River Discharge and the Flood Hydrograph

Understanding River Discharge

• River Discharge: Volume of water flowing in a river, measured in cubic meters per second (Q).
• High vs. Low Discharge:
  • High discharge: Large volumes of water in the river (flood conditions).
  • Low discharge: Small volumes of water (dried-up conditions).

The Flood Hydrograph (Storm Hydrograph)

• Tool to assess flood risk and how quickly rainfall enters the river channel.

Components of a Flood Hydrograph

Y-Axis (Vertical Axis)

• Precipitation:
  • Measured in millimeters.
  • Shown as a light green bar chart.
  • Example: Peak rainfall was around 45 mm.
• Runoff/River Discharge:
  • Measured in cubic meters per second (Q).
  • Shown as a dark blue line graph.

X-Axis (Horizontal Axis)

• Time: Events plotted over a timeline.

Interpreting the Hydrograph

• Rising Limb: Increase in river discharge following rainfall.
• Peak Discharge: Maximum river discharge at a given time.
• Falling Limb: Decrease in river discharge as it returns to normal.
• Lag Time: Time difference between peak rainfall and peak discharge.
  • Example: Peak rainfall at 7-8 o’clock, peak discharge at 10 o’clock.
  • Indicates how quickly rainfall enters the river system.

Additional Terms

• Antecedent/Base Flow: Conditions before the rainfall event, represented by a dark purple shaded area.
• Bankfull Discharge:
  • Represented by a straight line across the hydrograph.
  • Height at which flooding could occur (risk to local area).

Upcoming Content

• Next video will cover factors controlling the lag time.