1. Birla vishvakrma mahavidyalaya
Prepared by : Kachchhi Jateen M.
Enro no. : 140073106010
Batch: 3D2
Hydrology &water resources
engineering
Flood frequency analyses
Guided by : Dr. J. R. Pitroda
2. Introduction
Frequency:
“The frequency is the number of time that a given magnitude
flood may occur in a given period.
Knowledge of the magnitude and probable frequency of recurrence of
floods is necessary for
1) Economical planning and safe design of :
Bridges
Dams
Levees
Culverts,
Sewage disposal plants,
Other structures located along rivers and streams
3. 2) The effective management of
Flood plains
Flood defence scheme
3) To predict the possible flood magnitude over a certain time period.
4) To estimate the frequency with which floods of a certain magnitude may
occur.
Knowledge of flood frequency is necessary also to flood insurance and
flood zoning.
4. Either overdesign or underdesign of structures involves excessive costs
on a long-time basis. The initial cost of a bridge that is designed to pass
only a 5-year flood may be small, but the cost of rebuilding it at an
average 5-year interval would be large. A bridge at the same site which is
built to pass the 100-year flood might be extremely costly. Intermediate
design would provide a bridge at that site for the lowest average annual
cost.
For the flood frequency analysis use past flood data.
The discharge records or past data collected by the Geological Survey and
other Federal, State, and private agencies.
5. Flood frequency analysis uses historical
records of peak flows to produce guidance
about the expected behavior of future
flooding determination of the design flood.
• There are two methods of collecting the
flood peak data.
1) Annual-flood series
2) Partial-duration series
•An annual flood is defined as the highest momentary peak
discharge in a water year. The use of only one flood in each year is the
most frequent objection to the use of annual floods.
6. For the evaluation of flood frequency can be classified into two
approaches:
(1) Statistical approach estimates flood quantiles by applying
probability models.
(2) The design storm method uses the rainfall-runoff model.
• In statistical Method:
1) Probability plotting method
2) Weibull formula
3) Hazen method
4) California method
5) Gumbel’s method
• From this all method Weibull formula is most commonly used.
7. A flood frequency curve can be constructed by plotting a graph of
discharge versus probabilities.
This can easily be accomplished provided you have a data set of annual
peak discharge measured over a number of years.
We need to calculate the chronological order of each of the floods. Start
by ordering our data according to the magnitude of the flood, from
largest to smallest.
Number each of the floods in order, starting with the largest flood as
number "1".
The order of the flood is denoted by the letter "m". If you have 100 years
of records, you will calculate flood orders for m=1, m=2, m=3,....m=10.
8. We find the probabilities of flood by this equation,
P = m / (n+1)
Where , P= probability
m= the number we calculated
n=number of years in the record,
probabilities will go on the x axis; and discharge will go on the y axis.
title of graph “Flood Frequency Curve.”
Plot corresponding discharges and probabilities.
Draw a best fit line between the data set. The resulting line is the flood
frequency curve.
9. The likelihood of a particular event is calculated from its flood frequency
as follows:
F = P × 100
= [ m / (n+1) ] × 100
For example, the chance of a 50-year flood occurring this year is:
1/50 = 0.02
= 0.02 × 100
= 2% chance.
12. We also draw graph between Flood recurrence interval and discharge.
This plotting position is in terms of years.
The objective of the frequency analysis is to determine the magnitude of
the flood which will be equaled or exceeded once in a specified period of
years; this specified period of years is known as the recurrence interval.
“Recurrence intervals" or “Return period " is the time period over
which it is likely that a particular magnitude flood will occur.
Based on considerations of probability, several methods have been used
to compute the recurrence interval.
13. For example, if a town has a flood recurrence interval of 100 years, the
town on average will flood one time every 100 years.
Calculate the recurrence interval, which is the number of times in your
record that a flood of a given magnitude occurred.
Recurrence interval, which is the reciprocal of the annual exceedance
probability.
The formula for recurrence interval is.
T = 1/P
= (n+1)/m
14. Construct graph on the semi logarithmic paper.
Recurrence interval will go on the x axis; and discharge will go on the y
axis. title of graph “Flood Frequency Curve”
Plot corresponding discharges and recurrence intervals.
Draw a best fit line between the data set. The resulting line is the flood
frequency curve.
