Rivers, floods and management

Rivers, floods and management

Rivers, floods and management

The drainage basin hydrological cycle

Rivers, floods and management

The storm hydrograph

River discharge

Rivers, floods and management

What factors might result in a ‘flashy’

hydrograph?• short-lived but intense

rainfall

• impermeable underlying rock

such as granite

• steep relief

• urban area, where water is

channelled through drains to

the stream

• small round drainage basin

River discharge

Rivers, floods and management

• dense woodland vegetation

• high infiltration capacity of

soil

• permeable underlying rock,

such as limestone

• elongated drainage basin

What factors might create a hydrograph like

this?

River discharge

Rivers, floods and management

The long profile

Rivers, floods and management

The Hjülstrom curve

Rivers, floods and management

Amanda Barker

An upper course channel

Rivers, floods and management

Amanda Barker

A middle course channel

Rivers, floods and management

A lowland channel in flood

Amanda Barker

Rivers, floods and management

• vertical erosion

(by hydraulic

action and

corrasion) below

the waterfall

deepens the

plunge pool

• undercutting of

the softer rock

leads to eventual

collapse of the

resistant Whin

Sill and results

in headward

erosion

John

Pallister

High Force waterfall

Rivers, floods and management

The development of an oxbow lake

Rivers, floods and management

• the sediment load carried by the river is substantial

• the river has a large drainage basin

• the mouth of the river is located on a coastline where ocean

currents and tides are weak so the rate of deposition exceeds

the rate of sediment removal

Reasons for formation of a delta

Rivers, floods and management

• knick points, rapids

and waterfalls

• river terraces

• incised meanders, both

entrenched and ingrown

Features related to rejuvenation

Aerofilms

The city of Durham is built inside

an entrenched meander

Rivers, floods and management

• excessive precipitation

over a long period of

time

• intensive precipitation

over a short period of

time

• snowmelt

• climatic hazards such

as hurricanes

Flooding

• the nature of the

drainage basin

• relief

• rock and soil type

• natural vegetation

Physical causes of flooding

Rivers, floods and management

• Urbanisation creates impermeable surfaces so reducing

infiltration. Surface water is therefore transported more

rapidly through drains to the river.

• Deforestation reduces interception and increases runoff.

• Some methods of river management, such as channelisation,

increase flood risk downstream.

• Global warming leads to more extreme rainfall events.

Flooding

Influence of human activities on flooding

Rivers, floods and management

Flooding

Areas of high flood risk in England and Wales

Rivers, floods and management

Flooding

Short-term responses:

• media broadcasts of

information

• emergency relief, rescue,

shelter, food

• armed services for evacuation

and building emergency

defences

Long-term responses:

• government and/or foreign aid

• disaster planning for the

future

• flood management programmes

Responses to flooding

Rivers, floods and management

Flooding

• channelisation, to raise, straighten or strengthen banks

• dams and weirs

• diversion channels

• dredging

• retention basins and balancing lakes

• artificially raising the height of the floodplain

Examples of hard engineering

Rivers, floods and management

• afforestation

• floodplain zoning

• land-use management, e.g. contour ploughing

• wetland and riverbank conservation

• river restoration

• improved forecasting and warning

Flooding

Examples of soft engineering