1. HYDROSPHERE 2

2. DRAINAGE BASIN HYDROGRAPHS
HYDROSPHERE 2
DRAINAGE BASIN HYDROGRAPHS
Read page 7 of the booklet. It introduces you to two rivers that run side-by-side yet have different reactions to the same amount of precipitation.
In Standard Grade, you learned about the drainage basin of a river, and some associated terms.
In this section, we will be studying a typical drainage basin and learning how to ‘read’ the water flow in its streams.
These slide-shows are all on the Prepwork folder if you wish to copy any notes from them; we will not be stopping in class for you to do this

3. Construction and Analysis of hydrographs
HYDROSPHERE 2
Construction and Analysis of hydrographs
A hydrograph is a special kind of graph that records the speed that a river system removes the water that enters it.
It consists of a small bar graph showing the precipitation and a line graph showing the amount of water flowing past a point on the river.
Read about it on page 8 of the booklet.
When you look at a hydrograph, you should ask yourself certain questions- see the next slide.

4. 2. When it started was it heavy or light?
HYDROSPHERE 2
2. When it started was it heavy or light?
1. When did the precipitation start?
5. How long was it after the highest ( peak) precipitation until the river was at its highest?
3. How long did it precipitate for?
4. Did it stop suddenly or tail away?
7. How quickly did it return to base ( normal) flow?
6. How quickly did the river rise?
8. What was peak river flow?

5. This is what a hydrograph looks like.
HYDROSPHERE 2
Basin lag time
Peak flow
This is what a hydrograph looks like. 3
Rising limb
Overland flow
Recession limb 2 2 mm
Discharge (m3/s) 4
Through flow 1 3 2
Base flow
Hours from start of rain storm

6. Hours from start of rain storm
HYDROSPHERE 2
The next slides will demonstrate the hydrographs elements. With each new element there is a term and its definition to copy. 3 2 1
Discharge (m3/s)
Hours from start of rain storm

7. Rainfall shown in mm, as a bar graph
HYDROSPHERE 2
Rainfall shown in mm, as a bar graph 3 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

8. Discharge in m3/s, (cumecs), as a line graph
HYDROSPHERE 2
Discharge in m3/s, (cumecs), as a line graph 3 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

9. The rising flood water in the river
HYDROSPHERE 2
Rising limb
The rising flood water in the river 3
Rising limb 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

10. Maximum discharge in the river
HYDROSPHERE 2
Peak flow
Peak flow 3
Maximum discharge in the river
Rising limb 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

11. Falling flood water in the river
HYDROSPHERE 2
Peak flow
Recession limb 3
Falling flood water in the river
Rising limb
Recession limb 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

12. HYDROSPHERE 2
Basin lag time
Basin lag time
Peak flow
Time difference between the peak of the rain storm and the peak flow of the river 3
Rising limb
Recession limb 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

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

14. + = Overland flow* + Through flow* Storm Flow 3 2 Discharge (m3/s) 1
HYDROSPHERE 2
Basin lag time
Peak flow (of storm flow)
Overland flow* 3 +
Through flow*
Rising limb +
Overland flow
Recession limb = 2 mm
Storm Flow
Discharge (m3/s) 4
Through flow 1 3 2
Base flow
Hours from start of rain storm
*See next slide for definitions

15. Volume of water reaching the river from surface run off
HYDROSPHERE 2
Through flow
Overland flow
Volume of water reaching the river from surface run off
Volume of water reaching the river through the soil and underlying rock layers

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

17. HYDROSPHERE 2
Analysis

18. Interpretation of Storm Hydrographs
HYDROSPHERE 2
Interpretation of Storm Hydrographs
Basin lag time
You need to refer to:
Peak flow 3
Rising Limb
Rising limb
Overland flow 2
Recession limb mm
Recession Limb
Discharge (m3/s) 4
Through flow 1 3
Lag time 2
Base flow
Rainfall Intensity
Hours from start of rain storm
Peak flow compared to Base flow
Recovery rate, back to Base flow
Copy this list

19. HYDROSPHERE 2
Read and discuss the next few slides as a class.
The following are some theoretical interpretations of influencing factors
BUT…………
when interpreting hydrographs all factors must be considered together !

20. HYDROSPHERE 2
Area
Large basins receive more precipitation than small therefore have larger runoff
Larger size means longer lag time as water has a longer distance to travel to reach the trunk river
Shape
An elongated basin will produce a lower peak flow and longer lag time than a circular one of the same size- can you explain why?

21. HYDROSPHERE 2
Slope
Channel flow can be faster down a steep slope therefore steeper rising limb and shorter lag time
Rock Type
Permeable rocks mean rapid infiltration and little overland flow therefore shallow rising limb, less channel-fill and a long lag time.
Impermeable rocks mean faster channel-fill, steeper rising limb and a shorter lag time.

22. Drainage Density ( number of stream channels)
HYDROSPHERE 2
Soil
Infiltration is generally greater on thick soil, although less porous soils (eg. clay) act as impermeable layers
The more infiltration occurs the longer the lag time and shallower the rising limb
Drainage Density ( number of stream channels)
A higher density will allow rapid overland flow

23. Land Use Tidal Conditions
HYDROSPHERE 2
Land Use
Afforestation - intercepts the precipitation, creating a shallow rising limb and lengthening the lag time
Urbanisation - concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the lag time.
Tidal Conditions
High spring tides can block the normal exit for the water, therefore extending the length of time the river basin takes to return to base flow

24. The effects of urbanisation on lag times.
HYDROSPHERE 2
The effects of urbanisation on lag times.

25. HYDROSPHERE 2
Compare the lag times of the two land uses receiving the same rainstorm!

26. Precipitation and temperature
HYDROSPHERE 2
Precipitation and temperature
Short intense rainstorms can produce rapid overland flow and steep rising limb
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
NOW TRY THE EXERCISE ON THE NEXT SLIDE !

27. Read pages 9 and 10 of the booklet.
HYDROSPHERE 2
Read pages 9 and 10 of the booklet.
Draw up a table like the one shown below- take a whole page, layout in landscape.
Fit the phrases from the handout into the correct place in the table. This should take no more than half a lesson!
Gives reduced discharge gives short lag time gives longer lag time
The items down the side are the headings of the factors in the booklet

28. There is a homework exercise to do for one week today.
HYDROSPHERE 2
There is a homework exercise to do for one week today.
Take copies of the handouts and listen to the instructions carefully! 4

29. Influence each other Change throughout the rivers course
HYDROSPHERE 2
Remember; these influencing factors will-
Influence each other
Change throughout the rivers course 5

30. HYDROSPHERE 2
END OF PART TWO