Drainage basins and flooding: the drainage basin as an open system (inputs, stores, transfers, outputs), the storm hydrograph, the physical and human causes of river flooding, the impacts of flooding, and a UK flood event.

What this dot point is asking

This is part of Eduqas GCSE Geography A (C111) Theme 1, Landscapes and Physical Processes, assessed in Component 1. Eduqas expects you to understand the drainage basin as an open system (inputs, stores, transfers and outputs), to read and interpret a storm hydrograph, to explain the physical and human causes of river flooding, to describe the impacts of flooding (social, economic and environmental), and to study one UK flood event in detail.

The drainage basin as an open system

A drainage basin is the area of land drained by a river and its tributaries, separated from the next basin by a ridge of high land called the watershed. Eduqas treats it as an open system with four parts.

The storm hydrograph

A storm hydrograph shows how a river's discharge (the volume of water passing a point, in cumecs) responds to a rainfall event over time.

• The rising limb shows discharge increasing after rain; the peak discharge is the maximum; the falling (recession) limb shows it returning to normal.
• The lag time is the gap between the peak rainfall and the peak discharge. A short lag time with a steep rising limb and high peak means a flashy river that floods easily.
• Lag time is shortened by impermeable rock, steep slopes, saturated or frozen ground, sparse vegetation, urbanisation and intense rainfall, because all of these speed water to the channel.

Physical causes of flooding

Some flood causes are natural.

• Prolonged rainfall saturates the soil until no more can infiltrate, so extra rain runs straight off.
• Intense rainfall (a heavy storm) falls faster than the ground can absorb it.
• Snowmelt releases a large volume of water quickly in a thaw.
• Impermeable rock (clay, granite) stops infiltration, so water runs off the surface.
• Steep relief speeds overland flow into the valley.

Human causes of flooding

People can make floods more likely and more damaging.

• Urbanisation replaces permeable ground with impermeable concrete and tarmac and adds drains, so water reaches rivers faster.
• Deforestation removes trees that intercept rain and take up water, so more reaches the channel sooner.
• Building on the floodplain puts homes and businesses in harm's way and covers the natural flood store with hard surfaces.
• Reduced dredging of channels lowers their capacity, so they overtop sooner.

The impacts of flooding

Eduqas classifies flood impacts under three headings.

• Social: homes flooded and people made homeless, lives lost, water supplies contaminated, disruption to schools and hospitals, stress and ill health.
• Economic: the cost of repairs, lost business and farmland, damaged roads, railways and bridges, and rising insurance premiums.
• Environmental: soil eroded from fields, rivers polluted by sewage and chemicals, and habitats destroyed (though floodplains are also enriched by silt).

A UK flood event

Eduqas requires one UK flood studied in detail. A common choice is the 2015 Cumbria floods, when Storm Desmond dropped record-breaking rainfall (over 340 mm in 24 hours at Honister) onto already saturated, steep, impermeable upland catchments. Rivers including the Eden and Kent burst their banks, around 5,000 homes flooded in places such as Carlisle and Kendal, bridges were destroyed, and the damage ran to hundreds of millions of pounds. Another popular study is the 2014 Somerset Levels floods, where prolonged rain, a low flat landscape and reduced dredging left land underwater for weeks. Learn the causes, the impacts and the responses for your chosen event.

Try this

Q1. Describe the four parts of the drainage basin system. [4 marks]

• Cue. Inputs (precipitation), stores (interception, soil, groundwater), transfers (infiltration, throughflow, channel flow) and outputs (discharge, evapotranspiration).

Q2. Explain how urbanisation increases flood risk. [4 marks]

• Cue. Impermeable surfaces and drains stop infiltration and speed runoff, shortening lag time and raising peak discharge.