Flooding 2

Key idea: 1. Rivers flood due to a number of physical

and human causes. 2. Flooding appears to be an increasingly

frequent event

Hydrograph data to plot

Thinking through Geog

Lesson objectives

• To know the terms:Flooding, hydrograph, rising limb, falling limb, lag time, drainage basin

• To be able to explain the factors that cause flooding

• To know the frequency and location of flood events in the UK over the past 20 years

• Precipitation• Interception• Surface storage• Infiltration• Soil moisture• Percolation• Groundwater• Transpiration• Evaporation• Surface run off• Through flow• Groundwater flow• Water table

Starter activity….

• Textbook page 110 task 2

• Discharge• Drainage basin• Storm hydrograph• Flashy• Relief• Impermeable• Porous• Pervious• Deforestation• urbanisation

A flood occurs when…

…..the discharge is so great that all the water can no longer be contained within the channel so that the river overflows its banks

So what factors affect river discharge?

Factors affecting discharge

•Amount and type of rainfall

•Temperature

•Previous weather conditions

• Relief

•Drainage Density

•Rock Type

• Soil

• Land Use

• Slope

Most floods occur because of the weather:

Amount and type of rain

• Prolonged rain – long and continuous periods of rainfall lead to saturated ground

• With drizzle, there is time for water to infiltrate soil and underlying rock

• Heavy rain which causes large amounts of run-off in a short period of time

Temperature• High temperatures lead to greater evaporation

and transpiration rates so river levels decrease

• Snow melt – in spring a sudden increase in temperature which melts snow and ice. - In winter the water cannot seep into frozen ground

• Previous weather conditions

Textbook page 109

• Rock type• Rock type determines how much water

infiltrates and how much stays on the surface.

• Relief• Relief affects the rate at which water runs

off the land surface and into rivers. Steep slopes give faster run off due to gravity

• Gentle slopes allow time for infiltration

Human activities can make the flood risk worse …

• Building on a floodplain. Tarmac and concrete are impermeable which means that more water runs off the surface and faster.

• Deforestation reduces interception by the trees and so more rainwater reaches the surface quicker increasing rates of surface run off

• Textbook page 110 task 3

• You are to produce a double page spread around the title ‘ The reasons river discharge fluctuates’

• In the central box give the meaning of ‘river discharge’• Place the factors around the central title• Add information to each factor to explain how discharge

is affected.• Illustrate each factor with diagrams, sketches or photos.• Your finished work should be informative, accurate,

interesting, colourful and original

Your turn

What factors increaseThe risk of flooding?

* Impermeable rock* Hard dry soil

* Very wet soil

*steep slopes

* Cutting down trees

Building in the drainage basin

* Many tributaries

Which are physical and which are human factors?Plenary

Homework….

Card sort activityCut out the two hydrographs – one for

a ‘flashy’ river and a gentle graphNow cut out the individual cards and

place them under the appropriate hydrograph.