3.1.1.2 Runoff variation and the flood hydrograph

What is a flood hydropgraph include features

A flood hydrograph shows the amount of rainfall and river discharge in an area.

A flood hydrograph is a graph that shows a rivers discharge plotted over time in response to a rainfall event. Discharge is measured in Cubic metres per second (cumecs) Made up of baseflow and stormflow

● Rising Limb: The line on the graph that represents the discharge increasing. 

● Falling Limb: The line on the graph that represents the discharge decreasing. 

● Lag Time: The time between peak rainfall and peak discharge.

 ● Baseflow: The level of groundwater flow.

 ● Stormflow: Comprised of overland flow and throughflow. 

● Bankfull Discharge: The maximum capacity of the river. If peak discharge exceeds this then the river will burst its banks and be in flood.

● Peak Discharge:Peak discharge is the maximum amount of water in a river after a rainfall event, if this level surpasses the bankfull discharge then a flood will occur where the river overtops its banks.

What are the two types of Hydrographs

Which is most likely to occur during a storm event?

Flashy hydrograph (most likely to occur during a storm event)

Subdued hydrograph

Draw a flashy hydrograph and outline common features

bonus: What is an anthropogenic reason for ‘Flashy hydrographs’

• Short lag time

• Steep rising and falling limb

• Higher flood risk

• High peak discharge

Urbanisation - Impermeable surfaces such as concrete roads and laying drains reduces lag time. Less vegetation means less infilitration

Urbanisation is the main human impact on a storm hydrograph. Surface runoff increases when areas are  urbanised due to the removal of top soil and vegetation. As roads, pavements and buildings are constructed the surface becomes impermeable. Laying drains leads to the rapid transportation of water to river channels which reduces the lag time.

Draw a subdued hydrograph and outline common features

bonus: Outline a reason for more subdued hydrographs in rural areas?

• Long lag time

• Gradually rising and falling limb

• Lower flood risk

• Low peak discharge

Rural areas with predominantly permeable rock increases infiltration and decreases surface runoff. This increases lag time. The peak discharge is also lower as it takes water longer to reach the river channel.

What are the purposes of flood hydrographs [4 marks]

• A flood hydrograph shows the amount of rainfall and river discharge in an area. Discharge is measured in Cubic metres per second (cumecs) Made up of baseflow and stormflow

• Flood hydrographs provide a clear visual representation including: lag time, bankfull discharge and peak discharg

• Flood hydrographs help assess flood risk and predict potential flood impacts

• Can help to design flood control measures

What human and physical factors affect runoff variation

A variety of physical & human factors affect runoff (the flow of water over the Earth’s surface). Including:

Physical

Time of year

In summer, runoff levels are often low due to reduced rainfall. However, intense sunlight can bake the soil, making it hard for rain to soak in. In contrast, seasons like winter with high levels of precipitation (rainfall and snow) can lead to increased surface runoff.

Topography & relief

In “v” shaped valleys on steep slopes, runoff will increase due to gravity.

On flatter surfaces runoff is less likely to happen as water will be able to infiltrate into the soil more easily.

Storm conditions

Intense storms with heavy rainfall can lead to soils quickly becoming saturated. This can happen because of prolonged rainfall or intense rainfall.

HUMAN

Agricultural land use

Crops can intercept precipitation and reduce runoff.

Urban land use

Changing greenfield surfaces to impermeable concrete and tarmac as construction takes place can increase the level of runoff in an area