L7 hydrograph

Hydrology
CEN 363
Credit : 3.00
Md. Mehedi Hassan Masum
Lecturer
Department of Civil Engineering
Port City International University
Hydrograph
Department of CEN, PCIU

1. Hyetograph
2. Runoff
3. Classification of runoff
4. Hydrograph
5. Hydrograph Vocabulary

 A hyetograph is a graphical representation of the
relationship between the rainfall intensity and
time.
 It is the plot of the rainfall intensity drawn on the
ordinate axis against time on the abscissa axis.
 The hyetograph is a bar diagram.
HYETOGRAPH

 The area under the hyetograph gives the total
rainfall occurred in that period.
 This chart is very useful in representing the
characteristics of storm, and is particularly
important in developing the design storm to
predict extreme floods.
HYETOGRAPH

Runoff is that portion of rainfall that is not
evaporated.
The runoff is defined as the portion of the rainfall
that makes its way towards river or ocean as surface
or subsurface flow.
The discharge flowing in a river is the runoff
from the basin drained by that river.
Runoff is usually expressed as volume per unit
time, the common unit being m3/s or cumec.
Runoff

Runoff
When rainfall starts, some gets caught in retention
storage depressions where it will be evaporated again,
but some enters the ground (infiltration, including some
from detention storage) where it will contribute to the
channel flow at some later time.

 Surface runoff:
Water flows over the land and is first to reach the
streams and rivers which ultimately discharge the
water to the sea.
 Inter flow or subsurface flow:
A portion of rainfall infiltrates into surface soil and
depending upon the geology of basins, runs as subsurface
runoff and reaches the streams and rivers.
 Ground water flow or base flow:
It is that portion of rainfall which after infiltration,
percolates down and joins the ground water reservoir
which is ultimately connected to the ocean.
Classification of Runoff

Hydrograph
 A hydrograph is a graphical plot of discharge of a natural
stream or river versus time.
 It shows variations of discharge with time, at a
particular point of a stream.
 It also shows the time distribution of total runoff
at the point of measurement.
 Discharge is plotted on Y-axis and the
corresponding time is plotted on X-axis.
 Flood analysis and derivation of unit hydrograph a single
peaked hydrograph is required.
 Widely used method of analyzing surface runoff.

Hydrograph
Hydrograph determines the peak flood magnitude of flood
for the design of hydraulic structures i.e. a dam, spilway,
bridge, culvert, etc. Hydrographs are also described in terms
of the following time characteristics :
1) Time to Peak, tp:
Time from the beginning of the rising limb to the
occurrence of the peak discharge.
The time to peak is largely determined by drainage
characteristics such as drainage density, slope, channel
roughness, and soil infiltration characteristics. Rainfall
distribution in space also affects the time to peak.

Hydrograph
2.) Time of Concentration, tc:
Time required for water to travel from the most
hydraulically remote point in the basin to the basin
outlet. For rainfall events of very long duration, the
time of concentration is associated with the time
required for the system to achieve the maximum or
equilibrium discharge.
The drainage characteristics of length and slope, together
with the hydraulic characteristics of the flow paths,
determine the time of concentration.

Hydrograph
3.) Lag Time, tl:
Time between the center of mass of the effective rainfall
hyetograph and the center of mass of the direct runoff
hydrograph.
The basin lag is an important concept in linear modeling
of basin response. The lag time is a parameter that appears
often in theoretical and conceptual models of basin
behavior. However, it is sometimes difficult to measure in
real world situations. Many empirical equations have been
proposed in the literature. The simplest of these equations
computes the basin lag as a power function of the basin
area.
.

©Microsoft Word clipart
ComponentsofHydrograph

Component of Hydrograph

• Rising limb: The rising limb of hydro graph, also
known as concentration curve, reflects a prolonged
increase in discharge from a catchment area, typically in
response to a rainfall event
• Recession (or falling) limb: The recession limb extends
from the peak flow rate onward. The end of stormflow
(aka quickflow or direct runoff) and the return to
groundwater-derived flow (base flow) is often taken as
the point of inflection of the recession limb. The
recession limb represents the withdrawal of water from
the storage built up in the basin during the earlier phases
of the hydrograph.

• Peak discharge: the highest point on the hydro
graph when the rate of discharge is greatest
• Lag time: the time interval from the center of mass
of rainfall excess to the peak of the resulting
hydrograph
• Time to peak: time interval from the start of the
resulting hydro graph
• Discharge: the rate of flow (volume per unit time)
passing a specific location in a river or other
channel

Construction of Hydrograph
Construction
Of
Storm (flood)
Hydrographs

Hydrograph
Record of River Discharge (the level of water flowing down a
river channel) over a period of time, they show how certain
rivers respond to a rainstorm.
River Discharge (the level of water flowing down
a river) (is calculated)
= cross sectional area X rivers mean (average)
velocity
(at a particular point in its course)
Storm Hydrographs
Show the change in discharge caused by a period of rainfall

Why Construct & Analyze
Hydrographs ?
• To find out discharge patterns
of a particular drainage basin
Help predict flooding events,
therefore influence implementation
of flood prevention measures

0 12 24 36 48 30 72
3
Hours from start of rain storm
2
1
Discharge(m3/s)
The discharge
Discharge of
the river is
measured in
cumecs - this
stands for
cubic metres
per second

0
12 24 36 48 30 72
3
2
mm
1
Discharge(m3/s)
Rainfall shown
in mm, as a bar
graph

0
12 24 36 48 30 72
3
2
mm
1
Discharge(m3/s)
Discharge in
m3/s, as a line
graph

0
12 24 36 48 30 72
3
2
1
Discharge(m3/s)
mm
Rising limb
The normal
(base) flow of
the river
starts to rise
when run-off,
ground and soil
water reaches
the river.

0 12 24 36 48 30 72
3
2
1
Discharge(m3/s)
mm
Peak flow
Peak flow
Maximum
discharge in
the river, the
time when the
river reaches
its highest
flow

0 12 24 36 48 30 72
3
2
1
Discharge(m3/s)
mm
Peak flow
Recession limb
shows that
water is still
reaching the
river but in
decreasing
amounts

0 12 24 36 48 30 72
3
2
mm
1
Discharge(m3/s)
Basin lag time
Peak flow
Basin lag time
The time it
takes for the
water to find
its way to the
river

0 12 24 36 48 30 72
3
2
mm
1
Base flow
Discharge(m3/s)
Basin lag time
Peak flow
Base flow
Normal
discharge of
the river

0 12 24 36 48 30 72
3
2
mm
Through flow
1
Base flow
Discharge(m3/s)
Overland
flow
Basin lag time
Peak flow
Overland flow
+
Through flow
=
Storm Flow

Factors

Some Factors influencing
Storm Hydrographs
• Area
• Slope
• Rock Type
• Land Use
• Soil
Precipitation / Temp

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
Slope Land Use Precipitation / Temp
Rock Type Soil

Slope

Channel flow can be faster down a steep slope
therefore steeper rising limb and shorter lag time
Area
Rock Type Soil

Rock Type

Permeable rocks mean rapid infiltration and little
overland flow therefore shallow rising limb
Area
Rock Type Soil

Soil
Infiltration is generally greater on thick soil

The more infiltration occurs the longer the lag
time and shallower the rising limb
Area
Rock Type Soil

Land Use

Urbanisation - concrete and tarmac form
impermeable surfaces, creating a steep rising limb
and shortening the time lag

In wooded areas, trees intercept/absorb the
precipitation, creating a shallow rising limb and
lengthening the time lag
Area
Rock Type Soil

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 Precipitation / Temp
Soil Slope Land Use

Remember!
These influencing factors will:
Influence each other
Change throughout the rivers
course

Separation of base flow from direct
run off

 1) Straight line method :
Figure shows the two points Aand C which mark
the beginning and end of the direct run off.
 2)Two tangent method:
This is the most widely used method in whic
the base flow is separated from direct runoff
by two lines.

 The first line ABis obtained by extending the base flow
curve existing prior to the commencement of the surface
runoff, till it intersects a vertical line, drawn from the
peak.
 Point Band C are joined by straight line.
 Empirical equation to locate the point C
𝑁 = 0.83𝐴0.2
Where
N =time interval in days from peak
A=area of drainage basin (𝐾𝑚2)

researchgate.net/profile/Mehedi_Masum
For further query feel free to contact:
mehedi.ce.cuet@gmail.com

L7 hydrograph

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L7 hydrograph