Flooding_intro Flashcards
What are type A,B and C flooding?
A - flash
B - long duration
C - long duration & high magnitude
What is an example of a type A flood?
Boscastle 2004
What was the combination of factors that lead to flooding in boscastle?
weather conditions (relatively short time
scale intense rainfall) & physical setting (small, steep catchment with impermeable geology)
How much rain was their at boscastle?
75mm in ~2hrs
What were the effects of the boscastle flood?
200 homes/ businesses damaged
No fatalities
How is flood risk usually attributed?
On a catchment basis
What is an example case study for a type B flood?
Gloucetershire 2007
What was the cause and effect of glocestershire 2007?
78mm rain ~12hrs
widespread flooding across the Uk (5000 homes, no fatalities)
What is another name for type C floods?
Mega floods
What is usually the cause of type C floods?
Failure of a retaining structure (glacial outbursts)
When is an example of a type C flood?
Middle Pleistocene – Northern
Europe
What were the effects of the Middle pliestocene type C flood?
Evidence of large-scale scours, streamlined hills, giant bars and run-up deposits (initial wide spreading of flood waters). And trench-like channels (concentrated flow of later drainage)
Set the drainage pattern of all subsequent glacial retreat/post-glacial drainage
What were peak discharges from the Mid pleistocene type C?
465,000 to 673,000 m*3/s
what as the flow depth of the Mid- pleistocene type C?
87m deoth with velocity of 7m/s
What might some future causes of mega flood be?
Failure of infrastructure
Climate change glacial retreat
How can intensity of rainfall events be calculated?
divide rainfall (depth of water) by duration
What is the weighting of intensity and duration like for the different flood types?
A and B
Intensity is important in flash floods (type A), whereas overall rainfall is more important for long duration floods (type B)
What is the return period/ probability of rain fall events?
The liklihood of a similar scale event occuring in an area again (1 in 100yr event)
What is the tempora spacong of floods like?
Independant and random
WHat are structures in vulnerbale flood areas designed to be like?
withstand a specific max flood for a specific time (return periods can calculate failure)
How much of total water does flooding account for?
2% of 0.9% of 3% of all water
What is the fundamental component of flooding?
precipiation
What parts of the water balance equation can be altered to reduce the effects of flooding?
Evapotranspiration
Change of water storage
What is the weakness of the water balance equation?
1 dimensional only looks at one point in time and space not accouting for when and where the rain falls
What are the 3 main types of runoff?
Over land flow
Throughflow
Ground flow
O and T are quick flow G is base flow
Will water that ends up in ground flow influence the discharge of the food event?
No as the water will be moving at a much flow rate
What are the types of overland flow?
– Infiltration excess flow (Hortonian)
– Saturation excess flow
– Return flow
What are the ypes of throughflow?
Unsaturated throughflow
– Saturated throughflow
– Pipe flow
WHat is pipe flow?
Floe through relic root systems which connect pore spaces
What influeneces throughflow?
Soil texture and pipe flow
What can evapotranspiration also cover?
Infiltration and canpoy stroage in leaves
Taking water from the subsurface storage via roots (allows more infiltration)
Will the same type of flood happen in different catchments ?
No as there will be different triggers, topography and nature of precipitation
WHat are some examples of less common causes of floods? (anything other than rain)
Snow and ice melt
Ice jams (large ice block dam)
Landslides (landsldie dams)
Failure of dams and control works
Why are floods from structure failure seen as more devestating/ unexpected?
Due to the comfort in the thinking that the measures will remain in place
What are flood intensifying factors?
Driver (volume of water)
Basin factors stable i.e. area
Basin factors variable i.e. Climate and geology
Drainage network
Channel factors
What is the boscastle catchment like?
SMall with narrow steep valleys typical of cornish area
What is the probability of the boscatle flood like over the whole or cornwall?
Relatibely high with many areas being sensitive to a similar event
How does the distribution and intensity of rainfall in a catchment affect flooding?
If lower intensity rain is in the process of enetering the channel and then a high intenisty period occurs and overland flow takes place both rain packets will reach the flood area at the same time
What type of flood was Valencia?
Type A - with flood water being able to carry and stack cars
Dirty flood (sewage and lots of sediment in the water)
Why was valencia primed for a flood event?
The ground was already saturated pre-event
WHat flood type is the UK most used to experiencing?
Tybe B
Why is type B flooding so damaging?
As rainfall occurs for a long time over a wide part of the catchment with each time a head waters meet the volume combines
Why is prediction of type B floods easier?
As you know how much rain is in the system so can tell what water levels will be at a certain point alomg the river
What was the cause for the pleistocene mega flood?
Glacial retreat which created a melt lake which when ice had retreated enough exposed a topogrpahic low allowing spillage
WHy is return period and risk used for structural deisgn for flooding?
depeding on lifetime will need ridgity to different levels of event i.e., dams with long life time 1 in 10000 yr event
