Flood Hydrographs

What is a hydrograph?

Hydrographs illustrate river discharge and rainfall over time. Over the short term, a flood or storm hydrograph (figure 1.) can show short-term variations in discharge and rainfall. They cover a relatively short time period, usually hours or days rather than weeks or months. Storm hydrographs allow us to investigate the relationship between a rainfall event and discharge.

A flood hydrograph shows the amount of rainfall and river discharge in an area. The discharge of a river is the volume of water passing a point each second. It is expressed in cumecs (cubic metres per second). River discharge is displayed as a line graph. Precipitation is shown as a bar graph and is usually indicated in millimetres.

Figure 1. A storm hydrograph

The starting and finishing levels show the base flow of a river. The base flow is the water that reaches the channel through slow throughflow and permeable rock below the water table. As stormwater enters the drainage basin, the discharge rates increase. This is shown in the rising limb. The highest flow in the channel is known as the peak discharge. The fall in discharge back to base level is shown in the preceding limb. The lag time is the delay between maximum rainfall and peak discharge.

The shape of a hydrograph varies in each river basin and each storm event.

Flood hydrographs can predict flooding by showing how different precipitation levels affect a river during a storm.

Hydrographs can be of different shapes. The characteristics of a river, its drainage basin and the amount of water entering the system affect the shape of a flood hydrograph.

A gentle hydrograph shows the river is at low risk of flooding. These types of hydrographs have a gentle rising limb and a long lag time, which means it takes longer for the peak rainfall to reach the river channel, so the river discharge is increasing slowly.

A gentle hydrograph

Flashy hydrographs have a steep rising limb and a small lag time. This indicates that river discharge increases rapidly over a short period, indicating rainwater reaches the river very quickly. This means the river is more likely to flood.

A flashy hydrograph

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

Physical Factors Affecting Storm Hydrographs

A range of physical factors affect the shape of a storm hydrograph. These include:

Large drainage basins catch more precipitation and have a higher peak discharge than smaller basins. Smaller basins generally have shorter lag times because precipitation does not have as far to travel. The shape of the drainage basin also affects runoff and discharge. Drainage basins that are more circular in shape lead to shorter lag times and a higher peak discharge than long and thin ones because water has a shorter distance to travel to reach a river.
Drainage basins with steep sides tend to have shorter lag times than shallower basins. This is because water flows faster on the steep slopes down to the river.
Basins with many streams (high drainage density) drain more quickly and have a shorter lag time.
If the drainage basin is already saturated, surface runoff increases due to the reduction in infiltration. Rainwater enters the river quicker, reducing lag times, as surface runoff is faster than baseflow or through flow.
If the rock type within the river basin is impermeable, surface runoff will be higher, and throughflow and infiltration will also be reduced, meaning a reduction in lag time and an increase in peak discharge.
If a drainage basin has a significant amount of vegetation, this will have a significant effect on a storm hydrograph. Vegetation intercepts precipitation and slows the movement of water into river channels. This increases lag time. Water is also lost due to evaporation and transpiration from the vegetation. This reduces the peak discharge of a river.
The amount of precipitation can affect the storm hydrograph. Heavy storms result in more water entering the drainage basin, resulting in a higher discharge. The type of precipitation can also have an impact. The lag time is likely greater if the precipitation is snow rather than rain. This is because snow melts before the water enters the river channel. When snow melts rapidly, the peak discharge could be high.

Human Factors Affecting Storm Hydrographs

A range of human factors affect the shape of a storm hydrograph. These include:

Drainage systems that humans have created lead to a short lag time and high peak discharge as water cannot evaporate or infiltrate into the soil.
Areas that have been urbanised result in increased use of impermeable building materials. This means infiltration levels decrease and surface runoff increases. This leads to a short lag time and an increase in peak discharge.

Key Terms
Hydrograph – a graph that shows river discharge and rainfall over time.

Flood – when the capacity of a river to transport water is exceeded, and water flows over its banks.

Base flow – The base flow of the river represents the normal day-to-day discharge of the river and is the consequence of groundwater seeping into the river channel.

Storm flow – storm runoff resulting from storm precipitation involving surface and through flow.

Bankful discharge – the maximum discharge that a particular river channel can carry without flooding.

Peak discharge – the point on a flood hydrograph when river discharge is at its greatest.

Peak rainfall – the point on a flood hydrograph when rainfall is at its greatest.

Lag time – the period between the peak rainfall and peak discharge.