Human Activity & Storm Hydrographs

• There are many human activities which change the river landscape

• This impacts on the storm hydrograph, changing the lag time and discharge 

Human Factors Affecting Storm Hydrographs

• Deforestation, urbanisation and agriculture all increase the overland flow 

• Overland flow reaches the river more rapidly than through flow or groundwater flow

  • This means that the lag time is reduced

• They also increase the amount of water which reaches the river 

  • This increases the level of discharge leading to a steep rising limb

• The combination of these factors leads to a storm hydrograph which has a rapid response to a rainfall or storm event

• Afforestation increases the time the lag time and increases infiltration so less water will reach the river

Image showing comparison between urban and rural hydrography 

Worked example

Explain two ways in which human activities can affect storm hydrographs

(4 marks)

• You will not be awarded marks if you make a point and then for the second part write the opposite. For example, you might refer to deforestation increasing discharge and decreasing the lag time. If you then write about afforestation decreasing discharge and increasing the lag time, you will only be awarded 2 marks

Answer:

• More urbanisation (1) increases rates of run-off (or equivalent idea e.g. reduces infiltration) and/or more water in the river/higher discharge (1)

• Changing agricultural practices/ land use (1) such as ploughing land previously left in grass which will increase runoff (1)

• Deforestation/Afforestation (1) affects interception and infiltration rates so changing run-off and thus lag-times (1)

• Climate change my increase/decrease storminess (1) which will affect lag-times because of changes in infiltration/run off relationships (1)

• Human management of rivers e.g. levees (1) maintains more water in the channel (1)

Exam Tip

In 4 mark answers take care to not simply write the opposite for the second set of two marks. If you do this you are not showing the breadth of knowledge expected and will only be awarded marks for the first part of your answer. See the above worked example which illustrates where this could happen.

Case Study: River Ouse, York

• York is a city located in the north of England

  • The city's population is just over 200,000

• York regularly floods: Serious flooding has occurred in 1947, 1978, 1991, 1995, 2000 and 2015

• The floods in 2000 were the worst on record 

  • The Ouse rose almost 5.4 meters above its normal level

  • 540 properties were flooded and 320 were at serious risk

  • 18,700 hectares of farmland were affected

  • The overall cost of the floods was estimated to be over £12 million including lost income from tourism

Causes of 2015 flood

Physical and Human Causes of 2015 Flood