DRAINAGE BASIN HYDROGRAPHS

1
DRAINAGE BASIN HYDROGRAPHS
Read page 6 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
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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 7 of the
booklet. When you look at a hydrograph, you
should ask yourself certain questions- see the
next slide.
3
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?
4
Basin lag time
This is what a hydrograph looks like.
Peak flow
3
Rising limb
Overland flow
Recession limb
2
mm
Discharge (m3/s)
Through flow
4
1
3
2
Base flow
0 12 24 36 48 30 72
Hours from start of rain storm
5
The next slides will demonstrate the hydrographs
elements. With each new element there is a term
and its definition to copy.
6
Rainfall shown in mm, as a bar graph
3
2
mm
Discharge (m3/s)
4
1
3
2
0 12 24 36 48 30 72
Hours from start of rain storm
7
Discharge in m3/s, (cumecs), as a line graph
3
2
mm
Discharge (m3/s)
4
1
3
2
0 12 24 36 48 30 72
Hours from start of rain storm
8
Rising limb
The rising of the flood water in the river
3
Rising limb
2
mm
Discharge (m3/s)
4
1
3
2
0 12 24 36 48 30 72
Hours from start of rain storm
9
Peak flow
Peak flow
3
Maximum discharge in the river
Rising limb
2
mm
Discharge (m3/s)
4
1
3
2
0 12 24 36 48 30 72
Hours from start of rain storm
10
Peak flow
Recession limb
3
Falling flood water in the river
Rising limb
Recession limb
2
mm
Discharge (m3/s)
4
1
3
2
0 12 24 36 48 30 72
Hours from start of rain storm
11
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
0 12 24 36 48 30 72
Hours from start of rain storm
12
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
0 12 24 36 48 30 72
Hours from start of rain storm
13
Basin lag time
Peak flow (of storm flow)
Overland flow
3

Through flow

Rising limb
Overland flow

Recession limb
2
Storm Flow
mm
Discharge (m3/s)
Through flow
4
1
3
2
Base flow
0 12 24 36 48 30 72
Hours from start of rain storm
See next slide for definitions
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Through flow
Overland flow
Volume of water reaching the river through the
soil and underlying rock layers
Volume of water reaching the river from surface
run off
15
The completed hydrograph !
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Analysis
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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

0 12 24 36 48 30
72
Hours from start of rain storm

• Peak flow compared to Base flow

• Recovery rate, back to Base flow

Copy this list
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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 !
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The next few slides are for those seriously into
interpreting hydrographs! They are rather
sophisticated and may get you a bit bogged-down.
They will be useful if you get a question that
asks the importance of certain factors in
interpreting a hydrograph.
They are probably best studied at home with no
distractions!
Make of them what you can!
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Area

• Large basins receive more precipitation than
  small therefore have larger runoff

• Larger area 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?

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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.

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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

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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

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The effects of urbanisation on lag times.
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Compare the lag times of the two land uses
receiving the same rainstorm!
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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 !
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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!
The items down the side are the headings of the
factors in the booklet
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There is a homework exercise to do for one week
today. Take copies of the handouts and listen to
the instructions carefully!
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Remember these influencing factors will-

• Influence each other

• Change throughout the rivers course

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END OF PART TWO