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What you should know by the end of this lesson:
What you should know by the end of this lesson:
Be able to interpret a hydrograph
Be able to identify how hydrograph shapes are influenced
Background
Background
A hydrograph (sometimes called a storm graph) is a graph showing the rate of flow (discharge) versus time past a specific point in a river. The rate of flow is typically expressed in cubic meters (m3).
They are used to show how rivers respond to a flood.
Hydrographs are used to help predict flooding events, therefore influencing the implementation of flood prevention measures.
Hydrographs
Hydrographs
Digital Workbook and Paper Jotter
Workbook Page 7 Task 4
Use the information from below to annotate (label) an explanation for each of the labels on the model hydrograph
Flood Hydrographs in Action
Flood Hydrographs in Action
Watch the clip and think about how the basin characteristics could be contributing to the hydrograph shape in this clip.
Influences on Hydrographs
Influences on Hydrographs
Digital Workbook and Paper Jotter
Workbook Page 8 Task 5
Write an explanation for each section to explain how a drainage basins characteristics can affect a hydrograph
A basin with few tributaries relies heavily on soil- and ground-based water movement. A basin with many tributaries has a steep rising limb as water uses the channel to reach the trunk stream quickly.
The way in which the land is used will also have an influence on the hydrograph. Vegetation intercepts precipitation and allows evaporation to take place directly into the atmosphere. This reduces the amount of water available for overland flow.
The large number of impermeable surfaces in urban areas encourages run off into gutters and drains carrying water quickly to the nearest river.
Forest areas will intercept the precipitation, creating a shallow rising limb and lengthening the lag time. Vegetation will intercept precipitation and store and absorb it, therefore preventing the water frommoving quickly into the river.
Permeable rocks mean rapid infiltration and little overland flow, resulting in a shallow rising limb. Impermeable rocks will lead to an increase in overland flow and a steep rising limb.
Large basins receive more precipitation than smaller basins, therefore they have a larger runoff. A larger size also means a longer lag time as water has a longer distance to travel to reach the main river.
Where valley side gradients are steep, water runs off faster, reaches the river more quickly and causes a steep rising limb. Prolonged heavy rain causes more overland flow than light drizzly rain.
Influences on Hydrographs
Influences on Hydrographs
Digital Workbook and Paper Jotter
Workbook Task 6 Page 9:
Read the scenarios and for each decide if it will cause a long or short lag time
Write the answers in your jotter (in full sentences)
Give a justification for your answer.
Hydrograph Scenarios
a rainstorm in an area underlain by granite
a long period of 'drizzly' rain during summer
a snowstorm in the Highlands of Scotland
a thunderstorm in a city
a rain event in an area which is heavily forested
a period of showers in an area where there is much newly ploughed, fairly level, arable land
Answers
Answers
Short lag time. Granite is an impermeable rock leading to water running off quickly
Long lag time. A long period of rain will saturate the soil preventing infiltration and slowly lag time
Long lag time then fast. Snow will likely fall then be stored on the surface. As it melts it will flow quickly into the river system due to the steep nature of the highlands.
Short lag time. Heavy rain from a thunderstorm in an impermeable urban area will allow water to flow quickly back to the river system.
Long lag time. Heavily forested areas will intercept rainfall and slow overland flow.
Long lag time. Light showers with ploughed fields will encourage infiltration, the flat land will prevent overland flow slowing entry to river systems.
Past Paper Question - How to mark
Past Paper Question - How to mark
This exam question is based on an example from the 2019 Paper. It is out of 10 marks.
a)Describe the changes in the discharge level.
b)Suggest reasons why these changes may have occurred.
Task:
In your workbook, there is a 10 mark model answer to this question. Using at least 3 different colours, highlight where each mark was awarded.
Answer
a) The hydrograph shows that rainfall begins at 1100 increasing steadily until 1500. Rainfall then reduces before rising again to peak rainfall at 2100. Rainfall then reduces before fully stopping at 0200.
There is a slow rise in discharge until 1400. There is a gentle rising limb until 2200 when it suddenly becomes much steeper leading to a peak discharge of 85 cumecs at 0200. This means there is a lag time of 5 hours. After the peak discharge, there is a steep recession limb from 0200 until 0600 when the river has returned to normal.
b) This rain may have been intercepted by vegetation or when the water has fallen it has infiltrated and been stored in the soil. This could be why there is a delay in the rainfall reaching the river. The river begins to rise as the amount of storage has reached capacity or throughflow has occurred with the water reaching the river. There could be a limited number of tributaries in this basin that will transport water slowly to the main river channel. This could also be a large catchment area which will also mean a slow travel time to the main channel. The river basin may also have a gentle relief that slows transportation through to the main river channel.
Marking Information
Well done you have finished lesson 3 of Hydrosphere
Well done you have finished lesson 3 of Hydrosphere
Return to the topic launch page and attempt the lesson quiz
Return to the topic launch page and attempt the lesson quiz
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