1 db how rainfall reaches the river

1. FloodingLO: Understand what is meant by the term ‘flood’
and why rivers flood

2. Queensland, NE Australia

3. 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
capacity
And this is what it looks like (Calgary,
Canada, 2013).

4. 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 river
Throughflow is slower, but not as slow as …
Groundwater

5. 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?

6. 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]

7. 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.

8. 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 river
Throughflow 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 go
2: 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
learn
Speed (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 flow
Volume – 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.

9. 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)