1. Today we are learning this content:

2. What are the physical and human causes of flooding? Bullet point
Physical Factors that have an effect on drainage basins and storm hydrographs
What are the physical and human causes of flooding? Bullet point
What do you think ‘flash flooding’ means?
A flash flood is a rapid flooding of geomorphic low-lying areas: washes, rivers, dry lakes and basins. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, tropical storm, or meltwater from ice or snow flowing over ice sheets or snowfields.

3. Storm Hydrographs Very important you know what they look like
How to read them
You must know the various
labels and where to put
them
Storm hydrographs show the variation of discharge within a short period of time, normally an
individual storm or a group of storms not more than a few days in length. They can be
described as ‘flashy’ (short lag time, high peak and steep rising limb) or ‘flat’ (long lag time, low
Peak, gently rising limb)
The storm hydrograph records the changing discharge of a river or stream in response to a
specific input of precipitation. A hydrograph is a graph showing the rate of flow (discharge)
versus time past a specific point in a river

4. Interpreting a storm hydrograph
Before the storm starts the main supply of water to the river or stream is through groundwater flow/base flow
As the storm develops the water comes from different routes:
Infiltration through the soil become throughflow
Some flows over the surface as overland flow – this reaches the river quickly

5. Key terms you need to know
Rising limb
The time interval between peak rainfall and peak discharge
Peak discharge
The normal day-to-day discharge of the river
Lag time
The part of a storm hydrograph in which the discharge starts to rise
Falling/recessional limb
The time when the rainfall reaches its highest
Base flow
The time when the river reaches its highest flow
Peak rainfall
The part of a storm hydrograph where the water in the river starts to decrease
Use your key terms to label up the storm hydrograph diagram

6. Storm hydrograph
___________: the highest amount of rainfall per time unit
______________: shows how quickly the discharge rises after a rain storm
_______________: the highest recorded discharge following a rainfall event
_______________: the time difference between peak rainfall and peak discharge
_______________: shows the reduced discharge once the main effect of runoff has passed.
_______________: the normal flow of a river when its water level is being sustained by groundwater flow
_________________: (does not always appear) marks the level of discharge above which flooding will occur.
Match the keyword from the hydrograph to the definition

8. Apply to demonstrate
Using page 17 of Hodder
Answer the questions
Within the handout

9. Questions
How does the shape of the basin affect the shape of a storm hydrograph? Circular basins have shorter lag time, elongated basins have long lag time and delayed response hydrographs
How does the steepness(relief) of the profile affect the shape of a storm hydrograph? High steep slopes cause surface run off, low gentle slopes that allow infiltration and percolation
How does the drainage density affect the shape of a storm hydrograph? High drainage density means more streams and rivers per unit area, so water will move quickly to the measuring point. Low density means few streams and rivers per unit area, so water is more likely to enter the ground and move slowly through the basin.
How does geology affect the shape of a storm hydrograph? Impermeable rocks, such as granite, which restrict percolation and encourage rapid surface run-off. Permeable rocks such as limestone, which allow percolation and so limit rapid surface run off
How does the soil type affect the shape of a storm hydrograph? Low infiltration rate such as clays soils (0-4 mm), high infiltration rate such as sandy soils(3-12mm)

10. Use the statements to complete the grid showing how different factors determine the shape of a hydrograph
Apply to demonstrate

11. AFL
Apply to demonstrate
‘Flat’
river
‘Flashy’
river

12. The impact of urbanisation on hydrological processes
Apply to demonstrate
Use page 18 of Hodder to create a mind map of the impact urbanisation has on hydrological processes.
The impact of urbanisation on hydrological processes

13. Flooding and Urbanisation
Why does urbanisation increase the risk of flooding (6 marks)
Human factors increasing flood risk:
urbanisation, because towns and cities have more impermeable surfaces such as tarmac and this involves removing vegetation and soil.
Deforestation, because trees prevent sediment runoffs and forests hold more water than farms or grasslands. , Removing trees reduces the amount of water intercepted(catches the water) and increases run-off as there is a lack or drainage.

14. Synpotic Themes
Through their control of land use and land-use changes in a considerable number of countries, planners are major players. The decisions they make, for example about the location and amount of urban growth, can clearly affect the behavior of drainage systems. Their decision will impact most on run-off, water budgets and storm hydrographs.
Hydrographs are also likely to be affected by climate change, with a general prediction of more intense rainfall in many northern hemisphere areas which would create ‘flashy’ hydrographs and more flooding, while some tropical areas may become drier and hydrographs subdued (‘flat’ ‘subdued’ or ‘delayed’), with consequent navigation and water supply issues.

15. Review: Factors that affect the discharge of a river and the shape of the storm hydrograph.  
On the left hand side of the table add the word ‘HUMAN’ or ‘PHYSICAL’ for each factor.