This document discusses factors that affect the shape of a hydrograph during a rainstorm, specifically mentioning basin lag time. It notes that a longer lag time and shallower rising limb can occur if snow is on the ground acting as a storage area, or if the ground is permeable. A steeper rising limb and shorter lag time may result from intense rainfall, saturated ground from previous rain, steep slopes promoting rapid overland flow, or impermeable surfaces from urbanization.

1. •Rainfall Intensity
•Rising Limb
•Recession Limb
•Lag time
•Peak flow compared to Base flow
•Recovery rate, back to Base flow
You need to refer to:
Basin lag time
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Discharge(m3/s)
Base flow
Through flow
Overland
flow
mm
4
3
2
Peak flow
Interpretation of Storm
Hydrographs

2. Recap on Hydrographs
• Have a go at the Living Hydrograph exercise
…where do you think each statement would be
best placed on the graph?
• Now watch this video

3. Two different Hydrographs A & B – can
you describe them?

4. Factors effecting
flood hydrographs

5. 1. Amount and type of precipitation

6. 1. Amount and type of precipitation
• Short intense rainstorms can produce
rapid overland flow and steep rising limb.
• Snow will be stored on the ground and not
effect the hydrograph until it melts. This
means in spring there may be steep rising
limbs.

7. 2. Temperature

8. 2. Temperature

9. 2. Temperature

10. 2. Temperature
• Snow on the ground can act as a store
producing a long lag time and shallow rising
limb. Once a thaw sets in the rising limb
will become steep.
• If there have been extreme temperatures,
the ground can be hard (either baked or
frozen) causing rapid surface run off.

11. 3. Previous weather conditions

12. 3. Previous weather conditions
• If it has rained a lot before the storm, the
ground will be saturated. This would
encourage more surface runoff, causing a
steeper rising limb on the hydrograph.

13. 4. Relief

14. • Overland flow can be faster down a steep
slope therefore steeper rising limb and
shorter lag time
4. Relief (shape of the land)

15. 5. Rock Type

16. 5. Rock Type
• Permeable rocks mean rapid infiltration
and little overland flow therefore shallow
rising limb and longer lag time.

17. Land Use

18. Land Use

19. 6. Land Use
• Urbanisation - concrete and tarmac form
impermeable surfaces, creating a steep
rising limb and shortening the time lag
• Afforestation (planting trees) -
intercepts the precipitation, creating a
shallow rising limb and lengthening the lag
time

20. Hydrographs
What could cause 2 peaks in a
hydrograph?