What is the relationship isbetween precipitation andrunoff?Flood Hydrographs.
Learning goalsKnowledgeTo know what a flood hydrograph isUnderstandingTo understand the factors affectinghydrograph shapeSkillsUse key words to describe a flood hydrographHydrograph interpretation
Back to Back• The aim of the task is to draw a diagramas accurately as possible only from youpartners description.• In pairs you will need to decide who willdescribe the diagram and who will drawit.• You will have 5 minutes to complete thediagram.
Lets think…?What happens to rainwater once it has leftthe cloud?
Rainwater will:• Be lost through the system throughevapotranspiration• Be held in storage in lakes, the soil orunderground• Flow into a river to return, eventually tothe sea as run-off.
In other words…• The amount of rainwater that reaches theriver will be:Precipitation – (evapotranspiration + storage)So, river runoff will be less than precipitationIf we have excess precipitation then, we get?Flooding
What is a hydrograph?• A hydrograph shows variations in a river’s discharge over ashort period of time, usually during a rainstorm.• River discharge is the amount of water passing a given pointin the river at a particular time.• The shape of a hydrograph can be influenced by many things
What does a hydrograph look like?Label yourhydrographwith thefollowingsentences.
HydrographsHydrographs are graphs which show discharge (the amount of waterpassing a particular point in a river at a particular time).What affects the shape of ahydrograph?Land useType and amount of precipitationGeology and soil Gradient of the valley sides
• The line graph shows the discharge.• The bar graph shows the rainfall.• When a storm begins, discharge does not increaseimmediately as only a small amount of rain will falldirectly into the channel.• The first water to reach the river will come fromsurface run-off.• Water arriving in the river later comes from through-flow.• The increase in discharge is shown by the rising limb.• The decrease in discharge is shown by the fallinglimb.• The gap between the time of peak (maximum) rainfalland peak discharge (highest river level) is called lagtime.• A river with a short lag time and a high discharge ismore likely to flood than a river with a lengthy lagtime and a low discharge.
Let’s re-cap– Hydrograph– Surface runoff– Lag time– Antecedent rainfallChoose TWO of the following key terms and inpairs tell your partner all you know about;
1) What type of flow (choose from surface runoff,throughflow and groundwater flow) causes the risinglimb?3) What is the lag time?2) Which is the last type of flow to reach the river?RisinglimbFallinglimbHydrographs:Surface runoffGroundwater flow4 hours
The way ahydrograph lookswill vary accordingto land use/type:
How do trees reduce flooding?trees intercept the rainevapotranspirationreduces theamount of waterthat reaches theriverwater is taken through theroots and so less enters theriverSo theremoval oftreesincreasesthe risk offlooding.
Urbanisation•Water cannotinfiltrate (soak) intopavement•Where does it gothen?•Will this reach theriver quicker or moreslowly?As urban areas grow, risk offlooding increases.
What do hydrographs look like?Flashy SubduedWhat do you think the differences are betweenthe hydrographs?
Task• Look at ‘The shape of the hydrograph’worksheet• Your task is to complete the table usingthe information provided to describethe differences between the 2hydrographs• You may work in pairs
Have achieved what we set outto do?• Can you recognise a flood hydrograph?• Do you know how a flood hydrographworks?• Can you use key words to describe aflood hydrograph?• Do you know what types of humanbehaviour can affect a flood hydrograph?
HomeworkComplete the flood hydrographs questionsheet for . . . . . . . . .
Flood hydrographsWhat can you remember?• What does a flood hydrograph looklike?• Can you label the rising and fallinglimb?• Can you label lag time?
Task• Complete the graph using the dischargedata.• On the completed graph:– Label the peak discharge– Indicate the lag time– Label rising and falling limb• Calculate the lag time.
TaskOn file paper/in your books explain howthe different factors have affectedthe shape of the hydrographs.
Task – flood hydrograph: living graphWhat is a living graph?1. Use the rainfall data to complete a rainfallbar chart2.Use the discharge data to complete adischarge line graph3.Label your hydrograph with the 8 labels4.Cut out and stick on the information about Mr& Mrs Jones (this is the living bit)
On your hydrograph label thefollowing parts:• Peak rainfall• Peak discharge• Lag time• Rising limb• Falling limb
Compare these two hydrographs.Which one has the largest lag time? Explain your answer.Hydrographs
a bThe hydrographs ‘a’ and ‘b’ have been produced from the same storm event butfrom different drainage basins.Which of the following river basin descriptions are more likely to have producedhydrograph ‘a’ and which are more likely to produced hydrograph ‘b’?Urban areaRural areaImpermeable bedrock Clay soilGentle valley gradientsDeforested river basinChalk bedrockHydrographs
1) What type of flow (choose from surface runoff, throughflow and groundwaterflow) causes the rising limb?3) What is the lag time?2) Which is the last type of flow to reach the river?RisinglimbFallinglimbHydrographs
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