The Storm Hydrograph

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The Storm Hydrograph
Specification Focus Factors affecting
discharge the storm hydrograph. Bar and line
graphs

• Learning Outcomes
• recognise the regimes of different rivers and
  explain the differences.
• be able to draw and label a storm hydrograph.
• explain the physical and human factors that
  alter the storm hydrograph.

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Starter

• You have 5 minutes to (from memory)
• Draw a simple water budget graph.
• Label the points where there is water surplus,
  soil moisture utilisation and soil moisture
  recharge and explain these terms.

Stop!
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River Regimes
A river regimes shows the annual pattern of
discharge (water level) in a river. Copy the
definition into your notes
River Regime the variability in the discharge of
a river during the year in response to
precipitation, temperature, evapotranspiration
and drainage basin characteristics

• Rivers can have distinct patterns of
• discharge depending on a range of
• factors.
• These include climate, drainage
• basin characteristics and human
• activities

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Vegetation and Land use
Rainfall
Climate
Human activities
Size and shape of river basin
Shape of the land
Soil type and depth
Bedrock
How will these factors influence water flows
within a drainage basin? Write down some ideas
then discuss them with the person next to you.
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River Regimes
Discharge (Q)
J F M A M J J A S
O N D
An Alpine river The drainage basin contains
glaciers and snowfields, which feed the river.
Snow falls in the winter and melts in the
summer. This is reflected in the discharge of the
river.
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River Regimes
Discharge (Q)
J F M A M J J A S
O N D
A river in the Sahara Desert The climate is very
dry as most of the drainage basin is desert, so
river levels are very low or non existent. It
only rains occasionally, but when it does rain it
rains a lot in a short period of time. This can
cause a flash flood, and temporarily raise the
river levels very rapidly.
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River Regimes
Discharge (Q)
J F M A M J J A S
O N D
A medium river in England This drainage basin
has well drained soils and is covered by
vegetation. The rock underneath is permeable
chalk so the river levels remain relatively
constant....
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River Regimes
Discharge (Q)
J F M A M J J A S
O N D
A large river in the Mediterranean The
Mediterranean climate has hot, arid summers but
there can be intense downpours of rain during
the winter. The rock underneath this drainage
basin is impermeable chalk.
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Hydrographs

• The water levels in a river can be represented
  in a special type of graph, the hydrograph. You
  already know one type of hydrograph

Long period of time (a year) Shows response to
climate
River Regime
Short period of time (hours days) Shows
response to a single weather event
Storm Hydrograph (or Flood Hydrograph)
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Hydrographs

• The amount of water flowing in a channel is
  called the discharge and is calculated by
  multiplying velocity by cross sectional area of
  the river

Q (discharge) V (velocity) ? A
(cross sectional area) Q VA
Use your textbook to 1) write a definition for
discharge 2) give the name of the unit it is
measured in and 3) explain how the discharge of
a drainage basin can be calculated
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The Storm Hydrograph
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The Storm Hydrograph
The time from peak rainfall to peak discharge is
the LAG TIME.
The soil becomes saturated and overland flow and
through flow reach the river and discharge
increases. Overland flow arrives first.
The discharge starts to fall slowly as water is
added from through flow and groundwater flows
which are much slower.
The base flow supplies the river with water
between storms and keeps it flowing in summer.
Rainfall is intercepted or infiltrated into the
soil moisture store
Start of the storm there is a slow rise in
discharge, as only a small amount of water falls
into the channel
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Key words you need to know!
The Storm Hydrograph
Estimate of the water that flows into a river
from groundwater stores.
LAG TIME
The time at which river flow is at its maximum.
BASEFLOW
Rate at which river Q increases after a weather
event.
PEAK RAINFALL
Period of time between peak rainfall and peak
discharge
RISING LIMB
The time at which precipitation is at its
maximum.
RECEDING LIMB
The rate at which river discharge falls.
PEAK DISCHARGE
ANTECENDENT DISCHARGE
The discharge before the storm event.
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Hydrographs
Use the data below to construct your own Storm
hydrograph
Day Rainfall (mm) Discharge (cumecs)
1 0 2
2 5 2
3 30 3
4 45 4
5 30 8
6 0 12
7 0 14
8 0 12
9 0 8
10 0 6
11 0 4
12 0 3
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12 108642
Label the 6 key words to your graph
RISING LIMB
BASEFLOW
PEAK DISCHARGE
PEAK RAINFALL
LAG TIME
RECEDING LIMB
Then add the following labels to make a living
hydrograph
Lukes new carpet is ruined as water starts to come in under the door.
The football referee is deciding if the match should be postponed.
The Old Bridge at Singleton is re-opened as river levels start to subside
Lucy hears a flood warning and places sandbags by the door.
A canoe race is restarted as the river returns to safe levels
Extension activity What is the peak discharge?
What is the lag time? Add your own labels to the
graph.
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It should look something like this!
Lag time
Falling Limb
Rising Limb
Peak rainfall
Peak discharge
Baseflow
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Factors affecting The Storm Hydrograph
Imagine that these two areas have had the same
amount of rain over a 24-hour period. How would
the hydrographs be different for these two areas?
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VEGETATION COVER This varies seasonally. The type
and amount will affect interception and
stemflow/throughfall. Overland flow is reduced.
Lag time will be increased.
ROCK TYPE Impermeable rocks prevent groundwater
flow and encourage through flow and overland
flow. These rocks will decrease lag time.
Permeable rock will have the opposite effect.
CLIMATE The distribution of rainfall over the
year and the temperatures will affect the lag
times.
LAKES RESERVOIRS These will store floodwater
and thus reduce lag time and control river
response to heavy rainfall.
FACTORS WHICH INFLUENCE STORM HYDROGRAPHS
SLOPES Steep slopes will encourage overland flow
and gentle slope will slow run off down.
DRAINAGE BASIN Higher drainage density greater
risk of flood. Smaller the basin faster the
water reaches the channel.
RAINFALL INTENSITY DURATION Intense
rain will increase overland flow and reduce lag
times. Gentle rain over a longer time will allow
more infiltration.
SOIL TYPE DEPTH Deep soils store more water,
pipes in the soil encourage through flow. Soils
with small pore spaces will reduce infiltration
and increase overland flow.
LAND USE Impermeable surfaces created by
urbanisation will reduce infiltration and
encourage overland flow. Different types of crops
affect interception rates e.g. cereals 7-15.
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Flashy or Delayed?
High Discharge
Gentle rising limb
Gentle falling limb
Steep falling limb
Long lag time
Steep rising limb
Short lag time
Low discharge
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Flashy or Delayed?
You need to decide whether these factors would
cause a flashy or a delayed hydrograph
First sort them into 2 piles. Then you can mix
them up again and test each other on individual
factors. For each one you have to explain WHY
the factor would cause a flashy or delayed
hydrograph
High drainage density
Low drainage density
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Answering Extended Questions

• What is the question asking? Think command
  words, limits, examples
• Structure beginning (introduction), middle
  (argument), end (conclusion).
• PEE paragraphs Point, evidence/example and
  explanation.
• Evaluate show youve considered more than one
  view and show how the evidence affects your
  argument.
• Topic sentence There are several theories why
  flood damage is increasing in the UK.
• Evidence examples, facts, figures
• Diagrams only if useful! Good to example key
  processes or concepts that are hard to concisely
  explain.
• Timing writing for about 15 mins
• Conclusion draw together your argument and
  answer the question.
• Make a rough plan of key points

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The Storm Hydrograph
Specification Focus Factors affecting
discharge the storm hydrograph. Bar and line
graphs
What have we learnt today? - With the person
next to you decide on ? One thing you learnt ?
One thing you already knew ? One thing you found
difficult

• Learning Outcomes
• recognise the regimes of different rivers and
  explain the differences.
• be able to draw and label a storm hydrograph.
• explain the physical and human factors that
  alter the storm hydrograph.

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Homework

• Read pages 6-7 about the Carlisle floods
• Complete the questions on page 8 pay close
  attention to question 5 which should be written
  as an extended answer (essay).
• Complete homework sheet 1