(1.3.2 B) Hydrographs + Somerset Levels

What is a river regime?

A graph showing the variability in a river’s discharge throughout the course of a year.

What does a hydrograph show?

The variation of a river’s discharge over a short period of time, usually following a rainstorm.

Features of storm hydrographs: Approach Segment

The river’s discharge before it rains.

Features of storm hydrographs: Rising Limb

The increase in rainwater reaching the river- the discharge.

The steepness depends on the speed of water transfer.

Features of storm hydrographs: Peak Discharge

The highest level of discharge

Features of storm hydrographs: Lag Time

The period between maximum precipitation and peak discharge.

This shows how long it takes for the water to reach the river.

Features of storm hydrographs: Falling/Receding Limb

Point where discharge levels are decreasing. Falls less steeply than rise as throughflow is still being released into the channel.

Features of storm hydrographs: Bank full Discharge

Capacity of the river- if water reaches higher than this, the surrounding land will be flooded

What is a flashy hydrograph?

• steep rising limb

• dominated by quick flow processes

• likely to flood

What is a steady hydrograph?

• shallow rising limb

• dominated by slow flow processes

• unlikely to flood

Give an overview of the area of Somerset Levels.

• coastal, low-lying farmland and wetland area

• Majority of land at or just above sea level

• Mainly used for agriculture, grazing and some settlements

• Dec 2013 - Feb 2014, high levels of severe flooding.

Somerset Levels Flooding: Human Factors

• Not dredging → build up of silt → reduced capacity → flooding

• changes to agricultural practices → grassland became cereal plants → lowered retention and interception → bare in winter → higher discharge due to surface run-off

• Land drained, irrigated and modified for agriculture → lowered infiltration → fast water movement

Somerset Levels Flooding: Physical Factors

• coastal, low lying, flat area → prone to flooding

• tides and storm surges from Bristol come in pushing rainfall back up the river → flooding

• 200% increased level of rainfall from average for Jan + wet since October

Somerset Levels Flooding: Social Impacts

• Disrupted work and schooling

• Increase in crime and theft immediately after

• Mental and emotional health took a toll

• Volunteers, rescue crews and Marines assisted, floods on news - prompting action

Somerset Levels Flooding: Environmental Impacts

• Standing water prevented oxygen from reaching the ground → toxic ground

• Wildlife in the area suffered, drowning and destroyed habitats

Somerset Levels Flooding: Economic Impacts

• Estimated £16M in property damage

• Lost homes, property, livestock and farmland

• House prices and valuations decreased and insurance rose as people grew wary and reluctant

• Government pledged money to prevent a repeat of the event