1 Hydrographs Response of a given catchment to rainfall input. A hydrograph is a graph showing discharge versus time. Hydrograph Record of River Discharge over a period of time River Discharge = cross sectional area rivers mean (average) velocity X (at a particular point in its course) Storm Hydrographs Show the change in discharge caused by a period of rainfall Hydrograph :applications To find out discharge patterns of a particular drainage basin Help predict flooding events, therefore used for flood prevention measures 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Base flow Through flow Overland flow Basin lag time mm 4 3 2 Peak flow 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s)
12
Embed
Hydrographs Hydrograph River Discharge Storm Hydrographs · Hydrographs Response of a given catchment to rainfall input. A hydrograph is a graph showing discharge versus time. Hydrograph
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1
Hydrographs
Response of a given catchment to rainfall input.
A hydrograph is a graph showing discharge versus time.
Hydrograph
Record of River Discharge over a period of time
River Discharge
= cross sectional area rivers mean (average) velocity
X
(at a particular point in its course)
Storm Hydrographs
Show the change in discharge caused by a period of rainfall
Hydrograph :applications
To find out discharge patterns of a particular drainage basin
Help predict flooding events, therefore used for flood prevention measures
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (
m3/s
)
Base flow
Through flow
Overland flow
Basin lag time
mm
4
3
2
Peak flow
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (
m3/s
)
2
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (
m3/s
)
mm
4
3
2
Rainfall shown in mm, as a bar graph
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (
m3/s
)
mm
4
3
2
Discharge in m3/s, as a line graph
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (
m3/s
)
mm
4
3
2
The rising flood water in
the river
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (
m3/s
)
mm
4
3
2
Peak flow
Peak flow
Maximum discharge in the river
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (
m3/s
)
mm
4
3
2
Peak flow Falling flood water in the
river
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (
m3/s
)
Basin lag time
mm
4
3
2
Peak flow
Basin lag time
Time difference
between the peak of the rain storm
and the peak flow of the
river
3
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (
m3/s
)
Base flow
Basin lag time
mm
4
3
2
Peak flow
Base flow
Normal discharge of the river
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (
m3/s
)
Base flow
Through flow
Overland flow
Basin lag time
mm
4
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
Storm Hydrographs
•Rainfall Intensity
•Rising Limb
•Recession Limb
•Lag time
•Peak flow compared to Base flow •Recovery rate, back to Base flow
Basin lag time
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (
m3/s
)
Base flow
Through flow
Overland flow
mm
4
3
2
Peak flow
Time Characteristics of hydrograph
1. Time to peak
– • From beginning of rising limb to peak discharge
– • Function of basin characteristics
– • Drainage density, slope channel size, roughness and soil
infiltration characteristics
2. Time of concentration
– • Time required for the farthermost rain to reach the outlet
3. Lag time or Basin lag time
– • Between centre of mass of rainfall and runoff hydrograph mass
– • Since it is very difficult to find the centre then it is the time
between centre of mass of effective rainfall to peak discharge
4. Duration of rainfall
5. Base time of hydrographs
Storm rainfall:
Hydrograph :
0
10
20
30
40
50
60
70
80
90
100
1 2 3 4 5 6 7 8 9
Series1
14 15 16 17 18 19 20 21 22
?
Precipitation and river response
4
Understanding hydrological
responses
Peak discharge?
Lag time?
Rate of recession?
Factors affecting flood hydrographs
Channel characteristics
– Cross section,
– roughness
– Storage capacity.
Infiltration characteristic
– (a) Land use and cover
– (b) Soil type and geological condition
– (c) Lakes, swamps and other storage
Climatic factors
– (a) Shape
– (b) Size
– ( c) slope
– (d) Nature of the valley
– (e) Elevation
– (f) Drainage density
Physiographic Factors
Factors affecting flood hydrographs
Storm characteristics
– precipitation,
– Intensity,
– duration,
– magnitude and
– movement of storm.
Initial loss
Evapotranspiration
Climatic Factors
Factors influencing Storm Hydrographs
• Area
• Shape
• Slope
• Rock Type
• Soil
• Land Use
• Drainage Density
• Precipitation / Temp
• Tidal Conditions
Stream order and flood response Regional
climate
and flood
response
5
Geological substrate and flood
response Basin geometry and flood
response
circular
elonga
ted
effect of
storm
path?
Land use and flood response
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 Soil Precipitation / 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
Area Rock Type Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions
Slope
Channel flow can be faster down a steep slope therefore steeper rising limb and shorter lag time
Area Rock Type Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions
6
Rock Type
Permeable rocks mean rapid infiltration and little overland flow therefore shallow rising limb
Area Rock Type Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions
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
Area Rock Type Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions
Land Use
Urbanisation - concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the time lag
Afforestation - intercepts the precipitation, creating a shallow rising limb and lengthening the time lag
Area Rock Type Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions
Drainage Density
A higher density will allow rapid overland flow
Area Rock Type Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions
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
Area Rock Type Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions
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
Area Rock Type Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions
7
Base flow separation
To draw surface runoff hydrograph, it is
required to separate base flow
Methods
1. Straight line method
N=0.83A0.2 Line AB
2. Line ACB
3. Line AFE
Effective rainfall or Rainfall Excess
The direct runoff,
(initial losses and
infiltration losses are
separated).
Called as effective
rainfall hyetograph or
hyetograph of rainfall
excess
It is the rainfall which
contributed for direct
runoff
Example
A storm over a catchment of area 15 sq.km had a 12
hrs duration. The mass curve of rainfall is given. If φ
index is 0.3 cm/h, determine the effective rainfall
hyetograph and the volume of direct runoff from the