4.3.2 Human Activity and River Landscapes

Human Factors Affecting Storm Hydrographs

Deforestation

Human Factors Affecting Storm Hydrographs

Deforestation

Impact	Effect on Hydrograph
Lack of trees reduces interception and infiltration, increasing overland flow	Discharge ↑ Lag time ↓

Human Factors Affecting Storm Hydrographs

Afforestation

Impact	Effect on Hydrograph
An increased amount of trees increases interception and infiltration reducing overland flow	Discharge ↓ Lag time ↑

Human Factors Affecting Storm Hydrographs

River management

Impact	Effect on Hydrograph
Dams can be used to control the level of discharge downstream	Discharge ↓ Lag time ↑

Human Factors Affecting Storm Hydrographs

Urbanisation

Impact	Effect on Hydrograph
Impermeable concrete and tarmac increase overland flow  Water flows into the drains reaching the river rapidly	Discharge ↑ Lag time ↓

Human Factors Affecting Storm Hydrographs

Agriculture

Impact	Effect on Hydrograph
Bare soil and ploughing increase overland flow, especially where ploughing is downslope Replacement of pasture (grassland) with arable crops also increases overland flow	Discharge ↑ Lag time ↓

Human Factors Affecting Storm Hydrographs

Climate change

Impact	Effect on Hydrograph
Rising global temperatures may increase storm frequency and intensity, increasing precipitation  Increasing drought which leads to baked soil and periods of extreme cold which freeze the soil which means that water cannot infiltrate	Discharge ↑ Lag time ↓

Human Factors Affecting Storm Hydrographs

Summary

Image showing comparison between urban and rural hydrography

• 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

Case Study: River Ouse, York

WHERE IS YORK?

• York is a city located in the north of England

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

What is the flood history of York?

• 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

What were the Physical causes of the 2015 flood in York?

Before reaching York the Ouse is joined by a number of large tributaries (Swale, Ure and Nidd) which flow from the Yorkshire Dales

Annual precipitation levels in the Yorkshire Dales are between 600-1000mm. In 2010 one month's rain fell in just 24 hours

Yorkshire Dales are upland areas with steep slopes

Land around York is low-lying and flat

York lies on the confluence of the rivers Foss and Ouse

What were the human causes of the 2015 flood in York?

Farming in the Vale of York and grazing in the Yorkshire Dales means interception is reduced due to decreased tree cover and lack of other vegetation

Increased urbanisation around the city with new housing developments, retail parks and industrial areas have increased impermeable surfaces

Erosion of peat in upland areas reduces the amount of moisture held in the soil

Failure of the Foss Barrier increased flooding in the east of the city

Deforestation in upland areas