FloodingLO: Understand what is meant by the term ‘flood’

and why rivers flood

Queensland, NE Australia

What is a flood?A flood happens when there is too much water for the river to hold, its banks burst and the water flows out over the land.

A river has too much water in its channel so it exceeds its bankfull capacityAnd this is what it looks like (Calgary, Canada, 2013).

How rainfall reaches the river

What three processes cause the water to reach the river?

Which is the fastest?

Which is the slowest?

Surface runoff – the more there is, the quicker the water gets to the riverThroughflow is slower, but not as slow as …Groundwater

Key factors that affect flooding

• Precipitation (R: Wiki pic)

• Speed• Volume • So … it’s the total amount of precipitation; the

speed with which that precipitation gets to the river; and the volume of the precipitation that gets to the river• Characteristics of each drainage basin will affect the

speed and volume – can you define this term?

What affects the speed & volume?• Heavy rain• Gradient of valley (steep v gentle slopes)• Type of rock or soil (impermeable or permeable)

Extension• Can you think of any others?• Urbanisation; lack of/dying vegetation (without interception*,

soil will fill with water); depth of soil (shallow soils fill: common in urban + mountainous areas); amount of tributaries feeding in• *The process and the amount of rain or snow stored on leaves and

branches and eventually evaporated back to the air. (source)

• PARAGRAPH TASK• Write up in short paragraphs your explanation of the multiple

causes of flooding [1 paragraph per cause]

Remember…The more surface run-off there is

↓the faster the water will get to the river; also the

more water gets to a river

↓the more likely it is that there will be a flood.

1: What three processes cause the water to reach the river?Which is the fastest?Which is the slowest?

Surface runoff – the more there is, the quicker the water gets to the riverThroughflow is slower, but not as slow as …Groundwater but there are limits to the capacity of this + throughflow which, when reached, will boost surface runoff as water has nowhere to go2: What are the three factors causing more water to reach a river than it can hold within its banks?Precipitation – the processes above will influence how much impact heavy rainfall has; in geography always look for the links between concepts you learnSpeed (flow rate is a linked term) is crucial (steepness is a factor) – remember that this can lead to oxbow lakes, removing curves that slow down the river flowVolume – a shallow river has less potential energy; the deeper it is the great the energy carried, the bedload, the erosive impact and the potential to undercut river banks

Total recall?

3: What human factors will increase surface runoff?Urbanisation (concreting, tarmacking, buildings) and deforestation (or generally removing vegetation), often linked to industrial farming practices.

BBC guide: google bbc gcse flooding

Start researching … Boscastle flood, 2004 (essay on the causes of this flood written in lesson Thursday 6th April) – more on this next week …(I will also blog on this - stgeog.blogspot.lu)