Construction And Analysis Of Hydrographs. 0 12 24 36 48 60 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Base flow Through flow Overland

Construction

And

Analysis

Of

Hydrographs

0 12 24 36 48 60 72

Hours from start of rain storm

3

2

1

Dis

charg

e (

m3/s

)

Base flow

Through flow

Overland flowR

isin

g

limb

Recession

limb

Basin lag time

mm4

3

2

Peak flow

Floo

d Hyd

rogr

aph

HydrographRecord of River Discharge over a period of timeRiver Discharge= cross sectional area

rivers mean (average) velocity

X

(at a particular point in its course)

Storm HydrographsShow the change in discharge caused by a period of rainfall

WhyConstruct & Analyse

Hydrographs ? To find out discharge patterns

ofa particular drainage basin

Help predict flooding events,therefore influence implementation of flood prevention measures

Construction

Of Storm (flood) Hydrographs

0 12 24 36 48 60 72


3

2

1

Dis

charg

e (

m3/s

)

Base flow

Through flow

Overland flowR

isin

g

limb

Recession

limb

Basin lag time

mm4

3

2

Peak flow

Floo

d Hyd

rogr

aph

0 12 24 36 48 60 72


3

2

1

Dis

charg

e (

m3/s

)

0 12 24 36 48 60 72


3

2

1

Dis

charg

e (

m3/s

)

mm4

3

2

Rainfall shown in

mm, as a bar graph

0 12 24 36 48 60 72


3

2

1

Dis

charg

e (

m3/s

)

mm4

3

2

Discharge in m3/s, as a line graph

0 12 24 36 48 60 72


3

2

1

Dis

charg

e (

m3/s

)

Ris

ing

lim

b

mm4

3

2

Rising limbThe rising

flood water in the river

0 12 24 36 48 60 72


3

2

1

Dis

charg

e (

m3/s

)

Ris

ing

limb

mm4

3

2

Peak flow

Peak flowMaximum

discharge in the river

0 12 24 36 48 60 72


3

2

1

Dis

charg

e (

m3/s

)

Ris

ing

limb

Recession

limb

mm4

3

2

Peak flow

Recession limbFalling flood water in the

river

0 12 24 36 48 60 72


3

2

1

Dis

charg

e (

m3/s

)

Ris

ing

limb

Recession

limb

Basin lag time

mm4

3

2

Peak flow

Basin lag timeTime

difference between

the peak of the rain

storm and the peak

flow of the river

0 12 24 36 48 60 72


3

2

1

Dis

charg

e (

m3/s

)

Base flow

Ris

ing

limb

Recession

limb

Basin lag time

mm4

3

2

Peak flow

Base flowNormal

discharge of the river

0 12 24 36 48 60 72


3

2

1

Dis

charg

e (

m3/s

)

Base flow

Through flow

Overland flowR

isin

g

limb

Recession

limb

Basin lag time

mm4

3

2

Peak flow

Overland flow

Through flow

+

=Storm Flow

Volume of water reaching the river

from surface run off

Overland flow

Through flow

Volume of water reaching the river

through the soil and underlying rock

layers

0 12 24 36 48 30 72


3

2

1

Dis

charg

e (

m3/s

)

Base flow

Through flow

Overland flowR

isin

g

limb

Recession

limb

Basin lag time

mm4

3

2

Peak flow

Analysis

Factors influencingStorm Hydrographs

• Area

• Shape

• Slope

• Rock Type

• Soil

• Land Use

• Drainage Density

• Precipitation / Temp

• Tidal Conditions

Interpretation of Storm Hydrographs

•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


3

2

1

Dis

charg

e (

m3/s

)

Base flow

Through flow

Overland flowR

isin

g

limb

Recession limb

mm

4

3

2

Peak flow

Following, are some theoretical

interpretations of influencing factors

BUT…………

When interpreting hydrographs all factors

must be considered together !

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

Area Rock Type Drainage Density

Shape SoilPrecipitation / Temp

Slope Land Use Tidal Conditions

Shape

Elongated basin will produce a lower peak flow and longer lag time than a circular one of the same size




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




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




Land Use

Urbanisation - concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the time lag. Steeper recession limb. WHY?

RISING LIMB / LAG TIME:

Shallower in rural areas – increased interception

Vegetation intercepts and stores water slowing it’s passage into the river system

In Urban areas concrete, tarmac etc. channels water quickly into drains and straight into river / sewage system

Land Use


STORAGE:

Falling limb will be steeper on an urban hydrograph due to lack of infiltration, storage

In rural areas water will continue to flow into the river for many hours from underground flow and through flow

The return to base flow is much slower in rural areas

Land Use


Afforestation - intercepts the precipitation, creating a shallow rising limb and lengthening the time lag. Shallower recession limb. WHY?




Drainage Density

A higher density will allow rapid overland flow




Precipitation & Temperature

Short intense rainstorms can produce rapid overland flow and steep rising limb

If there have been extreme temperatures, the ground can be hard (either baked or frozen) causing rapid surface run off 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




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




Remember these influencing factors will:

Influence each other

Change throughout the rivers course

ANALYSING HYDROGRAPHS

1. Discharge does not increase immediately (only a small amount of precipitation falls directly into the river)

2. Water reaches the river from overland flow / surface run-off

3. This is then supplemented with water from throughflow

4. Rising limb shows overland flow

5. Recession limb indicates there is still water in the system – e.g. throughflow

6. Rivers with a short lag time and high discharge are most likely to flood

7. Two basins which receive the same amount of rain can have very different flood hydrographs

ANALYSING HYDROGRAPHS

1. ____ ____ will be higher in larger basins

2. If slopes are steep, ______ will be lower, therefore peak flow will be greater

3. If basin is flatter, there will be more infiltration resulting in

peaks

4. The _______ the slope – the shorter the lag time

A combined atmosphere / hydrosphere question can appear:

• ITCZ

• Hydrographs

Peak Flow

infiltration

lower

steeper

FLOOD HYDROGRAPHS

TASK:

Using pages 43 55

Answer questions 1 5 on page 68-69 under the heading ‘Flows and Flood Hydrographs’

PAST PAPER QUESTIONS

2006 – Hydrosphere

Remember to double marks when looking pre-2008

Documents

Construction And Analysis Of Hydrographs. 0 12 24 36 48 60 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Base flow Through flow Overland