- maintains relative normality of river
- reduces frequency of floods
What are some disadvantages of river bank conservation?
- reduces natural ecology of river bank
- labor intensive
What is the case study for flood mitigation?
- Jakarta
- Bangkok
- Mississippi
