Flooding and Hydrographs

Flooding and Hydrographs

This lesson will enable you to better understand flood hydrographs.

But first...

Let's see what you can remember in our recap quiz!

Open Ended

1. Name two human causes of flooding. Challenge: How do these factors cause flooding?

Open Ended

2. Name two physical causes of flooding. Challenge: How do these factors cause flooding?

Open Ended

3. What is flooding?

Open Ended

4. What happens on the inside bend of a meander?

Open Ended

5. Name all of the four types of erosion along with a definition of each process.

Open Ended

What do you already know about flood hydrographs?

What is a flood hydrograph?

This type of hydrograph is known as a storm or flood hydrograph and it is generally drawn with two vertical axes.

One is used to plot a line graph showing the discharge of a river in cumecs at a given point over a period of time.

The second is used to plot a bar graph of the rainfall event which precedes the changes in discharge.

Step-by-step guide in understanding flood hydrographs

Step 1

Rainfall in mm is recorded onto the hydrograph from the start of the rainfall/storm event.

Step 2

When precipitation first starts there is little increase in discharge as rain is being intercepted by vegetation and infiltrated into the ground

Step 3

As the soil becomes saturated, through flow and surface runoff reaches the river, resulting in an increase in discharge shown by the rising limb

Step 4

At some point the river will reach its peak flow and the discharge will begin to fall back to normal. This is called the falling limb.

Step 5

The lag time will show the difference in time between the peak of the rainfall and the point when the water in the river is at the highest level.

There are many factors that affect the lag time.

What have we learnt?

Time for a quick definitions quiz...

Multiple Choice

What is peak rainfall?

• The period of rising river discharge following a period of rainfall

• The hour of greatest rainfall during a storm

• The point when the amount of water in the river is at the highest point

• The period of time between peak rainfall and peak discharge

Multiple Choice

What is peak discharge?

• The period of time when the rivers discharge is falling

• The hour of greatest rainfall during a storm

• The point when the amount of water in the river is at the highest point

• The period of rising river discharge following a period of rainfall.

Multiple Choice

What is lag time?

• The base flow of the river

• The period of time when the rivers discharge is falling

• The point when the amount of water in the river is at the highest point

• The period of time between peak rainfall and peak discharge

Multiple Choice

What is the falling limb?

• The period of time when the rivers discharge is falling

• The period of rising river discharge following a period of rainfall.

• The period of time between peak rainfall and peak discharge

• The base flow of the river

Multiple Choice

What is the rising limb?

• The period of time when the rivers discharge is falling

• The period of rising river discharge following a period of rainfall.

• The hour of greatest rainfall during a storm

• The point when the amount of water in the river is at the highest point

Multiple Choice

What is normal flow?

• The point when the amount of water in the river is at the highest point

• The period of time between peak rainfall and peak discharge

• The base flow of the river

• The period of time when the rivers discharge is falling

What factors affect lag time?

Open Ended

Which factors would lead to a longer lag time? (the water takes longer to reach the river)

Open Ended

Which factors would lead to a shorter lag time? (the water moves quickly to the river)

Factors affecting lag time- rocks

Impermeable rocks (e.g. granite) will not allow water to pass through, resulting in large amounts of surface runoff and a greater flood risk as rivers respond quickly 

Results in a short lag time.

Factors affecting lag time- rocks

Permeable rocks and soil have a high infiltration capacity and will absorb water quickly, reducing overland flow.

Results in a longer lag time

Factors affecting lag time- rainfall

Heavy rain results in rapid saturation of the upper soil layers and the excess water therefore reaches streams quickly as surface runoff.

Results in a short lag time

Factors affecting lag time- rainfall

Slow Light Rain can be absorbed by infiltration and the river takes longer to respond to rainfall as water takes longer to pass through the drainage basin via through flow and groundwater flow.

Results in a longer lag time

Factors affecting lag time- land gradient

Drainage basin with a steep gradient will result in greater overland flow.

Results in a shorter lag time 

Factors affecting lag time- land gradient

Drainage basin with a less steep gradient allowing more time for infiltration to occur.

Results in a longer lag time.

Factors affecting lag time- saturation

If the soil has already been saturated by heavy rain its infiltration capacity will be reduced and further rain will go as surface runoff.

Results in a short lag time

Factors affecting lag time- saturation

If the soil is dry it will be able to absorb more water during infiltration and therefore the lag time will be longer.

Results in a longer lag time

Factors affecting lag time- deforestation

Areas cleared by deforestation will respond quickly to rainfall due to the reduced interception

Results in a short lag time

Multiple Choice

Saturated soil will lead to a short lag time

• True

• False

Multiple Choice

Heavy rainfall will lead to a longer lag time

• True

• False

Multiple Choice

Steep slopes will lead to a longer lag time

• True

• False

Multiple Choice

Permeable rocks will lead to a longer lag time

• True

• False

Multiple Choice

Deforestation will lead to a longer lag time

• True

• False

Exam Question

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