1. Higher Geography
Hydrosphere
Hydrographs [Date] Today I will: - Be able to construct and understand flood hydrographs

2. Higher Geography Hydrosphere
What is a hydrograph? A hydrograph is a graph showing the rate of flow (discharge) versus time past a specific point in a river. The rate of flow is typically expressed in cubic meters (m3). They are used to show how rivers respond to a flood.
Hydrographs are used to help predict flooding events therefore influence implementation of flood prevention measures.

3. Higher Geography
Hydrosphere

4. Higher Geography
Hydrosphere

5. Higher Geography
Hydrosphere

6. Hours from start of rain storm
Basin lag time
Peak flow 3
Rising limb
Overland flow
Recession limb 2
Discharge (m3/s) mm 4
Through flow 1 3 2
Base flow
Hours from start of rain storm

7. Hours from start of rain storm3 2 1
Discharge (m3/s)
The discharge of the river is measured in cumecs - this stands for cubic metres per second
Hours from start of rain storm

8. Rainfall expressed in mm3 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

9. Discharge in m3/s, as a line graph2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

10. Hours from start of rain storm
The rising limb is when the normal (base) flow of the river starts to rise when run-off, ground and soil water reaches the river. 3
Rising limb 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

11. Hours from start of rain storm
Peak flow 3
Peak flow is when the Maximum discharge in the river, the time when the river reaches its highest flow
Rising limb 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

12. Hours from start of rain storm
Peak flow
The recession limb shows that water is still reaching the river but in decreasing amounts 3
Rising limb
Recession limb 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

13. Hours from start of rain storm
The basin lag time is the time it takes for the water to find its way to the river
Basin lag time
Peak flow 3
Rising limb
Recession limb 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

14. Base Flow is the normal discharge of the river
Basin lag time
Base Flow is the normal discharge of the river
Peak flow 3
Rising limb
Recession limb 2 mm
Discharge (m3/s) 4 1 3 2
Base flow
Hours from start of rain storm

15. + = Overland flow Through flow 3 Storm Flow 2 Discharge (m3/s) 1
Basin lag time +
Peak flow 3
Through flow =
Rising limb
Overland flow
Recession limb
Storm Flow 2 mm
Discharge (m3/s) 4
Through flow 1 3 2
Base flow
Hours from start of rain storm

16. Reasons for Differences in Flood Hydrographs
Higher Geography
Hydrosphere
Read the information on page 230 and 231 of the NWW Textbook. Copy the table below into your jotter and fill in the information gathered.
Reasons for Differences in Flood Hydrographs
LONG LAG TIME
SHORT LAG TIME

17. Can you label 1 to 8 accurately?
Higher Geography
Hydrosphere
Can you label 1 to 8 accurately? 7. 1. 6. 8. 5.
River
discharge (cumecs)
Rainfall (mm)
Peak rainfall
Falling limb
Rising limb
Lag time
Peak discharge
Time 4. 2. 3.